Submarine Lithification of Holocene Reef Hardground: North Jamaica
J. Todd Mitchell
Submarine cementation and organic binding of skeletal reef debris have brought about the formation of an extensive carbonate hardground, which is exposed on the submerged seaward slope of the reefcrest at Discovery Bay, Jamaica. The hardground, dated as Holocene by carbon-14, is comprised of the fragmented and bioeroded remnants of a diverse coral-algal community. Mg-calcite and aragonite, the dominant cements, are common in a variety of growth forms and occupy intraskeletal and interparticle voids.
In order of decreasing abundance, Mg-calcite cements exhibit the following growth forms: (1) cryptocrystalline (micritic); (2) submicrocrystalline rhombic; (3) microcrystalline, stubby-to-acicular isopachous; and (4) microcrystalline blocky mosaic. Mg-calcite peloids, spherical bodies 20-60 µm in diameter, are ubiquitous as geopedal pore-fill. Aragonite cement is most common as acicular needles 50-100 µm long which grow as syntaxial fringes on skeletal substrates. Less common aragonite cement forms are microcrystalline blocky mosaic cements and radial fibrous aggregates. Adjacent pore spaces generally exhibit different cement fabrics, indicating that pores have unique microenvironments.
Electron microprobe analysis revealed dolomite dispersed in the peloidal pore-fill of a coral fragment sampled from 5 cm beneath the hardground surface. 14C dating, field relationships, isotopic and trace element analysis suggest that trace quantities of Holocene dolomite are forming in the shallow, subtidal reef environment.
The formation of the extensive coral pavement is a consequence of the interplay between the destructive forces of catastrophic storm events and bioerosion on one hand and the constructive effects of submarine cementation and organic binding on the other hand. Thriving but mechanically fragile coral communities are "bulldozed" by hurricanes. Bioerosion and mechanical abrasion further reduce the volume of the coral framework, ponding silt and sand-sized sediment in the voids between and within skeletal fragments. Meanwhile, submarine cementation and organic binding lithify the flattened framework. The wide scatter of 14C dates indicates that the extensive hardground is probably the result of many such events.
AAPG Search and Discovery Article #91038©1987 AAPG Annual Convention, Los Angeles, California, June 7-10, 1987.