Geology Reference
In-Depth Information
7
Tide-Dominated Deltas
Steven L. Goodbred, Jr. and Yoshiki Saito
Abstract
Among tidally infl uenced sedimentary environments, tide-dominated deltas are
perhaps the most variable and diffi cult to characterize. This variability is due in
part to the major role that fl uvial systems play in defi ning their delta, with rivers
differing widely in discharge, sediment load, seasonality, and grain size. Tide-
dominated deltas also tend to be large systems that can extend hundreds of kilo-
meters across and along the continental margin. The associated sediment transport
regimes are typically high energy, but they vary considerably at the scale of tidal
cycles and seasonal river discharge. As a consequence of varying transport energy,
the sedimentary successions formed in tide-dominated deltaic settings tend to be
heterolithic, with interbedded sands, silts, and clays and both fi ning- and coarsen-
ing-upward facies associations. The deltaic nature of tide-dominated deltas that
distinguishes them from other tidally infl uenced settings is defi ned by the cross- or
along-shelf progradation of a clinoform, or 'S' shaped, sedimentary deposit. Under
the infl uence of strong bed shear in tidally dominated margins, this prograding
clinoform is often separated into two distinct units, one associated with the suba-
erial deltaplain and one with an offshore subaqueous delta. Onshore, the large,
fertile deltaplains built by many modern tide-dominated deltas, especially in South
and East Asia, are heavily populated and sustain large economies, making them
global important settings. However, the reduction of fl uvial inputs by damming
and water extraction, as well as intense agricultural, urban, and industrial land
uses, threaten the stability and sustainability of these environments.
7.1
Introduction
River deltas are variably defi ned by their geography,
morphology, or stratigraphy, but are most generally
considered to be a sedimentary deposit formed by a
river at its mouth. Here, to distinguish deltas from riv-
ermouth estuaries that also receive fl uvial sediment
