PLEISTOCENE REEF FACIES DIAGENESIS WITHIN TWO SHALLOWING-UPWARD SEQUENCES AT COCKBURNTOWN, SAN SALVADOR, BAHAMAS

X-ray diffractometry and petrography disclose sharp diagenetic contrasts between reef sediments in a carbonate core retrieved near Cockburntown on western San Salvador, Bahamas. The 15-meter core contains two stacked shallowing-upward sequences of lagoon, reef, beach, and dune sediments capped by red micritic paleosols. These sequences are linked to glacioeustatically-determined episodes of transgression and regression during the late Quaternary.

Both of the core's reef facies are dominated by the Scleractinian coral Acropora cervicornis. These coral fossils are commonly overgrown by coralline algae (up to 5cm thick) of either granular or distinctly laminated microtextures. X-ray diffractometry indicates mineralogic alteration from aragonite to calcite with increased core depth that likely resulted from regional fluctuation between marine and meteoric environments.

Petrographic investigation reveals pore-lining micritic cement within reef sediments in the core's upper sequence. This facies is attributed to the 5e interglacial highstand and is characterized by nearly pristine fossils. Corals within the core's lower reef facies were deposited during a pre-5e highstand and subsequently subjected to meteoric diagenesis during at least two sea level lowstands. Resultant petrographic features include vuggy porosity and cement fabrics ranging from micritic pore linings to complete drusy infillings. Acropora cervicornis in the pre-5e reef facies has altered to chalky textures of varying extents, with diagenesis likely controlled by original taphonomic conditions, meteoric environment dynamics, and maturity-linked primary skeletal porosity.

Coralline algae show almost no alteration in the upper reef. In the lower reef, coralline algae show a lesser extent of alteration than does Acropora cervicornis. Obvious diagenetic characteristics of the algae include cementation and moderate amounts of boring, vugs, and possible iron-manganese fracture fills.