Geology Reference
In-Depth Information
freshwater environment. - J. Sed. Petrol.,
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, 1058-1071
Wright, V.P., Platt, N.H. (1995): Seasonal wetland carbonate
sequences and dynamic catenas: a re-appraisal of palus-
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Further reading
: K 030
Modern carbonate coastal and offshore environments
can be described by two models, the beach-barrier is-
land-lagoon model and the beach-strandplain model
(Reinson 1984, Tucker 1990). The first model devel-
ops inregions with moderate to high wave energy, rela-
tively high tidal range and high carbonate sand pro-
duction resulting in barriers and sand shoals and ridges.
The beach-strandplain model is also characterized by
high wave energy and high sand production rate, but
low tidal range. Most modern carbonate shoreface-fore-
shore-backshore environments are dominated by skel-
etal grainstones composed of skeletons of organisms
living in these environments and in small coastal patch
reefs; ooid grainstones are common in some regions.
Coastal lagoons differ in sediment production and
biota. Protected lagoons with varying salinities behind
beach barriers (islands or off-shor sand bars) are sites
for the deposition of fine-grained sediments forming
packstone through to mudstones; the biota are low-di-
versity. Open lagoons connected to the open sea via
tidal inlets and with a normal salinity contain a diverse
fauna and a variety of sediment types. Freshwater
marshes, tidal flats, brackish water and salt ponds or
sabkhas may occur adjacent to the lagoons.
2.4.2 Transitional Marginal-Marine
Environments: Shorelines and Peritidal
Sediments
Marginal-marine environments (deltas, beaches and
barrier-islands, estuaries, coastal lagoons and tidal flats)
occur in a narrow nearshore zone along the boundary
between continental and marine depositional realms.
The shoreline is marked by great environmental insta-
bility resulting from intensive interaction between high-
energy forces related to waves, tides, wind, and cur-
rents and constantly changing sea levels. Water energy,
sediment transport and resulting sediment structures
change across the shoreline from offshore to shoreface
and foreshore environments. Carbonate deposition takes
place in the coastal shoreline zone at the beach, in
coastal lagoons behind barriers, and within the periti-
dal zone. Sediments formed in arid and humid shore-
line environments differ in depositional patterns, com-
position and biota (Sect. 15.5).
Key areas for the understanding of modern carbon-
ate shoreline sedimentation are the Yucatan Peninsula,
Florida Bay, the Bahamas, Belize shelf, the Arabian
Gulf, and western Australia.
Excellent reviews of the environments, depositional
patterns and facies sequences in these areas are given
by Inden and Moore (1983) and Tucker (1990). Par-
ticularly handy overviews of modern peritidal carbon-
ates were presented by Shinn (1983), Hardie and Shinn
(1986), and Pratt et al. (1992).
2.4.2.2 Peritidal Environments
Peritidal carbonates are sediments formed 'around the
tides' (Folk 1973, Wright 1984) and include deposits
formed in supratidal, intertidal and shallow subtidal ar-
eas (Box 2.3). The term describes sedimentation on low-
energy tidal zones, especially on tidal flats accreting
from the shorelines of land areas or around islands, and
on shelves, platforms and ramps. Hardie and Shinn
(1986) defined ten basic features characterizing tidal-
flat deposition:
2.4.2.1 Beach (Foreshore), Barriers and
Coastal Lagoons
(1) Carbonate tidal flats occur in settings which are
protected from open ocean waves. Protection from the
open ocean can be caused by wide shelf lagoons, damp-
ening the incoming waves (e.g. Andros tidal flats on
the Great Bahama Banks), a position behind barrier is-
lands that separates back reef lagoons from the open
ocean (e.g. Abu Dhabi, Persian Gulf), or within re-
stricted embayments (e.g. Shark Bay, Australia).
(2) The basic physiography of the tidal-flat system
is determined by the scale of the tidal ranges.
(3) Tidal flats are subdivided into three depositional-
ecological zones by the daily oscillations of the tides
(Box 2.3).
(4) Tidal flats are partly sea and partly land. They
are affected by steep gradients of environmental con-
The beach is strongly wave-dominated and constantly
changing. Sediments are produced by the influx from
rivers, erosion of cliffs and headlands, erosion of the
sea floor, accumulation of shells and the deposition of
unconsolidated sediment by tides. A common feature
of low-ltitude warm-water beaches is the presence of
beachrock
(Scoffin and Stoddard 1983). Beachrock
originates from the rapid cementation of sand grains
and gravels by aragonite and Mg-calcite crystals grow-
ing in intergranular pores. Beachrocks are formed both
in tropical (see Pl. 3/7, 8; Pl. 33/3, Pl. 35/1, 2) and tem-
perate environments (Pl. 31/3, 4, Pl. 92/2).
