Page 220 - Sedimentology and Stratigraphy

Estuaries 207

the sediment spreads out into a thin, low-angle cone currents may be periodically strong enough to trans-
of detritus dipping very gently landwards. Bedforms port and form ripples. These ripple-laminated sands
on the flood-tidal delta are typically subaqueous will occur interbedded with mud that drapes the rip-
dunes migrating landwards, which result in cross- ple forms.
bedding with onshore palaeocurrent directions
(Boothroyd 1985). Ebb-tidal deltas form on the sea-
ward margin of the tidal channel as water flows out of 13.5 COASTAL SUCCESSIONS
the lagoon when the tide recedes. Building out into
deeper water they are thicker bodies of sediment than The patterns of sedimentary successions built up at a
flood-tidal deltas and the direction of bedform migra- coast are determined by a combination of sediment
tion is seawards. The size and extent of an ebb-tidal supply and relative sea-level change. (As will be seen
delta is limited by the effects of reworking of the in Chapter 23, these two factors are in fact dominant in
sediment by wave, storm and tidal current processes controlling the large- and medium-scale stratigraphy
in the sea. in all shallow marine depositional environments.)
Prograding barriers and strand plains are those
that build out to sea through time as sediment is
13.4.3 Macrotidal coasts added to the beach from the sea. A barrier will become
wider, and the inner margins may become more sta-
Coasts that have high tidal ranges do not develop bilised by vegetation growth. A prograding strand
barrier systems because the ebb and flood tidal plain will result in a series of ridges parallel to the
currents are a stronger control on the distribution of coastline, chenier ridges (Fig. 13.7), which are the
sediment than wave action. A depositional coast in a relicts of former beaches that have been left inland
macrotidal setting will be characterised by areas as the shoreline prograded (Augustinus 1989).
of intertidal mudflats that are covered at high tide Retrograding barriers form where the supply of
and exposed at low tide. Water flooding over these sediment is too low to counteract losses from the
areas with the rising tide spreads out and loses beach by erosion. Removal of sediment from the
energy quickly: only suspended load is carried across front of the barrier reduces its width and, in turn, its
the tidal flats, and this is deposited when the water height. This makes the coast more susceptible to
becomes still at high tide. The upper parts of the tidal washovers of sand occurring and the lagoon (or a
flats are only inundated at the highest tides. The marsh behind a strand plain) will become partly filled
incoming tide brings in nutrients and tidal flats are in. By this process the beach system will gradually
commonly areas of growth of salt-tolerant vegetation move landward.
(xerophytes) and animal life is often abundant Under conditions of slow relative sea-level rise the
(worms, molluscs and crustaceans in particular). beach may also move landward, but a lagoon will
The deposits of this salt marsh environment also expand, flooding the adjacent coastal plain in
(Belknap 2003) are therefore predominantly fine- response to the sea-level rise. Through time these
grained clay and silt, highly carbonaceous because transgressive barrier systems will build up a succes-
of all the organic material and the animal life results sion from coastal plain deposits at the base, overlain
in extensive bioturbation. The vegetation on the tidal by lagoon facies and capped by beach deposits of the
flats tends to trap sediment, and mud flats are com- barrier system (Fig. 13.12). A similar transgressive
monly sites of net accumulation. Tidal flats are often situation at a strand plain will result in coastal plain
cut by tidal creeks, small channels that act as deposits overlain by beach deposits.
conduits for flow during rising and falling tides: the
stronger flow in these creeks allows them to trans-
port and deposit sand, resulting in small channel 13.6 ESTUARIES
sand bodies within the tidal-flat muds. Coarser sedi-
ment is also introduced onto the tidal flats during An estuary is the marine-influenced portion of a
storms, forming thin layers of sand and bioclastic drowned valley (Dalrymple et al. 1992). A drowned
debris. Flaser and lenticular bedding (4.8) may occur valley is the seaward portion of a river valley that
on the lower parts of the tidal mudflats, where becomes flooded with seawater when there is a