COMPLEX CARBONATE SEDIMENT DEPOSITION AND ACCRETION CONTROLLED BY HIGH-FREQUENCY SEA-LEVEL OSCILLATIONS: EXUMA CAYS, BAHAMAS
The numerous and varied islands of the Exuma Cays are composed of Holocene (<6,000 ybp), MIS 5e (~125,000 ybp), and older Pleistocene strata. These stratigraphic units are laterally accreted forming the complex island stratigraphy of the Exumas. For example, field mapping and ground-truthing of satellite imagery of one key island (Hawksbill Cay) documents patterns of Holocene and Pleistocene facies accretion laterally; 38% of the island is Pleistocene at the surface and Holocene ridges were deposited around the Pleistocene topography in a complex fashion. Holocene ridges have elevations from near sea level to +12 m (mean elevation of +7.7 ± 2.5 m) while Pleistocene landforms have elevations from near sea level to +19 m (mean elevation of +9.3 ± 3 m). This complex pattern of carbonate sediment deposition and accretion extends along the entire Exumas windward margin.
Likewise, the vertical record displays complicated facies successions that can be unraveled by accepting the notion of oscillating sea level. Fourteen cores from +1 m to a maximum depth of -23 m drilled in the Exuma Cays spanning ~100 km northwest to southeast display complex vertical stratigraphic heterogeneity featuring overstepping wedges prograding eastward towards Exuma Sound. For example, in the core from Darby Island, two MIS 5e subtidal units are separated by an exposure horizon. In other cores, MIS 5e beach to eolian dune transitions occur at different depths, documenting pulsed down-stepping of sea level. In addition, the complex topography of the Pleistocene creates complicated antecedent topography for Holocene sediment accretion. The combined product of high-frequency orbital sea-level changes and suborbital oscillations is a complex lateral and vertical stratigraphic architecture that juxtaposes grainstones deposited in different environments.