Page 203 - Sedimentology and Stratigraphy

190 Deltas

the form of a sandwich of steeply-dipping conglom-
eratic strata between layers of horizontal beds of
conglomerate and sandstone (Fig. 12.14). The
height of the foreset is determined by the depth of
water the delta is building into, and ranges from a
few tens of metres to over 500 ms (Fig. 12.14). These
thick packages of steeply dipping strata are unique to
Gilbert-type deltas: the only deposits that even
approach this angle of deposition are on alluvial
fans, and these are at a lower angle than the 308
recorded in Gilbert-type deltas. They are typically
found at the edges of basins that have active faulted
margins such as rift basins (24.2.1), where uplift of
the land at the margin creates steep topography to
supply the gravel and the basin is subsiding to form a
deep, steep-sided basin.

12.4.5 Process controls:
river-dominated deltas
A delta is regarded as river-dominated where the
effects of tides and waves are minor. This requires
a microtidal regime (11.2.2) and a setting where
wave energy is effectively dissipated before the
waves reach the coastline. Under these conditions,
the form of the delta is largely controlled by fluvial
processes of transport and sedimentation. The unidi-
rectional fluvial current at the mouth of the river
continues into the sea or lake as a subaqueous flow.
The channel form is maintained, with well-defined
subaqueous levees and overbank areas (Fig. 12.15).
Bedload and suspended load carried by the river is
deposited on the subaqueous levees, building up to
sea level and extending the front of the delta basin-
wards as thin strips of land either side of the main
channel to form the characteristic ‘bird’s foot’ pattern
of a river-dominated delta (Bhattacharya & Walker
1992). A common feature of fluvially dominated del-
tas is channel instability due to the very low gradient
on the delta plain, resulting in frequent avulsion of
the major and minor channels. The course of the river
changes as one route to the sea becomes abandoned
and a new channel is formed, leaving the former
channel, its levees and overbank deposits abandoned.
Repeated switching of the channels on the delta top
builds up a pattern of overlapping abandoned lobes
(Fig. 12.16).
Fig. 12.13 A schematic sedimentary log of a Gilbert-type The deposits of river-dominated deltas have well-
coarse-grained delta deposit. developed delta-top facies, consisting of channel and