Page 201 - Sedimentology and Stratigraphy

188 Deltas

Fig. 12.11 A modern Gilbert-type coarse-
grained delta.

lake or sea, where they are reworked by waves or tidal supply of sediment, the rate at which the delta pro-
currents. Documented modern examples are all from grades will depend on the thickness of the sediment
basins where the tidal range is small and wave action pile that must be created to reach sea level. Delta
is the main mechanism for distribution of clasts in progradation will hence occur at a greater rate if it
shallow water. The energy associated with waves is is building into a shallow sea or lake (Fig. 12.8), and
strongly depth-dependent (4.4) and so there is a sort- the area covered by a delta lobe will be greater
ing of the sediment into different grain sizes according because it forms a thin, widespread body of sediment.
to water depth. The largest clasts remain in the shal- In contrast a delta building into deeper water will
lowest water where the wave action is strongest, form a thicker deposit that progrades at a slower
while smaller clasts are carried by waves further off- rate (Collinson et al. 1991).
shore into slightly deeper water. Across a gently slop- A delta building into shallow water will tend to
ing shelf there will be a progressive fining of the clast have a large delta-plain area. If the climate is suitable
size as the water depth increases and hence the for abundant plant growth, peat mires may develop
energy of the waves decreases (Fig. 12.7). on parts of the plain away from the delta channels
Progradation of a coarse-grained delta across a and delta successions that have developed in a
shallow lake or sea floor results in a coarsening-up shallow-water setting may therefore include coal
succession from finer sands deposited furthest offshore beds. The delta-front facies will all be deposited in
through coarser sands, granules, pebbles and even shallow water, and hence will be strongly influenced
cobbles or boulders at the top of the delta-front by processes such as wave action (Fig. 12.9). Sandy
succession, which is then overlain by coarse fluvial and gravelly deposits are therefore likely to be rela-
or alluvial fan facies of the delta top (Fig. 12.7). tively well sorted.
Coarse-grained deltas that display these characteris- In deeper water, a greater proportion of the sedi-
tics have been classified as ‘shelf-type fan deltas’by ment will be deposited in the lower part of the delta
Wescott & Ethridge (1990). slope as a thicker coarsening-up succession is gen-
erated during delta progradation (Fig. 12.10). The
area of the delta top will be relatively small, with less
12.4.3 Water depth: shallow- and potential for the development of widespread fine-
deep-water deltas grained delta-plain facies and mires. Wave-reworked
mouth-bar facies will be limited in extent because of
A delta progrades by sediment accumulating on the the small area of shallow water where wave action
sea floor at the delta front where it builds up to sea is effective. The delta slope will be extensive and a
level to increase the area of the delta top. For a given potential site for gravity flows: coarser deposits may