Sea-Level Oscillations during the Last Highstand (MIS 5e): Evidence from the Bahamas

Increasing evidence indicates that sea level during the last interglacial 125,000 ybp was not a single rise and fall but fluctuated several times. Bahamian Pleistocene islands provide evidence that during the sea level highstand of marine isotope stage (MIS) 5e, several oscillations occurred. By using the elevation of reef terraces, beach deposits, and documenting the transition from marine to eolian environments in cores and outcrops, Pleistocene sea-level positions can be reconstructed for the MIS 5e interglacial. Sea level was as low as -12 m based on the lowest beach-eolian transition in cores from the Exumas and as high as +6 m based on the beach deposits at Clifton Pier, New Providence.

The Exuma Cays are composed of a 170 km long and 5-10 km wide chain of carbonate islands composed primarily of Holocene (<6,000 ybp) and MIS 5e (~125,000 ybp) strata. Cores drilled at Shroud Cay, Warderick Wells, and O’Brien’s Cay in the northern-central Exumas document that the lowest occurrence of MIS 5e eolian dunes is approximately -12 m below present sea level. Core analyses in conjunction with island mapping reveal that there was not just one rise and fall but instead there were several fluctuations. Carbonate dunes do not migrate and start to cement soon after they form. Therefore, one ridge of eolian dunes implies one sea-level position in a typical carbonate coastal system. At two locations within the Exuma Cays Land and Sea Park, there are five parallel north-south trending eolian dune ridges that were deposited during MIS 5e. Base levels of the eolian deposits become deeper towards Exuma Sound suggesting a pulsed downstepping of sea level.

Shallow-water reefs are present at +1.5 m above present sea level on Rocky Dundas in the northern-central Exumas as well as on Darby Island and Norman’s Pond Cay in the southern Exumas. In the central Exumas, Bitter Guana Cay features shallow-water reef facies, including Acropora palmata, at approximately 1.8-2.8 m below present sea level. On New Providence Island, outcrops at Clifton Pier contain beach deposits at +6 m above present sea level. Combining the record in the Exumas with New Providence, the maximum oscillation during the MIS 5e highstand is approximately 18 m. Surficial mapping and core borings record extremely short sea-level changes which lends itself to improved understanding of the depositional anatomy and heterogeneity along the windward margin.
 

AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California