Facies Succession of Pliocene-Pleistocene Carbonates, Northwestern Great Bahama Bank

Abstract

Preliminary study of nine core borings from the northwestern Great Bahama Bank has yielded the first regional information on depositional facies and stratigraphy of pre-Holocene carbonates to depths of 75 m below sea level. The cross section of borings extends 120 km from Tongue of the Ocean across Andros Island and the shallow bank to Orange Cay on the western bank margin facing the Straits of Florida. The depositional facies present an atoll-like cross section with thick marginal accumulations of coral-coralline algal limestones at least 5 km wide both on the windward (east) and the leeward (west) margins. The discovery of these reef rims confirms earlier inferences that the Bahama Banks are Tertiary-Quaternary atolls. Although both margins have reef rims, there is a distinct asymmetry of facies within the bank: grainstones are more abundant on the windward margin and wackestone-mudstones are widespread over the interior and leeward part of the bank. The succession in borings from the interior of the bank can be subdivided into three intervals: the upper two intervals whose aggregate thickness averages 43 m are irregularly cemented, unstratified packstones and wackestones in which peloids and ooids are the predominant grains. The third, lowermost interval is predominantly stratified skeletal packstone with subordinate amounts of skeletal wackestone; it has an abundant fauna of bivalves, Foraminifera, and, commonly, branches of the finger-sized coral Stylophora spp. As the change from nonskeletal to skeletal limestones is relatively rapid and traceable for 40 km, it is used to define the base of a new formation, the Lucayan Limestone. The upper boundary of this formation is the limestone surface exposed on the Bahama Islands or cov red by soft Holocene sediments on the submerged banks. The Lucayan Limestone is dated as late Pliocene-Pleistocene on the basis of the disappearance of Stylophora spp. The known uppermost range of Stylophora spp. in the western Atlantic is through the upper Pliocene. Additional support for this dating comes from the distribution of discontinuity surfaces that are considered to be the record of subaerial exposure of the marine deposits. In the Lucayan Limestone, the frequency of discontinuity surfaces per meter of core is twice that of the pre-Lucayan unit; this increased frequency of subaerial exposure is consistent with the assignment of the Lucayan Limestone to the late Pliocene and Pleistocene. The major change in facies from stratified, skeletal, pre-Lucayan limestones to the unstratified, nonskeletal Lucayan Limestone is interpreted as the result of apparent shallowing of the bank triggered by the more frequent fluctuations of sea level and possibly by changes in the elevation of high stands; both began in the late Pliocene and continued throughout the Pleistocene.