Carbonate Platforms in The Geological Record
Sedimentary sequences show carbonate platforms as old as the Precambrian, when they were formed by stromatolitic sequences. In the Cambrian carbonate platforms were built by archaeocyatha, metazoa similar to porifera. During Paleozoic brachiopod (richtofenida) and stromatoporoidea reefs were erected. At the middle of the Paleozoic era corals became important platforms builders, first with tabulata (from the Silurian) and then with rugosa (from the Devonian). Scleractinia become important reef builders beginning only in the Carnian (upper Triassic). One of the best examples of a carbonate platform is the Dolomites, deposited during the Triassic. This region of the Southern Alps contains many well preserved atolls, including the Sella, Gardenaccia, Sassolungo and Latemar, the latter being a backstepping type platform. The middle Liassic "bahamian type" carbonate platform of Morocco (Septfontaine, 1985) is characterised by the accumulation of autocyclic regressive cycles and spectacular supratidal (top of sequences) deposits and vadose diagenetic features with dinosaur tracks. The Tunisian coastal "chotts" and their cyclic muddy deposits represent a good recent equivalent (Davaud & Septfontaine, 1995). Such cycles were also observed on the mesozoic Arabic platform, Oman and Abu Dhabi (Septfontaine & De Matos, 1998) with the same microfauna of foraminifera in an almost identical biostratigraphic succession.
In the Cretaceous period there were platforms built by bivalvia (rudists).
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