Paleoecology of the Capitan Reef


Rachel Wood


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Geologic Framework of the Capitan Reef

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Although the rate of accumulation, topography, and stratigraphic setting of the Permian Capitan reef are similar to that of modem coral reefs, both the ecology of the reef communities and the relative sources of carbonate in their final geological expression are profoundly different. Like modem coral reefs, the Capitan reef was constructed by a patchy mosaic of many different communities, but most were dominated by relatively short-lived aggregations of metazoans (particularly calcified sponges and bryozoans), and some were strongly differentiated into distinct open surface and cryptic subcommunities. Although our understanding of the distribution of these communities is poor, much of the deeper water Middle Capitan reef framework appears to have been constructed initially by a scaffolding of large frondose bryozoans. This framework created cryptic niches that were colonized by a diverse community dominated by solitary (cylindrical) sphinctozoan sponges and bryozoans that grew downwards from the roofs of the cavities. Bathymetrical ly shallow areas of the Middle and Upper Capitan reef were characterized by large, platy calcified sponges. In the Upper Capitan, the platy inozoan sponge Gigantospongia discoforma reached up to 2 m in diameter, and individuals projected from the reef slope to form the ceilings of substantial open cavities. The undersurfaces of these large sponges were colonized by an extensive cryptic community, including downward-growing branching sphinctozoans up to 0.5 m long. In the absence of both the physical and biological destructive forces prevalent on modem reefs, much of the relatively fragile Capitan reef-building community remained in growth position. It is unlikely, however, that such a community could have withstood highly turbulent and agitated waters. Additional strength was imparted to the framebuilding community by encrustations of Archaeolithoporella and Tubiphytes, followed by the precipitation of abundant micrite of probable microbial origin. This cavernous framework was subsequently partially filled with internal sediment and Areheolithoporella, and botryoidal aragonite and other submarine cements. These cements and sediments account for up to 70% of the reef rock. So unlike modem coral reef carbonates, most of the Capitan Reef was derived from probably inorganic sources, and moreover none of the organic carbonate—with the exception of phylloid algae and possibly Archaeolithoporella—is interpreted to have formed as a result of light-enhanced (photosynthetic) calcification. Such observations suggest that models that simulate modem coral reef growth, which is dependent upon the distribution of light, may not be applicable generally to Paleozoic reefs.

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