Page 221 - Sedimentology and Stratigraphy

208 Clastic Coasts and Estuaries

valleys have become flooded and these provide a
spectrum of morphologies and process controls that
can be used to construct models for estuarine sedi-
mentation. Two end members are recognised (Dal-
rymple et al. 1992): wave-dominated estuaries
and tide-dominated estuaries, with a range of inter-
mediate forms in between. In addition to these two
basic process controls, the volume of the sediment
supply and the relative importance of supply from
marine and fluvial sources also play an important
role in determining the facies distributions in an
estuarine succession. The extent of estuarine deposits
will depend upon the size of the valley and the depth
to which it has been flooded. Modern estuaries range
from a few kilometres to over 100 km long and from
a few hundred metres to over 10 km wide. The thick-
ness of the succession formed by filling an estuary is
typically tens of metres.
Sedimentation in an estuary will eventually result
in the drowned valley filling to sea level and, unless
there is further sea-level rise, the area will cease to
have an estuarine character. If there is a high rate of
fluvial sediment supply, deposition will start to occur
at the mouth of the river and a delta will start to form.
Under conditions where the marine processes are
dominant, the river mouth will become an area of
tidal flats if tidal currents are strong, or the sediment
will be reworked and redistributed by wave processes
to form a strand plain. An estuary is therefore a
temporary morphological feature, existing only dur-
ing and immediately after transgression while sedi-
Fig. 13.12 A schematic graphic sedimentary log of a
ment fills up the space created by the sea-level rise.
transgressive coastal succession.
The presence of estuarine deposits therefore can be
used as an indicator of transgression – see Chapter 23
relative rise in sea level (a transgression, 23.1.3). for further discussion of the relationship between sea-
They are regions of mixing of fresh and seawater. level changes and facies.
Sediment supply to the estuary is from both river
and marine sources, and the processes that transport
and deposit this sediment are a combination of river 13.6.1 Wave-dominated estuaries
and wave and/or tidal processes. An estuary is differ-
ent from a delta because in an estuary all the sedi- An estuary developed in an area with a small tidal
mentation occurs within the drowned valley, whereas range and strong wave energy will typically have
deltas are progradational bodies of sediment that build three divisions (Figs 13.13 & 13.14): the bay-head
out into the marine environment. A stretch of river delta, the central lagoon and the beach barrier.
near the mouth that does not have a marine influence
would not be considered to be an estuary.
Bay-head delta
Estuaries are common features at the mouths of
rivers in the present day because since the last glacial The bay-head delta is the zone where fluvial pro-
period there has been a relative rise in sea level. cesses are dominant. As the river flow enters the
During this Holocene transgression many river central lagoon it decelerates and sediment is