Page 225 - Sedimentology and Stratigraphy

212 Clastic Coasts and Estuaries

indicators. Muddy tidal flat deposits rich in organic
material may contain sandy sediment deposited within
tidal creeks, at the highest tides and during storms.

13.6.3 Recognition of estuarine
deposits: summary

There are many features in common between the
deposits of deltas and estuaries in the stratigraphic
record. Both are sedimentary bodies formed at the
interface between marine and continental environ-
ments and consequently display evidence of physical,
chemical and biological processes that are active in
both settings (e.g. an association of beds containing a
marine shelly fauna with other units containing root-
lets). The key difference is that a delta is a prograda-
tional sediment body, that is, it builds out into the sea
and will show a coarsening-up succession produced by
this progradation. In contrast, estuaries are mainly
aggradational, building up within a drowned river
channel. The base of an estuarine succession is there-
fore commonly an erosion surface scoured at the
mouth of the river, for example, in response to sea-
level fall. It may be difficult to distinguish between the
deposits of a tidal estuary and a tide-dominated delta if
there is limited information and it is difficult to estab-
Fig. 13.18 A graphic sedimentary log of tidal estuary lish whether the succession is aggradational and
deposits. valley-filling or progradational.

ebb tide. Herringbone cross-stratification will only
form in areas of overlap between banks of cross-beds 13.7 FOSSILS IN COASTAL AND
of different orientation, or if the currents change posi- ESTUARINE ENVIRONMENTS
tion. Where tidal currents are strongest the dune bed-
forms are replaced by upper flow regime plane beds Beaches are high-energy environments, continually
that form horizontally laminated sands. washed by waves, which move the sediment around
subjecting the clasts to abrasion. The supply of shelly
material from the sea is often abundant, but much of it
Successions in tide-dominated estuaries
will be broken up into fragments that may be identifi-
A succession formed in a tide-dominated estuary will able in only a general sense as pieces of mollusc, coral,
consist of a combination of tidal channel, tidal flat and echinoderm, etc. Only the most robust organisms
tidal bar deposits. The proportions preserved of each remain intact, and among these are thick-shelled mol-
will depend on the position in the estuary, the strength luscs such as oysters, which are also found living in
of the tidal currents and the amounts of mud, sand and high-energy, shallow water of the shallow subtidal
gravel available for deposition (Fig. 13.18). The base of zone. The abundance of bioclastic debris in beach depos-
a tidal channel is marked by a scour and lag, and will its will depend on the relative proportions of mineral
typically be followed by a fining-upwards succession of grains and shelly material supplied to the beach.
cross-bedded sands, which may show mud drapes, For organisms living in a lagoon, both hypersaline
inclined heterolithic stratification. Channel and bar and brackish conditions require adaptation that only
deposits may also show bi-directional palaeocurrent a limited number of plants and animals achieve.