Geology Reference
In-Depth Information
5
Processes, Morphodynamics, and
Facies of Tide-Dominated Estuaries
Robert W. Dalrymple, Duncan A. Mackay,
Aitor A. Ichaso, and Kyungsik S. Choi
Abstract
As defined in this chapter, an estuary forms during a shoreline transgression and then
fills during a progradational phase that is transitional to a delta. The spatial distribu-
tion of processes, grain sizes and facies within tide-dominated estuaries is predict-
able in general terms. Tidal currents dominate sedimentation along the axis, with
wave-dominated sedimentation occurring along the flanks of the estuary in its outer
part. Tidal energy increases into the estuary but then decreases toward the tidal limit,
with a gradual transition to river-dominated sedimentation at its head. The interac-
tion of the tidal wave with the morphology of the estuary, and with river currents,
causes the outer estuary to be flood-dominant, with a net landward movement of
sand. By contrast, the inner estuary is ebb-dominant, creating a bedload convergence
within the estuary. The axial sandy deposits are typically finest at this location. In
transgressive-phase estuaries, the main channel shows a low—high—low pattern of
sinuosity, with the tightest bends (sinuosity t 2.5) occurring at the bedload conver-
gence. These bends experience neck cutoff in the transition to the progradational
phase of estuary filling. The estuary-mouth region is characterized by cross-bedded
sands deposited on elongate sand bars, although wave-generated structures can be
important in some cases. Estuaries that are down-drift of major rivers have anoma-
lously muddy outer estuarine deposits. Further landward, upper-flow-regime paral-
lel lamination can be widespread. The margins of the inner estuary are flanked by
muddy salt-marsh and tidal-flat deposits that can contain well-developed tidal
rhythmites and evidence of seasonal variations in river discharge.
5.1
Introduction
The term 'estuary' is fraught with confusion, with two
overlapping but distinct definitions. The broadest defi-
nition is that of Pritchard (
1967
) that states that an
estuary
is “
a semi-enclosed coastal body of water in
which the salinity is measurably diluted by fresh water
derived from land drainage
”. In this definition, the
key element is the presence of brackish water; the spe-
cific geographic, geologic or stratigraphic context is
