Page 208 - Sedimentology and Stratigraphy

Syndepositional Deformation in Deltas 195
Fig. 12.20 A schematic graphic sedimentary log of wave- result is the formation of mud diapirs (Hiscott 2003)
dominated delta deposits.
a delta ‘cycle’ will be controlled by the depth of water
in the receiving basin (see above) and may range from
a few metres to tens or hundreds of metres in thick-
ness (Elliott 1986).
Variants on the idealised delta cycle are frequently
encountered (Fig. 12.23). A complete succession from
offshore fine-grained deposits up to the delta channel
fill will be seen only at the point where the axis of the
lobe has built out basinward. In other positions, the
top of the cycle will vary from delta-plain carbona-
ceous mudstones, to interdistributary bay deposits or
mouth-bar sands. In a hinterland direction, subsi-
dence will not be great enough for fully marine con-
ditions to develop at the base of each delta cycle, and
only the upper parts of the typical succession may be
seen (Elliott 1986).
In addition to the trends that represent the progra-
dation of delta lobes, smaller scale grain-size patterns
are also present. The filling of an interdistributary
bay results in a coarsening-up succession, but this
will normally be on a scale that is an order of magni-
tude smaller than the main delta cycle. Small-scale
fining-up trends are formed by the filling of distribu-
tary channels when they are abandoned.
12.6 SYNDEPOSITIONAL
DEFORMATIONINDELTAS
The delta front is a slope that can vary from about 18
in mud-rich settings to over 308 in coarse-grained
deltas. Even the very low angle slopes are potentially
unstable and mass movement of loose, soft sediment
on the delta slope is common. Debris flows, slumps
and slides (6.5.1) that consist of remobilised delta-
front deposits reworked and remobilised occur and
may be seen as part of the succession in deltaic facies.
The slumps and slides can be large-scale, involving
the movement of bodies of sediment tens of metres
thick and hundreds of metres across. The surfaces on
which the slides move are like faults, and these
features are often regarded as growth faults,
synsedimentary deformation structures (18.1.1)
(Bhattacharya & Davies 2001). Further instabilities
also arise as a result of the relatively rapid accumula-
tion of sediment on a delta: coarser, and relatively
denser sediment of the delta top is built up on top of
muddy, wet and less dense delta-front facies and the
(18.1.4).