Suborbital Sea-Level Oscillations during the Last Interglacial Highstand (MIS 5e): Evidence from New Providence Platform, Bahamas
Jackson, Kelly L.; Eberli, Gregor; Reid, Samuel B.; Harris, Paul (Mitch); McNeill, Donald F.
Evidence from New Providence Platform, Bahamas, indicates that sea level oscillated a minimum of 10 m during the last interglacial highstand (MIS 5e) 115-125 ka ago, creating early and late substages within 5e. A 10 m oscillation exposed Great Bahama Bank, creating two separate depositional cycles within 10,000 years. This highstand oscillation requires a suborbital forcing mechanism of much shorter duration than Milankovitch frequencies. It is important because it contradicts the preconceived notion that precession is the controlling factor of high-frequency sequences and the building blocks of carbonate cycles that we assume are reservoir flow units.
Positions of reefs, beach, and eolian deposits in core and outcrop on New Providence and the Exuma Cays, Bahamas, provide evidence that document the amplitude of the MIS 5e sea-level oscillations. Sea level first rose to +7.6 m above present then dropped to +7.0 m, documented by downstepping beach ridges on New Providence. Next, sea level oscillated a minimum of 10 m documented by a calcrete in the subtidal deposits adjacent to the beach on New Providence as well as calcretes separating subtidal deposits at -2 m in cores from the Exuma Cays. Sea level rises again to form the younger MIS 5e highstand represented by a beach ridge at +5.1 m on New Providence Island and Exumas reefs up to +1.5 m above modern sea level. Parallel downstepping beach to eolian dune transitions in Exumas core provide evidence for a pulsed downstepping of sea level at the end of MIS 5e. The lowest occurrence of this transition is approximately -12 m below present sea level.
Recognition of suborbital sea-level oscillations has two major implications. First, sea-level oscillations within interglacials indicate that a mechanism exists that can produce sea-level fluctuations during times when the Earth is considered ice free, i.e. the greenhouse world. This might explain the cyclic nature of the Cretaceous and Triassic strata. The short duration of these oscillations produces uncertainty into the commonly accepted notion of Milankovitch cyclostratigraphy in carbonates. Second, 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, and produces an even more complex heterogeneity that must be unraveled in subsurface reservoir analysis.
AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013