Back-Reef and Fore-Reef Analogs in the Pleistocene of North Jamaica: Implications for Facies Recognition and Sediment Flux in Fossil Reefs

Abstract

The 125,000 y.b.p. (Sangamon) Falmouth Formation is an emergent fringing-reef complex exposed along the north coast of Jamaica. Q-mode cluster analysis using constituent composition of Falmouth Formation rocks collected near Discoveiy Bay was employed to differentiate two distinctive facies. These are a dense, well-lithified skeletal packstone containing abundant calcareous algae (Halimeda and coralline algae) and molluscs, and a poorly lithified skeletal grainstone composed primarily of sand-sized coral fragments and coralline algae with only minor amounts of Halimeda. The composition of sediments within these facies is comparable to the composition of back-reef and shallow (5-8 m deep) fore-reef sediments of the Holocene Jamaican fringing-reef system. These results contradict the pervasive idea that sandsized, reef-derived sediments are ineffective as environmental indicators owing to post-depositional transport away from the reef system. Petrographic and X-ray analyses of the mineralogy of Falmouth Formnation limestones reveal that back-reef packstones retain much of their original aragonite and high-Mg calcite. In fore-reef grainstones, however, better sorting (increased permeability) results in dissolution and leaching of these metastable phases and reprecipitation of low-Mg calcite. Thus, variability in original sediment texture may create a preservational bias in the fossil record against the more permeable fore-reef deposits.