A look at intertidal through subtidal deposits in outcrop
This is part of the How To…series on describing sedimentary rocks – outcrops of shallow marine deposits and their sedimentary structures.
Here are some annotated, close-up outcrop images showing details of sedimentary structures – mostly bedforms from ancient shoreface, tidal flat and channel, and shallow shelf environments. The two guides to crossbed nomenclature have been borrowed from a previous post on crossbed terminology (link below). Most of these images have been cropped from originals featured in the Atlas of shelf deposits, and Atlas of beach-lagoon-bar-estuary-tidal flat deposits. The Atlases also show examples of modern analogues to these structures.
Related links in this series on outcrop descriptions
Click on an image to zoom in, then back arrow to return to this page
The first four diagrams show some basic sediment descriptors and terminology, and a typical stratigraphic column drawn from outcrop data. These are your starting points for describing and interpreting sedimentary rocks and sedimentary structures in outcrop, hand specimen, and core.
A few references on published sedimentary structure atlases and glossaries
J.C. Harms, 2003. Primary Sedimentary Structures. Annual Review of Earth and Planetary Sciences v. 7(1):227-248
A.W. Martinius and J.H. Van den Berg. 2011. Atlas Of Sedimentary Structures In Estuarine and Tidally-Influenced River Deposits of the Rhine-Meuse-Scheldt System. Their Application to the Interpretation of Analogous Outcrop and Subsurface Depositional Systems EAGE Publications. Available for download
We introduce some basic hydraulics of sediment movement, bedforms and the concept of Flow Regime.
Ripples and dunes form when a fluid (usually water or air) flows across a sediment surface. Structures formed by air flow are called subaerial ripples or dunes; those in water have the qualifier subaqueous. These structures are given the general name bedform. The construction of bedforms requires certain conditions:
– The sand must be cohesionless (i.e. grains do not stick together).
– Flow across the sediment surface must overcome the forces of gravity and friction, and
– There is a critical flow velocity at which grain movement will begin; this also depends on the mass of individual grains, and to some extent their shape.
Ripples and dunes form under a relatively limited range of flow conditions. We can illustrate this in a graph of flow velocity against grain size, plotting areas on the graph that correspond to bedform growth. Most of the data for plots like this are derived from flumes where experimental flow conditions can be monitored closely. The plotted distribution shows that bedforms can be categorized according to flow and sediment conditions. This partitioning of bedforms was used to construct the Flow Regime hydraulic model, first published in the now classic 1965 paper by J.C. Harms & R.K. Fahnestock and used widely ever since.
The Flow Regime model considers three fundamentally different states of flow:
No bed movement where there is too little energy in the system to initiate and maintain sand grain movement,
A Lower Flow regime in which all common bedforms develop. Here, plane bed (basically parallel, planar laminae with no ripples) represents the lowest velocity, or energy conditions where sediment movement is initiated. It has been observed in flumes and in natural channels that the size of bedforms increases from ripples to large subaqueous dunes in concert with flow velocity. Dune type also changes from two dimensional structures (straight crests and planar crossbed bounding surfaces), to three dimensional structures that have sinuous, arcuate and lunate outlines and spoon or scour-shaped bounding surfaces (commonly seen as trough crossbeds).
An Upper Flow Regime where the power of stream flow washes out ripples and dunes, replacing them with plane bed (this kind of plane bed commonly has parting lineations), plus antidunes, and erosional chutes and pools.
As stream flow increases the transition from Lower to Upper flow regime produces one of the more interesting bedforms – antidunes. They are mostly found in shallow channels (e.g. fluvial and tidal channels). You can recognize that this transition has taken place when you see standing surface waves – watch closely and you will see the waves migrate upstream. Antidunes are the bedforms that develop immediately below standing waves (the two are in-phase). If high flow is maintained, the antidunes will also migrate upstream. However, once flow slackens they tend to wash out; the preservation potential of antidunes is low.
The example above shows standing waves in a tidal channel on an out-going tide. Tidal flow is to the left; standing wave migration is to the right (Mangawhai Heads, North Auckland).
Hydraulic jumps: The transition from Lower to Upper flow regime passes with a change in bedform, in particular washing out of subaqueous dunes, but there is no sudden break in surface flow – the transition is reasonably smooth. This is not the case for an Upper to Lower flow regime transition that is marked by an abrupt increase in water level and turbulence – a hydraulic jump. Hydraulic jumps can be thought of as standing waves. They are caused by a reduction in Upper Flow Regime velocity, a change in stream-bed gradient or water depth, or combinations of all three.
You can generate a hydraulic jump in your kitchen or laundry sink, so long as the sink floor is reasonably flat. Turn on the tap until you see a flow pattern like the one in the image above. Flat laminar flow is generated by the downward force of tap water – this is the plane bed. The hydraulic jump is located where the flow changes abruptly, with an increase in surface amplitude. Beyond the jump is lower flow regime flow.
We can use the Flow Regime concept in the field as a quantitative indicator of changes in paleoflow in time (i.e stratigraphically) and space (laterally). For example, a stratigraphic sequence that shows a layer of ripples overlain by a layer of trough crossbeds indicates that flow velocities, and hence stream power increased abruptly. What kind of paleoenvironment might this have occurred in? This is one of the central questions for any sedimentological analysis.
Here are a couple of important references:
Ashley, G.M. 1990. Classification of large-scale subaqueous bedforms: A new look at an old problem. Journal of Sedimentary Petrology, v.60, p. 160-172.
Harms, J.C. and Fahnestock, R.K. 1965. Stratification, bed forms, and flow phenomena (with an example from the Rio Grande). S.E.P.M. Special Publication 12, p. 84-115.
The Atlas, as are all blogs, is a publication. If you use the images, please acknowledge their source (it is the polite, and professional thing to do).
The term ‘shelf’ is used here loosely – it covers a range of submarine settings, mostly shallower than about 300m, from the upper slope to shoreline, the shoreface, fairweather and storm wave-base. There is some overlap with the ‘Paralic’ category, but the context of the shallowest examples (like beach, shallow subtidal) is in their relationship to their deeper counterparts. The separation of the ‘Shelf’ and ‘Paralic’ categories is a bit artificial, and one of convenience.
This link will take you to an explanation of the Atlas series, the ownership, use and acknowledgment of images.
Click on the image for an expanded view, then ‘back page’ arrow to return to the Atlas.
The images:
Coarsening- and bed-thickening upwards shelf (about mid shelf) to shoreface cycle, Jurassic Bowser Basin, northern British Columbia. The coarser facies contains hummocky crossbeds (HCS) at storm wave-base, and subaqueous dune-ripples above fairweather wave-base. There are numerous trace fossils indicative of high energy,
such as Ophiomorpha, Rosellia, and Thalassinoides.
Coarsening=upward cycle at about outer- to mid-shelf – some HCS at the top of the sandstone. This is a more seaward cycle to that shown above. Jurassic Bowser Basin, northern British Columbia.
This shale to thinly bedded sandstone cycle occurs close to the shelf edge, at the transition to slope deposits. There are a few bottom current ripples, but no HCS or larger dune structures. Jurassic Bowser Basin, northern British Columbia.
The chert-pebble conglomerate accumulated in a shelfbreak gully. The uninterrupted transition from shale-dominated slope to shelf is located immediately to the right of the gully margin. Jurassic Bowser Basin, northern British Columbia. Details of the gullies have been published here: Shelfbreak gullies; Products of sea-level lowstand and sediment failure: Examples from Bowser Basin, northern British Columbia. 1999, Journal of Sedimentary Research 69(6):1232-1240
Hummock cross stratification (HCS) in a typical lower shoreface shelf cycle (storm wave-base), Jurassic Bowser Basin, northern British Columbia. Hammer rests on a thin pebbly debris flow that immediately underlies the HCS unit. It is generally thought that HCS forms during storms, from the combination of a unidirectional flowing bottom current, possibly as a sediment gravity flow, that is simultaneously moulded by the oscillatory motion of large storm waves.
Possible swaley bedding, formed in much the same way as HCS, but where the hummocks have been eroded leaving the concave-upward swales. Jurassic Bowser Basin, northern British Columbia.
Storm rip-ups of shelf muds in a mid-shelf cycle. Jurassic Bowser Basin, northern British Columbia.
Many shelf cycles in the Bowser Basin succession, terminate abruptly and are overlain by a bed of fossiliferous (ammonites, trigoniids and other molluscs), pebbly, mudstone. This marks the transition form a highstand (HST) to succeeding transgression; the mudstone is the TRansgressive Systems Tract (TST).
Transition from a sandy HST, to fossiliferous mudstone (small ammonite near the lens cap) of the TST. The top of the TST corresponds to a maximum flooding surface (MFS) – the stratigraphic record of maximum transgression. Jurassic Bowser Basin, northern British Columbia.
The upper portion of this coarsening upward shelf cycle, the highstand systems tract, contains low-angle planar lamination and some hummocky cross-stratification (HCS). The base of the transgressive unit (TST) is an erosional surface. Jurassic Bowser Basin, northern British Columbia.
Two views of a lenticular, trough crossbedded pebbly sandstone that has cut into the top of a shelf cycle. This has been interpreted as a lowstand fluvial channel, that traversed and eroded the shelf as it was exposed during falling sea level. This was one mechanism for transporting gravel and sand to the slope and deeper basin, via shelfbreak gullies (like the one pictured above). Jurassic Bowser Basin, northern British Columbia.
The same fluvial, lowstand channel shown in the images above. The channel is about 2m thick. Jurassic Bowser Basin, northern British Columbia.
Panorama of a slope-shelfbreak gully-shelf-to fluvial transition, beautifully exposed at Mt Tsatia, Jurassic Bowser Basin, northern British Columbia. Conglomerate on the immediate right are equivalent to the rusty beds near the opposite summit. The shelfbreak is located at the top of the wedge-shaped gully (corresponds to the top of the waterfall) – below the gully are slope deposits. The thickness of strata in this view is more than a kilometre.
A really nice (folded) succession of coarsening upward shelf cycles, Eocene Eureka Sound Group, South Bay, Ellesmere Island. The Eocene shelf was laterally equivalent to river-dominated deltas(Iceberg Bay Fm.) to the north and east.
Coarsening upward mid-shelf – shoreface cycles at South Bay, Ellesmere Island (same location as image above). Small subaqueous dunes, ripples and HCS are common.
Coarsening upward muddy shelf cycles, mostly below storm wave-base, but the occasional cycle extending into lower shoreface (some HCS). Eocene, Eureka Sound Group, Ellesmere Island
Downlap of muddy outer shelf siltstone and mudstone, Eocene Strand Bay Fm, Ellesmere Island
Sandy, Paleocene shelf dunes forming part of large sandwave complexes. Most of the crossbeds are the planar, or 2D type. The right image shows detail of crossbed foresets, with some reactivation surfaces (probably tidally induced); crossbed is about 40cm thick. There is some indication here of tidal (flood-ebb) couplets. Expedition Fm, Eureka Sound Group, Ellesmere Island.
Sandwave complex on a Paleocene sandy shelf, made up of multiple dunes. Eureka Sound Group, Ellesmere Island.
The abrupt, corrugated surface here is a Late Pleistocene wave-cut platform, eroded across Pliocene mudstones (Tangahoe Fm). The wave-cut platform and overlying estuarine-dune sands are part of the Rapanui Formation, near Hawera, New Zealand. The eroded corrugations and channels contain wood, shells and pebbles.
Late Miocene – Early Pliocene coarsening upward shelf cycles, from outer-mid shelf siltstone-sandstone, to shoreface, tidally induced sandy coquina sandwaves (left image). The 3 images show part of the highstand systems tract. The carbonate facies are part of the classic, cool-temperate water limestones of Wanganui Basin, New Zealand. Matemateaonga Fm, Blackhill.
Thick HST calcareous sandstone – limestone, Late Miocene – Early Pliocene Matemateaonga Fm, Blackhill.
Large planar crossbeds in shelf sandwaves (HST), overlain by a pebbly shellbed deposited during the next transgressions (TST). Late Miocene – Early Pliocene Matemateaonga Fm, Blackhill.
Typical transgressive systems tract (TST) shellbed, Late Miocene – Early Pliocene Matemateaonga Fm, Blackhill.
Detail of shelf dune foresets with backflow ripples climbing up foreset dip. Late Miocene – Early Pliocene Matemateaonga Fm, Blackhill.
Subtidal sandstone with lenticular and wavy bedding deposited during ebb-flood tides. Late Miocene – Early Pliocene Matemateaonga Fm, Blackhill.
Large planar crossbedded calcareous sandstone, formed either as shelf sandwaves or platform of a tidal inlet flood delta. Late Miocene – Early Pliocene Matemateaonga Fm, Blackhill.
This collection of images spans the shallowest marine environments including beach, lagoon-bay with all the associated environments such as sand-spits and barrier-bars, tidal flats, estuaries and coastal dunes. Where possible I have paired modern analogues with ancient examples.
The Atlas, as are all blogs, is a publication. If you use the images, please acknowledge their source (it is the polite, and professional thing to do).
This link will take you to an explanation of the Atlas series, the ownership, use and acknowledgment of images.
Click on the image for an expanded view, then ‘back one page‘ arrow to return to the list
The images:
Tairua estuary, east coast Coromandel Peninsula, New Zealand. Two images taken during falling tide, exposing attached and semi-detached sand-shell bars. The main channel is at bottom of each image.
Raglan ebb tidal delta, bedforms on the platform attached to the permanent shoreface
The estuary on the south side of Galway Bay, County Clare, Ireland, near New Quay. The boulder-cobble beach consists almost entirely of Burrens limestone (Carboniferous). Left view from Abbot Hill.
Mudflats and algae, Kinvara, at the head of the estuary, south side Galway Bay.
Karst in Burren Limestone at Flaggy Shore, New Quay, County Clare, has been accentuated by salt corrosion and mechanical erosion. It is overlain by boulders of locally derived Carboniferous limestone
Boulder storm ridge at Black Head, County Clare – the heart of the Burrens. All boulders are locally derived Carboniferous limestone
Potholes in Burren Limestone, Flaggy Shore, County Clare.
Sea grass, ripples, and burrow excavations in a tidal pool, Flaggy Shore, County Clare
Large, 2D dunes, intertidal Minas Basin, Fundy Bay
Shallow subtidal to intertidal, 2D subaqueous dunes, Rowatt Fm, Belcher Islands (Aphebian, about 2 billion years old). Hammer for scale.
Cross-sectional view of 2D subaqueous, intertidal dunes, showing complex migrating dune-formed crossbedding, and dune reactivation, Rowatt Fm, Belcher Islands (Aphebian, about 2 billion years old). The sands are mixed siliciclastic-carbonate (dolomite).
Reactivated 2D dunes with superposed ebb tide ripples, Minas Basin, Fundy Bay
Proterozoic tidal inlet, showing cross-section of reactivated subaqueous dunes (mid image), possible herringbone crossbeds, and smaller ripples. Rowatt Fm, Belcher Islands (about 2 billion years old). Lens cap bottom right.
Multiple dune sets, intertidal, Minas Basin, Fundy Bay
Sandy tidal flat ripples, Minas Basin, Fundy Bay
Paleocene, straight crested and bifurcating intertidal ripples, Expedition Fm, Axel Heiberg Is;and, Canadian Arctic
Straight-crested ripple train in Paleocene intertidal deposits, Expedition Fm, Axel Heiberg Island, Canadian Arctic. Hammer left-mid image.
Flood tide ripples over-ridden by smaller ebb tide ripples sets. Minas Basin, Fundy Bay
Interference ripples in Proterozoic tidal flat facies, Belcher Islands
Large 2D and 3D dunes, and superposed run-off ripple sets, Minas Basin Fundy Bay
Tidal flat, interference ripples, Minas Basin Fundy Bay
Ebb tide run-off & reactivation of 3D dunes, Minas Basin Fundy Bay
Large 3D dunes, Minas Basin Fundy Bay
2D flood tide dunes and small ebb tide ripples, Minas Basin Fundy Bay
2D and 3D intertidal dunes, Minas Basin Fundy Bay
Eroded salt marsh cycles, Minas Basin Fundy Bay
Small meandering tidal channel in very muddy estuarine tidal flats, Whitford Estuary, south Auckland. Bank failure is common.
Salt marsh, sedges and small mangroves being transgressed and eroded by tidal flat. This is a modern example of a ravinement surface. Whitford, south Auckland
Eroded salt marsh deposits, transgressed by sandy tidal flat – beach. The erosion surface is a modern, active ravinement surface. Galveston, Texas.
Two examples of Paleocene tidal bedding (mostly lenticular and wavy bedding) interfingering with lagoon and marsh. Eureka Sound Group, Ellesmere Island
Paleocene tidal bedding interfingering with marsh-lagoon-bay sediment. On the right, the thicker sandstones may represent storm washovers into the bay. Eureka Sound Group, Ellesmere Island
Coarsening- and sandier-upward bay or lagoon subtidal to beach, cut by small tidal channels (lenticular sandstones). Eocene, Eureka Sound Group, Ellesmere Island.
Ebb tidal delta at the mouth of Waikato River, south Auckland.
Paleocene subaqueous dunes up to 2m amplitude, in tidal inlet-delta, overlain by thin tidal flat-salt marsh deposits. Expedition Fm, Axel Heiberg Island, Canadian Arctic
Large within-channel dunes in a tidal inlet associated with a sand spit facies; the spits and bars were attached to (paleotopographic) headlands across an unconformity eroded into Ordovician carbonates. Paleocene, Eureka Sound Group, Ellesmere Island.
Two views of the unconformity between Ordovician carbonates and Paleocene estuarine-tidal channel-spit facies. Eureka Sound Group, Ellesmere Island.
Typical beach stratification in an eroded berm; primarily laminated sets with low-angle truncations, parallel, or slightly inclined to the beach face.
Tidal inlet standing waves (antidunes) in an outgoing tide, Mangawhai Heads, north Auckland. The antidunes migrate up-current (against the current) and gradually build until they break, subsequently reforming.
Proterozoic tidal channel – inlet trough crossbeds; this outcrop gives a 3-dimensional view of individual sets. Paleoflow was into the image. Rowatt Fm. Belcher Islands.
Flaser, lenticular and wavy bedding in late Pleistocene deposits near Ihumatao, Auckland. White muddy sediment overlies and envelopes grey sandy ripples, and fills troughs between ripples.
Coastal dunes, Galveston coast, Texas
Washover fan breaching coastal dunes, Galveston coast, Texas
Paleocene washover fan sandstone associated with barrier island and tidal inlets (see images above from the same formation), Expedition Fm, Axel Heiberg Island, Canadian Arctic.
Stacked storm deposits associated with an upper tidal flat, each layer consisting of ripped up muds. Rowatt Fm, Belcher Islands. Proterozoic.
Mudcracks in salt marsh, Kaiua, NZ
Proterozoic supratidal desiccation cracks and voids in multiple layers of delicately laminated dololutite. Some curled slabs may be coated with crpytalgal laminae. A layer of storm-derived lutite rip-ups at the bottom of the image. Rowatt Fm. Belcher Islands. See below for a modern analogue.
Desiccated, curled, algal mats from a salt marsh near Galveston Texas. The mats are easily disturbed during high or storm tides.
Mangroves: Left image: stabilizing shell banks (storm ridges) adjacent a tidal Inlet, Auckland Harbour; Right image: in a salt marsh, Kaiua, bordering Hauraki Gulf.
Everglades Mangroves, Florida. A tangle of roots and pneumatophores that are living quarters for so many species. On the right, an epifauna of barnacles, small snails and bryozoa.
Everglades alligators, including the little guy on its parent’s head.
Gravel bar formed at the intersection between a high energy, West coast New Zealand beach, and the Tangahoe River mouth
Ripples on a sand flat, with concentration of heavy minerals from local rhyolites and dacites. Little Bay, Coromandel.
How Geologists Interpret Ancient Environments. 2 Ruffles and desiccation
Nearly all sedimentary rocks contain structures – fabrics, planes, contortions. If properly identified these sedimentary structures provide important clues to how the original sediments were deposited.
There are many different kinds of sedimentary structures formed by layers of sediment oriented at different angles, or layers that have been contorted and squished, structures formed by wetting and drying of sediment, structures formed by slip and slide, and by animals leaving tracks and traces as evidence of their activity.
All of these structures can be thought of as contributing to the architecture of sediments and sedimentary rocks.
We are going to examine two of the more common kinds of sedimentary structure – Ripples, and Mud Cracks (sometimes called Desiccation Cracks).
