Page 192 - Sedimentology and Stratigraphy

12

Deltas

The mouths of rivers may be places where the accumulation of detritus brought down by
the flow forms a sediment body that builds out into the sea or a lake. In marine settings
the interaction of subaerial processes with wave and tide action results in complex
sedimentary environments that vary in form and deposition according to the relative
importance of a range of factors. Delta form and facies are influenced by the size and
discharge of the rivers, the energy associated with waves, tidal currents and longshore
drift, the grain size of the sediment supplied and the depth of the water. They are almost
exclusively sites of clastic deposition ranging from fine muds to coarse gravels. Deposits
formed in deltaic environments are important in the stratigraphic record as sites for the
formation and accumulation of fossil fuels.

12.1 RIVER MOUTHS, DELTAS body into the lake or sea. In contrast, an estuary is a
AND ESTUARIES river mouth where there is a mixture of fresh water
and seawater with accumulation of sediment within
The mouth of a river is the point where it reaches a the confines of the estuary, but without any build-out
standing body of water, which may be a lake or the into the sea. ‘Ordinary’ river mouths are settings
sea. These are places where a delta may form (this where there is no significant mixing of waters and
chapter), an estuary may occur (next chapter) or any sediment introduced by the river is reworked and
where there is neither a delta nor an estuary. This carried away by processes such as waves and tides.
variation depends on the morphology of the river
mouth, the supply of sediment by the river and the
processes acting in the lake or sea. A delta can be 12.2 TYPES OF DELTA
defined as a ‘discrete shoreline protuberance formed
at a point where a river enters the ocean or other Even a cursory survey of modern deltas reveals that
body of water’ (Fig. 12.1) (Elliott 1986; Bhattacharya they are widely variable in terms of scale, processes
& Walker 1992), and as such it is formed where and the nature of the sediment deposited. A stream
sediment brought down by the river builds out as a feeding into a lake may create a sediment body that is