CONSTRUCTION OF A COMPREHENSIVE MODERN LAGOONAL PATCH REEF MODEL FOR INTERPRETATION OF ANCIENT BIOHERMS

Modern reefs have served as the basis for interpretation of ancient carbonate buildups for more than 100 years. Although some ancient reef structures can be identified as fringing or barrier reefs; many organic structures do not appear to fit the model of modern forereef environment of fringing or barrier reefs. Data collected over the past thirty years suggest that lagoonal patch reefs may serve as a more useful model for ancient bioherms.

Geologists have essentially ignored lagoonal patch reefs as modern analogs in ancient bioherm studies because lagoonal reefs occur on muddy substrates in areas of low visibility. However, many ancient bioherms developed with fine clastic material deposited around the carbonate buildup and mixed with reef-derived carbonate sediments. These bioherms are found associated with micrite or calcareous shales, which are texturally and compositionally similar to those sediments being currently deposited in modern lagoonal environments. The muddy environment reduces light penetration and results in a biotic zonation that is also suggestive of ancient bioherms.

Biotic and bathymetric surveys initiated in 1974 at Discovery Bay, Jamaica, suggested that lagoonal patch reefs have a unique sponge-dominated biotic zonation that contains corals characteristic of greater depths on the forereef of the fringing-barrier reef. These biotic zones are remarkably similar to those observed in ancient bioherms. Further, similarities in developmental stages also suggest that bioherms can be modeled by lagoonal patch reefs. Initiation of substrate stabilization, diversification, and ultimate termination is the general developmental sequence of ancient bioherms. The Red Buoy reef in Discovery Bay experienced a sudden influx of fine sediment during 1977, 1980 and 1988 resulting in the developmental phases of termination and reinitiation, which are identified in ancient bioherms.

This investigation examines, in detail, modern lagoonal patch reefs in Discovery Bay, Jamaica, with particular attention to documenting historical bathymetric changes, biotic zonation, and sedimentological history since 1974. A comprehensive lagoonal reef model can provide greater understanding of the genesis of ancient bioherms and increase understanding of facies relationships.