Page 205 - Sedimentology and Stratigraphy

192 Deltas

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Fig. 12.16 When a delta channel avulses a new lobe starts to build out at the new location of the channel mouth. The
abandoned lobe subsides by dewatering until completely submerged. Through time the channel will eventually switch back to a
position overlapping the former delta lobe. This results in a series of delta-lobe successions, each coarsening-up.

12.4.6 Process controls: build up and out to form a new beach (Fig. 12.19).
wave-dominated deltas Wave-dominated delta deposits display well-developed
mouth bar and beach sediments, occurring as
Waves driven by strong winds have the capacity to elongate coarse sediment bodies approximately
rework and redistribute any sediment deposited in perpendicular to the orientation of the delta river
shallow water, especially under storm conditions. channel. This is in contrast to the river-dominated
The river mouth and mouth-bar areas of a delta are delta deposits, which would be expected to show
susceptible to the action of waves, resulting in a mod- less continuous mouth bars, and a higher proportion
ification of the patterns seen in river-dominated of channel and overbank deposits forming the
deltas (Bhattacharya & Giosan 2003) (Fig. 12.18). delta lobes. Delta-front and prodelta deposits may not
Progradation of the channel outwards is limited significantly differ between these two delta types
because the subaqueous levees do not form and bed- (Figs 12.17 & 12.20).
load is acted upon by waves as quickly as it is depos-
ited. Any obliquity between the wind direction and
the delta front causes a lateral migration of sediment 12.4.7 Process controls:
as the waves wash material along the coast to form tide-dominated deltas
beach spits and mouth bars that build up as elongate
bodies parallel to the coastline. Wave action is effec- Coastlines with high tidal ranges experience onshore
tive at sorting the bedload into different grain sizes and offshore tidal currents that move both bedload
and the mouth-bar deposits of a wave-influenced delta and suspended load. A delta building out into a region
may be expected to be better sorted than those of a with strong tides will be modified into a pattern that is
river-dominated delta. different to both river- and wave-dominated deltas
Progradation of a wave-dominated delta occurs (Fig. 12.21). First, the delta-top channel(s) are subject
because the wave action does not transport all the to tidal influence with reverses of flow and/or periods
material away from the region of the river mouth. of stagnation as a flood tide balances the fluvial dis-
A net supply of bedload by the river results in a series charge. This may be seen in strata as reversals of
of shore-parallel sand ridges forming as mouth bars palaeocurrent indicated by cross-stratification, and