Sedimentology and Diagenesis of Windward-Facing Fore-Reef Calcarenites, Late Pleistocene of Barbados, West Indies

John D. Humphrey, Tod N. Kimbell

Late Pleistocene reef terraces in southeastern Barbados developed extensive fore-reef sand facies during deposition in response to high-energy windward-facing conditions. Sedimentology and diagenesis of these deposits illustrate significant contrasts with previous studies from the leeward west coast. These calcarenites are dominantly skeletal packstones with less common grainstones and wackestones present. The fore-reef sand facies occurs within progradational reef sequences, being conformably overlain by deep-water head coral facies. Medium-bedded, laterally continuous sand sheets retain original depositional slopes, dipping seaward at 10°-15°. These fore-reef deposits, in places, are over 30 m thick (average 20 m) and developed rapidly during late Pleistocene lacio-eustatic sea level highstands. Sedimentation rate ranges from 2 to 5 m/1,000 years. Areal extent of fore-reef calcarenites in southeastern Barbados is estimated to be 8-10 km².

Lithologically, the packstones are composed of an abundance of coralline red algae and the benthic foraminifer Amphistegina sp. Other volumetrically significant allochems include echinoids, mollusks, rhodoliths, peloids, and micritized grains. Micrite in the wackestone and packstone lithologies is likely derived from intense physical/mechanical abrasion of shoal-water reef facies.

Diagenesis of these lithologies reflects a complex interplay of meteoric, mixing zone, and marine environments as a result of glacio-eustasy. Differences in diagenetic character are derived from differences in terrace ages, terrace geometry, a paleotopographic control on meteoric ground-water distribution, and high-energy coastal conditions. Diagenetic fabrics include equant, blocky meteoric phreatic calcite; limpid dolomite of mixing zone origin; and peloidal and isopachous fibrous cements from marine precipitation.

AAPG Search and Discovery Article #91022©1989 AAPG Annual Convention, April 23-26, 1989, San Antonio, Texas.