6.3 SEDIMENTARY MODELS
199
lying flood basins on either side of the alluvial plain. Flood basin deposits are generally
fine-grained sands, clays, and silts. They are interlaminated, cross laminated, and characteristically desiccation-cracked. Flood basin deposits are often burrowed, frequently
pierced by plant roots, and, under suitably waterlogged conditions, may become peatforming swamps and marshes. This assemblage of lev6e, flood basin, and swamp sediments is collectively referred to as an overbank deposit, to distinguish it from the assemblage of channel deposits.
One further type of overbank deposit to mention is the crevasse-splay. Rivers at bank
full scour channels through the lev6es termed "crevasses"; lobes of fine sand are deposited where these debouch into the flood basins. These crevasse-splays are similar to,
though smaller than, the lobes of deltas.
Returning now to processes of channel sedimentation, recall that the classification in
Table 6.5 differentiates active and abandoned channel types. The sedimentary processes
of channels just described may be terminated abruptly by switching or avulsion of the
channel course. This can happen in several ways. A sinuous river channel can ultimately
meander back on itself to short circuit its flow by necking, as it is termed. Another type
of channel diversion occurs when a river has raised itself up above the floodplain by lev6e
building to the extent that its floor is above the level of the floodplain. Channel diversion of this type can often occur on a huge scale with resultant catastrophic flooding and
loss of life. Both types of channel switching are characteristic of meandering river systems. In some situations the lower end of the abandoned channel may still open out as
a backwater into the main channel. After a time, however, this may become blocked to
form an isolated ox-bow lake, as it is picturesquely termed, lying within the floodplain
away from the present active channel. Abandoned channel deposits are similar to those
of flood basins. They are laminated, cross-laminated, desiccation-cracked, burrowed
fine sands, silts, and clays, which are occasionally pierced by rootlets and interbedded
with peats. Abandoned channel deposits are distinguishable from flood basin deposits,
however, by their channel-shaped geometry, and by the fact that they abruptly pass down
into channel-floor lag gravels instead of passing down gradually into cross-bedded point
bar sands.
This review of the processes and products of modern alluvial sedimentation is based
on a wide range of publications. Particularly important studies have been documented
by Leopold et al. (1964), Allen (1965), Miall (1978, 1981), Cant (1982), Collinson and
Lewin (1983), and Collinson (1996).
The upward-fining sequences of meandering alluvial deposits tend to be repeated
rhythmically. Classic examples of this phenomenon are described from the Devonian
Old Red Sandstone of the North Atlantic margins of the Appalachians, South Wales,
and Spitzbergen (Allen and Friend, 1968; Allen, 1964; Friend, 1965, respectively). The
cycles vary in average thickness from 1 or 2 m, up to 20 m.The genesis of the finingupward sequence is implicit in the sedimentary model of the laterally migrating channel. The cause of its repetition has been a subject for debate (Miall, 1981).
Four fining-up cycles in the Quaternary alluvium of the Mississippi River Valley are
correlative with eustatic sea level changes during the ice age (Turnbull et al., 1950). There
is also a strong presumption that erratic subsidence of basins bounded by active faults
may cause repeated regrading of floodplains, thus generating thick piles of alluvium with
upward-fining cyclothems. Cyclic climatic changes, especially precipitation, may effect
runoff and river discharge patterns. These can cause pulsating fluctuations in sediment
199
lying flood basins on either side of the alluvial plain. Flood basin deposits are generally
fine-grained sands, clays, and silts. They are interlaminated, cross laminated, and characteristically desiccation-cracked. Flood basin deposits are often burrowed, frequently
pierced by plant roots, and, under suitably waterlogged conditions, may become peatforming swamps and marshes. This assemblage of lev6e, flood basin, and swamp sediments is collectively referred to as an overbank deposit, to distinguish it from the assemblage of channel deposits.
One further type of overbank deposit to mention is the crevasse-splay. Rivers at bank
full scour channels through the lev6es termed "crevasses"; lobes of fine sand are deposited where these debouch into the flood basins. These crevasse-splays are similar to,
though smaller than, the lobes of deltas.
Returning now to processes of channel sedimentation, recall that the classification in
Table 6.5 differentiates active and abandoned channel types. The sedimentary processes
of channels just described may be terminated abruptly by switching or avulsion of the
channel course. This can happen in several ways. A sinuous river channel can ultimately
meander back on itself to short circuit its flow by necking, as it is termed. Another type
of channel diversion occurs when a river has raised itself up above the floodplain by lev6e
building to the extent that its floor is above the level of the floodplain. Channel diversion of this type can often occur on a huge scale with resultant catastrophic flooding and
loss of life. Both types of channel switching are characteristic of meandering river systems. In some situations the lower end of the abandoned channel may still open out as
a backwater into the main channel. After a time, however, this may become blocked to
form an isolated ox-bow lake, as it is picturesquely termed, lying within the floodplain
away from the present active channel. Abandoned channel deposits are similar to those
of flood basins. They are laminated, cross-laminated, desiccation-cracked, burrowed
fine sands, silts, and clays, which are occasionally pierced by rootlets and interbedded
with peats. Abandoned channel deposits are distinguishable from flood basin deposits,
however, by their channel-shaped geometry, and by the fact that they abruptly pass down
into channel-floor lag gravels instead of passing down gradually into cross-bedded point
bar sands.
This review of the processes and products of modern alluvial sedimentation is based
on a wide range of publications. Particularly important studies have been documented
by Leopold et al. (1964), Allen (1965), Miall (1978, 1981), Cant (1982), Collinson and
Lewin (1983), and Collinson (1996).
The upward-fining sequences of meandering alluvial deposits tend to be repeated
rhythmically. Classic examples of this phenomenon are described from the Devonian
Old Red Sandstone of the North Atlantic margins of the Appalachians, South Wales,
and Spitzbergen (Allen and Friend, 1968; Allen, 1964; Friend, 1965, respectively). The
cycles vary in average thickness from 1 or 2 m, up to 20 m.The genesis of the finingupward sequence is implicit in the sedimentary model of the laterally migrating channel. The cause of its repetition has been a subject for debate (Miall, 1981).
Four fining-up cycles in the Quaternary alluvium of the Mississippi River Valley are
correlative with eustatic sea level changes during the ice age (Turnbull et al., 1950). There
is also a strong presumption that erratic subsidence of basins bounded by active faults
may cause repeated regrading of floodplains, thus generating thick piles of alluvium with
upward-fining cyclothems. Cyclic climatic changes, especially precipitation, may effect
runoff and river discharge patterns. These can cause pulsating fluctuations in sediment