A look at fine-grained, high sinuosity rivers in outcrop
This is part of the How To…series on describing sedimentary rocks – describing and interpreting fine-grained fluvial (meandering river) deposits in outcrop.
The images shown here illustrate some of the outcrop features associated with fine-grained fluvial deposits; In most cases they relate to ancient high sinuosity (meandering) channel systems and floodplains.
The identification and analysis of fluvial deposits from the rock record has a long and distinguished history in Earth sciences, focusing on:
the ancient river systems themselves, their facies, channel dynamics, and development of depositional models,
sequence stratigraphic frameworks,
studies that place riverine sediment transport in the context of sedimentary basin evolution
the dynamics of source area uplift and erosion, and
analysis of source-to-sink, in other words, the journey that river-derived sediment travels.
The Landsat 7 image provides useful context for meandering river systems. The images are keyed to four environmental components of sinuous channels and floodplains.
Two of the images show deposits from the Geodetic Hills fossil forest, an exquisitely preserved forest that thrived on Middle Eocene floodplains in what is now the Canadian Arctic.
The first three 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.
Meandering rivers usually flow through a single channel
References
The literature on fluvial systems and their deposits is vast (actually, this also applies to most other sedimentological domains); the accumulation of two centuries of knowledge and excellent science. The few cited here provide a taste of this talent.
J.S. Bridge, 2006. Fluvial facies models: Recent developments. In: Posamentier H W, Walker R G. (Eds.), Facies models revised. SEPM Special Publication 84, 2006: 85–170. This is a revision of the iconic volume Facies Models, originally published by the Geological Association of Canada (Geoscience Canada) (1976-79). Currently Open Access
S.K. Davidson, S.Leleu, and C.P. North (Eds.). 2011. From River to Rock Record: The preservation of fluvial sediments and their subsequent interpretation. SEPM Special Publication 97. 21 papers on many aspects of fluvial sedimentology.
A.D. Miall, 2006. The Geology of Fluvial Deposits: Sedimentary Facies, Basin Analysis, and Petroleum Geology. Springer.
G.S. Weissmann et al. 2015. Fluvial geomorphic elements in modern sedimentary basins and their potential preservation in the rock record: A review. Geomorphology, v.250, p. 187-219
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.
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 glaciofluvial-periglacial category refers to pretty well anything sedimentological, that is associated with glaciations, glaciers, ice caps, and ice sheets. It includes the ice itself, outwash sediment in fluvial and lacustrine environments, and ice-related phenomena like permafrost and patterned ground. Most of my examples are from Canada, the Arctic, and Laurentide Icesheet locations like Ottawa, Ontario, and Fraser Lowlands, British Columbia.
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 Atlas
The images:
Terminal moraines ahead of a retreating Strand Glacier, Axel Heiberg Island, as it was in 1983.
Part of the Strand Glacier terminal moraine, the meltwater, and outwash channels. The sediment eventually finds its way to Strand Fiord.
Mixed sediment and ice at the snout of Strand Glacier, Axel Heiberg Island
Tanquary Fiord, Ellesmere Island – a typical Arctic fiord, rimmed by steep terrain and small, coastal fan deltas. The arrow locates a Geological Survey of Canada base camp in 1988.
The edge of an ice cap on central Axel Heiberg Island, eroded by the outwash stream. Deformation of ice is accentuated by dark mud and sand in the ice (clear ice lies above the darker foundation. Ice rheology here includes ductile and brittle deformation.
Aerial view of the ice cap edge shown in the image above.
Lateral moraine and crevasses at the Valley of the Six Glaciers, Lake Louise, Alberta
Polygonal surface in trundra formed by freeze and thaw of surface permafrost. This stuff is really difficult to walk on.
Thermokarst slumping, caused by melting permafrost
Deformed ground ice within the permafrost, probably caused by ice expansion. The trees above are 3-4m high. The ice fold was exposed in the slip face of a thermokarst slump. North Yukon.
Frost heaved blocks of greywacke; the shape of the blocks is governed by intersecting fractures in the bedrock. Belcher Islands.
Glacial striae on greywacke; scratches formed as rock fragments are dragged across the bedrock surface by flowing ice during the Last Glaciation (Laurentide Icesheet).
A beautiful U-shaped valley at Glen Rosa, Arran (Scotland), gouged by a glacier during the Last Ice Age.
Winter freezing of an estuary at Cape Cod, Massachusetts.
Late Pleistocene, crossbedded glacial outwash channel deposits; the channel base is lined with boulders. Bradner Road pit, Fraser Valley
Detail of trough crossbeds in a glacial outwash channel, Stokes pit, Fraser Valley, British Columbia.
Large foresets (4-5m thick) in glacial outwash, may have formed as a small Gilbert-type delta in an outwash lake of meltwater pond. The overlying topset sand is about 1.5m thick, that, in turn was overridden by a diamictite during ice advance.Bradner Road pit, Fraser Valley.
A complex array of trough and planar crossbeds in Late Pleistocene glacial outwash channels. Bradner Road pit, Fraser Valley.
Ice-contact deformation of outwash sands along small listric faults, that appear to detach at the contact with pebbly sand below. Field note book for scale. Bradner Road pit.
Detached slump block, draped by outwash gravels. The block is also cut be several small faults. Deformation was probably caused by ice loading. Bradner Road pit, Fraser Valley.
Crossbedded and rippled glacial outwash in a gravel pit at Kanata, Ottawa. Arrows point to small thrust faults (movement to the left) probably caused by ice loading prior to deposition of the thicker crossbedded unit above (the faults do not extend into the overlying unit).
Thick trough crossbeds in an outwash channel, Kanata, Ottawa. The channel seems to have been filled by at least three stages of sediment influx, and scouring of the channel floor.
Outwash sand, deposited in large migrating dunes (upper half of outcrop), and multiple sets of climbing ripples (about level with the geologist). The topmost layer contains sandy muds folded and contorted by ice loading. Kanata, Ottawa
A nice example of in-phase ripples in fine-grained, outwash sand, Kanata, Ottawa
Kink folds in semi-consolidated outwash sand, formed during ice-contact, Kanata, Ottawa
This collection of images from various modern and ancient fluvial settings, includes meandering (high sinuosity) and braided (low sinuosity) rivers. 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:
Where transverse sea-waves (generated by wind shear) meet riverine standing waves. Manawapou River, west coast NZ.
An Arctic braided river, Strand Fiord, Axel Heiberg I. Flow is seasonal, and at a maximum during spring-early summer thaw. The closer view (right) shows a bit more detail of within-channel bar surface features. The sediment load is mixed sand-gravel.
Bonnet-Plume River (Yukon) braided reaches, that, unlike high Arctic examples, are framed within vegetated overbank and some inactive gravel bars. The sediment load is mixed sand-gravel. The left image also shows an ephemeral meandering reach that cuts through inactive braid-bars.
Vegetated braid bar gravels exposed in a cut bank. Bonnet-Plume River, Yukon.
Gravelly, braided reaches of Ahuriri River, Otago, NZ. The headwaters are in the distant Southern Alps.
Gravel bar, Ahuriri River, Otago, New Zealand, with falling-stage sand deposits along the bar trailing edge (downstream edge).
Gravelly, braided glacial outwash near the terminus of Strand glacier, Axel Heiberg Island. Erosional chutes, that form during falling stage, are filled with crossbedded sand.
2-dimensional pebbly dunes, each with simple planar tabular crossbeds, formed along the margins of larger gravel braid bars during river falling stage. Monster River, Yukon.
Braid bar-top ripples (flow to the right), imprinted by rain drops. Monster River, Yukon.
Glacial outwash flow during spring thaw, between gravel bars, Strand Glacier (Axel Heiberg Island). The water is mud-laden. The standing waves are in-phase with antidune bedforms.
Strong imbrication of flat clasts along an exposed river bed. Flow was to the right.
Bank-full conditions in an ephemeral, gravelly, Franklin Pierce Bay (Ellesmere Island) stream, during a rare mid summer Arctic rainstorm. About 25mm fell in a matter of hours. The stream rose very quickly and forced us to move our camp. There were many rockfalls from nearby cliffs during the storm.
Eocene gravel bar deposits, Otto Fiord, Ellesmere Island. The sandy interval mid-picture was interpreted as small, crossbedded, falling stage sand bars, like those shown in the modern analogues.
Left: Thick, Eocene, syntectonic gravelly braided river deposits (Buchanan Lake Formation; Franklin Pierce Bay, Ellesmere Island). Sediment was shed from evolving thrusts during the Eurekan Orogeny. Right: Large gravel crossbeds (up to 3m thick) in the same formation.
Two examples of trough crossbedded sandstone in low sinuosity channels (braided) in the Middle Eocene Buchanan Lake Fm, Axel Heiberg Island (Arctic Canada). These channels are the more distal equivalents to syntectonic gravelly deposits (Eurekan Orogeny).
Festooned trough crossbeds in Proterozoic low sinuosity sandy channels, Loaf Fm. Belcher Islands (about 2 billion years old). The right image shows small ripples that probably developed during waning stream flow.
As river deposits settle and consolidate, the water between grains is forced out by the weight of the sediment (this is called dewatering). The process commonly disrupts and contorts the sedimentary laminae, forming structures that superficially resemble pillows; these structures are given the general name ball and pillow. Loaf Fm. Belcher Islands (about 2 billion years old).
Reddened (iron oxides), desiccated mudrocks interbedded with channelized sandstone in the Proterozoic Loaf Fm, Belcher Islands; proof that the level of oxygen in the ancient atmosphere had increased significantly.
Paleocene Sandy braid channel and associated bar crossbeds (Summit Creek Fm.) exposed in right bank of MacKenzie River near Fort Norman, Northern Canada.
Active sand bars attached to a semi-permanent, vegetated bar, mid-stream, Clearwater River, Alberta. This is a possible candidate for an anastomosing river.
Large chunks of rock can be carried significant distances across open water while embedded in the tangle of tree roots. A possible answer to the mystery of some drop-stones.
Small meandering stream, point bars, and oxbow lake, north of Calgary, Alberta.
Point bar and overbank deposits in a high sinuosity (meandering channel) in the Upper Cretaceous Dunvegan Formation, Peace River, northeast British Columbia. Flow was to the right.
Crossbeds in Upper Cretaceous Dunvegan Formation, Peace River, northeast British Columbia. Left: stacked planar tabular crossbeds (2D subaqueous dunes); Right: Trough crossbeds (3D subaqueous dunes). Both types are associated with the Dunvegan point bar (image above), and the sand-filled channel below.
Sandstone-filled channel in the Dunvegan Formation, Peace River, northern British Columbia. This view shows overall asymmetry of the channel.
Detail of sandstone channel cutting into slightly older, muddy overbank deposits. Upper Cretaceous Dunvegan Formation, Peace River, northeast British Columbia.
Sandy point bar deposits in high sinuosity channel (meandering), overlain by thin floodplain lignites in the Middle Eocene Buchanan Lake Fm, Geodetic Hills, Axel Heiberg Island. The lignites contain abundant, well preserved conifers (Spruce), Hickory, and Metasequoia fronds, cones and seeds. The point bar is about 4m thick.
Small crossbeds and laminated sandstone in point bar deposits, Middle Eocene Buchanan Lake Fm, Geodetic Hills, Axel Heiberg Island. There is abundant plant material throughout. Pen (mid image) is 15cm long.
Multiple sets of climbing ripples, Middle Eocene Buchanan Lake Fm, Geodetic Hills, Axel Heiberg Island. These form when a significant suspended load of fine sand settles and becomes part of the bedload on the channel floor.
Multiple thin lignite – subbituminous coal beds with exquisitely preserved tree trunks in growth position, Middle Eocene Buchanan Lake Fm, Geodetic Hills, Axel Heiberg Island. The succession here represents a stacking of flood plain and forested areas adjacent to meandering rivers. These deposits accumulated in a more distal position to the emerging mountain front during the Eurekan Orogeny. See my post on the Fossil Forests
Some of the exquisitely preserved plant material from the Geodetic Hills Fossil forest. Left: Metasequoia cones looking like they were buried yesterday; Right: Metasequoia fronds and hardwood leaves. Middle Eocene Buchanan Lake Fm, Geodetic Hills, Axel Heiberg Island.
Large meandering river point bars and overlying floodplain-swamp muds, associated with the Princess Coals, Carboniferous of Kentucky, near Rush (Highway I-64). Point bar ‘foresets’ consist of very laminated, rippled, and slumped, fine grained sandstone-mudstone. I visited these outcrops during an excellent AAPG field trip run by John Horne, 1984.
Detail of the inclined point bar layers show numerous discontinuities in sandstone lenses and wedges, and truncation surfaces that indicate shifting sediment distribution across the bar, and possibly some erosion. Carboniferous of Kentucky, near Rush (Highway I-64).
Slumping and rotation of laminated sandstone-mudstone in point bar foresets (Carboniferous of Kentucky, near Rush, Highway I-64). Small synsedimentary faults cut the middle layers.
Odd-ball (sic) structure sometimes found in fluvial deposits, are armoured mudballs. These form when a chunk of sticky mud slumps from a channel margin, is rolled by currents along the channel floor, and in the process picks up small pebbles and bits of wood. The recent example on the left is from Mackenzie River, near Fort Norman. The Paleocene example, to the right of the lens cap, (right image) is from the Summit Creek Fm, in an outcrop fortuitously nearby the modern analogue – small pebbles impregnate the mud ball surface.
