Page 204 - Sedimentology and Stratigraphy

Variations in Delta Morphology and Facies 191

Fig. 12.14 A Gilbert-type coarse-grained
delta exposed in a cliff over 500 m high. The
exposure is made up mostly of foreset
deposits dipping at around 308: horizontal
topset strata form the top of the cliff and the
toes of the foreset beds pass into gently
dipping bottomset facies.

Fig. 12.15 A river-dominated delta with the distributary channels building out as extensive lobes due to the absence of
reworking by wave and tide processes. Low-energy, interdistributary bays are a characteristic of river-dominated deltas.

overbank sediments. The characteristics of these supply sediment into these bays and they gradually
facies will be essentially the same as those of a fill to sea level to become the vegetated part of the
similar fluvial system. The overbank areas of a delta plain. The filling of interdistributary bays
delta top may be sites of prolific growth of vegeta- results in small scale (a few metres thick) coarsen-
tion, leading to the formation of peat and eventually ing-up successions (Fig. 12.17). In front of the
coal. The channels build out to form the ‘toes’ of the channels, mouth bars form and are localised to
‘bird’s foot’, between which there are large interdis- areas in front of the individual delta lobes. Little
tributary bays. These bays are relatively sheltered redistribution of mouth-bar sediments by wave or
and are sites of fine-grained, subaqueous sedimenta- tidal processes occurs, so individual mouth-bar
tion. Crevasse splays from the distributary channels bodies are relatively small.