Abstract: Submarine Diagenesis in Relict Holocene Coral Reef, Southeast Coast of Florida--Diagenetic Control on Porosity and Stratigraphy
Robin G. Lighty
Offshore pipeline construction 40 km north of Miami, Florida, sectioned a well-preserved relict shelf-edge Acropora palmata coral reef, which is covered by a submarine lithified crust and has various forms of submarine cementation within the reef framework. These submarine cements provide an effective control on porosity as well as influencing future stratigraphic control on reservoir potential. Internal reef submarine cement is predominantly microcrystalline magnesium calcite in the form of rim cement, peloidal infill, coatings on grains, and dense crusts on the surface of corals. Acicular aragonite rim cement is present in minor amounts within the coral skeletal cavities and predates the magnesium calcite cement where present together. Submarine cementation generally oc urs in areas of high agitation and/or low sedimentation, preserving initial skeletal porosity in Acropora palmata and fore-reef brain corals through the formation of a dense protective crust (infill rim) of magnesium calcite around the coral skeletons. Submarine cementation infills and destroys initial porosity in the smaller branching coral A. cervicornis. Intermittent layers of heavily cemented reef rubble are present in the fore-reef talus, representing periods of little sedimentation or a hiatus between talus slumping.
Radiocarbon dates indicate that the reef flourished during the Holocene transgression, then died, and has had no significant accumulation to the framework during the past 6,000 to 7,000 years. As this relict shelf-edge reef submerged, bioerosion, sediment infill, encrusting organisms, and submarine cementation altered the top of the reef into a dense micritic layer, which has protected the internal-reef components from further alteration/cementation. The diagenetic "cap rock" ultimately may improve the reef's reservoir potential.
AAPG Search and Discovery Article #90968©1977 AAPG-SEPM Annual Convention and Exhibition, Washington, DC