A DIAGENETIC HISTORY OF NORTH ANDROS ISLAND, BAHAMAS

This study focuses on the diagenetic history of two 12.6-meter cores (C2N and CH69) collected from North Andros Island, Bahamas. Andros, the largest of the Bahamian islands, is comprised of Pleistocene rocks overlain by Holocene sediments. The Pleistocene cores examined in this study were collected 1.25 km apart with C2N located inland of CH69.

The general sequences represented in the cores are shallowing-upward marine deposits, with muddier lagoonal sediments overlain by cleaner ooid-rich deposits of a stabilized sand flat. The environments recognized in the North Andros cores are similar to those found on Joulter’s Cays, supporting the idea that the Holocene ooid shoal complex at Joulters is the modern analog to Andros Island.

Andros Island has been subjected to prolonged exposure and diagenesis since the sea level highstand 125,000 years ago when the rocks were deposited. Petrologic and macroscopic examination of the cores has revealed features such as cementation, dissolution and neomorphism that are indicative of changing diagenetic environments. The upper 4.5 meters of both cores contain patchy cements that indicate vadose conditions. The lower portions of the cores contain equant blocky cements indicative of the phreatic zone. Remnant vadose features deeper in the cores make precise boundaries difficult to place. Dissolution occurs throughout the cores and is evident by the presence of vugs and the dissolution of ooid laminae. Total and partial recrystallization of skeletal grains occurs in both cores.

Porosity of the cores varies from 0% to >30% and is moldic, vuggy, intragranular, and intergranular. Mineralogically, transformation of aragonite to calcite has occurred with depth and prolonged exposure to fresh water. Neomorphism is evident by the presence of microspar and pseudospar between 5.7-7.0 meters in C2N and below 7.0 meters in CH69. Preliminary results indicate that sea level fluctuations have exposed these rocks to freshwater, marine and mixing zone environments leaving diagenetic changes that are preserved in the rock record.