Braided gravel river draining into Canon Fiord, Ellesmere Island; autogenic storage and release from channel bars and the river mouth fan delta. Flow is seasonal and a maximum during spring-early summer thaw. The river is fed by small alluvial fans from the hills on either side of the valley.
This collection of images from various modern and ancient fluvial deposits, includes meandering (high sinuosity) and braided (low sinuosity) rivers. Where possible I have paired modern analogues with ancient examples.
The closer view of the river shown above shows a bit more detail of channel bar surface features exposed at low water. 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 view also shows an ephemeral meandering reach that cuts through inactive braid-bars (lower right).More detailed view of active mid-channel bars, channels, and chutes on Bonnet Plume River (Yukon). Dark grey patches on the bar tops are accumulations of wood and plant debris probably captured during the falling stage of river levels. The sediment is a mix of sand and gravel.Vegetated braid bar gravels exposed in a cut bank. Bonnet-Plume River, Yukon.Gravelly, braided reaches of Ahuriri River, Otago, NZ. Some bars are sparsely vegetated. 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) by flow diverted during exposure of the main bar top. The sand wedges contain small planar-tabular and ripple crossbeds.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. Sand patches on the bar top also represent the falling stage flow.Glacial outwash flow between gravel bars during spring thaw, Strand Glacier (Axel Heiberg Island). The water is mud-laden. Flow here has nicely formed antidunes (standing waves) indicating supercritical flow conditions.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.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.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.Thick, Eocene, syntectonic gravelly braided river deposits (Buchanan Lake Formation; Franklin Pierce Bay, Ellesmere Island). Sediment was shed from evolving thrusts during the Mid-Late Eocene (?) Eurekan Orogeny. Large braid-bar gravel crossbeds (up to 3m thick) in the Buchanan Lake Formation.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 braided river deposits (Eurekan Orogeny).The exposure of trough cross-stratification on bedding is expressed as festoons of intersecting and overlapping crossbed sets. The curved base and foresets provide an excellent opportunity to measure paleoflow directions on a large number of individual crossbeds, wherein each flow vector bisects the curved foresets. In this example, flow was approximately parallel to the hammer handle towards the top of the photo. Middle Eocene Buchanan Lake Fm. Axel Heiberg Island.Festooned trough crossbeds in Proterozoic low sinuosity sandy channels, Loaf Fm. Belcher Islands (1.8 to 2 billion years old).
Exhumed trough of a trough crossbed – the trough axis approximately in line with the hammer; paleoflow to lower left. The surface also contains small lunate ripples. Paleoproterozoic Loaf Fm, Belcher Islands, Hudson Bay.
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 1.8-2 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 to the point where ferric iron was stable.Paleocene Sandy braid channel and associated bar crossbeds (Summit Creek Fm.) exposed in right bank of MacKenzie River near Fort Norman, Northern Canada. The following image shows continuation of the exposure to the right.Large planar-tabular and trough crossbeds in Paleocene sandy channel deposits, right bank of MacKenzie River near Fort Norman, Northern Canada. A succession of actively migrating channels are indicated by discordant pebble lags, Active sand bars attached to a semi-permanent, vegetated bar, mid-stream, Clearwater River, Alberta. This is a possible candidate for an anastomosing river. The submerged bar top also contains smaller subaqueous dunes. Flow to the left.Active sand bars attached to a semi-permanent, vegetated bar, mid-stream, Clearwater River, Alberta. The submerged, active bar top contains smaller subaqueous dunes. Flow to lower left.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.A typical fining-upward unit in Cretaceous fluvial channel and point bar deposits, Dunvegan Fm. Peace River, Alberta. The stratigraphic column width represents relative grain size; the unit is about 6m thick. The more massive looking, pale brown sandstones below are crossbedded (mainly trough crossbeds) channel thalweg deposits.Stacked tabular crossbeds, a record of successive 2D bedform migration across a Cretaceous river channel sand bed (Dunvegan Fm. Alberta). Yellow arrows indicate crossbed set contacts. Foresets in all units have tangential basal contacts (red arrow). Some foresets also have tangential upper contacts and where these occur the foresets present a sinusoidal geometry. Flow was to the left.Small trough crossbeds tend to occur near the base of the paleochannel – some have pebble lags. Cretaceous Dunvegan Formation, northern Alberta.Crossbedded sandstone-filled channel in the Dunvegan Formation, Peace River, northern British Columbia. This view shows overall asymmetry of the channel. The channel has cut into muddy, carbonaceous, overbank-floodplain deposits.Sandy point bar deposits (sandstone below the coal beds) in high sinuosity channel (meandering), overlain by thin floodplain lignites in the Middle Eocene Buchanan Lake Fm, Geodetic Hills, Axel Heiberg Island. Point bar foresets dip left. The lignites contain abundant, well preserved conifers (Spruce), Hickory, and Metasequoia fronds, cones and seeds. The point bar is about 4m thick. It overlies floodplain mudstone with thin coal layers that represent short-lived paleosols.Small crossbeds and laminated sandstone show the complexity of bedforms that can develop on point bar foresets. There is abundant plant material throughout. Pen (mid image) is 15cm long. Middle Eocene Buchanan Lake Fm, Geodetic Hills, Axel Heiberg Island. 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 a post on the Fossil ForestsSome of the exquisitely preserved Metasequoia cones from the Middle Eocene, Geodetic Hills Fossil forest, Buchanan Lake Fm, looking like they were buried recently.Metasequoia fronds and hardwood leaves. Middle Eocene Buchanan Lake Fm, Geodetic Hills Fossil Forest, 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 laminated, rippled, and slumped, fine grained sandstone-mudstone. I visited these outcrops during an excellent AAPG field trip run by John Horne, 1984.Large meandering river point bar (left dipping beds) and overlying floodplain-swamp muds, associated with the Princess Coals, Carboniferous of Kentucky, near Rush (Highway I-64). Point bar ‘foresets’ consist of laminated, rippled, and slumped, fine grained sandstone-mudstone.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.Armoured mudballs, an odd-ball (sic) structure sometimes found in fluvial deposits. 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 armoured mudball, to the right of the lens cap, from the Summit Creek Fm – small pebbles pepper the mud ball surface.
