Surficial and Shallow Subsurface Geology of the Northern and Central Exuma Cays, The Bahamas

Abstract The surface and subsurface geology of the Exuma Cays in the central Bahama Islands records sea-level cyclicity that extends back to the Jurasssic, however the focus of this paper is the Plio-Pleistocene, since approximately 1–3 million years ago. During the 1990s, the first author surveyed a large number of the islands by boat, mapped the geology on 1:25,000 topographic maps with the aid of air photos, and more recently complemented these mapping studies using Google Earth Pro. In 1994, two 33 m cores were extracted from Norman's Pond Cay and Lee Stocking Island in the southern group of the Exuma Cays. The outcropping rocks throughout the Cays are characterized by a mix of nearly pure bioclastic and oolitic sediments and limestone units. Generally, the highstand limestone deposits are capped with lowstand terra rossa paleosols, red-stained micritic limestone, calcrete, or karst surfaces. From surface and subsurface geology, a minimum of seven stratigraphic units are recorded, yet many cycles are obviously missing. Three broad eustatic intervals are evident from the δ18O record. These are associated with the Pliocene-early Pleistocene (“PP” >1.2 Ma), Mid-Pleistocene Transition (“MPT” ∼1.2–0.7 Ma), and the mid-late Pleistocene (“MLP” ∼0.7 Ma to present). A combination of physical stratigraphy, geomorphology, paleomagnetism, and amino acid racemization (AAR) provide a general age framework for these deposits. Pliocene beds at -25 m in only one core appear to indicate slow subsidence of the Bahama Banks. An interval of prolonged bank marginal or lower sea stands of the MPT are indicated by a cluster of dense, clayey red paleosols. Many MLP rocks on the islands and in the cores retain sufficient amino acids to establish a relative chronology for the MLP. The geomorphic and stratigraphic succession from the Exumas shows very rapid progradation of the over-steepened bank margin toward Exuma Sound by several km since the Plio-Pleistocene. The purpose of this paper is to: 1) characterize the geology of the existing Exuma rocks and cays; 2) demonstrate the shallow-subsurface stratigraphy in two 33-m long cores; and 3) provide some examples of how the Exuma Cays and their margins have evolved over the past 1–3 million years.