Page 209 - Sedimentology and Stratigraphy

196 Deltas

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Fig. 12.21 A tide-dominated delta in a macrotidal regime will show extensive reworking of the delta front by tidal currents
and the delta top will have a region of intertidal deposition.

These deformation processes occurring during the The delta top contains both relatively coarse sedi-
formation of the deltaic deposit give rise to some ment of the distributary channel as well as finer
quite complicated sedimentary features within a grained material in overbank areas and interdistribu-
delta succession. Beds may be deformed on a scale tary bays. The channel may be recognised by its
ranging from slump folds a few centimetres across to scoured base, a fining-up pattern and evidence of
synsedimentary faults that rotate and displace flow, which will be unidirectional unless there is a
packages of strata tens of metres thick (18.1.1). strong tidal influence resulting in bidirectional cur-
Similar features can occur in other depositional envi- rents. The delta top will show signs of subaerial con-
ronments, but they are probably most common in ditions, including the development of a soil. Deposits in
deltaic facies, especially if the succession contains a the sheltered interdistributary bays may show thin
high proportion of muddy sediments that deform rela- bedding resulting from influxes of sediment from the
tively easily. delta top and symmetrical ripples due to wave action.
The shallower water deposits of the delta front may be
extensively reworked by wave and/or tidal action
12.7 RECOGNITION OF DELTAIC resulting in cross-stratified mouth-bar facies. The ge-
DEPOSITS ometry and extent of the mouth-bar sand bodies will
be determined by the relative importance of river, tidal
A key feature of many deltas is the close association of and wave processes. Deeper, lower delta slope deposits
marine and continental depositional environments. In and prodelta facies are finer grained, deposited from
delta deposits this association is seen in the vertical plumes of suspended material disgorged by the river,
arrangement of facies. A single delta cycle may show or as turbidites that flowed down the delta front.
a continuous vertical transition from fully marine Deltaic deposits are almost exclusively composed of
conditions at the base to a subaerial setting at the terrigenous clastic material supplied by rivers. How-
top. This transition is typically within a coarsening- ever, there are examples of deltas formed by lavas and
upwards succession from lower energy, finer grained volcaniclastic material building out into the sea, and
deposits of the prodelta to the higher energy condi- these are not fed by water, but by the volcanic pro-
tions of the delta mouth bar where coarser sediment cesses: the term ‘non-alluvial delta’ may be applied to
accumulates. these deposits (Nemec 1990a). Limestone beds are