Fluvial Environment

Velocity and DepositionStreams flow downhill from a source area to a lake or the sea. The gradient down which the stream flows is steeper in upper mountainous areas and shallower across the flood plain near the mouth of the stream. The velocity of water in a river is slower near a stream bed or bank than it is in the centre where flow is faster. This is due to friction with the channel floor and sides. The current tries to flow in a straight line so the bank is eroded where the current hits the outside of a river bend.Where velocity is reduced, deposition takes place. Reductions in velocity occur: where there is a sudden change in slope on the inside of meander bends where a tributary joins a river where rivers enter lakes or seas that do not have strong currents. Characteristic Products of Sedimentation in Fluvial EnvironmentsAlluvial fan breccias, conglomerates and arkosesWhere mountain streams flow onto a flat valley floor or plain there is a marked decrease in gradient and reduction in stream velocity and energy. Large volumes of sediment are deposited to form alluvial fans.Breccias form from angular and coarse scree fragments. Conglomerates composed of rounded pebbles form in the streams. Arkose is formed from sand sized material deposited rapidly in fairly arid conditions, as it contains more than 25% feldspar, which shows little sign of weathering. The usual source of the feldspar in granitic rock upstream.Channel Conglomerates and Sandstones in Meandering RiversMeandering streams flow in channels with looping curves. The bank on the outside of a meander bend is eroded and deposition occurs on the inside of the meander on a point bar. As a result, the channel migrates laterally. Meandering rivers and braided rivers result in upward fining cycles of sediments. Conglomerates form from the channel lagand may show imbricate structure if the clasts are disc or blade shaped. Sandstones form as the point bar deposits on the insides of meander bends in the channels. They are often cross bedded and moderately well sorted with sub-rounded grains. They can contain muscovite as well as quartz, so may be described as micaceous sandstones. The sandstones have rare masses of lignite, which are fossil tree ttunks washed into the rivers. The clays and mudstones frmed in the low energy environment of the flood plain.Flood Plain Clays and SiltsAt times of high flow, rivers flood the surrounding flat area adjacent to the channel, forming a flood plain. Clays rich in organic material are deposited from the suspended load of the river and may contain fossil plants. Siltstone is also common on the flood plain and may show very small scale cross bedding. The mudstones may show evidence of sub-aerial exposure to the elements, with dessication cracks and the development of soils. Any fossils will be terrestrial  in origin, for example, vertebrate footprints or plant fossils.

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