Getting sedimentary grains moving in a flowing medium
Formation of granular sedimentary layers and many of the bedforms within requires movement of grains from one location to some other location, as bedload or suspended load. Initiation and maintenance of grain movement in a fluid (air or water) requires that the shear stress derived from the flowing fluid must overcome the opposing gravitational, viscous, and friction forces acting on each grain. The critical or threshold shear stress is that where grain movement begins.
An understanding of the conditions required to initiate grain movement is important for engineering problems involving fluid flow, such as the stability of bridge abutments in rivers, and for sedimentological interpretations of ancient environments. Questions like “What flow conditions were required to move the range of clast sizes in a pebbly sandstone” are central to sedimentary facies analysis.
A diagrammatic representation of the various forces acting on non-cohesive grains in a flowing fluid (air, water). The small black arrows around the grain boundaries represent viscous shear. A typical velocity profile (left) shows how bulk flow velocity increases with distance from the bed (blue arrows); the resulting pressure gradient is responsible for the lift forces acting on each grain. Lift forces may also develop from turbulence. The velocity profile shows the important differences in fluid flow at the sediment-water interface compared with that above the bed. Relatively continuous grain motion along a granular bed will take place when the drag and lift components exceed the opposing gravitational, viscous, and grain contact forces.
The definition of initial grain movement is not as straight forward as it first appears. Does the threshold shear stress (or critical flow velocity) for grains on a sediment bed correspond to an initial nudge, or does the grain need to travel one or more revolutions of its circumference? In most natural sediment beds there is a range of grain sizes so which grain threshold stress applies – that responsible for movement of the coarsest, the mean, or median grain size? John Southard has given us an excellent summary of this problem.
Two graphical representations of these threshold conditions stand out; the iconic Shields and Hjulström diagrams. Both are empirical constructions derived from experimental data; both were developed in the 1930s – the original and modified forms of these diagrams are still used widely.
Shields diagram
Albert Frank Shields (1908 – 1974) was an American engineer whose experiments on the transport of granular sediment led to the formulation of the eponymous Shields Parameter (Θ) that expresses the shear stress (τ) required to initiate grain movement. The flume experiments were actually conducted in 1930s Germany and published in 1936 (Application of similarity principles and turbulence research to bed-load movement). The Shields diagram plots (Θ) against the Reynolds Number that describes the hydraulic conditions across the grain boundary. The original diagram specifies two fundamental domains: One of grain movement, the other where the threshold shear stress is not high enough to initiate movement. His experiments used grains of different densities – amber, lignite, granite, barite, and sand.
A modified version of Shields original 1936 graph (his Figure 6), showing the data envelope (grey) about his threshold curve (solid black line); the envelope encompasses grains of different densities. The threshold shear stress and boundary (grain) Reynolds Number are dimensionless. Shields added annotation to his original diagram – the bedforms he observed, the beginning of grain saltation, and bed erosion (he called it abrasion). The extended threshold curve at low Reynolds Numbers (dashed line), plus the turbulence boundaries were added by later workers (see Southard, 2021).
The Shields Parameter Θ can be written as:
Θ = τc.D2/(ρs – ρw)gD3
where τc = critical stress at the grain boundary; D = mean grain diameter, and ρ the density of the solid grains and water respectively. The value (ρs – ρw)g is the submerged specific weight of a grain. The numerator τc.D2 is proportional to the fluid force acting on a grain; the denominator (ρs – ρw)gD3 is proportional to the weight of the grains. Θ is dimensionless because the shear stress (i.e., the pressure exerted on a grain) can be written as τ = ρw.gz (also the general form of the equation used to calculate hydrostatic and lithostatic pressures), where z is a characteristic depth or thickness and has the same units as grain diameter.
The Shields diagram plots (Θ) against the grain Reynolds Number. Use of the Reynolds Number (Re) is important because it relates inertial forces and dynamic fluid viscosity to the two fundamental types of flow – laminar and turbulent.
Re = ρw VD/μ
where V the mean velocity reflects shear rate and inertia forces, and μ is fluid viscosity that measures the resistance to shear. Re is dimensionless. In Reynolds’ original experiments D was the flow tube diameter; in the Shields diagram it corresponds to mean grain diameter. Thus, at low Re values viscous forces suppress turbulence and the flow is laminar. At high Re values inertial forces exceed viscous forces and flow is turbulent.
There have been several replottings, modifications, and recastings of Shields’ 1936 diagram. R.A. Bagnold (PDF available) published a US Geological Survey Report in 1966 where he considered the initiation of grain movement and the maintenance of a suspension load for finer-grained particles that incorporates Shields’ criteria. His entrainment diagram (his Figure 8) plots the non-dimensional threshold shear stress against (dimensional) grain diameter.
Bagnold’s plot of non-dimensional threshold shear stress with actual grain diameter (modified here from his Figure 8), showing the domains of no grain motion, grain movement as part of the bedload, and the theoretical limits for particle suspension. The grey band that defines grain movement captures the general spread of experimental data. The lower bounding line is Shields threshold curve; the upper bounding line is Bagnold’s calculated threshold curve.
Recasting and replotting of the Shields criteria by M.C. Miller et al., 1977 is another frequently cited version. These authors extended the grain size Reynolds numbers by more than two orders of magnitude beyond those used by Shields. The non-dimensional variables are the same as in Shields’ original diagram. Their graph shows the spread of data (from various sources).
Miller et al., (1977) replotting of Shields’ non-dimensional threshold shear stress and the non-dimensional Reynolds Number over Re values about 2.5 orders of magnitude higher than those used by Shields. The authors used experimental data from multiple sources. The spread of data is indicated by the threshold envelope.
Hjulström’s diagram
Filip Hjulström’s (1902-1982) diagram is particularly useful for sedimentologists who prefer to think in terms of measurable flow velocities rather than shear stresses or shear velocities. It was published in 1939. It is still a popular reference. His diagram specifies three domains: erosion (net loss of sediment), deposition (net gain in sediment), and transport that may involve components of erosion or deposition.
[Hjulström, F. (1939). Transportation of detritus by moving water: Part 1. Transportation. In P. D. Trask (Ed.), Recent marine sediments. A Symposium: Tulsa, Oklahoma (pp. 5–31). Tulsa, OK: AAPG.]
Hjulström was a Swedish geographer who conducted experiments on flow and sediment transport for his PhD on the River Fyris. His experiments involved measuring the mean flow velocity at which grains of a specified diameter began to move; the experiments were conducted in a canal with water depth of one metre. Hjulström understood the problems of what constitutes a representative flow velocity – it can be taken as the surface flow velocity, the mean velocity over a specified depth, or more appropriately for this particular problem, the velocity at the grain boundary but this was difficult to measure (at least back in the 1939s). He chose mean velocity because it is relatively easy to measure. His experiments included a much greater range of grain sizes than those conducted by Shields – clay through cobble sizes.
A typical Hjulström plot delineating the principal domains of deposition, transport, and erosion. Modified from Nichols, 2009, Figure 4.5. I have added the region where particle cohesion influences erosion and transport. The domain boundaries, although drawn as solid lines, are in fact more fuzzy in their placement because of the spread of experimental data, and subtle but important differences among experimental conditions such as flow depths and, velocity profiles.
The boundaries that describe each domain are based on flow depths of one metre, and average grain density of 2.5 – 2.6 g/cm3. The boundaries will shift for different water depths and grain densities. For example, domain boundaries will tend to move upwards for carbonate-dominated sediment (grain density 2.7 – 2.8 g/cm3) because greater shear stress is required to initiate grain movement. The boundary between bedload and suspension load transport will be diffuse depending on the textural and mineralogical character of the sediment, as well as the flow characteristics (e.g., compare the hydraulic behaviour of mica flakes to that of spherical quartz grains).
The lower curve separates depositional flows from transport flows. The upper V-shaped curve reflects the increasing cohesion of finer grained particles, particularly the clay and fine silt fractions of muds. Cohesion in this case is primarily a result of the large surface area relative to particle size, plus the residual electrical charges on clay mineral surfaces. Cohesive forces are also responsible for the elevated flow velocities required to erode consolidated muds. In fact, erosion of consolidated muds commonly produces chunks, or mud rip-ups rather than individual clay-silt particles.
Reading this graph is relatively straight forward as long as you are aware of the caveats such as grain shape and density, and whether flow is laminar or turbulent. For example, for any grain diameter, follow the vertical axis to determine the flow dynamics. It is also a useful diagram to compare the flow requirements for maximum clast sizes in a deposit with the requirements for mean grain size of that deposit.
Using Hjulström’s diagram to determine ball-park flow velocities for crossbedded, pebbly, glaciofluvial sandstone, where grain sizes range from medium sand (0.25 – 0.50 mm) to the upper size limit for pebbles (64 mm). The range of flow velocities for each size range is relatively broad, but the overall magnitude of velocities needed to transport these sediments ranges over about 2 orders of magnitude, from about 0.02 m/s to 3 m/s. Our paleoenvironmental interpretation of this deposit needs to account for this range of flow velocities, and for the rapid changes in flow energy from one crossbed to the next. For scale, the red dot is 26mm.
Modern Scleractinian coral reefs: One of the most diverse ecosystems on Earth. Descriptions and credits posted in the following text.
A description of the common skeletal elements of hard, stony corals
Hard corals are essential components of nearly all limestones. Colonial corals create the foundations for tropical reef associations (lagoon to deep forereef). They are also an important component of cool-temperate limestones, primarily as bioclasts of solitary corals.
Corals belong to the phylum Cnidaria, a group of animals that possess stinging cells, or nematocysts. The phylum includes hydrozoans like the siphonophores (Blue Bottles, Portuguese Man O War), true jellyfish and Sea Wasps (Box Jellyfish), a group of octocorals that includes soft corals, and the Subclass Hexacorallia that contains sea anemone taxa and the more familiar hard or stony Tabulate, Rugose, and Scleractinian corals. The latter three orders comprise the dominant species in the fossil record, as colonial reef-builders and solitary organisms. Modern reefs are dominated by the stony Scleractinians.
Modern Scleractinian corals, particularly the colonial reef-builders, depend for their survival on symbiotic, photosynthetic algae, or zooxanthellae. Thus, both algae and coral polyps thrive in water depths 30 m and less where solar energy in the photic zone is about 40% – 16% of incident light. Light intensity at these depths also depends on the amount of suspended sediment in the water.
Reef dwelling corals also do best in water temperatures at the high end of the 16o-32o C range, which generally restricts extensive reef development to the tropical latitudes. A few solitary and colonial Scleractinian species grow in colder waters, and at greater depths where the symbiotic algae do not rely on photosynthesis. Whether the extinct Rugose and Tabulate corals depended on this level of algal symbiosis is still debated.
All hard corals are marine. They are composed of calcium carbonate – Rugose and Tabulate corals of calcite, and nearly all scleractinian corals of aragonite although a small number of species have layered aragonite-calcite skeletons (Stolarski et al., 2020). Scleractinian corals are subject to recrystallization to low magnesium calcite during the early stages of diagenesis and burial.
Corals first appeared in the Early Cambrian but it wasn’t until the Ordovician that hard corals became major contributors to reefs (along with sponges, bryozoa, and other invertebrates), replacing the microbial stromatolite buildups that persisted through the entire Proterozoic. Tabulate corals appeared in the Early Ordovician, the Rugosa a bit later in the Mid-Ordovician. Both orders became extinct during the latest Permian mass die-off(along with many other invertebrates). The scleractinian corals first appeared in the Middle Triassic, rapidly becoming the dominant reef-builders we see today.
Skeletal morphology
The following skeletal structures are common to all or some of the three main hexacoral groups. Most of the external structures can be seen in hand specimens. Internal structures such as septa, tabulae, and dissepiments may be seen on broken or corroded specimens, on polished rock surfaces, or in thin section. Many of the photos were kindly provided by Annette Lokier, University of Derby.
Calice, calyx: The uppermost cup- or chalice-like depression of a corallum or corallite that provides space for the living polyp. The base of a calice is commonly bound by a tabula and dissepiments.
Coenosteum: (plural Coenostea) The porous calcareous structure that connects corallites in all three groups of colonial corals. It is part of the corallum in these colonies. In coenosteum-dominant species the corallites are spread out, connected by broad coenostea – the opposite applies to corallite-dominant species. The coenosteum structure is sponge-like and constructed of calcareous tubes, spines, the extensions of costae, and networks of small plates.
Columella: Present in Rugose and Scleractinian corals but not the Tabulates. In Rugose corals it is a central pillar-like structure from which the septa radiate towards the wall. In the Scleractinia, the columella, if present, is a mesh of tooth-like structures extending from the edge of the septa.
Corallum and corallite: There is potential for some confusion with these terms. Corallum (plural Coralla) refers to the whole skeletal structure of solitary corals (that are constructed by a single polyp). It is discoid, tube-, vase-, or cone-shaped. The term also applies to the entire structure of colonial corals – this makes sense if we consider the colony as a single community of polyps. Individual structures within a colony are called corallites and each corallite contains a single polyp.
The Miocene colonial Scleractinian coral Septastrea, that has broad coenostea between the corallites. Corallite theca (walls) are slightly raised above each coenosteum; each corallite has a cup-shaped calice with 12 primary septa. The entire colony is the corallum. Original photo courtesy of Annette Lokier, Derby University.
Costae: Rib-like structures aligned longitudinally along the epitheca of solitary corals; they are the extension of septa through the coral wall.
Dissepiments: Cup- or dome-shaped plates that grow between septa and separate the calice of a growing polyp from earlier growth stages. It has a similar function to the flatter tabulae. Both dissepiments and tabulae can occur together, with the former developed around the corallite periphery.
Transverse sections through a solitary Rugose corallum. Left: A sketch based on a sectioned corallum (not the one shown on the right), showing primary and secondary septa and dissepiments around the corallum periphery. Col = columella. Modified from Denayer and Webb,2015, Fig. 3. Right: A polished transverse section through Zaphrentites, showing the radial septa, the complex arrangement of peripheral dissepiments, and the central tabulae. Original photo courtesy of Annette Lokier, Derby University.
Epitheca: The thin, calcareous, outer wall of some solitary corals.
Fossula: (Plural Fossulae) Narrow gaps between the four quadrants of septa in Rugose corals; they are not always discernible, particularly if preservation is poor or there has been significant calcite recrystallization or neomorphism.
Growth lines: These are most prominent on solitary Rugose corals where they are expressed as raised concentric ridges on the epitheca. Their wrinkled appearance gave rise to the name of this order – from the Latin ruga. It is hypothesized that each line represents a day’s growth of the corallum. Statistical analysis has been used to estimate the lengths of solar years during the Paleozoic (e.g., Berkowski and Belka, 2008).
Horn corals: The common name for solitary Rugose corals having a horn-shaped corallum. They were the most common taxa of the Paleozoic Rugose order.
Septa: Vertical plates that radiate from the wall to the centre of a coral tube. They are secreted by the polyp as it grows from one calice to the next. In detail, septa may be laminated, perforated or spinose. Septa are prominent in Rugose and Scleractinian corals, but either absent or weakly developed in the Tabulates. In Scleractinian corals the septa are arranged in 6-fold symmetry, with primary septa the thickest and largest, and higher-order septa in sets of 12, 24 and so on in between. In Rugose corals the septa are arranged in quadrants separated by narrow gaps, or fossula; fossula are indistinct or poorly preserved in many species. The arrangement of septa in Rugose corals creates a bilateral symmetry.
Left: Diagrammatic representation of Scleractinian septa. Primary septa are the largest; 4th order septa the smallest and thinnest. Right: The arrangement of Scleractinian septa beautifully developed in the genus Fungia. There are 12 primary septa, with 2nd, 3rd, and 4th order septal plates inserted between. Note the ragged edges of the septal plates. Original photo courtesy of Annette Lokier, Derby University.
Left: Diagrams of two different septal arrangements in solitary Rugose corals. The example on the left is a more ideal representation that exemplifies the bilateral symmetry. Center: A transverse section showing a common septal arrangement in Rugose coral specimens. Col = columella. The dissepiments are also shown (orange). Right: A polished, transverse section through Dibunophyllum (Carboniferous, Derbyshire) revealing the septa and dissepiments. Original photo courtesy of Annette Lokier, Derby University.
Tabulae: (singular tabula) Flat or slightly curved horizontal plates secreted within the corallite or corallum by the growing polyp as it moves to a new calice – the tabula separates the polyp from the rest of the coral tube. Present in Rugose and a defining characteristic of Tabulate corals; they are not found in the Scleractinians. Cf. Dissepiments.
Left: A diagrammatic, longitudinal cross-section of a typical Rugose Horn coral showing the arrangement of dissepiments and tabulae around a central columella. Modified from Denayer and Webb,2015, Fig. 3. Right: A polished longitudinal section through the Silurian Rugose genus Ketophyllum. There has been some recrystallization of calcite and fracturing, but the central tabulae are still visible. Original photo courtesy of Annette Lokier, Derby University.
Theca: The solid calcareous outer wall of a corallite or solitary corallum, thickened in some species and thin or compressed in others. The theca may be covered by an epitheca.
Distinguishing among Scleractinia, Rugose, and Tabulate corals
Distinguishing Rugose from Tabulate corals is reasonably straight forward based on the presence of septa. However, Rugose corals bear a superficial resemblance to the Scleractinians, particularly with their septa. One could argue that if a specimen came from post-Permian rocks then it must be Scleractinian – this is NOT a practice that should be encouraged (for any fossil group). Identification of the taxonomic status of fossils should be based on morphological criteria and not a presumption of age or stratigraphic context.
Comparison of Scleractinian, Rugose, and Tabulate coral skeletal morphologies. As always, there are exceptions to these criteria. Information from multiple sources including: The Paleontological Society’s Digital Atlas of Ancient Life and Corals of the World.
Scleractinia
The defining characteristic of Scleractinian, or stony corals is the 6-fold radial symmetry of their septa. The primary septa (6 or 12 of them; some species have 8 or 10 primary septa) are the largest and thickest plates that radiate from the centre of the calice, commonly from a columella. A succession of smaller, higher-order septa are inserted between these plates: 2nd, 3rd, and 4th order respectively.
Solitary Scleractinian corals have flat, dome-, discoid-, or vase-shaped coralla; the Fungia specimen shown above is discoid. Successive stages of polyp growth are separated by dissepiments. Colonial corals display an amazing variety of structures depending on species and environmental conditions such as wave and current energy. Common structures are branched, columnar, encrusting, foliaceous (leaf-like), laminar or table-like, and massive or bulbous corals. Some of these forms are illustrated below. Zooxanthellate colonial corals (i.e., those requiring symbiotic, photosynthetic algae) are the primary reef builders in post-Permian strata.
Thriving Scleractinian reef corals are some of the most beautiful biological structures to have ever evolved. These examples show some of the basic growth forms: Left: Branching Acropora (center), columnar Porites (lower right), white encrusters (bottom center), and laminar or table corals – Acropora (left). Image Credit: National Marine Sanctuary of American Samoa. Top right: A bulbous cactus coral (?Isophyllia) surrounded by small encrusting corals, bryozoa, and the aragonite shedding alga Halimeda. Bottom right: A community of branching Porites, foliaceous corals (?Agaricia), encrusting forms, and a couple of Vase sponges (center). Both images from Palancar Reef, Cozumel National Park, courtesy Charlie Kerans.
A delicately branched, colonial Acropora. Individual corallites are 2 mm diameter. Original photo courtesy of Annette Lokier, Derby University.
A recent, colonial, bulbous Alveopora. Corallites are 2-3 mm diameter. The coenosteum is perforated. Although not visible here, the septa are porous, consisting of a meshwork of spines that connect at the columella. Original photo courtesy of Annette Lokier, Derby University.
Rugosa
This extinct order is generally known for its horn-shaped solitary coralla (hence the name ‘Horn coral’), although colonial forms were also important as Paleozoic reef-builders. The overall length of solitary forms ranged from a few centimetres to one metre. The Rugosa also grew septa that, depending on the degree of preservation, can appear very similar to the Scleractinia. However, Rugose septa are arranged in quadrants that impart bilateral symmetry. In many species, each quadrant is separated by a longitudinal spacing, or fossulae, although these structures are not always easily discerned. Successive stages of polyp growth were separated by tabulae and dissepiments.
A typical Horn Coral, Ketophyllum columnariina (Silurian) and although not in the best condition, some costae and growth lines or bands are preserved. This specimen is about 15 cm long. Original photo courtesy of Annette Lokier, Derby University.
Primary and secondary septa are nicely preserved in this Horn coral segment – Ketophyllum rugosa. Note the deep calice in the top view and prominent costae in the longitudinal view. Original photos courtesy of Annette Lokier, Derby University.
Lithostrotion, a Carboniferous colonial Rugose coral. Left: A polished slab from the Dinantian in Yorkshire Dales. Zoom in to see the septa in transverse sections and tabulae in longitudinal sections of individual corallites. Original photo courtesy of Annette Lokier, Derby University. Right: Several clusters of colonial Rugose corals from Dinantian limestones at Blackhead, Burrens (Ireland). Exposure is approximately along bedding.
Tabulata
This extinct order is quite different from Rugose and Scleractinian corals. Tabulates only occurred as colonial forms. Except for a few species, their corallites had no septa. Successive stages of polyp growth were separated by tabulae (hence the name) and dissepiments. Growth of colonies was mostly encrusting although some species developed branching habits. Well-known growth forms include the honeycomb corals (e.g., Favosites), and three- dimensional chain-link structures (e.g., Halysites).
Carboniferous Syringopora, a colonial Tabulate consisting of an array of corallite tubes. Zoom in to see faint tabulae in the corroded specimen left. Thick corallite theca best seen in the polished slab (right). There are no septa. Arrow (left image) locates crinoid ossicles. Original photo courtesy of Annette Lokier, Derby University.
Left: The massive or bulbous tabulate coral Michelinea. Corallites have deep calices and diameters of 8-10 mm in. Right: A bulbous Silurian (Wenlock) Favosites with much smaller corallites. Attached to this specimen are some bryozoa (Br – center), a small, ribbed brachiopod (Ba), and a crinoid ossicle (top right). Both photos courtesy of Annette Lokier, Derby University.
1969 and about to begin a BSc at Auckland University, Aotearoa-New Zealand (aiming to major in chemistry), I needed one more paper to complete the first-year syllabus. A friend suggested I try geology – “isn’t that fossils, rocks, dirt and stuff?” “Yeah, pretty interesting though”. “That’ll do!” I had made my choice. At that time BSc geology consisted of 2 papers each year – one paper (imaginatively called Geology A or B – I can’t remember which) included all ‘soft rock’ topics (sedimentology, paleontology, stratigraphy, geophysics etc.), and the other paper mostly ‘hard rock’ topics (igneous, metamorphic, structure, crystallography etc.). Students were immersed in everything – there were no options.
So, I majored in geology and chemistry for my BSc (Auckland University), but my passion was geology.
Fast forward another 2 years and completion of an MSc (geology, Auckland University, 1975), with a thesis on the sedimentology and stratigraphy of Quaternary aeolian, shallow marine, and fluvial deposits in subtropical northernmost New Zealand. Much of the exposure was coastal so in between measured sections and samples, I collected shellfish or threw out a fishing line to catch dinner.
From 2 million years to 2 billion years: It’s now January 1976, confronted by a minus 25oC Ottawa, Canada, on my way to start a PhD at Carleton University. I’d never seen so much snow. My supervisor was to be Alan Donaldson, a Precambrian seds guru, who was pretty keen on me doing a thesis on the sedimentology and stratigraphy of a Paleoproterozoic succession on Belcher Islands, Hudson Bay. This collection of elongate, squiggly islands is held up by a 7-9 km thick, 1.8 to 2 billion year-old succession of stromatolitic platform carbonates, shallow marine siliciclastics, a banded iron formation, a turbidite succession, red beds, and two spectacular volcanic successions. When I asked Al which formation I should work on he said “do the whole shebang“. Ok! A tough working environment but great exposure of fabulous rocks (lots of boat – Zodiac work over 0oC seawater and inclement weather – my assistant and I wore life jackets so that, we were told, they could find our bodies for insurance purposes). Al was a great supervisor.
So over two 10-week field seasons (1976-77) I measured 20,000 m of section, 2000 paleocurrent directions, and a slew of petrographic analyses. I defended in June 1979.
Next stop Calgary for a two-year stint with Gulf Canada Resources, then in 1981 landed a job with the Geological Survey of Canada. In the GSC Calgary office most of that work was centred on field mapping and stratigraphy-sedimentology of Upper Cretaceous – Paleogene clastic sediments on Ellesmere and Axel Heiberg islands – mostly shallow marine, fluvial and delta settings. There were other bits and pieces in the Alberta Front Ranges, Yukon, and Mackenzie River. A move to the GSC Vancouver office in 1989 saw the emphasis change to Mesozoic sedimentology of one of the largest Intermontane basins in British Columbia, Bowser Basin, a foredeep with sediment derived from an obducted slice of oceanic crust.
Between 1992 and 1993 I realized I needed a change in Earth Science emphasis, and a logical choice was hydrogeology where I could use my knowledge of sedimentary rocks (sediment body geometry, composition, porosity-permeability and so on) and geological mapping to characterize fluid flow at both a basin-scale, and near-surface groundwater aquifer scale.
This culminated in a 4-year GSC pilot project to map and characterize aquifers in the greater Vancouver – Fraser Valley – Delta region of southern British Columbia, coordinating the expertise of colleagues with the use of shallow reflection seismic, ground penetrating radar, electromagnetics, gravity, MODFLOW modelling, and GIS data management of water-well databases and subsurface aquifer maps. Inserted between these programs was a brief secondment to the Hungarian Geological Survey in 1992 to help develop their basin analysis projects.
I quit the GSC in 1997 and moved with my Canadian family back to Aotearoa – New Zealand to work a 4-year teaching stint at University of Waikato, working primarily on Late Miocene – Pliocene siliciclastics and cool-water carbonates in Whanganui Basin (west North Island). From there a part-time position at Auckland University, teaching post-graduate basin analysis and undergraduate hydrogeology, and supervising (mostly) groundwater-related theses. But this position also required a significant weekly commute, and with an evolving disenchantment of academia I decided to form my own consulting company in 2005 – a company of one – and never looked back!
As a consultant I worked lots of NZ coal and oil-gas well-site geology (yes, I know…!), geothermal hydrogeology (Taupo region), basin-scale CO2 sequestration evaluation (with GNS), lithium-bearing brines in the Chilean Altiplano (base camp at 4000 m – the geology here is amazing), and a bunch of smaller jobs mostly groundwater-related. I loved this stage of my life – I was my own boss!
Now I am retired, tending to our organic kiwifruit orchard – we’ve been at it 25 years (all fruit exported so keep an eye open for it on your supermarket shelves), and maintaining the geoscience website that you must have linked to because you are reading this.
So, almost 50 years as a geologist. Time during 40 of those years was also spent as Editor and Associate Editor, mostly for the Canadian Society of Petroleum Geology and the SEPM including 8 years on the SEPM Council, and reviewer for dozens of journal papers. I still get requests for paper reviews, but I politely decline, noting that it is someone else’s turn.
Would I do it all again? Damned right, although I would probably try to insert a chapter on planetary geology into that life.
Talus fan colluvium sourced from bluffs of Eocene conglomerate (Buchanan Lake Fm.), Emma Fiord, Ellesmere Island. Depositional mechanisms are primarily mass wasting (rock fall and slide) and slope wash during the Arctic spring thaw. The transfer of colluvium to the shoreline will produce significant improvements in sorting and stratification.
Colluvium – the source for most of Earth’s alluvium
Colluvium is a sedimentary deposit that occurs as a thin layer over much of Earth’s terrestrial surface. Colluvium deposition is dominated by hillslope processes where the dominant force is gravity. Deposits are commonly gravelly, poorly or unsorted, and non-stratified. Colluvium mantles bedrock and unconsolidated sediment alike. It is the ultimate source for most of Earth’s terrigenous, alluvial sediment.
The following historical notes are taken from an excellent review and summary by Miller and Juilleret (2020).
From an Earth science perspective there is a close spatial relationship between colluvium and alluvium – most alluvium is derived from colluvium. However, the fundamental differences between the two types of deposit are often blurred by historical and recent usage. Both terms have been in use at least since the 17th century. To this day, alluvium has consistently referred to the transport and deposition of sediment in flowing water. Channelized flow is frequently invoked, but modern usage includes sheetflood and sediment gravity flows, particularly debris flows that contribute coarse-grained sediment to alluvial fans.
Early use of the term ‘colluvium’ was reserved for soils and less palatable deposits such as garbage and sewerage. The intervening three centuries have seen changes in the meaning of the term, dispensing with the less desirable qualities but assuming more regional differences. During the 19th century it was also subsumed by an ecclesiastical term – diluvium.
Before Darwin’s opus and despite the published, rational theories espoused by James Hutton and Charles Lyell (early 1800s), the term diluvium was widely used by English, European, and North American geologists to describe the unconsolidated mud, sand, and gravel overlying bedrock. Deposition of diluvium was ascribed to the Noachian deluge. Those promoting the diluvium concept also identified water-laid alluvium as a separate and younger stratigraphic unit. The biblical roots of diluvium were inextricably linked to the widely held 19th century view of Earth’s age, calculated by Bishop Ussher at 4004 BC (or BCE). The term lost its popularity over the later decades of the 19th century as burgeoning geological observation and theory demanded different modes of thinking. For example, deposits identified as diluvium in Europe and Britain were reinterpreted as glacial drift.
The meaning of colluvium during the latter half of the 19th century had morphed to a combination of alluvium and diluvium. By the end of the 19th and beginning of the 20th century the scientific community (but not the creationists) had largely divested itself from the diluvial concept. At this time there were several definitions of ‘colluvium’, but two main streams of thought prevailed, streams that persist in 21st century definitions.
Modern usage
Two main depositional processes dominate modern definitions of colluvium:
Mass wasting, usually on hillslopes, where the primary mover of sedimentary fragments is gravity. Hillslope processes in include rockfalls, landslides, creep, freeze-thaw, and solifluction.
Slope wash by discontinuous, non-channelized sheet flow of water during precipitation events and spring thaw.
Common definitions of colluvium that emerge from these two processes are summarized in the table below – modified from Miller and Juilleret, 2020, Table 1 (see link above). The definitions tend to use one or combinations of both depositional processes.
Categories of sediment type and depositional process that can be used to formulate definitions of colluvium. Modified from Table 1, Miller and Juillerete, 2020 (see link above).
The differences among these categories depend very much on how the term colluvium has and is being used, geologically and/or technically.
In the absence of a single all-encompassing definition, Miller and Juillerete suggest the following criteria be used to “structure” future definitions:
“Colluvium = Transported regolith formed by the accumulation of clastic sediments by mass-gravity movement where water can function as a lubricant. Deposits exhibit heterogeneous particle sizes that frequently include angular fragments of rock, poor sorting, and little to no stratification. Usually found on slopes and at their bases.”
Colluvium lithofacies
Some examples of what I think constitute colluvium lithofacies are shown below. They emphasise the heterogeneity of the sediment, sediment texture, and absence of stratification. The examples include slope and base of slope colluvium derived from mass wasting, slope wash, and gravitational creep, but do not include sediment gravity flows.
Down-slope gravitational soil creep has folded and fragmented the upper surface of this Devonian shale, forming a colluvium layer consisting almost entirely of shale cleavage fragments. The fragmented shale is overlain by a modern soil that has been disrupted by various land use activities. In this example, use of the term colluvium can be confined to the overturned and fragmented shale (yellow bracket) (Category 1, mass wasting), or a combination of the fragmented shale and topsoil (Category 5). Both usages are correct, and the choice of one over the other will depend on geological or technical necessity. The reasons for choosing one or the other usages should be stated explicitly.
A non-sorted, non-stratified colluvium deposit consisting of angular fragments of andesite derived primarily by seasonal mass wasting of lava flows on the flanks of a Miocene volcano, Chilean Altiplano. The main depositional processes here are rock fall, rolling, downslope sliding, and creep (Category 1 in the table above). The basal layer (at the hammer) contains weak layering possibly derived from slope wash. The climate here is hyper-arid and at elevations greater than 4000 m a.s.l. winter freezing is severe. Most of the mass wasting occurs during spring thaw but sediment transport can also include intermittent sheet floods and debris flows.
Section through a boulder colluvial deposit on the flanks of an Eocene, andesite volcanic cone. The primary depositional mechanism was rock fall down a talus fan, such that the accumulation is thickest and coarsest at the base of slope (Category 1 in the table above). The angular fragments are close fitting, forming a clast-supported framework with poorly sorted, finer grained clastic material filling the interstices. The top 50-100 cm of the unit are cemented by very porous, ferroan calcite (orange). Chilean Altiplano. The climate here is identical to that in the previous image.
The upper part of a talus fan where blocks of columnar-jointed basalt accumulate via rockfall (Category 1 in the table above). The deposit here and farther down slope has clast-supported frameworks. Polygonal clast shapes are determined by the vertical and horizontal joint patterns of lava flows. The basalt flows are part of the Edziza Volcanic Complex (northern British Columbia) and are 1-2 million years old.
Talus fans exiting joint-controlled erosional notches in diabase sills that intrude Triassic shale, Axel Heiberg Island, Canadian Arctic. The uppermost sill is about 30 m thick. The dominant process here is rock fall (Category 1 in the table above), but slope wash (sheet wash) is also responsible for distributing talus during spring thaw (Category 2), so this colluvium might be categorised as type 3 (hybrid). The talus (colluvium) deposits consist of small shale fragments and larger clasts of diabase; the largest clasts are accumulating at the base of slope.
This view shows the transition from colluvium to alluvium via different landforms. The hill slopes flanking the braided river (centre-right) are underlain by steep dip-slopes of interbedded Paleocene-Eocene sandstone and shale. Small talus fans are accumulating at the base of these slopes. The colluvium is a mix of Category 1 and 2 types – mass wasting and slope wash (i.e., Type 3 – hybrid), the latter forming during the Arctic spring thaw. In the foreground (a) the talus fans merge at an abrupt break in slope into small alluvial fans. At (b) the talus fans merge abruptly with the braided river channels. Sediment distribution beyond the confines of the colluvium at (a) and (b) occurs primarily by channelized flow. Mt. Moore area, southern Ellesmere Island.
Anthropogenic colluvium (Category 5) created by excavation and redistribution of Late Pleistocene-Holocene flood-plain sand and gravel (flanking Waikato River). The original textural and stratigraphic characteristics of the flood plain deposits have been destroyed and replaced by a non-sorted, haphazardly stratified to non-stratified colluvium that is periodically redistributed by sheet flood runoff. The well-head right-centre is an artesian flowing outlet connected to the reservoir behind the embankment. Located near Huntly, New Zealand.
The sartorial spendour of spiny Murex – the real show-offs in the world of gastropods. From the left: Chicoreus ramosus (the Ramose Murex, Philippines); Murex pecten (Venus comb Murex, Philippines); Porieria zelandica (New Zealand) – the one I stood on!.
Modern and fossil critters, trace fossils, shell morphology
Many, but not all the terms included here are associated with articles posted on this website.
Adapical orientation(Cephalopod) Describes the facing direction of sutures and external ribs towards the apex or protoconch (the first chamber). It applies to straight and coiled forms.
Adductor muscle scars (Bivalve) : Scars where the animal’s adductor muscles are attached to the shell. Two muscle scars are present in most equivalve species near the anterior and posterior margins of both valves. Valves are opened and closed by the adductor muscles, in combination with tension in the ligament.
Aboral surface(Echinoderm) Defined where there is an opening for the anus, the term is used to orient specimens – there is no dorsal-ventral or anterior-posterior position because of the radial symmetry of most echinoderms.
Ambulacral grooves(Echinoderm) Grooves in the test surface that allow the tube feet to pass food to the mouth.
Ambulacral plates(Echinoderm) Polygonal-shaped plates, each a single calcite crystal, arranged petal-like as part of the five-fold symmetry. Ambulacra contain the pores that allow tube feet to extend; they do not contain spine tubercles. cf. interambulacra.
Ammonoid(Cephalopod) An extinct group of cephalopods that first appeared in the Devonian and disappeared at the end of the Cretaceous. They display a diverse array of coiling geometries. They are distinguished from Nautiloids by the position of the siphuncle that tracks the outer margins of chambers, and their complex septa.
Anal canal(Gastropod) The opening in the aperture margin, opposite the siphonal canal, that allows the egress of waste.
Angiosperm: Plants that produce flowers and seed-bearing fruits. They first appeared in the Late Cretaceous, about 125-100 Ma. The evolution of pollinating insects closely parallels this event. They are the largest group of all extant green plants.
Anterior(Cephalopod) The anterior position is shown with the shell upright and aperture downward and facing the observer, The opposite margin is posterior.
Anterior-posterior(Brachiopod) Posterior orientation is applied to the beak-hinged part of both valves. The opposite margin is anterior.
Anterior-posterior(Trilobite) Looking down on the carapace (dorsal view) the margin of the head – cephalon is anterior, and the tail – pygidium is posterior.
Aperture(Cephalopod)The opening of the last grown chamber, the body chamber, in which the animal resides, feeds, and swims. cf. aperture in gastropods.
Aperture: (Gastropod) The open end of a shell through which the animal feeds and excretes. The base of the aperture margin (when holding the shell upright) is commonly interrupted by a siphonal canal that channels water for respiration; the anal canal is located at the top of the aperture. It is usually protected by an operculum.
Aperture: (Graptolite) The open end of the theca that is secreted by graptolite zooids, through which the zooid feeding filters extend. Graptolite aperture margins are commonly adorned with nodes and spines.
Apical orientation(Cephalopod) Describes the facing direction of sutures and external ribs towards the aperture. It applies to straight and coiled forms.
Articulates (Brachiopod) Bivalved brachiopods where the valves possess a well defined hinge, teeth and sockets. This is the most abundant group of brachiopods. cf. Inarticulates.
Asteroids The common star fish, containing 5 arms, or arms in multiples of five. In some species the arms can be replaced if broken off or predated. Ambulacral and interambulacral plates are much smaller than in the echinoid cousins. Like the echinoids they use tube feet to move, pass food to the mouth, and for respiration. The oral surface is usually down.
Autotrophs: Organisms that derive energy from light or chemical reactions. Predominantly in the plant domain where the principal mechanism is photosynthesis. In the absence of light, chemotrophic organisms will obtain their energy and carbon for growth from chemical reactions with compounds such as sulphur and ammonia, or carbon dioxide. cf. heterotroph.
Axial lobe(Trilobite) The central, longitudinal, segmented part of the thorax, bound on each side by left and right pleural lobes. Left and right pleura are determined about an imaginary line of bilateral symmetry with the dorsal surface facing and pygidium down (or towards you).
Beak (Bivalve): The pointy end of the shell (always dorsal) that represents the initial stage of animal growth. It points towards the anterior margin.
Beak(Brachiopod) The pointy end of the pedicle and brachial valves, where shell growth was initiated (similar to pelecypods).
Benthic: (adjective) An ecological term applied to organisms that live on a sediment-water interface, or within sediment. It includes invertebrates, vertebrates, and plants (particularly algae and cyanobacteria). The most prolific benthic zones are located within the photic zone that constrains the limits of photosynthesis.
Bioimurration: The process where the skeletal or encrusting material (commonly calcium carbonate) overgrows another organism. The process has the potential to preserve fine details of the substrate structure – this is important where the substrate is easily biodegraded (e.g., plants).
Bioturbation: The reworking of sediment and soil by plants and the burrowing and grazing activities of animals, primarily invertebrates. Bioturbation destroys primary depositional structures and textures in sediment, creating new structures in the process (e.g., lebenspurren). It can also affect sediment-soil redox conditions and redistribute nutrients. Bioturbation has been important driver of invertebrate evolution.
Biserial: (Graptolite) Graptolites that grow two sets of theca on both side of their stipes.
Bivalvia: A major Class within the phylum Mollusca. The majority of bivalves (pelecypods) have two valves equal in size and shape and hinged along their dorsal margin. An (imaginary plane of symmetry separates the valves. Exceptions to this arrangement are commonly manifested as inequivalve taxa where the largest valves is cemented or attached to a substrate (e.g., oysters). Classification is based mainly on gill structure in living taxa and dentition. They are primarily filter feeders. Pelecypod shells consist of layered calcite or aragonite crystals.
Body chamber(Cephalopod) The last grown chamber that contains the aperture, in which the animal resides.
Body whorl: (Gastropod) Usually the largest whorl that contains the aperture and operculum.
Boring: (Trace fossils) Burrows drilled into hard substrates such as wood, bedrock, shells, carbonate hardgrounds, or amber. Common examples are the cylindrical or bulb-shaped tubes formed by common ship worms (Teredo), Pholad bivalves, and mollusc shell borings made by carnivorous gastropods. Microborings (mm to sub mm scale), for example in shells, can be produced by endolithic algae and certain species of sponge.
Brachial folds(Brachiopod) A distinctive fold (positive, raised, anticline-like structure) in the brachial valve; there is a corresponding sulcus (depression) in the pedicle valve. The axes of both structures are approximately parallel to the bilateral plane of symmetry. Both structures are common in the Rhynchonellids and Spiriferids. Cf. ribs which are superficial ornaments on the shell surface.
Brachial valve(Brachiopod) (dorsal valve) The smaller of the two valves. Sockets occur on this valve if dentition is present.
Brachidium(Brachiopod) An internal, coiled skeletal structure in many species, composed of calcium carbonate, that supports the softer lophophore used for feeding and respiration. The coil looks a bit like an expanded slinky. It will only be observed in broken or sectioned shells.
Burrow: (Trace fossils) The result of sediment excavation by an animal, mostly invertebrates, during any of several life activities (e.g., feeding). Burrows assume a variety of tube-shapes; straight, curved, or sinuous, single or branched, lined or unlined, actively back-filled or passively filled. The multiplicity of burrow types reflects animal behaviour.
Burrow lining: (Trace fossils) Sediment or fecal pellets added to a burrow wall as the animal moves or burrows. A good example is the lumpy, pellet-lined burrows of Ophiomorpha. Linings commonly have a different colour and/or texture to other burrow filling.
Calice, calyx: (Corals) The uppermost cup- or chalice-like depression of a corallum or corallite that provides space for the living polyp. The base of a calice is commonly bound by a tabula and dissepiments.
Callus: (Gastropod) A calcareous growth extending along the inner margin of the aperture that partly or completely covers the umbilicus.
Cephalon(Trilobite) The trilobite head that was composed of a central lobe, or glabella that divided the head laterally, fixed and free cheeks, and a pair of compound eyes. All these segments were fused together (no flexibility). The cephalon margin is anterior.
Cnidaria: The phylum of animals includes hard and soft corals, sea anemones, siphonophores (e.g., Blue Bottles), and true jellyfish. Most Cnidarian taxa use stinging cells (nematocysts) for defense or to capture prey. The phylum includes the abundant Scleractinia, Rugose, and Tabulate corals.
Coccoliths: Marine phytoplankton that secrete calcium carbonate skeletons; they are one of the main constituents in natural chalk. Coccospheres are algal cells surrounded by coccoliths arranged into spheres tubes and cup-shaped bodies, up to 100 microns in diameter. They are one of the culprits responsible for marine algal blooms.
Codiacean algae: A group of green algae that precipitate aragonite needles 2-3µm long. Two common species are Halimeda and Penicillus that, across carbonate platforms and reefs, produce large volumes of aragonite mud. Cf. coralline algae.
Coenosteum: (plural Coenostea) The porous calcareous structure that connects corallites in all three groups of colonial corals. It is part of the corallum in these colonies. In coenosteum-dominant species the corallites are spread out, connected by broad coenostea – the opposite applies to corallite-dominant species. The coenosteum structure is sponge-like and constructed of calcareous tubes, spines, the extensions of costae, and networks of small plates.
Coiling(Cephalopod) There is a range of coil geometries in nautiloids and ammonoids.
Orthoconic – straight shells
Evolute – Coils are in contact with their neighbours but do not overlap.
Convolute – Each new chamber partly overlaps earlier coils.
Involute – The latest part of the coil, including the body chamber, overlaps all previous coils.
Lituiticone – Coiling is initially tight and involute with later chambers more loosely coiled and hook-like.
Coiling:(Gastropod) Hold the shell upright with the aperture facing you. Apertures that open on the right have dextral coiling (the most common type); those that open on the left are sinistral. Coiling is centred around a central column – the columella. Most common is helical coiling. Shells where the body whorl completely envelops the earlier whorls have involute coiling. Planispirally coiled shells have bilateral symmetry (similar to ammonites). Non-coiled forms include the common limpet.
Columella: (Corals) Present in Rugose and Scleractinian corals but not Tabulate corals. In Rugose corals it is a central pillar-like structure from which the septa radiate towards the wall. In the Scleractinia, the columella, if present, is a mesh of tooth-like structures extending from the edge of the septa.
Columella(Gastropod) The central column that connects all whorls. It is only visible in broken shells, or along the inner margin of the aperture. It may be ornamented with raised ridges, or columellar folds.
Commissure(Brachiopod) The line of contact between the two valves. It may be smooth or disrupted by plications (folds in both valves).
Compound eyes(Trilobite) Eyes of great complexity that appear quite suddenly as fossils in the Early Cambrian. They are located either side of the glabella. The eyes were constructed from multiple prismatic calcite crystals, each crystal acting as a lens, to give an almost 360° field of view.
Coquina: A limestone made up of shells, shell fragments and other bioclasts, with a degree of sorting that indicates relatively high depositional energy. Where the fragments are mostly sand-sized, the Dunham limestone classification equivalent is grainstone.
Corallite: An individual skeletal tube that houses a single polyp, within a coral colony. Corallites are separated from each other by the coenosteum.
Coralline algae:Calcite and high magnesium calcite precipitating red algae, that build upon substrates such as bioclasts and rock surfaces and other algae. All begin life as encrusters, but grow to different forms such as articulated, flexible, bush-like branches, or nodular clusters around shells or pebbles (e.g. Lithothamnion). They are an important contributor to cool-water bioclastic limestones. Both types contribute to temperate and tropical carbonate sediment. They are important components of coral and bryozoan reefs.
Corallum: Corallum (plural Coralla) refers to the entire skeletal structure of a solitary coral (that is constructed by a single polyp). It is discoid, tube-, vase-, horn- or cone-shaped. The term also applies to the entire structure of corallites in a colonial coral.
Costae: (corals) Rib-like structures aligned longitudinally along the epitheca of solitary corals; they are the extension of septa through the coral wall.
Crawling traces: (Trace fossils) A behavioural trait exhibited by invertebrates that produce trails, grooves and burrows when moving from A to B – basically just getting somewhere. Traces are fairly simple, lacking systematic patterns.
Crinoids Probably the most spectacular group of echinoderms. There are two groups – the sea lilies that are attached to a substrate, and the feather stars that can move about. Both groups have a calyx, a cup like structure that contains the viscera and from which the arms extend. The calyx is made up of ambulacral and interambulacral plates. The calyx is attache to a stem made up of calcite discs – columnals. The flexible arms also consist of interconnected columnals.
Crossed lamellarstructure (Bivalve): Layers in molluscan shells where aragonite crystallites are organized in sheets. The crystallites in one sheet are oriented at relatively high angles to adjacent sheets. The overall effect can sometimes resemble a zig-zag pattern.
Cryptalgal laminates: A general term for laminated mats composed primarily of cyanobacteria, but like includes other microbes. The laminates may be flat and uniform, or tufted, pustulose, or polygonal, resulting from desiccation or, in arid environments, evaporite precipitation. In the rock record they are commonly found with stromatolites. The term microbialite is generally used in modern examples because there are several groups of microbes including bacteria, cycanobacteria, and red and green algae.
Cyanobacteria: Microscopic, single cell or colonial, prokaryotic organisms that today are aquatic and photosynthetic. They are likely the first known photosynthetic organisms on Earth, and were the primary builders of stromatolites and cryptalgal laminates (or microbialites) the oldest being about 3.4 Ga; as such they were responsible for producing free (molecular) oxygen in Earth’s ancient atmosphere. Precambrian fossil microbes, best preserved in cherts, are an assortment of filaments and coccoid colonies.
Declined: (Graptolites) Graptoloids where thecae have been added obliquely downward from the sicula, on stipes that have inverted V- or U-shapes.
Delthyrium(Brachiopod) A triangular shaped region just below the beak on the pedicle valve (posterior), through which the fleshy pedicle exits. It is analogous to the pedicle foramen.
Dendroid: One of the main Graptolite orders (the other is the graptoloids). They were the first graptolites, appearing in the Middle Cambrian, dying out late in the Carboniferous (outlasting the graptoloids). They were mostly sessile and grew bush or fan-like structures consisting of many stipes (branches), each stipe connected laterally by dissepiments (horizontal rods); a few species became planktic. Stipes were commonly bifurcate (unlike the graptoloids).
Dentition (Bivalve): An array of teeth and sockets along the dorsal hinge line.The teeth and sockets on one valve have corresponding sockets and teeth on the other valve. The two most common types are heterodont and taxodont dentition. Heterodont dentition (the most common) consists of two or three largish cardinal teeth and corresponding sockets that grow immediately below the beak. In many species lateral teeth also extend along the hinge line towards the anterior and posterior margins. Taxodont dentition consists of many small teeth and sockets arranged in a row, usually on either side of the beak.
Dentition(Brachiopod) Small knob-like structures on one valve, with corresponding sockets on the other valve.
Diatom: One of the largest groups of phytoplankton; single cell but commonly occur in chains or colonies, moving freely within the water column or attached to substrates (Benoiston et al., 2017, OA). They are composed of opaline silica, an amorphous, hydrated form of silica (SiO2). Diatoms occur in nearly all freshwater, fully marine and paralic waters (Armbrust, 2009), but in nearly all cases within the photic zone (they are photosynthetic autotrophs). They are important for the production of oxygen to both water and atmosphere.
Dissepiments: (Corals) Cup- or dome-shaped plates that grow between septa and separate the calice of a growing polyp from earlier growth stages. It has a similar function to the flatter tabulae. Both dissepiments and tabulae can occur together, with the former developed around the corallite periphery.
Dissepiments: (Graptolites) Horizontal rods of collagen or chitin that connect stipes, primarily in the dendroid graptolites.
Dorsal-ventral (Bivalve): The part of the shell containing the umbo, beak and hinge-ligament structures is dorsal; the opposite, outer shell margin is ventral.
Dorsal-ventral(Brachiopod) The alternative names for brachial and pedicle valves respectively.
Dorsal-ventral(Cephalopod) With the shell upright and aperture downward and facing the observer, the dorsal margin is at the top, the ventral at the bottom.
Dorsal-ventral(Trilobite) The carapace surface is dorsal, and the soft under-belly is ventral.
Dwelling traces: (Trace fossils) Mostly expressed as burrows and borings produced by invertebrates for somewhere to live. Commonly built by suspension feeders.
Echinodermata: Modern echinoderms include star fish (Asteroids), sea urchins and sand dollars (Echinoids, brittle stars or Ophiuroids, feather stars and sea lilies or Crinoids, and the oddball group, the Holothuroids or sea cucumber. Their ancestors first appeared in the Early Cambrian, and possibly the Ediacaran.
Echinoids A major group of echinoderms that includes extant sea urchins and sand dollars. They have no arms; instead they have ambulacral and interambulacral areas arranged in petal-like five fold symmetry. Most have spines. Most are grazers and hence the oral surface is usually down.
Ecosystem: The totality of organisms, micro- and macro-, plant and animal, their biological interactions (e.g., symbiotic relationships, food web), and their interactions and influence with the abiotic, physical and chemical environment. Common marine ecosystems include coral reefs, mangrove forests, and seagrassmeadows. Common terrestrial systems include boreal and tropical forests, and Arctic tundra.
Eelgrass: The common name for thin bladed seagrasses like Zostera and Posidonia.
Endemic (species): A species that is found naturally and is indigenous to only one region – cf. native species that are also indigenous but may occur in more than one region (more cosmopolitan).
Endolithic algae: Eukaryotic algae that live in micropores of skeletons and shelly material, and in pore throats of granular sediment. They are capable of dissolving calcium carbonate and promoting early diagenesis or weakening organic structures that leads to their fragmentation. They also play a role in micritisation of bioclasts. The term also applies to endolithic fungi and bacteria.
Epifauna: Marine and non-marine benthic organisms that live on a substrate – the sediment-water interface, shells, aquatic plants, other organisms. They may be permanently attached (e.g., barnacles, forams, calcareous algae), or mobile (e.g., gastropods, many bivalves, forams, ostracods).
Epiflora: Marine and non-marine benthic plants that live on a substrate – the sediment-water interface, shells, aquatic plants, other organisms. Common examples include macroalgae, calcareous red and green algae.
Escape structures: Fugichnia (Trace fossils) Burrows constructed by invertebrates to keep pace with sudden increases in sedimentation or erosion. Burrows may show packing structures, or spreiten as the animal moves from side-to-side, oriented depending on whether the animal is moving up of down through sediment.
Escutcheon (Bivalve): A narrow, oval-shaped depression along the dorsal margin of both valves. It is posterior to the beak and approximately parallels the hinge line and the ligament.
Ethology (Trace fossil): Applied to trace fossils, it is the study of animal behaviour as recorded by the traces and burrows it creates – it asks the question “what was the critter doing when it made a particular trace?”. Ichnology considers the following behaviours: grazing, feeding, crawling (moving from A to B), resting, dwelling, and escaping. Depending on its behaviour, the same animal can create different structures.
Euhaline Aquatic systems with salinity of 30.0-40 parts per thousand derived primarily from marine salts.
Extant: Means still living or surviving. It usually applies to biological forms, as in extant species.
Euxinic conditions: Ocean waters that are depleted in dissolved oxygen (anoxic) and are sulphidic. The sulphide is primarily dissolved H2S. Euxinia can occur in highly stratified water bodies, such as lakes and enclosed seas where there may be an the anoxic layer occurs beneath shallower waters with varying amounts of dissolved oxygen. However, euxinia may also have occurred in larger oceanic water masses in the geological past.
Facial suture(Trilobite) The suture on the cephalon separating the free cheeks from the fixed cheeks. During molting the free cheeks would break along the suture and separate from the cephalon.
Feeding trails: (Trace fossils) (Fodichnia) These trails are constructed by deposit feeders on or beneath the sediment surface. They tend to reflect regular patterns of the search for food. Zoophycus is an excellent example expressed as a corkscrew-like, arcuate pattern of spreiten around a central cylindrical burrow.
Fixed – free cheeks(Trilobite) Solid plates on either side of the glabella and separated by the facial suture. The free cheeks could separate from the cephalon during molting, leaving the fixed cheeks and the rest of the cephalon intact.
Foliated structure(Bivalve), Layers in molluscan shells where micron-sized calcite crystal plates create a fabric that approximately parallels the shell surface.
Fossula: (Corals; Plural Fossulae) Narrow gaps between the four quadrants of septa in Rugose corals; they are not always discernible, particularly if preservation is poor or there has been significant calcite recrystallization or neomorphism.
Fusiform: In paleontology, a reference to a form that is tapered, spindle, or leaf-shaped. An example is the graptolite Phyllograptus.
Gastropoda: The most abundant modern class of mollusc, that most commonly construct a single, helically coiled shell, or less commonly internally coiled or uncoiled shells (e.g. limpets). There are at least 60,000 modern representatives, mostly marine, but with fresh water and terrestrial representatives. They are herbivores or carnivores. Some species use their radula to cut through the shells of other molluscs. They first appear in the Cambrian.
Genal spine(Trilobite) A large spine extending from the outer, posterior, left and right margins of the cephalon.
Glabella(Trilobite) A longitudinal lobe that separates the cephalon bilaterally. It is flanked by the fixed cheeks and compound eyes.
Graptolite: An extinct subclass of the phylum Hemichordata, thought to be the earliest hemichordates that eventually gave rise to full chordates (including vertebrates). They appear in the Middle Cambrian and died out in the latter part of the Carboniferous. There are two main orders – the Dendroids and Graptoloids. Graptolites were composed of collagen and chitinous compounds. They are preserved mostly in shale lithologies. Graptoloids provide high resolution biostratigraphic subdivision of the Ordovician and Silurian.
Graptoloid: One of the main graptolite orders appearing early in the Ordovician. They were entirely marine planktic and probably the earliest metazoan representatives in the planktonic realm. They evolved rapidly in the Ordovician and Silurian. Representatives have one to four stipes on which individual zooids secreted collagen-like thecae (tubes), interconnected by a thread-like structure called a nema. Their closest living relatives are worm-like pterobranchs (a class in the hemichordate phylum). Graptoloid taxonomy is based primarily on the number, organisation and morphology of stipes, thecae, and sicula.
Grazing traces: (Trace fossils) (Pasichnia) Tracks of benthic animals searching for food; the traces . commonly reflect systematic patterns of search. Helminthoida is an iconic example where the animal behaviour is expressed as repeated, highly sinuous meandering or spiral trails across the sediment surface.
Growth lines-rings (Bivalve): Concentric bands or ridges that represent changes in growth and calcium carbonate secretion, centred about the umbo. Depending on the species, rings may be delicate lines, low-relief ridges, or contain more complicated ornamentation.
Growth lines (Brachiopod) Faint lines or bands that are concentric about the beaks of both valves, that represent stages of shell growth. Similar to bivalve and gastropod growth lines.
Growth lines: (Corals) These are most prominent on solitary Rugose corals where they are expressed as raised concentric ridges on the epitheca. Their wrinkled appearance gave rise to the name of this order – from the Latin ruga. It is hypothesized that each line represents a day’s growth of the corallum.
Growth lines:(Gastropod) Fine threads or raised ridges oriented axially on a whorl surface. They are oriented at a high angle to ornamental spiral threads and ribs.
Gymnosperm: An important group of plants that produce seeds not enclosed in a flower ovary of fruit (unlike angiosperms). From the Greek gymnos meaning naked, and sperma meaning seed. The group includes conifers, cycads, and Gingko.
Hemichordate: (Graptolite) A phylum of marine, bilaterally symmetrical, worm-like animals that have a relationship to echinoderms and to chordates, but lack a true notochord that in chordates always appears at embryonic stages of life and in vertebrates eventually gives rise to a backbone. From a geological perspective, the Hemichordate class Pterobranchia is the most important because it probably relates to graptolites.
Heterozoan: Heterotrophs are organisms that consumes plants or animals for energy and nutrients. The group includes a huge variety of invertebrate and vertebrate organisms. Invertebrate heterozoans are important contributors to tropical and temperate carbonates, particularly skeletal grainstones-packstones and reef structures. C.f.. autotrophs.
Hinge(Brachiopod) A narrow zone of articulation between the two valves. It may be straight (strophic hinge) or curved (astrophic hinge).
Hinge line (Bivalve): A curved or straight line located below the beak, that contains elements of dentition and ligament. If a resilifer pit is present, then the hinge line is divided into anterior and posterior segments.
Horizontal: (Graptolites) Graptoloids where thecae have been added to laterally, or horizontally from the sicula. These forms usually have two stipes.
Horn corals: The common name for solitary Rugose corals having a horn-shaped corallum. They were the most common taxa of the Paleozoic Rugose order.
Ichnocoenosis: (plural Ichnocoenoses) A trace fossil assemblage that represents a single benthic ecological community and reflecting/recording a set of physical, biological, and chemical conditions (e.g., (e.g., Curran and White 1991)). For example, a sandy open-sea high-energy beach may contain a specific set of filter-feeding bivalves in the low tide zone, and crab burrows higher up the beach. Lower energy beaches associated with lagoons will present a different set of benthic organisms that create their own set of traces and burrows.
Ichnofacies: Assemblages of trace fossils that recur in space and time, each assemblage representing a set of hydrologic, sedimentologic, and climatic conditions. For example deep water basin plain and base-of-slope environments commonly contain assemblages of grazing and feeding traces that reflect low hydrologic energy and long periods of reduced or non-deposition. In this case, a typical depositional setting is submarine fans. The most common ichnofacies have been given names that reflect the dominant kind of trace fossil; marine lithofacies include Trypanites, Glossifungites, Skolithos, Cruziana, Zoophycus, Nereites, and Teredolites (wood hardground). Each ichnofacies corresponds to a bathymetric range. Non-marine – continental examples are, Scoyenia, Mermia, and Coprinisphaera ichnofacies. The ichnofacies concept was championed by Adolf Seilacher, e.g., 1967.
Ichnology: The study of animal traces and trace fossils in terms of their environment, behaviour (ethology), and physical activity. A particular trace will reflect what the animal was doing at the time, how it was doing it, substrate composition (mud, sand), environmental conditions such as wave and current energy (water and wind), or salinity. Successions of trace fossils can also reflect changing environments (e.g., oxygenation, sediment flux, nutrients), substrate changes such as sea floor carbonate cementation, and evolving benthic communities.
Inarticulates(Brachiopod) Bivalved brachiopods composed of calcium phosphate, where the valves lack a well defined hinge, teeth or sockets. Lingula is the best known modern representative of this group.
Infauna: Marine and non-marine benthic organisms that live or feed within sediment, usually the upper few centimetres below the sediment-water interface. Common examples include molluscs and crustaceans. Infaunal activity produces bioturbation.
Interambulacral plates(Echinoderm) Polygonal-shaped plates, each a single calcite crystal, arranged petal-like as part of the five-fold symmetry, located between the ambulacral regions. These plates do not contain tube feet, but they do have tubercles to which the spines are attached.
Lebensspuren: German for “traces of life”, used primarily for the physical structures left from animal interaction with sediment. Basically it is synonymous with traces and trace fossils, although some reserve the term for modern structures.
Ligament (Bivalve): A tough, elastic, complex protein (called conchiolin) secreted by the animal that, in combination with the adductor muscles, keeps the two valves attached, and assists in articulation. When the valves are closed the ligament is under tension; when open the ligament is relaxed. The ligament itself has very low preservation potential.
Lophophore(Brachiopod) The soft organs that support respiration and feeding, that are held in place by the brachidium. Rarely preserved.
Lunule (Bivalve): A small heart-shaped depression immediately below and anterior to the beak. The size of the lunule varies considerably among species.
Macroalgae: A loose term that generally applies to large brown, red, and green algae, otherwise called seaweeds.
Mangrove: Flowering shrubs and small trees that are salt tolerant, living on sandy and muddy tidal flats and salt marshes. Most common between the subtropics but do extend beyond these latitudinal limits. They deal with salt uptake by excreting it from their leaves. They have complex root systems that help stability under conditions of shifting sediment and tides. Mangroves provide important habitats and breeding grounds, and also help protect coasts from storm wave surges and erosion. See also paralic, marsh.
Meniscus structure: (Trace fossils) Flat to curved structures formed as an animal back-fills sediment and fecal pellets in the burrow as it moves through the sediment. Menisci are usually repeated through the burrow.
Mollusca: One of the most numerous and diverse of all invertebrates, they are characterised by a mantle that protects the viscera (it may be external as in shell-less forms, or internal), breathing apparatus, and a radula (a saw-like structure that scrapes and cuts food, and is capable of cutting through the shells of other molluscs – only the bivalves lack one of these tools). There are six Classes: the limpet-like Monoplacophora, now found only in deep water; Amphineura that includes the common chitons; Gastropoda – marine, fresh water and terrestrial snails; marine Scaphopoda – that have the well-known tusk shape; Bivalvia – two-valved, marine and fresh water shells; and marine Cephalopoda, that includes the modern squid, octopus, cuttlefish, and Nautilus, and extinct ammonites.
Nacreous structure: (Gastropod-Bivalve) Layers in molluscan shells where patterns of micron-sized aragonite plates arranged parallel to the surface – where this layer occurs on the inside of a shell, it is easily recognized by the lustrous, rainbow-like colours e.g., abalone/Paua, mussels, oysters.
Native (species): A species that is indigenous to or has evolved in a particular region, but it may occur naturally in other regions – as opposed to ‘introduced’ or exotic’ which imply human intervention (cf. endemic). Native species can migrate or be distributed naturally by animals, air, water, or the longer-term plate tectonic rearrangements of oceans and landmasses.
Nautiloids(Cephalopod) Some of the oldest cephalopods, appearing in the Cambrian and surviving today as the genus Nautilus. They are characterised by chambers separated by simple flat to slightly curved septa, and a siphuncle that tracks through the centre of each septum. cf. Ammonoids.
Nema: (Graptolites) A thin collagen tube that connects the sicula to zooids in successive thecae through the centre of a graptolite stipe. The nema thread commonly extends from the latest nema along a stipe, or directly from the sicula to a float.
Nematocysts: Stinging cells, one of the defining structures of the phylum Cnidaria, are present in soft and hard coral polyps, sea anemones, jelly fish, and siphonophores like Bluebottlesand Portuguese Man of War. Their primary uses are to stun prey and for defense, but are also used for locomotion.
Niche: More correctly ecological niche applied to an organism, defines the physical (including location), chemical, and biological conditions in which an organism lives and multiplies. How the organism responds to the system in which it lives, and what it does to accomplish this, are of equal importance. Some organisms thrive in a very specific niche with a narrow range of biotic and abiotic conditions (e.g., Darwin’s finches on specific islands of the Galapagos; or reef corals that exist within a narrow seawater temperature range, light intensity, and symbiotic algae); others exist in a broad range of conditions (e.g., wide-ranging wolves, or blue whales).
Nuclear whorls:(Gastropod) The initial whorl or whorls secreted by the animal. They have low preservation potential. Together they constitute the protoconch.
Omission surface: Depositional surfaces swept bare by erosion or starved of sediment. Omission surfaces are important components of condensed stratigraphic sections. They are commonly modified by encrusting and boring organisms.
Operculum: (Gastropod) The tough chitinous or solid calcareous, moveable cap that covers the aperture and is connected to and protects the animal.
Ophiuroids Also called brittle stars, this group is superficially similar to star fish in having 5 arms and a very small test containing discs and plates. Each arm contains a calcite tube. Arms frequently break off.
Oral surface(Echinoderm) Defined where there is an opening for the mouth, the term is used to orient specimens – there is no dorsal-ventral or anterior-posterior position because of the radial symmetry of most echinoderms. The oral surface is down for species that graze, and up for most filter feeders.
Orientation (Bivalve): Valves are identified as left and right. To determine this, orient the shell upright with the beak pointing away from you. The valve on the left is left valve. The shell margin closest to you is posterior; away from you is anterior.
Ornamentation Bivalve): Most commonly manifested as raised ribs that radiate from the beak towards the ventral margin. Depending on the species, they can be presented as narrow, low-relief ridges to more prominent corrugations like those commonly seen in Pectens (scallops). The intersection of growth lines and radial ribs in some species can produce some spectacular nodular or spiny growths (e.g., Spondylus)
Orthocones(Cephalopod) Straight shelled cephalopods, including early nautiloids and ammonoids, and belemnites.
Ossicles(Echinoderm) Echinoderm tests consist of interlocking plates and discs bound by connective tissue. Many tests also bear spines. All these components are made of calcite – collectively they constitute the ossicles. Individual ossicles are composed of a single calcite crystal.
Pallial line – pallial sinus (Bivalve): The pallial line marks the edge of the mantle that in life covers the animal viscera. It is commonly etched into the shell and occurs on both valves. The pallial sinus is an indentation in the pallial line that marks the position of the animal’s siphons; it is usually posterior.
Pedicle(Brachiopod) A fleshy, stalk like structure that extends from the posterior end of the pedicle or ventral valve, that in some species is used for attachment to a hard substrate. Rarely preserved.
Pedicle foramen(Brachiopod) A round opening at the posterior end of the pedicle-ventral valve through which the pedicle stalk extends. It is analogous to the delthyrium.
Pendant: (Graptolites): Graptoloid constructions where thecae are added downward (almost vertically) from the sicula on the inside margin of the stipe. The sicula is located on the outside margin of the stipes. An example is Didymograptus murchisoni.
Pentaradial symmetry(Echinoderm) Ambulacral and interambulacral plates are arranged in 5-fold symmetry. Star fish (asteroids) are one of the more obvious examples of this arrangement, with 5, or multiples of 5 arms. All echinoderms except the holothurians (sea slugs) display the symmetry externally in their tests and arms; in the sea slugs it is internal.
Periostracum: (Gastropod) An organic layer secreted by many mollusc species on the outer layer of a shell, that helps protect the shell from abrasion – it has low preservation potential.
Periproct(Echinoderm) The opening for the anus on the aboral surface; usually opposite the peristome. It is on the upper surface for grazing echinoderms, and on the lower surface for filter feeders.
Peristome(Echinoderm) The opening for the mouth on the oral surface. In most forms it is on the side opposite the anus.
Photic zone: The uppermost layer of the oceans and lakes where light penetrates. About 45% of incident light penetrates to one metre depth; at 30 m depth the light intensity is about 16%. The base of the zone is at about 1% of incident sunlight. On average it is about 200 m deep. It is the layer where more than 95% of photosynthesis by marine organisms takes place.
Phototrophism: The growth of an organism in response to light – light intensity can be a limiting factor. Common in plants. In aquatic environments this includes algae, seagrasses, and heterozoan organisms that rely on photosynthesis – macro- brown algae, calcareous red and green algae, and various protozoa.
Phragmocone(Cephalopod) Cone-shaped depression in a belemnite rostrum (guard) that contains septa and body chamber. The septa were probably aragonitic.
Phytoplankton: Single-cell microalgae that contain chlorophyll and therefore require sunlight to generate photosynthesis reactions. They exist primarily within the photic zone of water bodies. The two main groups are diatoms and dinoflagellates, that occur in marine and non-marine waters. Both are critical components of the ocean/lacustrine food webs. Diatoms produce microscopic shell-like structures called frustules composed of amorphous silica. Dinoflagellates are mobile, using a whip-like structure called a flagellum. Dinoflagellates do not produce a mineralized shell. Phytoplankton produce nearly 50% of the oxygen in Earth’s atmosphere.
Pleural lobes(Trilobite) These are located either side of the axial lobe on the thorax. Each is segments, and each had two pairs of legs attached. Left and right pleura are determined with the dorsal surface facing and pygidium down (or towards you).
Plications(Brachiopod) Folds and corresponding sulci that are structural elements involving the whole shell – cf. superficial ribs on the outer surfaces of the shells.
Pneumatophore: On some mangrove genera (e.g., Avicennia) spongy growths extend from the tree roots vertically a few 10s of centimetres above the sediment surface; their primary function is to enhance gas exchange when exposed to air and water exchange when submerged. The pneumatophores also provide a substrate for epiphytic algae and diverse invertebrate species.
Polyp: (Corals) The living animal resides within the calice of a corallum or corallite, and responsible for secretion of the calcium carbonate in the construction of solitary and colonial corals. Each polyp has a central mouth surrounded by tentacles that contain nematocysts (stinging cells). Coral polyps live symbiotically with photosynthetic zooxanthellae (algae).
Posterior(Cephalopod) With the shell upright and aperture downward and facing the observer, the posterior margin is on the opposite side of the shell. The opposite margin is anterior.
Preservation potential: A nebulous expression that is generally used to express the relative potential for preservation of sedimentary structures and fossils. Thus, the soft part of an animal has very low potential because it degrades rapidly or is consumed by other critters; the shelly exoskeletons, shells, test, and internal skeletons have significantly higher potential. Likewise, plant leaves, flowers, fruits and seeds have relatively low potential – although pollen, because of its composition, are commonly well preserved.
Prismatic structure: (Gastropod-Bivalve) Layers in molluscan shells where high- or low-magnesium calcite or aragonite prisms are oriented normal to the shell surface. This is commonly the outer layer of a shell.
Propagule: Any part of a plant that can be used to propagate (seeds, nuts, cut stems, leaves, root sections). Mangroves are angiosperms and produce flowers and seeds within a fruit – commonly referred to as propagules. Seed distribution and germination have evolved, like other mangrove traits, to survive saline conditions where dispersal is primarily by wind and tidal currents. Propagules can survive many months in transit to locations where they can put down roots.
Protoconch(Cephalopod) The first chamber of a fledgling cephalopod. It is located at the apex of coiled forms, and at the pointy posterior end of phragmocones in belemnites. cf. gastropods.
Pterobranch: (Graptolite) One of the living classes of the Hemichordate phylum that includes worm-like colonies constructed by filter-feeding zooids – considered to be the modern analogue to graptolite colonies.
Pygidium(Trilobite) The tail (posterior) that is divided bilaterally and segmented like the axial lobe. The suture dividing the thorax and pygidium allowed flexibility.
Radula: (Gastropod) A ribbon-like structure with attached teeth, is part of the mouth in most molluscs except the bivalves. It is used for feeding. Some species of gastropod use it, in conjunction with ab acidic secretion, to bore though the shells of other molluscs – this action commonly leaves a small circular hole in the shell of the intended dinner partner. It is an important criterion for species identification. It has low preservation potential.
Reclined: (Graptolites): Graptoloids where thecae are added obliquely upwards from the sicula on the outside margin of the stipe. The sicula is located on the inside margin of the U- or V-shape stipes. An example is of Isograptus.
Resilifer pit (Bivalve): A triangular-shaped depression just below the beak and present in both valves. In life it is occupied by the resilifer, a fibrous tendon-like structure that participates in valve articulation.
Resting traces: (Trace fossils) (Cubichnia) The impressions of animals taking a break (or perhaps dead). They tend to reflect animal shapes such as starfish, or arthropods like Trilobites. They occur on bedding planes.
Rhabdosome: (Graptolites) Refers to the entire graptolite colony. It may consist of 1, 2, 3, or 4 stipes in graptoloids, or multiple stipes in dendroids.
Rhizome: Fibrous or woody plant structures that grow within a soil, from which stems, leaves, and roots extend. In plants such as seagrasses they can develop dense mats just below the sediment-water interface. They are one of the main mechanisms for expansion of plant growth.
Rostrum – Guard(Cephalopod) The solid calcite, bullet-shaped structure that is the most commonly preserved part of a belemnite. Rostrum cross-sections range from circular to oval. It was an internal structure that probably functioned to assist buoyancy.
Rugose corals: An important hard coral Order (Subclass Hexacorallia), that appeared during the Mid-Ordovician, becoming extinct at the end of the Permian. They are most commonly found as solitary, horn-shaped coralla, but colonial forms also occur. They have bilaterally symmetrical radial septa, unlike the Scleractinians. Calices are separated by tabulae and dissepiments.
Scandent: (Graptolites): Graptoloids wherein successive thecae were added vertically upward from the sicula, on the outside of the stipes. An examples is Climacograptus.
Scleractinian corals: The most important, extant, hard coral Order (Subclass Hexacorallia), primarily as tropical reef builders but also as solitary corals in colder environments. Polyp growth depends on symbiotic, photosynthetic zooxanthellate algae. Polys and the corallites they secrete have 6-fold radial symmetry, mostly expressed in 6 or 12 primary, radial septa with higher order septa inserted in between. Successive calices are separated by dissepiments. They first appear in the fossil record during the Early Triassic.
Seagrass: Seagrasses are monocotyledons, the group of angiosperms that evolved a tolerance to saline conditions from their Late Cretaceous terrestrial ancestors. They inhabit low to moderate energy, intertidal and shallow subtidal environments, and develop extensive root systems, produce flowers, and are pollinated while submerged. They are one of the most productive marine ecosystems, act as nurseries and habitats to many infaunal-epifaunal invertebrate and vertebrate species and dampen waves and tidal currents. Seagrass communities frequently coexist with mangrove forests, salt marshes, and coral reefs.
Septa(Cephalopod)(singular septum) Internal walls of (probable) aragonite separating the chambers. Their geometries range from simple flat surfaces, to surfaces containing highly intricate, three dimensional lobes and saddles, particularly in Mesozoic ammonites. The complexity of the septa is reflected in the shape of the corresponding sutures.
Septa: (Corals) Vertical plates that radiate from the wall to the centre of a coral tube. They are secreted by the polyp as it grows from one calice to the next. In detail, septa may be laminated, perforated or spinose. Septa are prominent in Rugose and Scleractinian corals, but either absent or weakly developed in the Tabulates. In Scleractinian corals the septa are arranged in 6-fold symmetry, with primary septa the thickest and largest, and higher-order septa in sets of 12, 24 and so on in between. In Rugose corals the septa are arranged in quadrants separated by narrow gaps, or fossula; fossula are indistinct or poorly preserved in many species. The arrangement of septa in Rugose corals creates a bilateral symmetry.
Septal neck(Cephalopod) A small aragonite tube extending from the convex side of the septum, through which the soft siphuncle tracks successive chambers.
Sicula: (Graptolites) The first cone-like tube secreted by a zooid on a graptolite stipe. All successive thecae along the stipe are constructed from this initial growth.
Siphonal canal:(Gastropod) When viewed upright, the opening in the aperture margin that provides access to and protects then siphons used for water intake. In some species, this structure is as long or longer than the remainder of the shell.
Siphuncle(Cephalopod) A soft tube that connects abandoned chambers and provides the animal with a means for regulating chamber buoyancy. In the ammonoids it is located along the out margin of coils. In nautiloids it passes through the centre of successive septa.
Spines(Echinoderm) Slender rods extending from tubercles on the interambulacral plates. Each rod is a single crystal of calcite. A system of pores extend along the length of the spines; in cross section they appear as circular to oval openings. Spines are very common in Echinoid species, particularly the sea urchins. Some spines are poisonous.
Spines(Trilobite) Spines of varying length and complexity are common outgrowths of pleural segments on the thorax and pygidium. In a few species, spines also grew on the upper surface of the thorax.
Spiral threads:(Gastropod) Ornamental fine or coarse lines and ridges that extend around whorls; they are generally parallel to sutures. The intersection of threads and growth lines is frequently the locus of ornamental spines and nodes.
Spire:(Gastropod) The pointy end of the shell containing all whorls extending from the body whorl. The spire tip contains the nuclear whorls, or protoconch.
Spreite: (Trace fossils) Plural spreiten. Curved or U-shaped surfaces that reflect the back-and-forth action of the burrowing, deposit-feeding animal as it moves progressively through sediment. The surfaces are stacked, and in 2-dimensional exposures appear as a series of grooves and ridges. Spreite stacks can have any orientation. For example, Diplocraterion stacks tend to be vertical, and Zoophycus stacks corkscrew or spiral. Spreite have greater three-dimensionality than meniscus structures. They also represent different animal behaviour.
Stipe: (Graptolites) The branch-like structure that defines graptolites. It commonly appears saw-toothed. Stipes grow as successive zooids secrete a theca (tube). Dendroids develop multiple stipes; graptoloids construct 1, 2, 3, or 4 stipes.
Stony coral: The name normally reserved for Scleractinian corals, although it is sometimes used for other hard corals like the Rugosa and Tabulates.
Sulcus (sulci)(Brachiopod) The negative, or depressed structure on the pedicle valve that sits opposite a corresponding fold on the brachial valve.
Sutures(Cephalopod) These are seen on the outer surface of shells and delineate the intersection of septa with the shell wall. They are relatively simple straight or curved lines in the nautiloids and early ammonoids, but became increasingly complicated with the addition of lobes and saddles on the septa, particularly in Cretaceous ammonites.
Sutures:(Gastropod) The contact between whorls. There may be some overlap from one whorl to the next. C.f.. involute coiling.
Tabulae: (Corals; singular tabula) Flat or slightly curved horizontal plates secreted within the corallite or corallum by the growing polyp as it moves to a new calice – the tabula separates the polyp from the rest of the coral tube. Present in Rugose and a defining characteristic of Tabulate corals; they are not found in the Scleractinians. Cf. Dissepiments.
Tabulate corals: An important hard coral Order (Subclass Hexacorallia), that appeared during the Early Ordovician, becoming extinct at the end of the Permian. The Order is entirely colonial where corallites form chain-link and honeycomb structures. Most corallites lack septa; a few species have poorly developed septa. Successive calices are separated by tabulae.
Theca: (Corals) The solid calcareous outer wall of a corallite or solitary corallum, thickened in some species and thin or compressed in others. The theca may be covered by a thin outer wall, or epitheca.
Theca: (Graptolites) (plural thecae) The collagen or chitin-like tube secreted by zooids along a Graptolite stipe. Each theca and its zooid along a stipe is connected by a nema.
Thorax(Trilobite) Located between the cephalon and pygidium, it contains an axial lobe and the two pleural lobes. Each lobe contained movable segments that provided flexibility, including the protective action of curling into a ball. The trilobite legs were attached to the pleura (but rarely preserved).
Trace fossil: Traces and burrows that record the activity and behaviour (ethology) of animals on and within a sediment substrate. Borings into hard substrates such as rock, wood or shelly material are also included in the definition. Traces are regarded as both fossils and sedimentary structures. Body fossils are rarely found within the structures they create. A single critter can create different traces, and different animals can create similar traces. Synonymous with the term ichnofossil.
Transpiration: Natural water loss as vapour from the exposed parts of a plant (mostly from leaves, but also flowers, stems, and branches). The process is used to remove excess water and as a cooling mechanism. The rate of transpiration is inversely proportional to the relative humidity of air, proportional to water uptake by the roots, and increases/decreases with temperature. Some plants, like salinity-tolerant mangroves, have evolved to reduce transpiration.
Trilobita: One of the earliest and an important class of complex arthropods that appeared in the Early Cambrian and died out during the Permian extinction event. As arthropods they possessed bilateral, trilobed symmetry of their head (cephalon), thorax, and tail (pygidium) segments. They had a carapace of calcium carbonate that over the lifetime of an individual could have been shed (molted) several times. The carapace had high preservation potential.
Tube feet(Echinoderm) Small, fleshy, tube-like structures that the animal extends through pores in the ambulacral plates. They are part of the animal’s vascular system. Their primary functions are to pass food to the mouth, respiration, and locomotion across a substrate.
Tubercles(Echinoderm) Raised knob-like, calcareous structures on the interambulacral plates to which spines are attached.
Umbilicus: (Gastropod) A cone-shaped opening through the centre of the columella. It is only visible in the body whorl. However, it may also be completely or partially covered by a shelly callus that extends from the inner aperture margin.
Umbo (Bivalve): The dorsal part of a valve where curvature increases significantly; this region also contains the beak.
Uniserial: (Graptolites) Stipes on which there is a single row of thecae.
Virgella: (Graptolites) A spine-like structure that extends from the base of, and is part of the sicula.
Wetland: The region between terrestrial and fully aquatic systems, where the watertable is very shallow or at the surface for a significant period such that hydrophytic plants thrive. Wetlands may be tidal or non-tidal. Wetland waters may be fresh, brackish (riverine, lacustrine), or partly saline from marine derived salts (e.g. estuarine, coastal plain, delta plain).
Whorl: (Gastropod) One complete shell revolution about an imaginary axis that extends from the top of the spire to the base of the aperture. Whorls may be inflated rounded) or flat.
Zooid: (Graptolites) Soft-bodied, filter-feeding, colonial, marine animals that are the primary builders of graptolites, secreting collagen or chitin-like tubes (thecae). Each theca is connected by a thread-like tube, the nema, that extends the entire length of the graptolite stipe. Modern Pterobranch colonies are also constructed by zooids.
Zooxanthellae: (Corals): Photosynthesizing algae that live symbiotically with coral polyps. Zooxanthellae use CO2 produced by the polyps to produce sugars and other nutrients via photosynthesis, that in turn are used by the polyp to grow and construct the coral skeleton.
The almost perfectly symmetrical profile of the Late Pleistocene stratovolcano Mt. Taranaki, the late afternoon sun emphasizing its ribbed carapace and incised mountain streams. The view is from Pouakai Track, where there is an overnight camp to complete the end of a fine day’s hike from the mountain flank. Hiked with my son Sam Ricketts, who took this photo.
Mountain streams – the ultimate source of sediment for many fluvial systems
The idea for writing this brief arose whilst struggling through some challenging trails on Mt. Taranaki (with son Sam – I was struggling, he wasn’t), a dormant stratovolcano that is responsible for the prominent topographic salient on North Island (NZ) west coast.
Published records of ancient fluvial depositional systems are replete with examples of mud, silt, sand, and gravel that are organized into lithofacies and assemblages of lithofacies. The important word here is “organized“:
– we recognise the physicality of an organized world, for any science, but in this case of sediments, sedimentary structures, and stratigraphic trends.
– Our methodological attachment to “organization” grants us a warrant to interpret this world and, at least in our own minds, create order out of apparent disorder.
The degree of organization we commonly observe in fluvial deposits (witness all those lovely models) belies their origins. Most fluvial drainage systems begin on steep slopes where water flow is usually confined to single, steep-sided, boulder-strewn channels (some rivers spill from lakes or springs). Flow is frequently torrential, but just as commonly reduces to a trickle – there is no significant base-flow that, in most low-gradient rivers, helps sustain the supply of water to channels when there is no overland flow. The bedforms that help us identify the different components of fluvial systems are absent or rare in mountain streams.
Despite their importance, mountain streams are rarely incorporated into fluvial models. At best, the models make brief reference to some distant source. I suspect this discrepancy is a function of their preservation potential rather than neglect. Mountain streams are entirely dependent on some pre-existing, steep, hill-slope topography. Confident recognition of mountain streams in the rock record would require identification of this paleotopography. There may be some relief from this dilemma where steep topography is associated with faults, for example in rift basins, or basins formed at the releasing bends of strike-slip faults. The Violin Breccia in Ridge Basin, California, may contain possible examples of this type of preservation (e.g., Crowell, 2003; Link and Crowell, 2003). Cretaceous, synrift, chaotic breccias in the Cauvery rift basin, SE India, have also been interpreted as base-of-slope talus fans associated with normal faults, that may also qualify as steep gradient stream-fed deposits (Chakraborty and Sarkar, 2018; Open Access).
The following notes focus on mountain streams but should apply to any steep-slope, coarse bedload channels.
Channel characteristics
Geomorphologists generally consider grades of 3% to be steep, but mountain streams can have average grades greater than 20%. Steep gradient channels are commonly interrupted by pools, boulder steps, and bedrock waterfalls. Channel gradients are therefore expressed as averages.
Channel widths are usually measured in metres. Under conditions of normal flow, water depths are less than the diameter of the largest boulders.
Mountain stream channels are commonly floored by bedrock. However, mountains continuously shed eroded and weathered rock fragments, and in the case of volcanic edifices this includes airfall debris (ash, lapilli, blocks) and the deposits of pyroclastic density currents. Streams that flow across this colluvium and airfall mantle will erode through it rapidly until they reach bedrock. Thus, stream banks and valley walls tend to be steep. Failure of slopes underlain by colluvium can change the course of channels as well as delivering important sources of clastic sediment.
A narrow, boulder channel with blocks of andesite to 4 m – the block indicated by the arrow is about 2 m long dimension. Summer flow is reduced to a trickle. The main source for these angular, disorganized blocks is a jointed andesite flow about 100 m upstream. Some clasts are also derived from the boulder colluvium exposed along the channel margins. Holly Hut track, Mt. Taranaki, New Zealand.
A common feature of mountain streams is the interruption of channel flow by pools, steps, and riffles arranged in staircase-like successions. Steps form at breaks in bedrock, or where boulders (and logs) are jammed across the width of a channel, creating temporary barriers to flow. Water will flow over the top of the steps during high precipitation or snow melt but will flow through the interstices between packed clasts during periods of low water. Pools develop on the upstream side of the steps. Riffles also alter channel flow but in this case the structure is a bar-like accumulation of gravel, such that flow tends to increase across the shoaling upstream slope but decrease on the downstream face of the gravel bar.
This boulder filled channel occurs on slightly lower gradient slopes about 2 km from the primary source of andesite flows. There is some evidence of clast rounding, although sorting is extremely poor. Water flow in this view is directed through large pore spaces in the gravel framework that exits into surface pools. The block indicated by the arrow is 1.5 m long dimension. Holly Hut track, Mt. Taranaki, New Zealand.
Flow characteristics
Flow velocities are highly variable because of pool-steps and riffles, boulders and logs that commonly litter channel floors, and the general bed roughness created by variable clast sizes. Flow rates as high as 5-8 m/s have been recorded. Flow can change abruptly from subcritical to supercritical both across a channel, and in the downstream direction. Supercritical flow over a riffle or boulder step rapidly changes to subcritical flow in the downstream pool where sand and finer grained sediment is deposited. The turbulence of hydraulic jumps at this transition can erode loose sediment at the base of the step, resulting in partial collapse or rolling of large clasts. Turbulence is also generated in plunge pools below larger boulders or bedrock walls. Flow over submerged boulders can produce stationary waves (standing waves), particularly during floods. Large, immovable boulders also play an important role in reducing flow energy because of frictional drag.
Flow velocities, even during floods, may not be sufficient to move some of the largest boulders, which begs the question – how did they get there? Three possible mechanisms are:
Large blocks have broken or toppled from a steep bedrock face, or from colluvium exposed on the adjacent channel margin, and rolled downslope.
The blocks were emplaced by debris flowsor lahars, the subsequent stream flow removing the finer-grained matrix.
On volcanic slopes, large ballistic blocks can be embedded in finer ash. If the softer ash is subsequently eroded, the blocks will join the channel bedload.
Colluvium exposed in a slump scarp on Mt. Taranaki stratovolcano contains two former channels (outlined) filled with a mix of poorly sorted gravel (clasts of andesite) ash and lapilli. The lower channel cuts into bedded ash-lapilli tephra that also contain some larger blocks (probably ballistics). Both channels are draped by airfall tephra layers. The boulder at left-centre is about a metre across (arrow). Holly Hut track, Mt. Taranaki, New Zealand.
Textural characteristics
The most obvious gravel components in mountain streams are cobbles and boulders. However, finer grained sediment is nearly always present – pebbles, sand, silt, and their volcaniclastic grain size equivalents are deposited in pools and in the lee of larger clasts. Thus, clast sizes along any channel reach can range over 3 or 4 orders of magnitude, from silt to boulders a metre and more in diameter. Sorting is at the extreme end or ‘poor’, with standard deviations ranging over 8-12 phi. Even the pockets of sand are poorly sorted.
Most gravel clasts derived from bedrock begin life in angular mode, although a degree of rounding can occur surprisingly rapidly within a few 100 m of transport downstream. Even large, immovable boulders develop some rounding on their upstream faces because of the high number of impacts during floods.
Streams that erode older sedimentary rock, particularly conglomerate, will potentially inherit some of the pre-existing textural properties. Thus, beds of older conglomerate that consist of rounded clasts may release their clasts intact. Likewise, joints and fractures in bedrock and lava flows will dictate the initial shape of clasts.
Angular, boulders, cobbles and pebbles of intensely fractured granodiorite overlying a channel base of the same rock type. The step in the foreground consists of bedrock and fitted boulders. Boulder orientation is haphazard. The larger boulders dampen flow during flood events and trap smaller cobble and pebble-sized clasts. Mt. Garibaldi trail, southern British Columbia.
Left: Bedrock floored boulder stream draining the outer slopes of the active volcano, Mt. Ruapehu, New Zealand. There are multiple bedrock and boulder steps and pools along its course. The channel margins are mantled by coarse- and fine-grained tephra, colluvium, and paleosols. Right: An exposed part of the active channel containing a mix of rounded and angular gravel clasts (andesite and basalt) and scattered pockets of sand. The summit is about 5 km away (line of sight), along which there are multiple bedrock (lava flows), tephra, and colluvium sources. Significant rounding of clasts can occur over that distance.
Sources of gravel
Mountain uplift is always accompanied by weathering and erosion of exposed bedrock – a continuous battle among competing processes and forces (tectonic uplift, denudation, isostatic adjustment). On the denudation side, fragmental debris is produced by the elements of weathering (e.g., precipitation, freeze-thaw, soil formation), and the forces of gravity, the latter witnessed as rock falls and topples, landslides, sector collapses, soil creep, and debris flows. Active volcanoes add all the products of airfall and pyroclastic density currents to this mix.
Some of this debris is added directly to existing mountain streams. However, most of the debris accumulates as colluvium on the mountain or steep hill-slope flanks, where it remains in storage until it is released to active stream beds. Depending on the local climate, soils formed over the colluvium will help stabilize this sediment mantle; the soils are a potential source of muddy sediment and a cause of mineral instability, dissolution, or replacement. Steep slope colluvium is frequently subjected to failure that can:
Supply fine and coarse-grained sediment to existing channels.
Focus overland flow and the development of new channels.
Bluffs that expose stacked, columnar jointed lava flows are a prime source of blocky fragments that tumble or slide directly into the narrow channel or become part of the slope colluvium. Supply of debris to the channel also occurs via gravitational failure of the colluvium – seven recent failures and their head scarps are visible here (arrows). The main bluff (left-centre) is about 75 m high. Holly Hut track, Mt. Taranaki, New Zealand.
As depositional slopes decrease…
Ultimately, the very coarse, poorly sorted deposits of mountain streams are destined for talus fans, alluvial fans, and a diverse array of river systems, but en route there are significant changes to the nature of the channels, the character of the sediments and their lithofacies:
One of the many mountain streams that drain the slopes of Mt Taranaki merging into a small, lower gradient alluvial fan near the base of the edifice. The outer edge of the fan (yellow arrow) interfingers with a swamp (brown hues). Unlike the mountain torrents, the alluvial fan is characterised by gravel bars and bedforms, and an improvement in clast sorting. Holly Hut indicated by the white arrow. Pouakai track, Mt. Taranaki, New Zealand. Photo by Sam Ricketts.
Mountain streams merge with alluvial fans, trunk, and distributive river systems. They can also flow directly into seas and lakes.
Channels tend to develop perennial flow with the downstream addition of tributaries and base-flow.
Base level becomes increasingly important as a determinant of channel sinuosity. Changes in base level, for example sea level, are unlikely to have much effect on steep gradient mountain streams.
Bedload commonly changes downstream from gravel-dominated to sand-silt-mud dominated (although not always uniformly).
The development of floodplains; not seen in mountain streams.
Downstream reduction in grain size. This may not be obvious where a drainage basin has multiple sediment sources and points of sediment entry along its rivers.
Downstream changes to clast rounding and sorting – these textural properties usually improve.
Mechanically unstable components are preferentially removed – this includes minerals possessing good cleavage, and clasts prone to breakage because of sedimentary layering (e.g., shale), penetrative cleavage, joints, or fractures.
Hydraulic sorting and winnowing of lower specific gravity minerals. The classic example of this process is the increase in the proportion of quartz grains at the expense of lithic, feldspar, and heavy mineral grains with distance and time from the primary source.
Potential chemical removal of less stable components (e.g., some ferromagnesian minerals, feldspars, carbonates). Development of soils on floodplains is an important part of this process, where changes in soil-water pH and REDOX conditions can affect mineral stability.
A submarine channel from thalweg to overbank pinchout margin (white arrow), about 20 m thick, incised into slope mudrocks and filled by debris flow conglomerates. Lower in the succession, a couple of overbank debris-flow lobes sourced and isolated from an older channel bankfull flood (yellow arrows). Mid Jurassic Bowser Basin, northern British Columbia.
A post on debris flows, mud flows, hyperconcentrated flows, mud torrents, gravel flows slurry flows, lahars.
General considerations – the rheological spectrum
The expression debris flow is the general name given to flows of solid framework debris supported by a viscous fluid. Geological debris is mostly rock and wood, but modern flows frequently contain old tyres, vehicles and any other jetsam strewn through a water course. Viscosity is developed primarily from mud suspensions in water (clay plus silt) and can vary from highly fluid to goopy. The spectrum of viscosities gives rise to different mechanical behaviours where low viscosity flows tend to be highly mobile and fast moving, and viscous flows more sluggish. Note however that these references to flow speed are relative – all debris flows move pretty quickly. Debris flows are one of natures more violent geological processes.
Viscous fluids have innate strength, called the yield strength where the fluid will not deform or flow below a critical stress (flow is a manifestation of deformation). Fluids (or solids) that behave in this manner are plastic, or hydroplastic. Yield strength depends on viscosity. Thus, the viscosity decreases at strain values beyond the yield strength. If stress is proportional to strain beyond the yield strength (the ideal case) then the material is a Bingham plastic. The relationship between shear stress and shear strain (deformation) is shown in the diagram below.
Generalized stress-strain relationships that express the rheological properties of different kinds of Earth fluids involved in sediment entrainment and deposition, focusing here on non-Newtonian fluids. For hyperconcentrated flows the fluid has no yield strength but viscosity depends on strain rate, such that an increase in flow velocity will increase shear stress, that in turn increases strain (deformation in the form of turbulence). For debris flows and viscous mud flows, deformation (i.e., flow) will not occur until the yield strength is exceeded. Modified from Middleton and Wilcock, 1994.
Evolution of flows
The names mud flow, hyperconcentrated flow, mud torrent, gravel flow, slurry flow, and lahar are often used synonymously with debris flow. However, each of these represents a distinct set of compositional and rheological properties of the muddy fluid and solid framework. The spectrum of flow types ranges from highly fluid mud-matrix dominated, to flows where the granular component dominates. One type of flow can also evolve to a flow having different mechanical and rheological properties, for example where granular debris is deposited such that the fluid becomes the dominant component, or in subaqueous flows where water is ingested through the flow head resulting in matrix dilution. Lahars are a case in point.
Schematic of subaerial and subaqueous debris flows, outlining the principal morphological features. Left diagram is modified from T. Pánek, 2020 (Fig. 10. PDF available); diagrams on the right are based on flume experiments conducted by Elverhoi et al., 2010, (Open Access). at St. Anthony Falls Laboratory, University of Minnesota, and represent two end member flow types – mud-rich and clast-poor flows (top diagram), and clast-rich flows. In all cases, dilution of suspended clastic debris takes place via deposition, ingestion of fluid at the flow head, and in the case for subaqueous flows, elutriation of fines to the overlying turbulent plume.
Lahars are terrestrial sediment gravity flows where the granular material consists of volcanic debris. They begin life as mobile, gravelly debris flows following major precipitation events, rapid melting of ice, or breaches of crater lakes or ponded water. The steep volcano slopes on which they form, plus their rheology ensure some of the longest flow-runout distances known – the 1877 event on Cotopaxi produced a lahar that traveled 325 km. Lahars are known to transform to hyperconcentrated flows when the deposition of coarse material leaves a mud-charged but highly mobile fluid phase (Mothes and Vallance, 2015, PDF available). For example, an eruption at Mt St. Helens in 1982 breached a lake, the sudden release of water producing a debris flow that transformed to a turbulent, hyperconcentrated flow after travelling 27 km from its source. Pierson and Scott(1985, PDF available) estimate that, at the point of flow transformation, the sediment concentration in the lahar was reduced to 57% (volume) because of dilution.
Flow units and surges
Flow unit is used to describe a single sediment gravity flow event preserved in a single bed.
However, the term becomes ambiguous for flows that develop multiple surges, a common feature of highly mobile flows – in this case should each surge be considered a ‘flow unit’ or ‘sub-unit’? Semantics aside, it is important to recognize this phenomenon in the rock record. Criteria to consider are:
The deposits of successive surges should have well-defined boundaries that represent the contact between the cessation of deposition in one surge and the beginning of the next surge. However, these boundaries may be diffuse if there is no grain size partitioning within or between surge units.
Fluid-sediment shear along depositional boundaries may generate clast alignment.
What is the nature of the upper and lower contacts of the main flow unit (that contains several surge subunits)? Can they be distinguished from internal surge boundaries? This problem is simplified if the underlying and overlying deposits are fundamentally different, such as hemipelagic mudrocks or turbidites in subaqueous environments, or paleosols and other evidence of subaerial exposure in terrestrial debris flows?
Is there a grain-size trend between successive surge subunits that would indicate the gradual loss of coarse size fractions as the flow evolves?
Here are a couple of links to videos of recent debris flows (from Dave Petley’s blog at AGU).
Time lapse of a surging flow, San Bernardino Mountains Sept 12, 2022, after significant rainfall.
Debris flow lithofacies
We can tease four types of flow and their resulting lithofacies from this broad spectrum. Note however that there are variations in flow behaviour and deposit between these basic types.
Hyperconcentrated flows
This category has the lowest matrix viscosity and therefore negligible yield strengths. They are highly fluid mixtures of gravel (usually finer grades), sand, mud and water that behave more like Newtonian fluids where clast support is maintained by turbulence. Flow velocities can be high enough (supercritical) to generate standing waves and antidune bedforms.
Lithofacies
Compared to many other debris flow types, deposits from hyperconcentrated flows are relatively thin – centimetres to decimetres. This is partly due to the ‘watery’ nature of the flows, where fluid drains rapidly once the coarser fraction has come to rest.
Normal grain size grading is common – a product of sediment fallout from turbulent suspensions. Turbulence is also possible in some mobile debris flows, but it is usually subordinate to the effects of matrix strength and dispersive pressures, the latter producing reverse size grading – reverse grading is a useful criterion to distinguish hyperconcentrated flows from other debris flows.
Alignment of clasts is also possible, an indication of shear along the depositional surface, but this fabric can occur in other debris flow types. Contacts at the base of flows may be scoured – another indication of turbulent flow.
A recent sheetflood deposit in Death Valley, where crude grading and some clast alignment indicate possible sedimentation from a hyperconcentrated flow. The resulting deposit is 20 cm thick.
Deposits from hyperconcentrated flows generated during flash floods in Gower Gulch, Death Valley. Flow units are outlined. Contacts at the base of each flow unit are scoured. Flow was right to left. Coin at centre-left (red) is 22 mm diameter.
Mud flows
Fluid viscosity and yield strength will increase as the proportion of mud increases in the fluid phase. In mud flows the fluid phase is proportionally greater than the solid framework. Framework clasts are supported not by turbulence, but primarily by matrix strength and buoyancy. The more viscous state of mud flows enables them to carry large blocks over great distances (kilometres to 10s of kilometres), but at a relatively slower pace compared with hyperconcentrated flows (the word ‘slower’ is meant in a relative sense – mud flows can travel at 10s of km/hour). Dispersive pressures are less important because the low concentration of clasts means few collisions.
Lithofacies
The diagnostic textural feature of this flow type is the proportion of mud matrix in which framework clasts are fully supported; they are also called pebbly mudstone. Mud flow mechanics dictate that normal and reverse grain size grading are absent or poorly developed. Deposit thickness can extend to two metres or more although thicker units may be composite.
Mud flow and matrix-supported debris flow units over a 1.9 m thick section displaying some of the depositional characteristics that represent the rheological differences in flow mechanisms. The mud flow units lack grading and clast alignment. The lowest mud flow unit contains some grain size segregation that may represent successive surges. The debris flows at the top contain reverse grading. All flow units lack normal grain size grading and scouring along their basal contacts – characteristics that help to distinguish them from hyperconcentrated flow deposits. The scale is divided into 10 cm intervals. The outcrop is part of an arid alluvial fan, Gower Gulch, Death Valley.
Mud flow deposits are generally clast poor – matrix-rich, as is the case in this Mid-Jurassic example from Bowser Basin, British Columbia. Here the pebbles are completely enveloped in a matrix of clay, silt, and sand. The outcrop is part of a submarine slope channel complex where channel-fill consists of mud flows, matrix-supported and clast-supported debris flows. The lens cap is 50 mm diameter.
Matrix-supported debris flows
This type of debris flow is characterised by approximately equal proportions of framework clasts and matrix. Dispersive pressures generated by clast collisions are the primary flow support mechanism. Matrix strength and buoyancy play a more subordinate role. This type includes lahars. Debris flows stall en masse rather than by gradual accretion across a depositional surface. In subaqueous environments, debris flows elutriate fine sediment to an overlying turbulent plume that can flow as a turbidity current far beyond the debris flow depositional limits.
Lithofacies
Clast size is highly variable, ranging from pebble to large blocks. Framework clasts are supported by matrix (there may be some clast-support), but unlike mud flows, the proportions of these two components are approximately the same. Reverse grading is common, a product of clast collisions, a textural characteristic that serves to distinguish these flows from hyperconcentrated flows and mud flows. Any clast alignment or crude stratification is generally confined to the flow base.
Another example where debris flow mechanism changes from one flow event to the next. Multiple debris flow units (arrows) contain crude reverse grading and approximately equal clast to matrix ratios. The intervening, finer grained mud flow contains a higher proportion of mud matrix and lacks reverse grading. There is little evidence for clast alignment. Clast size throughout ranges from fine pebble to boulder. This example is from the Early-Middle Miocene San Onofre Breccia, Dana Point, California. The notebook is 21 cm long.
Left: Flow upon flow upon surge of matrix-supported debris on the flank of an Early Miocene andesite volcano (Waitakere Arc), west Auckland, NZ. The cliff is 12 m high. The largest clasts are 75 cm long (arrows). Right: A closer view of a volcanic debris flow, clast-supported at its base becoming matrix-supported high in the flow unit (120 cm thick). Andesite clasts are angular to subrounded; molluscs, bryozoa, and solitary corals occur as framework and matrix fragments.
Left: A massive debris flow that is a composite of flow units or multiple surges; total thickness is about 11 m. Arrows point to large rafts of turbidite sandstone and deformed sandstone-mudstone. The entire unit is sandwiched between turbidites. Right: (inset) Closer view of the lowest debris flow, its scoured base, matrix-supported framework, and reverse grading. Clast size here is up to 20 cm. Lower Miocene Waitemata Basin, Waiwera, north Auckland, NZ.
The view of this lahar is approximately parallel to bedding. Most of the basalt clasts, up to boulder size, are embedded in the brown, muddy matrix. The boulder at left-centre is about 80 cm diameter. This and other lahars formed on the flanks of a Late Pliocene basaltic stratovolcano, Karioi, Raglan, NZ. The conglomerates also contain pockets of shallow marine bivalves and gastropods, indicating runout across a Late Pliocene beach or shoreface. At other localities nearby, lahars contain basalt blocks to 2.5 m across, indicating very energetic flows.
Clast-supported debris flows
The fluid phase is subordinate in these flow types and may act more as a lubricant than in a support capacity. Flow support is maintained primarily by dispersive pressures. Clast alignment and crude subparallel stratification is common, indicating significant shear across the depositional surface.
Lithofacies
Clast-supported frameworks and a paucity of matrix define the textural character of these flows. Reverse grading is common. Elongate clasts tend to be aligned parallel to bedding, and in some cases imbricated.
Well-rounded pebbles of radiolarian chert form a clast-supported framework where there is crude alignment of clast long-axes parallel to bedding, imparting a degree of stratification. This submarine debris flow was less cohesive than the matrix-supported types, where the proportion of interstitial fluid was subordinate to framework; dispersive pressures were the dominant forces supporting the flow. Clast alignment and stratification indicate significant shear along the aggrading depositional surface. The boundaries between flow units are subtle, almost gradational (arrows); they are interpreted as surge boundaries. This type of debris flow is commonly interbedded with more cohesive debris flows and turbidites. Bar scale subdivisions in centimetres. Jurassic, Bowser Basin, northern British Columbia.
Support mechanisms and rheology commonly change from one debris flow to the next. In this example from the Late Cretaceous Pigeon Point Formation, California, the lower clast-supported debris flow contains minimal matrix, and is overlain by a much finer grained, pebbly mudstone that has normal grain-size grading and may have been deposited by a hyperconcentrated flow. The two flow units at the top of the outcrop have reverse grading in a matrix-supported framework where clasts were kept in suspension by a combination of dispersive pressures, matrix strength and buoyancy. Lens cap right centre is 50 mm diameter.
Common environments
Debris flows can potentially form anywhere there is loose, unconsolidated gravelly sediment, an initial sloping surface, and excess water to form a supporting muddy fluid. Common water sources are:
Direct precipitation that saturates soils and raises watertables (the overall effect is reduced shear strength in the alluvial-colluvial deposits).
Rapidly melting ice.
Breached water bodies such as elevated lakes and volcanic crater lakes.
Continuous and complete water saturation in submarine and lacustrine environments.
In terrestrial environments, debris flows form directly as lahars on volcanic edifices during and after eruptive episodes, as canyon- and steep hillside-fed floods to alluvial fans. They can also evolve from landslides. Initial slopes are usually several degrees at the point if initiation but can flatten at the limits of flow runout (kilometres to more than 300 km).
In submarine and lacustrine settings, slope failure and sediment gravity flow initiation can occur on slopes less than 1o. Debris flows (and other sediment gravity flows) are commonly generated from submarine landslides at fault margins, carbonate platform margins, oversteepened continental slopes, delta, and fan-delta slopes. They also develop on the margins of submarine gullies at the shelf-slope break, and submarine canyon walls. In most of these situations, debris flows will evolve to turbidity currents once the fluid phase is sufficiently diluted by deposition of clastic debris and ingestion of water at flow heads.
Pocket beaches blanketed by pebbles, cobbles and boulders, secreted away among rocky promontories, islets, nooks and crannies (north Auckland).
Pebble, cobble and boulder beaches and their shoreface equivalents.
The swimmability of a beach, if it was a popularity contest, is primarily a function of its wave climate and sandiness. A decent surf can be fun, notwithstanding the inevitable rip currents, but subdued waves are probably safer. A sandy beach is a lot easier on the feet (notwithstanding the heat in black sand exposed to the sun); pebble, cobble, and boulder beaches are less conducive to that mad-dash into the waves, particularly when clasts are encrusted with oysters and barnacles.
However, gravel beaches are more interesting from a (biased) sedimentological perspective. They elicit questions like:
Where did all the cobbles come from?
How are they moved across the beach?
Is the energy required for clast entrainment available in the current environmental conditions or are they relics of some past event?
Do the gravels extend to the adjacent shoreface and if so, how did they get there?
Consideration of the first question will often lead to answers to other questions. A few scenarios related to modern coastal deposits are listed below. Some of these will apply to ancient examples.
First cycle gravel derived from bedrock exposed along the coast, eroded and weathered by coastal processes (waves, precipitation, freeze-thaw, plant roots, salt crystal expansion).
First cycle gravel sourced directly from terrestrial drainage basins and transported by rivers, or across alluvial fans and fan deltas that drain to the coast.
Second cycle gravel derived from older deposits exposed at or near the coast. Examples include older fluvial, alluvial, glaciogenic, or volcaniclastic conglomerate and breccia.
First or second cycle gravel moved across a shelf during sea level lowstand and subsequently stranded there once sea level rises. These deposits are palimpsest and available for reworking on the shoreface and beach.
First cycle here refers to the bits of eroded or weathered bedrock that are delivered to a depositional sink, in this case a beach or shoreface. En route the fragments may be rounded and sorted.
Second cycle gravel clasts are derived from older conglomerates or breccias that formed in unrelated depositional systems. For example, gravels sourced from a glacial till and deposited on a beach would be considered second cycle. But gravels reworked from floodplains into adjacent braided channels would be considered first cycle because they are part of the same depositional system and only stored temporarily on the floodplain.
Beach and storm ridge gravels
Most modern, active gravel beaches are located on high energy coasts exposed to constant wave attack. Clast size is highly variable and depends on the composition and fabric of bedrock in the case of first cycle gravels, and on inherited clast size, shape, and stratigraphic distribution in second cycle gravels. Thus, clasts derived from homogenous bedrock will be less prone to breakage and size diminution than those derived from bedrock that contains strong sedimentary or structural fabrics.
Gravel beach slopes are usually steep, >5o; beach grade usually steepens towards its landward extent. Gradients also increase with clast size and wave steepness. There is a general tendency for clast size grading up the beach face, where clasts are largest near the low tide zone, becoming finer up the beach, but there are many exceptions to this where the trend is interrupted by storm waves and surges. Size grading may be less pronounced up narrow beaches. Clast size grading can also occur along the length of a beach – coarser grained at either end, finer in the middle, although smaller pocket beaches (beaches between closely spaced rocky promontories) are less likely to exhibit this trend.
An active gravel beach on the Hauraki Gulf coast (Kaiaua), New Zealand. Clast size ranges from pebble to boulder. A storm ridge (berm) has developed at the upper limit of the gravels where clasts are mixed with broken bivalve and gastropod shells. The largest clasts have been dumped by storms on the landward margin of the berm. The gravel composition is almost 100% greywacke and derived from bedrock farther inland.
Invertebrate shells and calcareous frameworks like bryozoa, coral, and calcareous algae are common in beach gravels, but they tend to be broken or seriously abraded. Shell fragments may be trapped between larger clasts. Large clasts that move infrequently may be encrusted with by bryozoa, barnacles, calcareous algae (including rhodoliths), and seaweed.
Storm waves move gravel from the shallow shoreface to the beach and beyond the high or spring tide limit onto storm ridges or berms. The ridges are low amplitude mounds, a few centimetres to decimetres high. They generally have gently rounded surfaces on the seaward margin but may be steeper landward. Shelly material tends to be broken. Low tide and high tide berms may also develop, the latter associated with the fairweather limit of wave swash.
The most common lithofacies are low-angle and tabular crossbeds, and massive bedded gravels.
Gravel beach and storm ridge opposite the pub at New Quay, County Clare, Ireland. More than 95% of the clasts are limestone, derived from Late Pleistocene glacial tills and glacial outwash streams. The glaciogenic deposits were in turn derived from Carboniferous limestone that underlies the glacio-karst landscape known as the Burrens. The line of seaweed marks high tide – the storm berm lies immediately landward.
Massive-bedded gravel lithofacies
Cobble and boulder beaches on will seldom have recognisable stratification. They tend to be poorly sorted because smaller clasts (and shells) are trapped in the framework interstices. Massive bedding may pass laterally to low-angle crossbedding where there is a concomitant decrease in grain size (parallel to and up the beach face).
The internal organisation of clasts in storm ridge gravels, regardless of their grain size, tend to be massive because there is little opportunity for reworking and sorting. However crude tabular crossbedding can develop on the landward margins of the ridges if the landward dipping lee faces are steep enough to generate clast cascades.
Low-angle crossbedded gravel lithofacies
Low-angle stratification in fine gravels (mostly pebble grades) forms in the swash-backwash zone. The crossbedding is like that formed in sand although the laminae and crossbed set boundaries tend to be more diffuse (because of grain size). Foreset dip and crossbed set truncation angles are usually less than 10o. Crossbedding is more definitive in pebble-sand mixtures.
A shallow excavation revealing low-angle crossbedding in Holocene beach or shallow shoreface deposits (foresets have shallow dip to the right). Bivalves and gastropods are common in small scour pockets, scattered along crossbed foresets, or in concentrations nestled against large cobbles or boulders. The modern shoreline is about 100 m behind the trees. Hauraki Gulf coast (Kaiaua), New Zealand.
Tabular crossbed lithofacies
Crude tabular crossbedding can form on the landward margins of gravel bars and storm ridges where gravel clasts cascade down steepened slopes.
Gravel, or mixed gravel-sand beaches and bars are common at river mouths like this one – Tangahoe River on the north Taranaki coast, New Zealand. In this case gravel is sourced from the river. However, most of the sorting, rounding, and redistribution of gravel clasts takes place under continuous wave wash across the beach.
With a landward advancing shoreline and sufficient supply of sediment, storm ridge gravels will also progress landward. At this locality they are transgressing a salt marsh. The gravel pulses have developed steep lee faces that are potentially preserved as tabular crossbeds. Minas Basin, Nova Scotia.
Shoreface gravels
Beach gravels may extend in an unbroken sheet across the shoreface or occur as more isolated deposits separated by sand and gravelly sand. There is a reasonable expectation that gravel clast size will decrease with depth in concert with decreasing wave orbital velocities, although storm waves may completely disrupt this pattern. Palimpsest gravel deposits may also disrupt this trend.
Some of the sedimentary lithofacies encountered in sand-prone shoreface environments will also apply to their gravelly counterparts. Included in this array are tabular, trough, and low-angle crossbed lithofacies, and massive or poorly stratified gravel lithofacies. Hummocky and swaley cross stratification are mostly represented in mixed gravel-sand sediments. Diverse trace fossil assemblages, macro- and microfauna, and in some cases framework biofacies such as corals and bryozoa, can be added to this list. The influence of tides across a shoreface can also be demonstrated if reactivation surfaces, tidal bundles and herringbone crossbeds are present, if not in the gravels, then in the associated sand lithofacies.
The movement of gravel swaths or sheets across a shoreface will also be influenced by changes in relative sea level. Modern and ancient conglomerate packages appear to develop most extensively during progradation. However, episodes of transgression will drive the shoreline landward, and along wave-prone coasts this will be accompanied by the landward transfer of some beach and shoreface gravel. In this case, the expected stratigraphic trends should contain remnant beach and/or shoreface gravels truncated by an erosional ravinement surface that in turn is overlain progressively by a gravel lag and fine-grained, deeper shoreface or outer shelf deposits. A maximum flooding surface will mark the transition to subsequent regression.
Three examples of beach-shoreface stratigraphy are outlined below.
A wave-cut platform
The photo below shows an excellent example of a shore platform eroded into Pliocene mudstone (Tangahoe Formation). The wave-cut platform is a ravinement surface that developed during a Pleistocene interglacial rise in sea level. The platform contains prominent gutters and potholes. It is overlain by semiconsolidated, brown pebble conglomerate and sand (Rapanui Fm.) deposited about 120,000 years ago as the shoreline and shoreface migrated landward. The lower conglomerate, about 1.5 m thick, contains crude subhorizontal and low-angle crossbeds, and a few tabular crossbeds. Larger clasts have been trapped in the gutters, along with shells and wood. The sands higher in the succession contain flaser and lenticular crossbedding indicative of tidal current reversals. The outcrop is located along the north Taranaki coast, New Zealand.
A Late Pleistocene guttered and potholed wave-cut platform on Pliocene mudrocks (Tangahoe Fm.) formed by ravinement during landward migration of the shoreline about 120,000 years ago. Ravinement may have removed an earlier subaerial unconformity. The overlying conglomerate and sandstone accumulated on a shallow shoreface and beach during the subsequent regression. The gutters are about 80 cm deep.
Falher Member, Alberta Basin
Shoreface conglomerate beds are well known in the subsurface and outcrop in Alberta Foreland Basin. One unit, the Albian Falher Member contains several important gas fields and hence has been the subject of much sedimentological scrutiny. Throughout the Falher Member, there are conglomerate lags at transgressive ravinement surfaces, and thicker bedded units of progradational clast and mud-supported pebble conglomerate. For example, the Falher G unit, exposed in the Front Ranges, contains a 12 m thick conglomerate that prograded under relatively high rates of sediment supply, such that the shoreline trajectory moved upward and seaward (Zonneveld, 2004, PDF available). Minor transgressive episodes moved some gravel landward along wave-prone coasts, where the conglomerates overlie ravinement surfaces.
Falher Member conglomerate units are interbedded with sandstone and pebbly sandstone. Together they contain a typical suite of shelf-shoreface trace fossils and sedimentary lithofacies including tabular and trough crossbedding in the conglomerates, and in the sandstones and pebbly sandstones hummocky and swaley crossbedding and current ripples. (Casas and Walker, 1997, PDF available; Zonneveld, 2004).
Waitemata Basin
The Lower Miocene Waitemata Basin (Auckland, New Zealand) is known primarily for its fill of sediment gravity flow deposits (turbidites and debris flows). However, the base of the succession contains a succession of bioclastic sandstone, limestone, and conglomerate (Kawau Formation) deposited in beach and shoreface – shallow shelf environments. The strata drape paleotopographic highs and fill the lows on a Mesozoic-Upper Paleozoic greywacke basement surface. The formation thickness varies from zero to 20-25 m, reflecting the commonly abrupt changes in paleotopography and paleoslope – the paleogeographic picture is one of drowned valleys, resulting in an embayed, rocky coast with numerous pocket beaches, sea stacks and an offshore area dotted with greywacke-cored islands (Ricketts et al.,1989 PDF available). The deposits represent the initial stage of basin subsidence and consequent transgression. They are abruptly overlain by bathyal turbidites, a transition that indicates rapid basin subsidence.
Pocket beaches can be mapped, at some localities, between basement (paleo)sea stacks and rocky promontories. Beach deposits contain pebbles, cobbles and a few boulders of reworked greywacke in massive, subhorizontal and low-angle crossbed lithofacies. The conglomerate lithofacies are interbedded with sandstone beds that contain mostly low-angle crossbedding and some tabular crossbedding. At a few localities there is a demonstrable pinching out of beds where they drape paleotopographic basement highs. Rhodolith concentrations are sometimes nestled against boulders or bedrock protuberances.
Greywacke pebbles and cobbles overgrown by calcareous algae (rhodoliths) are common additions to the coarse-grained lithofacies. Additional skeletal material in the mix includes barnacles, bryozoa, and a few solitary corals, all of which indicate shallow marine conditions where gravel clasts were subjected to relatively intense wave and/or tidal current activity. Matheson’s Bay, north Auckland.
Shoreface conglomerates, sandstones, and bioclastic limestones also contain low-angle crossbedding, and some tabular crossbeds centimetres to a few decimetres thick. Common fossils include barnacles, gastropods, bivalves (including large oysters), brachiopods, solitary corals, bryozoa, rhodoliths, and benthic foraminifera. The shoreface deposits tend to have greater lateral continuity than the pocket beaches because they overlie a more subdued paleotopography.
Trace fossils are common. One form in particular is presented in outcrop cross-sections as vertical-sided, semi-circular scoops 10-20 cm wide, and on bedding as circular to oval structures filled with fine pebbles and sand. In some sections there are 4-6 excavations per linear metre. They are interpreted as ray feeding holes by Murray Gregory et al., (1979).
Left: Tabular crossbedded pebble conglomerate and pebbly sandstone, crossbed sets 20-60 cm thick, are pocked by pebble-filled, vertical sided scours – possible ray feeding excavations. A few of the foresets are outlined. Outcrop is 3.3 m high. Right: Closer view of possible ray feeding holes. The tops of these structures coincide with bedding. Most are filled by fine pebble conglomerate, which creates good visual contrast with the host sandstone. The vertical, non-deformed sides in most suggest the holes were filled rapidly. Lens cap centre right is 50 mm diameter. Both photos of the Early Miocene, basal Waitemata Basin. Both located at Matheson’s Bay, north Auckland.
Lower Miocene shoreface-shelf pebble conglomerate and pebbly sandstone onlapping greywacke basement, Matheson’s Bay, north Auckland. The predominant lithofacies is low-angle crossbedding, with subordinate tabular crossbedding. The unconformity (dashed line) places Early Miocene shoreface deposits over Mesozoic – upper Paleozoic greywacke basement. Pockets of boulders overlie the unconformity.
Braided rivers are a common sight in mountainous, eastern Canadian Arctic, this one draining into Strand Fiord, Axel Heiberg Island. Flow is seasonal, and at a maximum during spring-early summer thaw. Late summer low water levels expose large mid-channel gravel bars, dissected by chutes during peak flow runoff. Partially submerged active bars are developed around more sand prone tabular bedforms during late summer low-flow conditions (this view). The floodplain has only sparse vegetation – bank collapse supplies slugs of gravel to the active channels.
Featuring tabular, trough, and horizontally crossbedded gravels
General occurrence
Tabular and trough crossbedding in gravel and mixed sand-gravel deposits are comparable to their sandy counterparts in some respects:
The basis for defining them as tabular and trough bedforms uses the same criteria, namely the 2D and 3D geometry of their lower crossbed set boundaries – planar and spoon-shaped respectively.
Trough, tabular (and horizontal bedded) gravels are closely associated in fluvial-alluvial depositional systems.
They are a response to bedload transport of sediment, under conditions that vary from subcritical to supercritical flow.
The main point of departure from these commonalities is the significantly greater shear stresses required to entrain clasts >2 mm diameter, particularly for pebble and coarser clast sizes. In many cases, such flows will be Froude supercritical. Furthermore, in natural channels and sheet flow there is much variation in flow velocity (and bed shear) from one part of a channel to another, where flow fluctuates between supercritical to subcritical. Whereas the bedload movement of sand may be relatively continuous under subcritical (lower flow regime) conditions, the transport of gravel will likely be discontinuous.
Tabular and trough crossbedded gravel lithofacies
Tabular and trough crossbeds are assigned to separate lithofacies, but here are treated together.
External structure
Tabular and trough crossbedded gravel bedforms tend to be larger than their sandy counterparts by virtue of their grain size. The best place to observe them in their entirety is in river and alluvial fan channels during low flow conditions. In coarse-grained braided systems, tabular crossbeds are one of the foundational structures of within-channel bars. Note however that most channel bars are complex amalgams of overlapping, discordant, and truncated tabular and trough crossbed sets rather than a single bedform, because of changing flow directions and flow competences as channel discharges wax and wane. Unlike their sandy counterparts, identification of 2D and 3D bedform geometries is more difficult. In reality, confident identification of either tabular or trough gravel bedforms requires cross-section exposure of foresets and crossbed set boundaries.
Internal structure
Tabular foresets are generally planar or slightly concave upward, dipping 20o-30o. Trough crossbed foresets are more spoon-shaped, mimicking to some extent the trough-like lower boundary. Crossbed set thickness generally ranges from a few decimetres to 3 metres and more. Grain-size grading is common where the largest clasts occur at the base of foresets, becoming finer-grained up dip. Clast-supported frameworks predominate. Successive crossbed sets of both types truncate earlier formed sets.
Unfortunately, in real world exposures crossbedding can be difficult to discern because the grain-size range along a single gravel foreset and among groups of foresets commonly ranges over two and even three orders of magnitude (keep in mind the Wentworth grain-size scale is logarithmic). If you want to decipher sedimentary structures in gravel deposits, you really do need to stand back from the outcrop.
Two sedimentary structures provide some relief from this dilemma.
Thin sandy deposits on foresets provide good contrast in grain size within the gravel accumulations, and
Imbrication or alignment of platy and oblate clasts along foreset planes.
A massive conglomerate of probable fluvial origin, but lacking obvious crossbedding. Framework is clast-supported but sorting is extremely poor, with clasts ranging from small pebbles to 150+ mm diameter. The conglomerate is interbedded with other coarse-grained, crossbedded units. Buchanan Lake Fm, Ellesmere Island (?Eocene).
Large tabular crossbeds in an Eocene (?) conglomerate, with sets up to a 120 cm thick. Crossbed set contacts are mostly planar. Foresets are more easily identified on weathered exposure.
Imbrication of platy and elongate clasts accentuates crossbed foresets and set boundaries. In this modern example, there is a strong imbricated fabric atop an inactive gravel bar. River flow was to the right.
Formation – hydrodynamic conditions
Tabular crossbedded pebble conglomerate in scoured contact with underlying tabular crossbedded lithic sandstone. Flow in both units was to the left. The apparent dip of gravel foresets is 20-25 deg (dashed lines outline general trends). The lowermost conglomeratic tabular bed can be traced laterally for about 8 m. It is overlain and locally scoured by tabular and trough crossbedded conglomerate. Lower Cretaceous Elk Fm, southern Alberta.
Tabular and trough bedforms in gravels develop from bedload transport – a combination of traction carpet and saltation load. For this to take place, flow velocities must be high enough to initiate (entrain) and maintain grain movement. For entrainment to begin, the shear stresses generated by flow of water over a sediment bed must be greater than the combined forces of gravity and frictional drag; these are referred to as critical stresses. Some of the variables that influence critical stress include:
Flow characteristics at the water-bed interface. In most natural flows this will involve turbulence.
Flow velocities.
Clast diameter; large clasts will have a greater surface area exposed to flow than their smaller neighbours.
Clast density.
Clast shape; spherical clasts will roll more easily than platy clasts of the same diameter and density (Cassel et al., 2021, Open access).
Overall bed roughness, or the degree to which some grains stand proud against their nearest neighbours, is determined by grain size, sorting, and clast packing.
Bed roughness can have a significant impact on clast entrainment. In a gravel bed where size sorting is relatively poor (a common feature of gravels), smaller clasts trapped by larger neighbours will not move until the larger clast is dislodged. This means that some sections of a gravel bed may be static, while other sections are on the move.
Assuming entrainment is in full swing, clasts will roll, bounce, and jostle their way along the top of the submerged bar or bedform. Clasts will cascade down the lee face. Tabular crossbedding is the most common type formed in this way. Any entrained sand will probably be swept beyond the lee face but may fall out of suspension in the backflow eddies because of reduced flow competence.
The rate at which bars and bedforms migrate downstream is also dependent on sediment supply (e.g., Nelson and Morgan, 2018). There are three main sources of coarse- (and fine) grained material:
Newly introduced sediment from upstream sources such as alluvial fans during floods or peak flow during spring thaw.
Erosion and collapse of channel margins (e.g., older channel deposits). Supply of this kind will be discontinuous.
Reworking of material from within-channel bars during peak flow as channels migrate laterally or on bars dissected by chutes.
Tabular crossbedded pebble conglomerate in sets up to 1.5 m thick. You can gain a sense of the truncation and erosion by successive bedforms by tracing crossbed set boundaries through the exposure. Overall, the channel fill thickness exceeds 8 m. This does NOT necessarily mean the channel depth exceeded 8 m deep at the time of deposition but could imply a continuity of accommodation space generation in a channel that maintained its location. Late Jurassic, Bowser Basin, northern British Columbia.
Common environments
We generally associate these lithofacies with gravel-bed fluvial channels in low sinuosity river systems. Alluvial fans may also contain ephemeral, braided channels. In both cases, within-channel transverse, point, and longitudinal bars consist mostly of amalgamated, tabular crossbedded lithofacies, with the trough crossbedded lithofacies created by scouring in pools that form at the junction of active channels (see N. Smith, 1974 for one of the first descriptions of these bar types – PDF available). These conditions also apply to glaciofluvial environments, and to fluvial channels on continental shelves or platforms that are exposed during sea level lowstand.
Fluvial channels that incised shelf deposits during a sea level lowstand (now exposed as a coarsening upward parasequence), produced spoon-shaped scours filled by trough crossbedded conglomerate and pebbly sandstone (TrXB), and a few tabular crossbeds (TabXB) that represent migration of 2D bedforms over the channel floor. Callovian Bowser Basin, northern British Columbia.
Fluvial channels that supply sand to the topset beds of coarse-grained fan deltas contain both lithofacies. Larger versions of the tabular crossbedded gravels also resemble the foreset layers of Gilbert deltas.
Beach storm ridge gravels tend to be structureless internally but may develop crude tabular crossbeds if the gravels are pushed landward. In this case the foresets will dip landward. Associated lithofacies and biofacies will provide a reasonable basis for making the distinction between beach settings and fluvial, glaciofluvial, and fan delta environments.
Horizontally bedded gravel lithofacies
The lithofacies is included in this post because in coarse-grained fluvial deposits it is associated with the crossbed lithofacies, as a bedform itself and as a transitional or precursor bedform to tabular crossbedded gravels (Hein and Walker, 1977 PDF available; Miall, 2006).
External structure
The lithofacies consists of flat bedded, low relief gravels deposited as diffuse sheets. They are generally a few decimetres thick.
Erosional cross-section through a subhorizontal gravel sheet, deposited on an alluvial fan that periodically discharges into Peel River, Yukon. This view shows the crude low angle layering in very poorly sorted gravel. Framework clast sizes range from fine pebble to 150-200 mm diameter cobbles. Matrix is a mix of fine and coarse sand. Flow was left to right. Abandonment of the bar was accompanied by erosion and cannibalization of gravel that was deposited on younger bars farther down the fan slope.
Internal structure
Stratification mimics the external bedform geometry and thus tends to be flat with very low-angle, downstream dip. Stratification tends to be crude and in coarser gravels may not be easily identified, although it may be enhanced by clast imbrication. Clast-supported frameworks predominate. In most deposits the sand matrix has infiltrated clast interstices during low flow conditions.
Formation – hydrodynamic conditions
Based on their observations of bar development in Kicking Horse River, Hein and Walker (1977) inferred that these bedforms developed during peak flows as a gravel lag one to two clasts thick. If the supply of clasts is high then the sheets aggrade, but their overall geometry does not change. The overall effect is the development of flat, horizontal or low angle stratification. If clast supply rates remain high, then the gravel sheet will also expand downstream.
In the Hein and Walker model, lower rates of sediment supply also result in vertical aggradation of the bedform but in this case a lee or slip face develops at the downstream end of the gravel sheet. Clasts that tumble down the slip face will contribute to the development of tabular foresets. Thus, if the Hein and Walker hypothesis is correct, horizontal bedded sheets and bars can act as a precursor to tabular crossbedded bars.
A cartoon version of the Hein and Walker (1977) model for development of gravel sheets and tabular crossbeds. Modified from their Fig 6. The image of a modern braid channel (right) helps put these two bedforms into context. The example shows low-flow conditions (mid-summer on Axel Heiberg Island, Canadian Arctic) where bar tops are exposed. Low-flow exposure accentuates the changing locus of channels, and bar-top chutes that formed during a lowering of water levels. Observations from the channel bank identified tabular gravel and sand bedforms in the active channels.
Common environments
Like the tabular and trough crossbed lithofacies, there is a general expectation that these gravel sheets occur in coarse-grained river and ephemeral alluvial fan channels, including channels that transport sediment across fan delta topsets.
Low-angle and horizontally bedded gravels also occur in marine shoreface and beach deposits. However, the association with other lithofacies and biofacies in these environments should serve to distinguish them from fluvial-dominated settings.
The beach next to the pub at New Quay, County Clare, Ireland, is composed of well-rounded pebbles, cobbles, and boulders that extend through the swash-backwash zone to a spring tide – storm ridge above the line of brown seaweed. Most of the clasts were sourced from local, Late Pleistocene glaciogenic deposits that in turn were derived from the nearby glacio-karsted Burrens, a glaciated terrain underlain by Carboniferous limestone.
Conglomerates are eye-catching. They tend to stand out, physically in outcrop and visually – the puddingstones of geological cuisine. They constitute only a few percent of the entire sedimentary crust, a paucity that belies their geological importance wherein they provide insights into:
Depositional processes: clast entrainment in fluid and viscous sediment gravity flows requires significantly higher shear and buoyancy forces than is required for sandy sediments.
Depositional environments: e.g., coarse-grained fluvial-alluvial depositional systems, or the rheological differences among different kinds of debris flows, lahars, and pyroclastic density currents.
Glaciogenic deposition, particularly diamictites, or ice-contact and outwash gravels.
Palimpsest accumulations on continental shelves and platforms.
Provenance, sediment source-to-sink and distribution pathways,
Slope failure, such as landslides (terrestrial and submarine), talus fans, and rock falls.
Explosive volcanism, pyroclastic density currents.
Tectonism and volcanism, mountain uplift and erosion, unroofing stratigraphy.
Evaluating plate tectonics settings based on all the above insights.
Grain size
The names conglomerate, breccia, gravel, rudstone and so on are used for deposits having clasts coarser than 2mm, the upper size limit for coarse sand in the Udden-Wentworth grain size scale. Gravel is a catchall term for anything coarse-grained. Conglomerate is reserved for rocks where clasts are rounded; breccia refers to a deposit of mostly angular clasts. The terms conglomerate and breccia can be used for sedimentary rocks of any composition, although carbonate and volcaniclastic lithologies also have specific nomenclature. Rudstone and floatstone are specific to gravelly carbonate clasts, while the terms lapilli and block are used to describe the grain sizes of primary (juvenile) volcaniclastics that are coarser than ash, and deposited by airfall, pyroclastic density currents, and as ballistics. A list of the gravel clast size ranges is shown below.
Grain size qualifiers for common rock types. Conglomerate and breccia qualifiers can be used for any lithology, whereas the qualifiers for carbonates and primary volcaniclastic deposits are more specific. The Udden-Wentworth grain size scale is used across all sedimentary lithologies.
Composition
Conglomerates and breccias are composed of large rock fragments. They are derived from pre-existing rocks during exposure to the elements of weathering, erosion, corrosion (chemical change, dissolution), slope failure, and volcanic eruption. Clast composition can be any rock type across the gamut of sedimentary (including older conglomerates), igneous and metamorphic terrains. Lithified gravels are commonly cemented by quartz, clays, and various carbonate minerals, and less commonly by iron oxide and evaporite minerals.
Conglomerate clasts tend to be polymineralic reflecting their original source rock. For example, an andesite clast will contain ferromagnesian minerals such as amphiboles, pyroxenes, and plagioclase phenocrysts, and some groundmass, all with varying degrees of chemical alteration, some of which may be inherited from the volcanic source rock, and some formed during diagenesis. In contrast, sand grains derived from the same rock would include single crystals of the phenocrysts, plus grains that sample the groundmass and counted as lithic fragments. The provenance value of individual sand grains is more ambiguous than is usually encountered with conglomerate clasts.
Rounded and subrounded pebbles and cobbles of andesite. The whole rock, polymineralic composition of each clast is a good representation of their source rock composition. Bar scale (lower left) is 100 mm long.
Hydrodynamic requirements for transport and deposition
We can identify four fundamentally different regimes in which gravel clasts are entrained, transported, and eventually deposited, based primarily on the fluid medium and flow rheology:
Water flow, or fluidal conditions; this includes the familiar fluvial channel and near-shore – beach environments.
Terrestrial and subaqueous sediment gravity flows in which fluid viscosity, buoyancy, turbulence, and dispersive pressures from grain-to-grain collisions, are strong determinants of the depositional product (e.g., mobile versus cohesive debris flows, grain flows).
Dry slope failure and rock-face collapse where entrained clasts are not continually immersed in water or sediment-water mixtures. The most common examples of this type of deposit are dry landslides, and talus fans adjacent steep mountain slopes.
Gravels deposited by Newtonian water–fluidal flows
Water that washes shorelines and flows through terrestrial and paralic channels behaves as a Newtonian fluid. Newtonian fluids have negligible shear strength, which means they deform instantly any kind of stress is applied.
Generalized stress-strain relationships that express the rheological properties of different kinds of Earth fluids involved in sediment entrainment and deposition, focusing here on Newtonian fluids where stress is directly proportional to strain. This means that, in the absence of yield strength, deformation or shear occurs instantly a stress is applied. Modified from Middleton and Wilcock, 1994.
Stand next to a gravelly stream that is in flood (at a safe distance) and you will hear the sound of cobble impacts as they bounce along the bed. Transport of sediment via a traction carpet or suspended load occurs under both Froude subcritical and supercritical conditions. Bedload conditions apply to gravels in much the same way as they do for sandy sediments, except that shear stress over the sediment bed (and therefore velocities) must be greater to initiate (entrain) and maintain clast movement; gravel is rarely moved as a suspended load. In gravel bed river channels, the saltation load is most likely to occur during flood conditions (and probably supercritical flow). These conditions also apply to the wave swash zone on gravel beaches, and to some dilute pyroclastic surges in which turbulence is the dominant flow-supporting mechanism.
Bedload transport of sediment takes place via a traction carpet at the sediment-water interface and a saltation load, mechanisms that apply to sandy and gravelly deposits. However, entrainment of gravel clasts requires significantly greater shear stress at the bed surface. Small clasts that become wedged between larger clasts will not move until their larger neighbours have been dislodged. Unlike sand where grain movement is relatively continuous, gravel transport along a bed may take place in a series of ‘bursts’. Smaller cobbles and pebbles may saltate under supercritical flow conditions.
Gravels deposited by non-Newtonian flows
Sediment gravity flows consist of solid framework clasts dispersed in a viscous fluid. Fluid viscosity is developed primarily by clay suspensions in water and can vary from highly fluidal and mobile, to ‘goopy’ and sluggish. Viscous fluids have innate strength, called the yield strength, where the fluid will not deform or flow below a critical stress (flow is a manifestation of deformation). Fluids (or solids) that behave in this manner are called plastics, or hydroplastics.
For non-Newtonian fluids, the viscosity will decrease when strain values exceed the yield strength (because yield strength depends on viscosity). If stress is proportional to strain beyond the yield strength (the ideal case) then the material is called a Bingham plastic. The relationship between shear stress and shear strain (deformation) is shown in the diagram below.
You can demonstrate this stress-strain relationship with a simple experiment. Consider a jar of yoghurt. In its unshaken state, place a small, flattish object on the top surface (the object must be denser than the yoghurt, but be sensible – a button, small shell, etc.). If the object is not too heavy it will sit on the surface. In this condition the yoghurt has sufficient strength to prevent it sinking (sinking would be a manifestation of deformation). Remove the object and shake the yoghurt. Gently place the object back on the surface – it will probably sink because the viscosity is now significantly less than its original state (it is more fluid). The yoghurt has behaved as a non-Newtonian fluid.
Generalized stress-strain relationships that express the rheological properties of different kinds of Earth fluids involved in sediment entrainment and deposition, focusing here on non-Newtonian fluids that exhibit plastic, or hydroplastic behaviour. All hydroplastic fluids have viscosity-dependent yield strength. Deformation of an ideal plastic or hydroplastic is proportional to stress once the yield strength has been reached (the straight red line segment on the graph). Modified from Middleton and Wilcock, 1994.
Sediment gravity flows generally behave to a greater or lesser extent as hydroplastics. We commonly define two end-member types of debris flow, but acknowledge there are many variations in between:
Debris flows that are predominantly matrix-supported
The classic pebbly mudstone contains clasts dispersed and supported by a muddy matrix (the matrix may also contain sand and small pebbles). These are cohesive flows, where the matrix has sufficient strength to support the framework clasts and the flow itself; turbulence is not involved in clast support.
It is possible for a cohesive debris flow to evolve to a more mobile, less cohesive flow by ingestion of water through the head of the flow. Terrestrial and submarine landslides may transform to cohesive or mobile debris flows (and turbidity currents).
Another set of forces comes to play as the proportion of framework clasts to matrix increases – these are dispersive pressures resulting from clast collisions. Reverse grading of framework clasts is a common manifestation of these mechanics, where there is an increase in clast size towards the top of the flow. Reverse grading may occur at the base of a debris flow or be present throughout the flow.
Reverse grading can be demonstrated in a simple experiment. Fill a jar (not that jar!) with different size beads or small pebbles (dry). Shake the jar gently from side to side – the larger objects will gradually work toward the top of the pile. The larger objects are subjected to more collisions and proportionally higher dispersive pressures.
Left: A classic example of pebbly mudstone from Pigeon Point, California, one of the localities where matrix-supported conglomerates were first described and interpreted as the products of sediment gravity flows. In this example, nearly all the pebbles and cobbles are enveloped in mudstone. The bed represents deposition by a cohesive debris flow. The base of the bed is indicated by a black arrow. The large cobble at lower right was deposited at the top of the underlying bed. Lens cap lower centre is 50 mm diameter. Arrow top right indicates stratigraphic top. Right: Well rounded pebbles of radiolarian chert form a clast-supported framework where there is crude alignment of clast long-axes parallel to bedding, imparting a degree of stratification. This debris flow was probably more mobile than the pebbly mudstone opposite, where clast alignment and stratification indicate shear along the aggrading depositional surface. This type of debris flow is commonly interbedded with more cohesive debris flows. Bar scale in centimetres. Jurassic, Bowser Basin, northern British Columbia.
Debris flows that are predominantly clast-supported
Dilution of the matrix generally results in flows having greater mobility (less cohesive and lower yield strength). Debris flows that contain a high proportion of clast-supported frameworks represent an end member in which shear at the base of the flow results in planar fabrics where elongate clasts are deposited parallel to bedding and may even show imbrication (see the previous image).
There is much variation in matrix viscosity (and yield strength) and framework clast concentrations between the two end-member debris flow types. For example, terrestrial lahars and mud flows can be highly mobile (flow velocities of 10s of kilometres per hour) and capable of transporting vehicle-size blocks – they are very destructive. The more mobile flows commonly are supported by a combination of mud matrix strength, dispersive pressures, and turbulence.
Pyroclastic density currents
Deposits of this type have been described in several posts (linked below). Pyroclastic density current (PDC) is a general term that encompasses all ground-hugging, gravity-driven mixtures of gas and volcaniclastic debris derived from explosive eruptions. Included in this mix are pyroclastic flows, nuée ardentes, ignimbrites, pyroclastic surges and base-surges, and block and ash flows. Flow velocities are commonly several 10s to 100s of km/hour; they tend to be warm (up to 700o C – 1300oF). Flow support mechanisms include dispersive pressures in the more concentrated ignimbrites, turbulence in dilute surge and base-surge type PDCs, fluid pressure (from the superheated steam and gas), and fluidization (produced by incineration of vegetation and vapourised groundwater).
Gravels deposited in dry conditions
This group includes terrestrial landslides, rock falls, and talus fans. In each type, movement of clasts may be lubricated by water, for example along a glide plane, but the rocky material is not immersed in water during transport; interstitial fluids do not participate in the movement of sediment – it’s all driven by gravity in a dry(ish) state. Clasts tend to be highly angular and very poorly sorted; there is little opportunity for reworking. Talus fans may show some degree of down-slope size grading, with the largest fragments at the base of slope.
Part of the mountain rock wall and run-out of Frank Slide, a massive, dry rock slope failure that buried part of the Alberta mining town of Frank in 1903. The failure surface can be seen in the limestone ridge behind. Landslide deposits in the foreground are angular and very poorly sorted. Coal mining may have contributed to weakening of fractured rock along the ridge. Image credit: Marek Ślusarczyk 2007, Wikipedia Commons.
