Fluvial sediments

Deposits formed by rivers. An alluvial river is one which flows within its own fluvial sediments, as distinct from one that has incised into the underlying bedrock. A river accumulates deposits because its capacity to carry sediment has been exceeded, and some of the sediment load is deposited. Rivers tend toward a state of dynamic equilibrium, in which they adjust their slope in response to changes in discharge and sediment load. The result is a channel profile that is steep in its source areas but flattens out downstream, and is graded to a slope of zero where the river discharges into a lake or the sea. Fluvial sedimentary accumulations range from temporary bars deposited on the insides of meander bends as a result of a loss of transport energy within a local eddy, to deposits tens to hundreds of meters thick formed within major valleys or on coastal plains as a result of the response of rivers to a long-term rise in base level or to the uplift of sediment source areas relative to the alluvial plain. Both these processes perturb the graded profile so that it tends to rise, creating space, or accommodation, for sediment. The same processes control the style of rivers and the range of deposits that are formed, so that a study of the deposits may enable the geologist to reconstruct the changes in controlling factors during the accumulation of the deposits.  See also: Depositional systems and environments; River; Stream transport and deposition

Coarse debris generated by mechanical weathering, including boulders, pebbles, and sand, is rolled or bounced along the river bed and is called bedload. The larger particles may be moved only infrequently during major floods. Finer material, of silt and clay grade, is transported as a suspended load, and there may also be a dissolved load generated by chemical weathering. Mass movement of large volumes of sediment by sediment gravity flows, typically debris flows, may occur when rare flash floods mobilize debris that may have been accumulating in source areas for some time. Whereas the volume of sediment tends to increase downstream within a drainage system, as tributaries run together, the grain size generally decreases as a result of abrasion and selective transport. This downstream grain-size decrease may assist in the reconstruction of transport directions in ancient deposits where other evidence of paleogeography has been obscured by erosion or tectonic change.  See also: Mass wasting

 

Types of river

 

River type may be described by two main variables, sinuosity and channel multiplicity. These variables combine to form four end-member styles, discussed below, although there are many examples of rivers showing various styles intermediate between these end members.

 

Braided rivers

 

These typically occur in areas of high sediment load and variable discharge. They consist of several or many branching, unstable channels of low sinuosity, and are characterized by abundant coarse bedload, forming bars, islands, and channel-floor deposits. The channel complex typically occupies most of the valley floor, leaving little room for a floodplain. Glacial outwash streams and ephemeral streams draining mountainous areas in arid regions are normally braided, and may form broad sheets of sand or gravel crossed by networks of shallow, shifting channels.

 

Meandering rivers

 

These are single-channel streams of high sinuosity, in which islands and midchannel bars are rare. Sediment in these rivers range from very coarse to very fine. A significant proportion of the bedload typically is deposited on the insides of meander bends, forming point bars. The channel, with its coarse deposits, may be confined to a narrow belt within an alluvial valley, flanked by a broad floodplain, upon which deposition of fine-grained sediment takes place only during flood events—seasonally or at longer intervals.

 

Anastomosed rivers

 

These develop in stable, low-energy environments or in areas undergoing rapid aggradation. They consist of a network of relatively stable, low- to high-sinuosity channels bounded by well-developed floodplains. Channels are characteristically narrow and accumulate narrow, ribbonlike sandstone bodies.

 

Straight channels

 

These are rare, occurring mainly as distributaries in some deltas.

Rivers which emerge from a mountainous catchment area into a low plain drop their sediment load rapidly. The channel may bifurcate, becoming braided in character. A distinctive landform, an alluvial fan, results.

 

 

Sedimentary facies

 

River deposits of sediment occur as four main types.

 

Channel-floor sediments

 

The coarsest bedload is transported at the base of the channel, commonly resulting in deposits of gravel (Fig. 1a), waterlogged vegetation, or fragments of caved bank material. In sand-bed rivers the channel floor commonly is covered by fields of large, sinuous-crested dunes or ripples (with amplitudes of 2 in. to 10 ft, or 5 cm to 3 m), which impart a trough–cross-bedded structure to the sand.

 

 

Fig. 1  Typical fluvial deposits. (a) Gravel and sand channel-fill and bar deposits exposed in a gravel quarry face about 40 ft (12 m) high, fluvioglacial outwash, Alberta, Canada. (b) Point bar, 13 ft (4 m) thick, Carboniferous, Alabama. (c) Typical floodplain deposits, Triassic, Arizona; outcrop is about 33 ft (10 m) high.

 

 

 

 

 

 

 

Bar sediments

 

Accumulations of gravel, sand, or silt occur along river banks and are deposited within channels, forming bars that may be of temporary duration, or may last for many years, eventually becoming vegetated and semipermanent. Bars attached to one of the channel banks are termed side or lateral bars. Those occurring on the insides of meander bends are termed point bars. They develop by lateral accretion as the meander widens or shifts in position by erosion on the outer bank of the bend (Fig. 1b and Fig. 2). Bars occurring within channels accrete by the addition of sediment on all sides, but most commonly preferentially on one side representing the inside of a bend in the adjacent channel, or at the downstream end of the bar. Such bars commonly have complex internal structures, reflecting many seasons of growth and intervals of erosion.

 

 

Fig. 2  Development of a fining-upward succession by lateral accretion of a point bar, such as that in Fig. 1b.

 

 

 

 

 

 

 

Channel-top and bar-top sediments

 

These are typically composed of fine-grained sand and silt, and are formed in the shallow-water regions on top of bars, in the shallows at the edges of channels, and in abandoned channels. Small-scale ripples, with amplitudes of less than 2 in. (5 cm), are typical sedimentary structures, together with roots and bioturbation structures.

 

Floodplain deposits

 

These are formed when the water level rises above the confines of the channel and overflows the banks (Fig. 1c). Much of the coarser sediment is deposited close to the channel, in the form of levees. Breaks in the channel bank, termed crevasses, permit the transportation of additional coarse sediment onto the floodplain, where it forms small deltalike bodies spreading out into the floodplain, termed crevasse splays. Much silt and mud may be carried considerable distances from the channel, forming blanketlike deposits. In swampy areas, floodplains may be the site of thick vegetation, which in time may be transformed into lignite and eventually into coal. Soils develop in response to weathering activity and plant growth, and may form distinctive brightly colored layers, termed paleosols. Nodular beds of calcium carbonate are a common component of paleosols, especially in relatively arid areas, where they form as a result of the evaporation of ground waters.  See also: Paleosol

 

 

Facies associations and sedimentary cycles

 

Fluvial sediments may be dominantly conglomeratic, sandy, or silty, depending on the nature of the sediment load of the river. This characteristic is mainly a function of slope and the proximity to sources, but is also a reflection of sediment availability. Certain source materials, such as fine-grained sediments or limestones, may yield coarse debris on erosion, but it is likely to be broken down into fine material or dissolved on prolonged transportation. Humid climates favor chemical and biochemical weathering processes, which yield a large suspended or dissolved sediment load. Coarse detritus is more typically the product of drier climates, in which mechanical weathering processes (such as frost shattering) are dominant.

The sedimentary facies described above were listed in approximate vertical spatial order, from channel floor to floodplain. This order is one of decreasing grain size upward, a feature which may commonly be observed in ancient fluvial deposits (Fig. 2). Such deposits may consist of a series of fining-upward successions, or cycles, each a few meters to few tens of meters in thickness.

There are a variety of causes of such cycles. The first that was recognized is the mechanism of lateral accretion, whereby point bars enlarge themselves in a horizontal direction as the meander bounding them migrates by undercutting the bank on the outside of the bend (Fig. 2). The depositional surface of the point bar may be preserved as a form of large-scale, low-angle cross-bedding within the deposit, its amplitude corresponding approximately to the depth of the channel. Similar cycles may be caused by the nucleation and growth of large compound bars or sand flats within braided channel systems. The accretion surfaces, in such cases, may dip in across- or down-channel directions. Individual flood events, especially on the sand flats of ephemeral stream systems, may form sheetlike flood cycle deposits up to a meter or so thick, the upward fining corresponding to decreasing energy levels as the flood waned. The gradual choking of a channel with sediment, and the progressive abandonment of the channel, will also generate a fining-upward cycle.

 

Regional controls

 

Tectonic activity in a fluvial catchment area may cause the generation of sedimentary cycles, which either fine or coarsen upward, depending on whether relief and slope are decreased or increased, respectively. Such cycles tend to be tens to hundreds of meters thick and to extend for several to many kilometers. They may have smaller cycles formed by channel fill and migration processes nested within them.

River systems are also affected by changes in base level, that is, by a rise or fall in the level of the lake or sea into which the river drains. A fall in base level may lead to widespread incision of channels along a coastal plain as they adjust to a lower river mouth. Between channels, sedimentation may cease, with the formation of widespread, well-developed paleosols. The same effect is brought about by peneplanation, that is, long-continued subaerial erosion in the absence of tectonic rejuvenation of the river system.

A rise in base level may flood the mouths of rivers, forming estuaries. However, if the sediment supply is adequate, sedimentation may be able to keep pace with such a base-level rise, with the river changing in style in response to changes in the balance between the rate of sediment input and the rate of base-level change (Fig. 3). These sedimentary responses to external forcing are part of a larger story concerning the regional and global controls of sedimentation. The regional erosion surface formed at a time of falling to low base level or deep peneplanation is termed a sequence boundary. It may be cut by incised channels that are, in turn, typically filled by coarse channel sediments (in sequence stratigraphic terminology, these deposits are classified as the lowstand systems tract). At the coast a rise in base level is commonly recorded by transgression (the transgressive systems tract). The coastal rivers at this time may be of anastomosed style. As base-level rise reaches its highest level, the rate of rise slows, and the rivers typically evolve into a meandering type (highstand systems tract). These changes may be reflected in the resulting sediments by changes in the geometry and spacing of channel sandstone and conglomerate bodies. Lowstand deposits commonly consist of coarse, laterally amalgamated channel deposits. A rapid rise in base level (the transgressive systems tract) may be marked by isolated channel sands spaced out within thick floodplain units. The spacing of such channel bodies becomes closer, and more units are amalgamated, in the highstand deposits, above.  See also: Floodplain; Sequence stratigraphy

 

 

Fig. 3  Response of river systems and their deposits to a cycle of fall and rise of base level. SB = sequence boundary. (Modified from K. W. Shanley and P. J. McCabe, Perspectives on the Sequence Stratigraphy of Continental Strata, American Association of Petroleum Geologists, vol. 78:560, 1994)

 

 

 

 

 

 

Far inland, base-level changes may not markedly affect the rivers unless they persist for very long periods of time. Changes in discharge and sediment yield in response to climate change are commonly more important in controlling river style and the resulting sediment types. For example, during the Pleistocene glaciation, rapid deposition of coarse sediments occurred at the margins of continental ice caps. Channel erosion, forming widespread surfaces of incision, occurred at times of change, between glacial and interglacial periods, because at these times river discharge tended to increase whereas sediment yield did not. Climatically driven erosion and deposition inland were therefore out of phase with the cycle of change generated by base-level change at the coast.

More than one forcing function, including climate change, base-level change, and tectonism, may be operating at any one time, resulting in complex patterns of cyclicity. The resulting sequences may be widespread. Reconstruction of this sequence stratigraphy may provide an essential mapping tool for those engaged in basinal exploration.

 

Tectonic setting of fluvial deposits

 

The thickest (up to 6 mi or 10 km) and most extensive fluvial deposits occur in convergent plate-tectonic settings, including regions of plate collision, because this is where the highest surface relief and consequently the most energetic rivers and most abundant debris are present. Some of the most important accumulations occur in foreland basins, which are formed where the continental margin is depressed by the mass of thickened crust formed by convergent tectonism. Examples include the modern Himalayan foreland basin of the Indus and Ganges valleys, the Devonian foreland basin west of the Appalachian Mountains, and the late Cenozoic foreland basin of France and Germany, north of the alpine mountain chain.  See also: Basin

Thick fluvial deposits also occur in rift basins, where continents are undergoing stretching and separation. The famous hominid-bearing sediments of Olduvai Gorge and Lake Rudolf are fluvial and lacustrine deposits formed in the East Africa Rift System. Triassic fault-bounded basins along the North American Atlantic coast and through western Europe are an older but comparable example. Fluvial deposits are also common in wrench-fault basins, such as those in California.

 

Economic importance

 

Significant volumes of oil and gas are trapped in fluvial sandstones. Major reservoirs include those of Triassic-Jurassic age in the North Sea Basin; Triassic sandstones of the Paris Basin; Permian-Triassic sandstones of Prudhoe Bay on the Alaskan North Slope; the Lower Cretaceous reservoirs of the giant Daqing field of the Songliao Basin, China; Jurassic sandstones of interior Australia; the heavy-oil sands of the Cretaceous Athabasca and related deposits in Alberta; and numerous large to small fields in mature areas such as the Alberta Basin (Cretaceous), the southern midcontinent (Pennsylvanian), and the Gulf Coast (Cretaceous and Oligocene fields).

Placer gold, uranium, and diamond deposits of considerable economic importance occur in the ancient rock record in South Africa and Ontario, Canada, and in Quaternary deposits in California and Yukon Territory. Economically significant roll-front uranium deposits occur in the Mesozoic deposits of the American Western Interior and elsewhere, primarily in fluvial facies.

Fluvial deposits are also essential aquifers, especially the postglacial valley-fill complexes of urban Europe and North America. Much work needs to be done to investigate the internal geometry of these deposits in order to resolve problems of domestic and industrial pollution that now interfere with the use of the ground water from these sources.

Andrew D. Miall

 

Bibliography

 

 

  • P. A. Carling and M. R. Dawson (eds.), Advances in Fluvial Dynamics and Stratigraphy, Wiley, 1996
  • C. R. Fielding (ed.), Current research in fluvial sedimentology, Sed. Geol., special issue, vol. 85, 1993
  • A. D. Miall, The Geology of Fluvial Deposits, Springer-Verlag, 1996
  •  Alifazeli=egeology.blogfa.com