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Deltaic Cycles and Stratigraphy 193

the formation of mud drapes (11.2.4). Overbank areas
on the delta top may be partially tidal flats, and all of
the delta top will be susceptible to flooding during
periods of high fluvial discharge coupled with high
tides. The tidal currents rework sediments at the river
mouth into elongate bars that are perpendicular to the
shoreline. These are modified mouth bars, which may
show bidirectional cross-stratification and mud drapes
on the cross-bed foresets due to the reversing nature of
the ebb and flood tidal currents (Willis et al. 1999)
(11.2.4).
The deposits of a tidally influenced delta can be
distinguished from other deltas by the presence of
sedimentary structures and facies associations which
indicate that tidal processes were active (reversals of
palaeoflow, mud drapes, and so on), and subaqueous
mouth bars will be elongate parallel to the river
channels. The overall succession of strata will
display the characteristic coarsening-up of a delta
(Fig. 12.22), a feature that allows it to be distin-
guished from other tidally influenced environments
such as estuaries, which have much in common in
terms of depositional processes. The main distinguish-
ing feature is that a delta is always a progradational
feature, whereas an estuary commonly forms as
part of a retrogradational, or transgressive, succession
(23.1.6).

12.5 DELTAIC CYCLES AND
STRATIGRAPHY
When the channel on the delta top changes course,
the former lobe is abandoned as a new site of
deposition is occupied. River-dominated deltas tend
to have the most frequent changes in position of the
active lobe, but avulsion of channel course also
occurs in other delta types. The deposits of an aban-
doned lobe will gradually compact as water depos-
ited with the fine-grained sediment escapes from the
pore spaces and the bulk density increases. This
compaction occurs without any additional load,
and results in the abandoned lobe subsiding below
sea level. The fall below sea level of the abandoned
lobe will be accelerated if the delta is located in a
region of overall subsidence or if there is a eustatic
rise in sea level.
The beds that mark the end of sedimentation on a
delta lobe are known as the abandonment facies
Fig. 12.17 A schematic graphic sedimentary log of river-
dominated delta deposits. (Reading & Collinson 1996). In the upper part of the