Sea-Level Oscillations during the Last Interglacial Highstand (MIS 5e): Evidence from the Bahamas
Kelly L. Jackson
University of Miami, CSL-Center for Carbonate Research Miami, FL USA
Evidence from the Bahamas indicates that the last interglacial highstand 125,000-115,000 years before present (MIS 5e) was not a single rise and fall but instead oscillated a minimum of 12 m over a few thousand years. Sea-level highstand oscillations of this magnitude require a suborbital forcing mechanism of shorter duration than the ~20, 40, and 100 kyr Milankovitch frequencies commonly assumed to drive sea-level fluctuations. These oscillations create complicated juxtapositions of facies and ages in carbonate coastal systems that directly question key assumptions in cyclostratigraphy. The understanding of the resultant architecture is essential to characterizing reservoir heterogeneity and flow behavior in carbonate grainstone strata.
Exposure horizons and lithologic changes in cores and outcrops combined with age dating in the Exuma Cays and New Providence, Bahamas, provide sedimentologic and stratigraphic evidence of MIS 5e sea-level oscillations. Cores from Darby Island feature two MIS 5e subtidal units separated by an exposure horizon. Beach deposits +7.6 m above present sea level on New Providence represent the older MIS 5e peak. A down-stepping beach ridge indicates a subsequent sea-level position at +7.0 m. A calcrete separating subtidal deposits adjacent to the beach documents the mid-MIS 5e sea-level drop. The Exumas calcrete associated with this fall separates subtidal facies at -5.2 m. Sea level rises again to form the younger MIS 5e highstand represented by a beach ridge at +5.1 m on New Providence and Exumas reefs up to +1.5 m above modern sea level.
AAPG Search and Discovery Article #90157©2012 AAPG Foundation 2012 Grants-in-Aid Projects