EARLY DIAGENESIS OF A PLEISTOCENE SHALLOW-WATER CARBONATE SEQUENCE: PETROLOGIC AND MINERALOGIC INDICATORS

Pleistocene rocks of North Andros Island, Bahamas, comprise shallow-water carbonates separated by paleosols. Petrologic and mineralogic evidence from cores and outcrops of the uppermost 15 meters of rock indicates that the sediments have undergone a minimum of three episodes of diagenesis. The data suggest that the diagenesis occurred during lowstands in sea level after the 5e, 7, and 9 highstands. Two of the lowstands are marked by the presence of a paleosol (laminated crusts, blackened grains, and root casts).

Distribution of cements record changing positions of the unsaturated/saturated zones associated with sea level changes. Modern vadose, phreatic, and mixing zones are present within the sequence. Meniscus and pendant cements indicative of vadose conditions are found within and below the modern vadose zone (down to depths of 7 meters). Blocky, isopachous rim and drusy cements characteristic of the phreatic zone are found both in the present phreatic zone and in zones that also contain vadose cements.

Within grainstones and well-washed packstones, dissolution of ooid nuclei and laminae and precipitation of cement between grains and within laminae are the major diagenetic features. Neospar (mainly microspar with rare pseudospar) occurs in muddy rocks (wackestones and packstones) that have been subjected to at least three episodes of diagenesis. Neospar is found in rocks buried as shallow as 5 meters. Successive episodes of diagenesis cause the rocks to become hard and dense. Vugs are more prevalent and larger after three episodes of diagenesis and occur rarely in rocks with only two episodes.

Mineralogic changes in the rocks include a decrease in aragonite from an average of 50% for one episode, to an average of 20% for two episodes, and to an average of <5% for three episodes of diagenesis. These results are consistent with aragonite distributions seen in equivalent rocks on San Salvador, Bahamas.