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.
Sediment that is moved along a substrate (e.g. the sea floor, river bed, submarine channel, wind-blown surface) will commonly generate structures that record its passing. Sedimentary structures that preserve directionality (paleoflow) are indispensable for deciphering whence the sediment came and where it went; for interpreting sedimentary facies (local scale) and sedimentary basins (regional scale). Paleocurrents are a measure of these ancient flows.
A single structure, such as a ripple will give a unique measure of paleoflow at a certain point in space and time. An important question for this single piece of data is – how relevant is it to the bigger picture of sediment dispersal? To get a sense of regional flow and sediment transport patterns, we need many measurements so that we can tease the overall pattern of flow from whatever local variations might exist.
We can illustrate this central problem by looking at flow in a fluvial meander belt with depositional settings like the main channel (arrows), point-bars and adjacent flood plain. This snapshot in time shows clearly the huge variation in local flow directions. We also need to account for other ‘snapshots’ in time, because even at a local scale (e.g. one meander bend and point-bar), the directions of flow and sediment transport will vary from flood to low water stage. We can try to circumvent this problem if we measure a large number of flow directions over an equally large area of the river and floodplain. In modern drainage basins this is straight forward but for the rock record, exposure is likely to be discontinuous and even structurally disjointed.
Structures indicating unique flow directions
Subaqueous dunes and ripples: These bedforms are built by 2-dimensional (straight-crested) dunes and ripples. Hence, the boundaries between adjacent crossbed sets tend to be planar (cf. trough crossbeds). Flow direction is approximately at right angles to dune or ripple crests.
Trough crossbed, or 3D subaqueous dunes Spoon-shaped troughs filled by migrating, sinuous dunes produce trough crossbedding. This kind of crossbed is common in confined, channelized flow (e.g. fluvial and tidal channels). The mean flow direction is along the axis of the trough.
Left: Festooned trough crossbeds exposed approximately parallel to bedding. Paleoflow is the direction of the hammer handle Proterozoic Loaf Fm.). Right: Cross-section view of multiple trough crossbeds – only apparent flow directions can be surmised from outcrop (Eocene Buchanan Lake Fm.).
A caution about wave-formed ripples; This bedform does not arise from bedload transport in flowing currents, but from wave orbitals. Wave ripples are not paleocurrent indicators. However, wave ripple crests will be oriented approximately parallel to the strike of the ancient shoreline.
Imbrication Flat and platy clasts are commonly oriented by strong currents, such that the ‘plates’ dip upstream. These fabrics are common in gravelly fluvial deposits.
Imbricated platy cobbles and pebbles in a modern stream. Flow is to the right.
Flute casts Flutes originate from erosion of a soft, commonly muddy substrate and are filled with sand – they are part of the overlying bed and are usually seen as casts on the sole of the overlying bed. Flow direction is towards the open, shallow end of the flute.
Large flute casts on a turbidite bed sole (Omarolluk Fm, Belcher Islands). Flow was from top left to bottom right
Structures indicating ambiguous flow directions:
Groove casts Objects dragged across a soft substrate by strong currents (e.g. bottom currents, turbidity currents) will scour linear grooves that become filled by the overlying sedimentary layer. Like flutes, they are usually seen as casts on the soles of beds. In the absence of other indicators, the two possible paleoflow directions are 180o apart.
Groove casts on a bed sole, indicate flow in either direction. other criteria, like flute casts, are need to specify unambiguous flow directions.
Parting lineation These are subtle structures 2 or 3 grains thick, that are visible only on exposed laminated bedding. The word ‘Parting’ refers to rock breakage along planar laminations. Parting lineation is attributed to high flow velocities where the long axes of sand grains become aligned (in Flow Regime terminology this corresponds to Upper Plane Bed conditions). Paleocurrents are measured parallel to the long direction of parting, but like groove casts, are ambiguous.
Paleoflow indicated in this parting lineation was either to the left or right.
Current alignment of elongate fossils, rod-shaped clasts, or bits of wood can generally be treated like groove casts in terms of their paleocurrent value. There are exceptions; for example turreted gastropods may be aligned with their apices pointing downstream. The example shown here shows fairly consistent alignment of Permian Fusulinid foraminifera parallel to the prevailing flow (but the actual flow direction is ambiguous).
The classic text that deals with paleocurrent analysis is – Potter, P.E. and Pettijohn, F.J. (1977) Paleocurrents and Basin Analysis. 2nd Edition, Springer-Verlag, New York, 425 p.
We are regular visitors to the beach; walks with the kids-grandkids, the dog, swimming, fishing, or just sitting and cogitating. It’s easy to get lost in the timeless rush of waves, their impatient foam. My mind reels at the thought that the sea has been doing this for more than 4 billion years. It’s a bit like getting lost in the night sky. There’s so much to discover.
Beaches are geological domains – part of a continuum that extends to the deep ocean, but a part that is easily accessed. Geological stuff happens there. My attention is always grabbed by the small streams that drain across beaches at low tide. Whenever we came across one of these my kids would scatter, lest they be regaled yet again about the fascination of miniature worlds. I admit it was a bit over the top, so it goes…
Some beach outflows come and go with the tides, others are more permanent leakage from inland drainage. Some trickle, others rush. They are all fascinating, as microcosms of grander floodplain or massive deltas. Project this microcosm to the real world of geological process, of cause and effect. In doing this, you are engaging in the scientific process of creating your own analogy, an insight into a larger universe.
The streams usually start afresh with each tidal cycle. As tides recede, stream flow begins to erode its channel, deepest at the top of the beach. The channels may be straight and narrow, or broad networks of braided sand. Continue reading →
A safe harbour offers a place of refuge. Those in peril (or evading taxes), running before a storm, crossing a figurative bar to welcome respite. Non-figurative harbours, the coastal kind, have traditionally provided safe haven for mariners escaping inclement weather or foes.
Harbours fill and are emptied of seawater on the tide. Sea water that enters or exits is commonly focussed through narrow inlets. Here, powerful currents are generated that carry fish, sediment, flotsam, and unwary boats. Filling on an incoming tide is like a cleansing, a renewal; outgoing tides reveal channel arteries that keep alive the bars and broad flats of mud and sand, textured Kandinsky-like.
Northern New Zealand’s west coast has 6 harbours distributed along a 300 km stretch of coast. Each is protected by large sand barriers that have built over the last 2-3 million years with sand moved inshore by successive rises and falls of sea level.
Many of New Zealand’s harbours are drowned valleys, where rising sea level (following the last glaciation) has inundated dissected landscapes. Rising seas have crept up valleys, leaving the exposed high ground to front an intricately embayed coastline, islands, and estuaries that extend their marine fingers far inland.
New Zealand’s west coast is open to large swells, generated by westerly winds across a 2000 km expanse of Tasman Sea. Sea conditions along this coast are often rough. Access to the open sea via harbour inlets, requires sailors to ‘cross the bar’ – the zone of shallow, constantly moving sand. Strong tidal currents, particularly out-going tides can increase wave heights even further, as well as making wave conditions in general very choppy. The sea condition can change rapidly. Many a boat has come to grief across these west coast bars, a mix of bad luck and poor judgement (NIWA has real-time images of current bar conditions at several locations).
The oceanographic and geological term for sand bars at the entrance to harbours and lagoons is tidal delta. Tidal deltas can form on the seaward margin, in which case they are called ebb tidal deltas (because they are downstream of the outgoing tide). Those that form inside harbours and lagoons are flood tidal deltas where sand is deposited by incoming tides.
Raglan Harbour is small but it sports a very nice example of a symmetrical ebb tidal delta. The delta extends 1.5 – 2 km from the harbour mouth. Darker hues (image below) that mark the main channel contrast nicely the shallower sand bars on either side over which waves tend to break. These marginal sand deposits are called swash bars.
Westerly swells approach the coast with relatively straight crests. As they pass over the shallow delta platform, they move at a slower speed because they interact (friction) with the sea floor. Some of the wave energy is transferred to the sea floor such that sediment is moved as ripples and dunes. Slowing waves also build in amplitude (height); this is the region where waves break. However, the same waves in the adjacent, deeper water are moving at a faster pace – trace the crests of each wave and you will see it ‘bending’ around the delta.
Most of the tidal delta remains submerged even at low tide. Parts of the swash bar that are exposed during low tide show evidence for sand movement, mostly as ripples, large and small. Sand is moved during flood and ebb tides. The shape of these sand bars changes from one tide to the next, demonstrating that this is a dynamic environment.
The Raglan tidal delta consists almost entirely of sand. In contrast, Raglan Harbour and its estuaries contain a high proportion of mud. So where does all that sand come from?
The tidal delta is part of a much larger system of sand transfer – supply and demand from the adjacent continental shelf to the adjacent beaches, shallow sand bars (commonly formed by rip currents) and sand dunes. Sand in the inshore region is also moved along the coast by long-shore currents and it is this sand that continually feeds the delta. The delta in turn, via its main channel, moves sand back onto the shelf, completing the cycle.
The beach south of the tidal delta continually changes its profile. At times the profile is an uninterrupted swath of black sand along most of its length (about 3 km). At other times a significant volume of sand has been removed exposing ancient boulder deposits from nearby Karioi volcano; sand removal frequently occurs during stormy weather. The sand dunes also participate in this budgeting exercise. Sand transfer from the beach (and dunes) is probably a combination of movement directly offshore by rip currents and wave undertow, and long-shore movement towards the delta. Sand replenishment and removal from the beach, and addition to the tidal delta, is part of a much larger system of sand supply and demand – nature’s sand budget.
Sand moved onto the swash bars helps to replace sand that is removed by the deep, fast-moving channel. Channel flows in narrow inlets like the one at Raglan are commonly 4-6 km/hr (1-2 m/second), which may not sound fast (try swimming against it) but is sufficient to move large volumes of sediment during each tidal cycle. There are some small sand bars in the harbour itself, but the channel is an effective flushing mechanism that prevents the estuaries and tidal flats from clogging up.
Changes in sea level have a profound impact on coastal sand systems. If sea level falls, the beach and dunes would follow the retreating shoreline, the harbour would eventually become the domain of non-tidal rivers and swamps, and the main channel would be free to meander over a broad expanse of exposed continental shelf. Tidal deltas might be more ephemeral structures, constantly on the move. This was probably the scenario during the last glaciation, when sea level was more than 100m below its present position.
Perhaps of more immediate concern is a rise in sea level (the present situation) which would erode older foreshore beach and dune deposits, and destabilise some cliff areas south of the Harbour. The Surf Club at the south end of Ngarunui Beach would need to move – yet again. The Harbour area flooded at high tide would increase, resulting in a greater volume of seawater entering and exiting the narrow inlet. To accommodate this, the inlet would need to expand, or the speed of current flow would need to increase. Changes such as these would have an immediate effect on the size and shape of the tidal delta.
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.
When James Hutton in 1785 presented to the Royal Society of Edinburgh his ideas on the uniformity of earth processes (over vast tracts of time), he did so with both feet planted firmly on good Scottish ground. Hutton’s Principle, for which Archibald Geikie later (1905) coined the phrase “The present is the key to the past” gave to geologists a kind of warrant to interpret the geological past using observations and experiments of processes we see in action today (see an earlier post for a bit more discussion on this philosophy). One wonders whether either of these gentlemen gave thought to the Principle being used to interpret processes elsewhere in our solar system.
There is of course, no logical reason why we cannot use terrestrial environments and physical-chemical-biological processes to unravel the geology on our solar neighbours. We may need to extend our thinking beyond purely earth-bound processes, but the Principle remains a starting point for scientific thinking, interpretation, and discovery. Mars provides the perfect opportunity for this scientific adventure.Continue reading →
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).