FLUVIAL ENVIRONMENTS | Sediments

672 FLUVIAL ENVIRONMENTS/Sediments Shane, P. A., Black, T., and Westgate, J. A. (1994). Isothermal plateau fission-track age for a paleomagnetic excursion in the Mamaku Ignimbrite, New Zealand, and implications for late Quaternary stratigraphy. Geophysical Research Letters 21, 1695–1698. Shane, P. A., Black, T. M., Alloway, B. V., and Westgate, J. A. (1996). Early to Middle Pleistocene tephrochronology of North Island, New Zealand: Implications for volcanism, tectonism and paleoenvironments. Geological Society of America Bulletin 108, 915–925. Shane, P. A., Froggatt, P., Black, T. S., and Westgate, J. A. (1995b). Chronology of Pliocene and Quaternary bioevents and climatic events from fission-track ages on tephra beds, Wairarapa, New Zealand. Earth and Planetary Science Letters 130, 141–154. Staudacher, T. H., Jessberger, E. K., Dominik, B., Kirsten, T., and Schaeffer, O. A. (1982). 40Ar/39Ar ages of rocks and glasses from the Nordlinger Ries Crater and the temperature history of impact breccias. Journal of Geophysics 51, 1–11. Storzer, D., and Poupeau, G. (1973). Ages-plateaux de mine´raux et verres par la me´thode des traces de fission. Comptes Rendus De L’ Academie des Sciences Paris, Se´rie D 276, 137–139. Storzer, D., and Wagner, G. A. (1969). Correction of thermally lowered fission-track ages of tektites. Earth and Planetary Science Letters 5, 463–468. Storzer, D., and Wagner, G. A. (1982). The application of fission track dating in stratigraphy: A critical review. In Numerical Dating in Stratigraphy (G. S. Odin, Ed.), pp. 199–221. Wiley, New Jersey. Van den haute, P. (1985). The density and the diameter of fission tracks in glass with respect to age interpretation. Nuclear Tracks 10, 335–348. Wagner, G. A., and Van den haute, P. (1992). Fission-Track Dating. Enke, Stuttgart. Walter, R. C., and Aronson, J. L. (1993). Age and source of the Sidi Hakoma Tuff, Hadar formation, Ethiopia. Journal of Human Evolution 25, 229–240.

Westgate, J. A. (1989). Isothermal plateau fission-track ages of hydrated glass shards from silicic tephra beds. Earth and Planetary Science Letters 95, 226–234. Westgate, J. A., Christiansen, E. A., and Boellstorff, J. D. (1977). Wascana Creek Ash (Middle Pleistocene) in southern Saskatchewan: Characterization, source, fission-track age, palaeomagnetism and stratigraphic significance. Canadian Journal of Earth Sciences 14, 357–374. Westgate, J. A., Easterbrook, D. J., Naeser, N. D., and Carson, R. J. (1987). Lake Tapps tephra: An Early Pleistocene stratigraphic marker in the Puget Lowland, Washington. Quaternary Research 28, 340–355. Westgate, J. A., Preece, S. J., Froese, D. G., Walter, R. C., Sandhu, A. S., and Schweger, C. E. (2001). Dating early and middle (Reid) Pleistocene glaciations in central Yukon by tephrochronology. Quaternary Research 56, 335–348. Westgate, J. A., Sandhu, A. S., Preece, S. J., and Froese, D. G. (2003). Age of the gold-bearing White Channel gravel, Klondike district, Yukon. In Yukon Exploration and Geology 2002 (D. S. Emond and L. L. Lewis, Eds.), pp. 241–250. Yukon Geological Survey, Whitehorse. Westgate, J. A., Shane, P. A., Pearce, N. J. G., et al. (1998). All Toba tephra occurrences across peninsula India belong to the 75 ka eruption. Quaternary Research 50, 107–112. Westgate, J. A., Stemper, B. A., and Pe´we´, T. L. (1990). A 3 m.y. record of Pliocene–Pleistocene loess in interior Alaska. Geology 18, 858–861. Wilcox, R. E., and Naeser, C. W. (1992). The Pearlette family ash beds in the Great Plains: Finding their identities and their roots in the Yellowstone Country. In Quaternary International: Tephrochronology: Stratigraphic Applications of Tephra Beds (J. A. Westgate, R. C. Walter and N. D. Naeser, Eds.), Vol. 13–14, pp. 9–13. Pergamon Press, Oxford, UK. Young, D. A. (1958). Etching of radiation damage in lithium fluoride. Nature 182, 375–377.

FLUVIAL ENVIRONMENTS Contents Sediments Responses to Rapid Environmental Change Terrace Sequences Deltaic Environments

Sediments A Aslan, Mesa State College, CO, USA ª

2007 Elsevier B.V. All rights reserved.

Introduction Rivers are a dominant feature of most landscapes and thus fluvial sediments are widespread. Fluvial deposits are represented by a continuum of sediment types that range from clay- to gravel-size particles, and include both terrigenous and organic deposits (see

Terrigenous Sediments). Fluvial sediments provide an important link between weathering and slope processes in source areas as well as deltaic and coastal processes within depositional basins (see Deltaic Environments). Investigations of fluvial sediments are legion; recent syntheses include those by Miall (1996), Blum and To¨rnqvist, (2000), and Bridge (2003). Two additional aspects of fluvial sediments deserve mention. First, these deposits represent economically important reservoirs of oil, natural gas, and water and, include significant coal reserves. Second, fluvial sediments provide critical records of past and present geologic processes and terrestrial

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environments. This chapter summarizes the major characteristics of fluvial sediments and discusses key differences between river systems of continental interiors and those associated with subsiding continental margins (see Responses to Rapid Environmental Change).

Fluvial Sediments Fluvial sediments consist predominantly of channel and overbank deposits (Allen, 1965). Channel deposits are dominated by sand and gravel and represent the bed load of rivers. Overbank deposits generally consist primarily of clay, silt, and lesser amounts of very fine to fine sand and represent suspended-load sediments that accumulate on floodplains. Organic sediments such as peat, and chemical sediments such as carbonate and evaporite

minerals, are also important constituents in some overbank settings. Channel Deposits Channel deposits consist of channel-bar and channelfill sediments (Bridge, 2003). Channel deposits commonly occur as isolated lenticular sand bodies (i.e., ribbon sands) or as thick sheet sands associated with channel belts (Figs. 1–3). Channel belts are floodplain areas containing active and abandoned channels and channel bars that record the activity of individual channel systems. Channel-bar deposits Channel bars typically form in areas where decreasing flow velocities lead to bed-load sedimentation, such as along the inner bend of river meanders (Fig. 4A). Channel-bar sediments are stratified sands and gravels, which are

Macon Ridge

20 km Mississippi R

Natchez N

Holocene Miss. R. channel belt 1

Holocene Miss. R. channel belt 2

Holocene Miss. R. channel belt 3

Holocene Miss. R. channel belt 4

Holocene flood basin

Pleistocene braid belt

Pleistocene undifferentiated

Abandoned Miss. R. channels

Figure 1 Geologic map of the Mississippi River floodplain near Natchez, Mississippi. Four Holocene Mississippi River channel belts are shown. Modified from Aslan and Autin (1999).

674 FLUVIAL ENVIRONMENTS/Sediments

Flood basin Mis

siss

ipp

i

R

Alluvial ridge

Flood basin

N 5 km

Figure 2 Satellite image of the Mississippi River floodplain south of Baton Rouge, Louisiana. Cultivated areas located adjacent to the river are on natural levees. Flow direction is to the southeast.

2 point bar

channel belt

1

braid bar

chute

1 groups of large-scale inclined strata sets (sandstone body): channel belt

2

set of large-scale inclined strata (storey): channel bar and fill

large-scale inclined stratum: accretion on channel bar simple

compound

small-scale cross strata: ripples planar strata

medium-scale cross strata: dunes

medium-scale cross strata superimposed on simple large-scale inclined strata: dunes on unit bar

Figure 3 Diagrams showing different scales of fluvial deposits. The upper diagrams show cross-sections through a channel belt composed of multiple depositional units. Large-scale inclined strata (individual examples shown by shaded portions of crosssections) are the dominant depositional unit of the channel-belt deposits. At the smallest scale, cross strata representing migrating dunes are present within the large-scale inclined strata. From Bridge (2003). Reprinted with permission from Blackwell Publishing.

arranged in sets of large-scale inclined strata (Bridge, 2003) (Fig. 3). Large-scale inclined strata consist of gently dipping cross strata that reflect episodic migration of major bar forms (i.e., point bars, side bars) in response to channel translation and/or expansion (Fig. 4B). The base of large-scale inclined strata sets is an erosional bounding surface, which often is overlain by rip-up clasts derived from fine-grained bed and bank materials. Within large-scale inclined strata, smaller-scale sets of cross-stratified sand and/or gravel are present (Figs. 3 and 4C). These smaller-scale packages of cross-stratified deposits represent bed forms such as dunes and ripples that migrate along the surface of major bars and the bed of channels. Common sedimentary structures associated with these smaller bed forms include small- to medium-scale trough and planar cross-bedding as well as small-scale ripple stratification (Bridge, 2003). Finer-grained sediments such as mud can be present in the upper portion of bar deposits, or occur as mud drapes that separate sets of cross strata. Channel-fill deposits Channel-fill deposits are highly variable; sediments range from coarse-grained sand and gravel to fine-grained mud and organic sediments. Specific characteristics of channel fills depend on flow conditions at the time of channel abandonment. In vertical profiles through channelfill deposits, grain size generally decreases upwards. Grain size also tends to decrease in a downstream direction. The dominance of coarser-grained bedload deposits or finer-grained suspended-load sediments depends on whether or not the channel-fill succession represents an active or passive channel fill. Active fills are typically associated with upstream segments of abandoned channels where flow reduction was gradual. In contrast, passive fills are often associated with the central or lower portions of abandoned channels where flow reduction was severe. Active fills are characterized by bed-load deposits that represent the migration of bar forms and smaller-scale dunes and ripples within the upstream portion of the abandoned channel. As flow wanes, active channel-fill deposits transition upwards to smaller grain sizes and thinner bed sets representing progressively smaller bars and bed forms. Passive fills are represented by clay, silt, and very fine sand deposited from suspension as well as organic sediment such as peat. These deposits are concentrated in the middle and downstream portions of an abandoned channel and are associated with rapid decreases in flow such as occurs during neck cutoff and formation of an oxbow lake.

FLUVIAL ENVIRONMENTS/Sediments 675

(A)

(B)

(C)

Figure 4 (A) Mississippi River and major point bar and chute-channel complex located several kilometers northwest of Vicksburg, Mississippi. The river is approximately 1 km wide. View is upstream. (B) Cross-sectional view through an eroding point bar of the Trinity River in Texas showing a set of large-scale inclined strata. The bank is approximately 6 m tall. Flow direction is to the left. (C) Stacked sets of planar cross-bedded sand representing migrating dunes on a point bar of the Trinity River in Texas. Flow is to the right.

Overbank Deposits Overbank sediments accumulate during floods in natural-levee, crevasse-splay, and flood-basin environments (Fig. 5). Sediment grain size and accumulation rate decreases away from the active channel (Fig. 6A). Avulsion, the abandonment of all or part of a channel belt in favor of a new course, also plays an important role in the accumulation of overbank deposits (Allen, 1965; Smith et al., 1989). In general, overbank sediments have received considerably less attention than channel deposits (Farrell, 1987). Modern examples of overbank deposits are numerous (Farrell, 1987; Smith et al., 1989; Aslan and Autin, 1999). Classifications of these deposits are presented in Platt and Keller (1992) and Miall (1996). Although overbank deposits are sometimes viewed as monotonous successions of fine-grained alluvium, the recognition of paleosols in ancient overbank sediments has proved useful for subdividing and interpreting these deposits (Kraus, 1999) (see Nature of Paleosols). Additionally, there is increasing recognition that fine-grained sediments in both modern and ancient settings that have traditionally been interpreted as products of repeated overbank flooding, may represent episodes of avulsion (Smith et al., 1989; Kraus, 1999; Morozova and Smith, 1999; Stouthamer and Berendsen, 2000; Slingerland and Smith, 2004). Natural-levee sediments Natural levees are asymmetric ridges that slope away from active and abandoned channels towards adjacent flood basins. These features are typically well developed along low-gradient sinuous rivers with large suspended-sediment loads that are subject to deep overbank floods such as the Mississippi (Fig. 2). Natural levees can rise up to 5 m above adjacent flood basins and are up to several kilometers wide on floodplains such as the Mississippi (Aslan and Autin, 1999; To¨rnqvist and Bridge, 2002). Natural-levee sediments are wedges of sand and silt with minor amounts of clay that typically thin and fine

Figure 5 Flooding of the Missouri River in August 1993. Floodwaters have completely inundated the floodplain near Jefferson City, Missouri. Trees on artificial levees outline the sinuous bankfull channel. Areas that are not flooded represent uplands along the margins of the river valley. View is downstream.

away from channel margins as a result of decelerating overbank flow (Fig. 6A). Typical facies are similar to those of upper channel-bar deposits (Bridge, 2003), and consist of horizontal planar sand, small-scale cross-stratified sand, and bioturbated mud (Farrell, 1987). Proximal natural-levee deposits are relatively thick, sandy, and thick bedded. Distal natural-levee

676 FLUVIAL ENVIRONMENTS/Sediments

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(B) Point bar

Natural levee

Point bar

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Crevasse splay Flood basin A

A′

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Increasing sedimentation rate, variable grain sizes

Decreasing sedimentation rate, grain size

Sand

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Figure 6 Floodplain maps and cross-sections showing (A) floodplain deposits formed by overbank flooding and sedimentation, and (B) floodplain deposits related to crevassing and avulsion. Note that avulsion deposits thicken away from the active channel belt. Modified from Aslan and Autin (1999).

sediments are thinner, contain a greater percentage of silt and clay, and show more evidence of bioturbation and pedogenesis. Natural-levee facies often occur as vertically stacked meter-thick, fining- or coarseningupward sequences, which reflect periods of episodic overbank flooding and levee abandonment or progradation, respectively (see Sequence Stratigraphy). Crevasse-channel and crevasse-splay sediments Crevasse channels are breaches in natural levees and transfer water and sediment from mainstem channels to flood basins. Crevasse channels often bifurcate (A)

into simple or complex distributary systems that discharge into flood-basin lakes or wetlands (Smith et al., 1989; Smith and Pe´rez-Arlucea, 1994; Pe´rezArlucea and Smith, 1999). The linear to lobate accumulation of sediment surrounding crevasse channels is referred to as a crevasse splay, and includes the distributary channel network (Fig. 7A). Where crevasse channels flow into lakes they form lacustrine deltas. Crevasse channels have natural levees as well as distributary- or terminal-mouth bars (Fig. 7B). Crevasse-splay channels can extend up to 10 km into flood basins (Smith, 1986; Smith et al., 1989). (B)

Figure 7 (A) Crevasse splay along Cano Macareo, a major distributary of the Orinoco Delta in Venezuela. Photograph was taken towards the end of the rainy season as water levels began to fall. (B) Sandy distributary mouth-bar deposits at the terminus of one of the crevasse channels shown in (A). The crevasse-splay system discharges into a seasonal wetland.

FLUVIAL ENVIRONMENTS/Sediments 677

Crevasse-channel deposits are typically lenticular bodies of sandy channel-bar and sandy to muddy channel-fill sediments. Grain sizes associated with crevasse-channel deposits are similar to those of the bed load fraction of the mainstem channel, and are somewhat coarser than other types of overbank sediments. However, muddy fine-grained crevasse-channel fills do occur (Smith and Pe´rez-Arlucea, 1994; Pe´rezArlucea and Smith, 1999). Stratification types include small- to medium-scale cross bedding and ripple lamination. Evidence of frequent changes in flow stage such as desiccation cracks and bioturbation features may be present (Bridge, 2003). Crevasse-channel deposit thicknesses will generally be smaller than the thickness of the main-channel sand bodies. Crevasse-splay deposits range from tabular to wedge-shaped packages of interbedded sand and silt with lesser amounts of mud. Individual crevassesplay sand bodies are generally 1 to 2 m thick. Strata are commonly bioturbated and ripple laminated (Farrell, 1987). Individual splay packages often show upward-coarsening or upward-fining sequences similar to natural-levee deposits (Pe´rezArlucea and Smith, 1999). Upward-coarsening sequences reflect crevasse-splay progradation whereas deposition of overbank silt and clay during waning flood stages produces upward-fining deposits. Crevasse-splay deposits thin or thicken away from channel margins depending on local floodplain topography (Willis and Behrensmeyer, 1994). Crevasse-splay sedimentation is particularly important within avulsion belts (Smith et al., 1989) and is

(A)

responsible for rapid fine-grained floodplain aggradation, a key attribute of avulsion. Flood-basin sediments Flood basins are broad seasonally flooded depressions that separate channel belts (Fig. 8). These areas are characterized by low relief, shallow water tables, and complex drainage channel networks. In humid settings, flood basins are vegetated and contain lakes and perennial or seasonal wetlands. In more arid environments, flood basins are sparsely vegetated and can have ephemeral lakes. Eolian processes may also be important in arid settings. Flood-basin deposits are generally tabular accumulations of clay and silt that bury channel deposits or interfinger with sand bodies representing minor floodplain channels and crevasse-splay deposits (Aslan and Autin, 1999) (Figs. 9 and 10). Flood-basin sediments are often cross-cut by channel-belt sand bodies (Stouthamer and Berendsen, 2000). In Quaternary settings, these deposits are typically meters to tens of meters thick, but can be hundreds of meters thick in pre-Quaternary alluvial successions. Slow sediment accumulation rates in flood basins lead to soil development. Bioturbation features such as root mottles and burrows along with soil nodules and mud cracks are common in these deposits. Flood-basin sediment characteristics largely depend on climatic conditions. In humid areas with a shallow fluctuating water table (i.e., seasonal wetlands), gray sediment colors and yellow-brown to red color mottles reflect episodes of seasonal wetting and drying. Authigenic minerals such as iron and manganese

(B)

Figure 8 (A) Seasonal flooding of the Orinoco Delta, Venezuela. Vast tracts of interdistributary flood basins are inundated for several months of each year. During this time, only the crests of natural levees of distributary channels are subaerially exposed. (B) Mud cracks developed in flood-basin silt and clay during the dry season on the Orinoco Delta. This area is completely inundated during the rainy season.

678 FLUVIAL ENVIRONMENTS/Sediments

Flood-basin mud

Channel-belt sand

Figure 9 Holocene organic flood-basin mud overlying channelbelt sand on the Mississippi River floodplain near Natchez, Mississippi. Flood-basin mud is approximately 2.5 m thick.

oxides, calcite, and gypsum can be common in these settings (Aslan and Autin, 1999). If smectitic clays are present, seasonal wetting and drying produces pedogenic slickensides and desiccation cracks. In more perennially saturated flood basins, elevated water levels and low clastic input leads to widespread peat accumulation (Flores, 1981). If clastic input is significant, bioturbated flood-basin sediments will often interfinger with laminated lacustrine clay and silt. Lacustrine strata are concentrated in topographic depressions where water levels are perennially elevated and reducing conditions prevail. These deposits often contain reducing minerals such as pyrite, vivianite, and jarosite. In more arid settings, flood-basin sediments and playalake deposits contain a greater percentage of mud cracks, bioturbation features, and authigenic minerals such as calcite, gypsum, and other evaporite minerals. Avulsion deposits Avulsion is common in many low-gradient alluvial river systems and is best known from studies of the Rhine-Meuse Delta, the Saskatchewan River, and the Mississippi River and Delta (Smith et al., 1989; Stouthammer and Berendsen, 2000; Aslan et al., 2005). A significant aspect of avulsion is that this process accounts for large volumes of fine-grained overbank sedimentation on floodplains (Smith et al., 1989; Aslan and Autin, 1999; Pe´rez-Arlucea and Smith, 1999). During an avulsion, water and sediment escape from the main channel and are transferred to the floodplain through a series of prograding crevassesplay complexes and/or anastomosing channels (Smith et al., 1989). These channels deposit silt and clay and lesser amounts of sand in local floodplain depressions such as wetlands and lakes. As these topographic depressions fill with sediment, the locus of floodplain deposition shifts downvalley. Because avulsive deposition occurs at all stages of flow and is

not restricted to relatively rare large-magnitude floods, this process results in rapid floodplain aggradation. Texturally, avulsive deposits strongly resemble fine-grained sediments that are typically associated with overbank flooding. However, the stratigraphic architecture of avulsive deposits differs from that of typical overbank deposits (Fig. 6B). Overbank flooding causes grain size and sediment accumulation rate to decrease away from the active channel. In contrast, avulsion deposit grain sizes do not vary systematically as a function of distance from the active channel. In fact, sedimentation rates can actually increase away from the active channel.

Fluvial Channel Patterns Fluvial channels have been traditionally subdivided into straight, meandering, and braided (Leopold and Wolman, 1957). More recent classifications recognize anastomosing channels (Miall, 1977, 1996; Rust, 1978) (Fig. 11A). These classifications emphasize two factors: channel sinuosity and bifurcation patterns. For example, braided rivers are characterized by channel bifurcation around bars or islands and low channel sinuosities, whereas anastomosing channels are separated by larger-scale flood basins (Schumm, 1977; 1985; Makaske, 2001; Bridge, 2003). Problems with existing channel classification schemes include measurements of the sinuosity of multi-channel rivers, calculations of braiding indices, and delineation of individual channels versus channel belts. Additionally, channel patterns are not mutually exclusive. For example, anastomosing rivers can have braided channel segments (Bridge, 2003). Discrimination of channel patterns using channel characteristics and hydrologic data can be problematic and remains an area of continued research (Lewin and Brewer, 2001; Van den Berg and Bledsoe, 2003). An alternative alluvial river classification scheme proposed by Makaske (2001) subdivides channel systems based on numbers of channel belts, braiding characteristics, and channel sinuosity (Fig. 11B). Although floodplain classification has received far less attention than channel classification, Nanson and Croke (1992) provide a recent classification of floodplain systems. Changes in channel patterns are commonly associated with one or more of the following variables: 1) discharge, 2) sediment load and type, 3) regional gradient, and 4) bed or bank materials (Schumm, 1977; 1985). In general, increases in discharge, bed load, and gradient favor the development of lowsinuosity or braided streams. In contrast, decreases in discharge and gradient and increases in suspendedsediment load favor the development of meandering

FLUVIAL ENVIRONMENTS/Sediments 679

(A)

A’

A

N

Natchez . pi R

sip

sis Mis

5 km

Holocene Miss. R. channel belt 1

Flood basin

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Holocene Miss. R. channel belt 2 Pleistocene deposits

Holocene Miss. R. channel belt 3 Active Mississippi River channel

Core location Abandoned Mississippi River channel

A′

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10 Elevation above mean sea level (m)

0 1 km

Brown silt and sand (natural levee)

Blue-gray massive mud (flood basin)

Gray sand and silt (channels and crevasse splays)

Gray mottled mud (flood basin)

Dark gray laminated mud (lacustrine)

Floodplain cores

Figure 10 (A) Map of Holocene floodplain deposits of the Mississippi River near Natchez, Mississippi. (B) Cross-section showing typical overbank sediments in the region. Brown silt and sand wedges represent natural-levee deposits. Gray sand and silt represent crevasse-splay sediments (thin sheets to lenticular bodies) whereas the thicker sand accumulations represent floodplain channels. Gray mottled mud and blue-gray massive mud represent flood-basin environments associated with predominantly oxidizing and reducing conditions, respectively. Dark gray laminated mud represents lacustrine strata. From Aslan and Autin (1999).

rivers or anastomosing systems. Anastomosing rivers are especially common where regional rates of aggradation are high and bank materials are cohesive (Smith, 1986; Makaske, 2001).

alluvial architecture, and degree of stratigraphic completeness. These differences are demonstrated by comparing fluvial sediments from eroding continental interiors with those associated with subsiding continental margins.

Sedimentary Records of Quaternary Fluvial Systems

Fluvial records from eroding continental interiors

Sedimentary records of Quaternary fluvial systems vary dramatically in terms of their abundance,

Fluvial sediments of continental interiors typically reflect alternating episodes of aggradation, incision,

680 FLUVIAL ENVIRONMENTS/Sediments Multiple thalwegs braided

One thalweg meandering

straight

One channel belt

Straight

B > 1.5

B < 1.0 Pind > 1.3

Meandering

Pind < 1.3 B < 1.0

Anastomosing

Anastomosing

Multiple channel belt

Bar surfaces covered during flood stages

Legend Floodbasin

Braided

(A)

(B)

B = Braid-channel ratio Channel belt Pind = Sinuosity Active channel

Figure 11 (A) Traditional classification scheme for alluvial channels. From Miall (1977). (B) An alternative classification of alluvial channels based on the number of channel belts present, braiding characteristics, and channel sinuosity. From Makaske (2001). Figures reprinted with permission from Elsevier.

and soil formation, and are located in river valleys. Fluvial sediments in these settings undergo extensive erosion and sediment reworking. Thus they have a low preservation potential and provide incomplete or discontinuous records of fluvial activity. The erosional nature of fluvial records in continental interiors stems largely from long-term rock and surface uplift driven by erosional isostasy or tectonic processes, which leads to fluvial incision (Pederson et al., 2002). Shorter-term episodes of aggradation and incision associated with glacial-interglacial cycles are superimposed on this long-term uplift and incision (see Glacial-Interglacial Scale Fluvial Responses) Collectively, these processes lead to the development of flights of down-stepping terraces

along many major river valleys of continental interiors (Fig. 12). Sand and gravel are common constituents of fluvial sediments in continental interiors, and are associated with channel bars and bed-load deposition (Fig. 13A). Overbank sediments are often represented by thin veneers (Fig. 13B). The predominance of coarse-grained sediment reflects the relatively steep slopes of rivers in these settings and low floodplain relief, which combine to inhibit the deposition and preservation of silt and clay. The relative paucity of overbank silt and clay in continental interiors has led some workers to suggest that overbank flooding plays a minor role in the development of floodplains in these settings (Wolman and Leopold, 1957).

(B)

(A)

Quaternary river gravels

Colorado River

Quaternary overbank silty sand Quaternary river gravels

Gunnison River Cretaceous Mancos shale

Figure 12 (A) River gravels associated with a Quaternary Gunnison River terrace near Grand Junction, Colorado. The terrace strath is cut into Cretaceous sandstone and is approximately 100 m above the modern Gunnison River. This terrace is part of a flight of downstepping Quaternary terraces in the region, all of which are associated with abundant gravel. View is upstream. (B) Quaternary river gravels overlain by reddish overbank silty sand deposits of a Quaternary Colorado River terrace near Grand Junction, Colorado. The terrace strath is cut into Cretaceous shale and is approximately 30 m above the modern Colorado River. View is upstream.

FLUVIAL ENVIRONMENTS/Sediments 681 (A)

(B)

Figure 13 (A) North Fork of the Gunnison River valley near Hotchkiss, Colorado. This river drains volcanic and sedimentary rocks of the central Rocky Mountains, and the channel and floodplain are represented by abundant gravel and sand. Flat benchlands in the background represent river terraces (arrows). View is upstream. (B) Cutbank along the Colorado River near Grand Junction, Colorado showing channel-bar gravel overlain by overbank silt and sand.

LITTLE CANEY RIVER 100°

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Figure 14 (A) Map showing locations of alluvial rivers that have undergone Holocene filling and entrenchment. BC ¼ Brushy Creek; CC ¼ Carnegie Canyon; CL ¼ Candy Lake; CR ¼ Colorodo River; DC ¼ Deleware Canyon; DM ¼ Double Mountain Fork; HC ¼ Hog Creek; EF ¼ Elm Fork of the Trinity River; KL ¼ Keystone Lake; LC ¼ Little Caney River; MC ¼ McGee Creek; NF ¼ North Fork of San Gabriel River; PR ¼ Pedernales River; RC ¼ Richland Creek; SS ¼ South Sulfur Spring. (B) Cross sections showing the Holocene alluvial stratigraphy at three locations in (A). Sand and gravel deposition and Holocene aggradation commenced 5 ka in all three locations, which produced depositional unit ‘B’ in the cross sections. From 2 to 1 ka, sedimentation slowed and a cumulic, organic-rich soil (Copan Soil) developed. At 1 ka, channel trenching occurred throughout the southern Great Plains. Channel trenching was followed by episodic floodplain sedimentation, which produced depositional unit ‘A’ in the cross sections. Ages are in radiocarbon years before present. Premiddle Holocene sediments (depositional unit ‘C’) are rare in the valley fills. These types of valley-fill fluvial deposits have low long-term preservation potential because of the narrow valley widths and the frequency of channel entrenchment and filling. From Hall (1990).

Fluvial sediments of continental interiors (e.g., Holocene arroyo fills) are often associated with complex alluvial stratigraphies that reflect alternating episodes of aggradation, entrenchment, and soil

formation (Fig. 14). Alluvial successions in the southern Great Plains of the United States provide excellent examples of complex fill histories, and contain regionally correlative stratigraphic units. For

682 FLUVIAL ENVIRONMENTS/Sediments

example, numerous Holocene valley-fill deposits in this region record aggradation from 5 to 2 ka, soil formation from 2 to 1 ka, and entrenchment since 1 ka followed by episodic floodplain sedimentation (Fig. 14B). The correlative nature of the alluvial stratigraphic units in these valleys, and similarities in the timing of the major aggradation and incision events suggest that climatic influences and changes in river hydrology have largely controlled the alluvial stratigraphy of the southern Great Plains (Hall, 1990) as well as elsewhere in the southwestern United States (Walters and Haynes, 2001). Narrow valley widths relative to channel widths cause rivers to rapidly rework older alluvial deposits in these settings. Thus the length of time represented by the fluvial sediments is limited. Fluvial records from subsiding continental margins Fluvial sediments of continental margins typically accumulate in subsiding sedimentary basins. For

example, the Gulf Coast region of the United States represents a subsiding passive margin that consists of stacked successions of fluvial-deltaic Quaternary units (Fig. 15). Because these sediments are accumulating in a subsiding basin, they have a high preservation potential and the sedimentary records of fluvial activity are more complete than those of continental interiors. Rivers in this region occupy broad alluvial valleys separated by low upland divides. Large floodplain widths relative to channel widths, gentle slopes, and depositional relief created by sedimentation along channels (i.e., elevated alluvial ridges) lead to the accumulation and preservation of significant volumes of fine-grained overbank deposits, in addition to coarse-grained channel sediments (Fig. 16). Late Quaternary fluvial sediments of continental margins predominantly occur as valley-fill complexes associated with glacial-interglacial cycles (spans approximately 100 ky) (Fig. 17). Packages of fluvial sediments accumulate in response to

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Figure 15 Geologic map of the Texas Gulf Coastal Plain showing the distribution of late Quaternary deposits and major river valleys. Modified from Blum and Price (1998).

FLUVIAL ENVIRONMENTS/Sediments 683

Holocene sediment

Paleosol

Late Pleistocene OIS 5a paleosol overlying flood-basin sediments

Flood-basin sediments

Late Pleistocene OIS 6 paleosol

Beaumont Alluvial Plains

Figure 16 Holocene and late Pleistocene fluvial deposits of the Colorado River of south Texas. Holocene overbank deposits veneer a late Pleistocene paleosol that represents the upper boundary of fluvial sediments deposited during the oxygen-isotope-stage (OIS) 5a highstand approximately 70 ka. Most of the sediments below this paleosol accumulated during a period of rising sea level. The light colored zone at the base of the section shows a paleosol that formed during OIS 6, which represented a period of alternating episodes of floodplain construction and valley incision.

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Figure 17 (A) Satellite image of the Lower Colorado River alluvial plain and the transect line and core-sampling sites for the cross section shown in (B). (B) Stratigraphic cross section showing the facies architecture of the late Pleistocene to Holocene Colorado River valley fill complex. The valley fill consists of a series of down-stepping terraces underlain by channel sands and separated from one another by paleosols and erosional unconformities. The sands range in age from approximately 60 to 20 ka. These coarse-grained basal valley-fill deposits are overlain by a mixture of channel and overbank sediments that were deposited during late Pleistocene to Holocene sea-level rise. From Blum and To¨rnqvist (2000). Reprinted with permission from Blackwell Publishing.

684 FLUVIAL ENVIRONMENTS/Sediments

changes in sea level and river hydrology that reflect glacial versus interglacial conditions (Blum and To¨rnqvist, 2000). In the example of the Colorado River of the Texas Gulf Coast region, the river incised and filled its current valley over the past 70 ka (Blum and Price, 1998). The valley fill consists of a basal Late Pleistocene sand-dominated unit that is overlain by a mixed sand- and mud-rich succession of Holocene age. The basal sand-dominated unit is comprised of a series of down-stepping terraces and well expressed buried soils (Fig. 17). During the Holocene, the valley filled with finegrained muddy overbank deposits that encase channel and minor overbank sand bodies. Similar muddy overbank sediments are thin or absent from the basal Late Pleistocene portion of the valley fill. This major difference in the abundance of overbank sediments in Holocene and Late Pleistocene portion of the valley fill could reflect less frequent episodes of deep overbank flooding during the Late Pleistocene (Blum and To¨rnqvist, 2000). Over longer time scales involving multiple glacial-interglacial cycles, avulsion during late stages of valley filling can lead to abandonment of an alluvial valley (Blum and To¨rnqvist, 2000) (see Glacifluvial Landforms of Deposition). Avulsion causes a shift in the location of the valley axis and future valley incision and filling, which causes the ages of valley-fill complexes to vary both

laterally and vertically (Fig. 18). In areas of active subsidence, alluvial valley-fill complexes locally onlap and cross cut one another. The basal valleyfill unconformities form a composite unconformity that is overlain predominantly by sandy channelbelt deposits with well expressed paleosols. In contrast, the upper portions of the valley fills are a combination of fine-grained overbank and sandy channel deposits.

Summary Fluvial sediments provide important records of terrestrial environments that are critical for interpreting geologic histories of continental settings. These sediments are also important for evaluating petroleum, coal, and water resources. Fluvial sediments are traditionally subdivided into coarse-grained channel deposits and fine-grained overbank sediments. Channel deposits consist of channel-bar and channelfill sediments, and accumulate in response to downstream progradation of large-scale bar forms and periodic channel abandonment. Overbank deposits accumulate in natural-levee, crevasse-splay, and flood-basin environments through a combination of overbank flooding and avulsion. Fluvial sediments vary in terms of abundance, architecture, and stratigraphic completeness between continental interiors and margins. In eroding continental interiors,

Active channelbelt Pleistocene highstand alluvial plain surfaces Pleistocene channelbelts

Holocene channelbelts

Onlap of successive 100 kyr alluvial plain

Holocene alluvial plain surface

10 m 0

basal valley fill unconformities 0

10 km

Alluvial plain paleosol

Figure 18 Model showing alluvial-plain construction and the distribution of valley-fill complexes along a subsiding passive margin. From Blum and To¨rnqvist (2000). Reprinted with permission from Blackwell Publishing.

FLUVIAL ENVIRONMENTS/Sediments 685

fluvial sediments are represented by flights of terraces underlain by coarse-grained deposits. Fine-grained overbank sediments are relatively rare. These fluvial sediments reflect long-term incision and common sediment reworking, and thus they preserve relatively incomplete records of fluvial activity. In subsiding continental margins, thick accumulations of both sandy channel and muddy overbank deposits are preserved. These thick fluvial successions provide relatively complete long-term records of fluvial activity. See also: Fluvial Environments: Responses to Rapid Environmental Change; Terrace Sequences; Deltaic Environments. Glacial Landforms, Sediments: Glacifluvial Landforms of Deposition. Glacial-Interglacial Scale Fluvial Responses. Paleoceanography, Physical and Chemical Proxies: Terrigenous Sediments. Paleosols and Wind-Blown Sediments: Nature of Paleosols. Quaternary Stratigraphy: Lithostratigraphy; Sequence Stratigraphy.

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