Page 246 - Sedimentology and Stratigraphy

Shallow Marine Carbonate Environments 233

15.3 SHALLOW MARINE CARBONATE Reef-forming organisms
ENVIRONMENTS
Scleractinian corals are the main reef builders in
modern oceans, as they have been for much of the
The character of deposits in shallow marine carbonate
Mesozoic and Cenozoic (Fig. 15.10), These corals are
environments is determined by the types of organisms
successful because many of the taxa are hermatypic,
present and the energy from waves and tidal currents.
The sources of the carbonate material are predomi-
nantly biogenic, including mud from algae and bac-
teria, sand-sized bioclasts, ooids and peloids and
gravelly debris that is skeletal or formed from intra-
clasts. Bioturbation is usually very common and
faecal pellets contribute to the sediment. A number
of different carbonate deposits are characteristic of
many shallow marine environments, for example
shoals of sand-sized material, reefs and mud mounds.
15.3.1 Carbonate sand shoals

Sediment composed of sand to granule-sized, loose
carbonate material occurs in shallow, high energy
areas. These carbonate shoals may be made up of
ooids (3.1.4), mixtures of broken shelly debris or may
be accumulations of benthic foraminifers (3.1.3).
Reworking by wave and tidal currents results in
deposits made up of well-sorted, well-rounded mate-
rial: when lithified these form beds of grainstone, or
sometimes packstone. Sedimentary structures may
be similar to those found in sand bodies on clastic
shelves, including planar and trough cross-bedding
generated by the migration of subaqueous dune
bedforms. However, the degree of reworking is often
limited by early carbonate cementation (18.2.2).
Extensive wave action tends to build up shoals that Fig. 15.8 Modern coral atolls.
form banks parallel to the coastline, whereas tidal
currents in coastal regions result in bodies of sediment
elongated perpendicular to the shoreline.
15.3.2 Reefs

Reefs are carbonate bodies built up mainly by frame-
work-building benthic organisms such as corals
(Figs 15.8 & 15.9) (Kiessling 2003). They are wave-
resistant structures that form in shallow waters
on carbonate platforms. The term ‘reef’ is used by
mariners to indicate shallow rocky areas at sea, but
in geological terms they are exclusively biological
features. Reef build-ups are sometimes referred to
as bioherms: carbonate build-ups that do not form
dome-shaped reefs but are instead tabular forms Fig. 15.9 Modern corals in a fringing reef. The hard parts of
known as biostromes. the coral and other organisms form a boundstone deposit.