Platform-Margin Reef Depositional Environments

Platform-margin reefs can form continuous barriers to isolated buildups along a platform margin. Tidal channels develop along continuous platform-margin reefs, which allow circulation between the open ocean and the platform interior.

Belize barrier-reef system.

James (1983) presented a model of a platform-margin reef that shows five distinct environments. A discussion of these environments is presented below.

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ROCK TYPES

Back-Reef Facies

Back-reef mollusk/coral rudstone from the Pleistocene of Barbados. The relatively poor sorting of clasts may indicate that storms affected this unit.

Back-reef stick-coral (Acropora cervicornis) bafflestone from the Pleistocene of Barbados.

Rudist-fragment rudstone from the back reef of the Lower Cretaceous Stuart City Trend in South Texas.

Reef-Crest to Reef-Front Facies

Reef-crest Acropora palmata framestone from the Pleistocene of West Caicos Island.

Massive coral framestone from near the reef crest in the Pleistocene of Barbados.

Massive coral framestone from near the reef crest in the Silurian of Ohio.

Requinid-rudist framestone from the reef front of the Lower Cretaceous Stuart City Trend in South Texas.

Late Frasnian Platform-Margin Reef in Australia

The “Classic Face” at Windjana Gorge in Australia is an example of a Late Frasnian reef-rimmed platform. The right-hand portion of the photograph is horizontally bedded shelf strata, with the middle portion being shelf-margin reef and the left-hand portion being forereef slope. Height of the canyon wall is 100 meters. (Click on red boxes to see example photographs.)

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