AAPG ANNUAL CONFERENCE AND EXHIBITION
Making the Next Giant Leap in Geosciences
April 10-13, 2011, Houston, Texas, USA
Stratigraphic Complexity of a Windward Platform Margin, Exumas, Bahamas
(1) Comparative Sedimentology Laboratory, University of Miami, Miami, FL.
(2) Chevron Energy Technology Company, San Ramon, CA.
Windward carbonate platform margins are often assumed to stack vertically but recent studies document a more complicated architecture of onlapping and overstepping wedges that stack laterally and prograde seaward. This notion is of academic interest to researchers interested in the evolution of carbonate platform margins, but it is also of critical interest to those in industry who are concerned with delineating the geologic framework of carbonate reservoirs for exploration and development-scale projects. Quaternary high-frequency sea level highstand variability is recorded along the Exumas windward margin of Great Bahama Bank. The Exumas include more than 350 islands (cays) featuring a complex juxtaposition of Pleistocene and Holocene grain-dominated stratigraphic units. Field mapping, coring, and age determinations provide insight into the lateral and vertical heterogeneity of depositional facies and diagenetic overprint resulting from sea level highstands with variable frequency and amplitudes. Deposition occurred during the Holocene (past 6 kybp), marine isotope stage (MIS) 5e (~125 kybp) when sea level peaked ~6 m higher than today, and during older sea level highstands that were higher (MIS 9 at 330 kybp and MIS 11 at 410 kybp) or lower (MIS 7 at 240 kybp) than today.
Geologic mapping of more than 15 cays in the Exuma Cays Land and Sea Park, covering 456 km2 and extending 35 km along the platform margin, documents the heterogeneity generated by the interaction of antecedent topography with younger accretion where facies juxtapositions are often abrupt and discontinuous. Holocene sediment accretion fills accommodation space between Pleistocene eolianite ridges with generally seaward prograding catenary beach ridges and back beach storm ridges. Catenary beach ridges prograde seaward 100 - 500 m and extend shore parallel up to 1 km. Cemented back beach storm ridges create vertical successions that are centimeters to several meters in scale. Holocene dune complexes (up to 30 m high) accrete vertically on pre-existing Pleistocene topography. Rock cores, up to 22 m long, document the interplay between Holocene and Pleistocene deposits and record the vertical successions and stratigraphic heterogeneity of Pleistocene highstand deposits from MIS 5, 9, and potentially 11. Cores feature carbonate grainstones deposited in subtidal, beach, and eolian environments, as well as calcrete surfaces that mark exposure horizons that separate the Pleistocene deposits.