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Windward Carbonate Margin Parasequence Geometry Linked to Precursor Topography, Exuma Cays, Bahamas

McNeill, Donald F.1; Hearty, Paul 2
1 CSL-RSMAS, University of Miami, Miami, FL.
2 Bald Head Island Cons., Bald Head, NC.

Grainy platform margins form the transition from the platform top to the flanks and basin. As such, the stratigraphy of these transition zones is complex, and critical to sequence correlation and connectivity of potential reservoir facies in oversteepened margins. To address this complexity we age date two cores and outcrops from the windward margin of the Exuma Cays. Pleistocene high frequency sequences (HFS) are used to estimate sea-level elevations and reconstruct antecedent topography. We then interpret and compare the modern deposits to those Pleistocene events. The surface geology and two 33-m cores from the Exumas contain 11 limestone-paleosol “couplets” back to Early Pleistocene. The couplets are composed of marine limestone bounded by paleosol-karst exposure surfaces. Amino acid racemization and magnetic reversals provide a functional chronostratigraphy. At the base of the bankward core, couplets C1 to C3 contain the extinct coral Stylophora, which ranges into the Early Pleistocene. The overlying C4 complex consists of several meters of multiple, dense reddish paleosols and thin intertidal lenses. These C4 paleosols bracket the Brunhes-Matayuma boundary (0.78 Ma) and equate with a succession of much lower highstands. Couplets C5 and C6 represent deposition during MIS 13 and 11, respectively. The prominent MIS 11 interglacial left little evidence at the margin (thin eolian ridge), probably due to deep platform flooding and little antecedent topography. Highstands at MIS 9 and 7 (C7 and C8) resulted in sediment wedges that onlapped only the outer margin, as they did not flood the platform top. Higher MIS 5 sea level deposited a broad wedge of sediment in multiple ridges several km wide and flooded the deep platform interior. These results confirm stratigraphic variability reflecting sea-level amplitudes. They provide constraints on HFS correlation and quantify 2D geometries of the grainy deposits. Flooding events that failed to overtop preexisting topography result in wedge-shaped units on the seaward margin and do not have a correlative platform top unit. Conversely, highstands that overstep precursor topography to flood the platform produce a second wedge of sediment that extends bankward. These HFS's are thicker at the margin, may host eolianites, and have a correlative platform interior unit. On the modern margin the Holocene unit is laterally disconnected with platform interior deposits due to interruption by precursor topography.

 

AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009