Page 150 - Sedimentology and Stratigraphy

River Forms 137

(Fig. 9.15). The abandoned meander loop becomes
isolated as an oxbow lake (Fig. 9.15) and will remain
as an area of standing water until it becomes filled up
by deposition from floods and/or choked by vegeta-
tion. The deposits of an oxbow lake may be recognised
in ancient fluvial sediments as channel fills made up
of fine-grained, sometimes carbonaceous, sediment
(Fig. 9.16).

9.2.3 Ephemeral rivers

In temperate or tropical climatic settings that have
rainfall throughout the year, there is little variation in Fig. 9.16 A channel is commonly not filled with sand: in
river flow, but in regions with strongly seasonal rain- this case the form of a channel is picked out by steep banks
fall, due to a monsoonal climate, or with seasonal on either side, but the fill of the channel is mainly mud.
snow-melt in a high mountain or circumpolar area,
discharge in a river system can be variable at different
times of the year. During the dry season, smaller sediment many kilometres along normally dry chan-
streams may dry up completely. In deserts (8.2) nels cut into an alluvial plain. The deposits of these
where the rainfall is irregular, whole river systems ephemeral flows are characteristically poorly sorted,
may be dry for years between rainstorm events that consisting of angular or subangular gravel clasts in a
lead to temporary flow. Many alluvial fans (9.5) are matrix of sand and mud. Gravel clasts may develop
also ephemeral. imbrication, horizontal stratification may form and
In upland areas with dry climates, weathering the deposits are often normally graded as the flow
results in detritus remaining on the hillslope or clasts decreases strength through time. Longitudinal bars
may move by gravity down to the valley floor. Accu- may develop and create some low-angle cross-stratifi-
mulation may continue for many years until there is a cation, but other bar and dune forms do not usually
rainstorm of sufficient magnitude to create a flow of form. The deposits are restricted by the width of the
water that moves the detritus as bedload in the river channel but the channel may migrate laterally or
or as a debris flow (4.5.1). The flow may carry the there may be multiple channels on an alluvial plain

!

Fig. 9.15 Depositional architec-
ture of a meandering river: sand-
stone bodies formed by the lateral
migration of the river channel
remain isolated when the channel
avulses or is cut-off to form an
oxbow lake.