GEOLOGICAL EVIDENCE FOR A HIGHER THAN PRESENT SEA LEVEL DURING THE MIDDLE HOLOCENE FROM NW POINT, MAYAGUANA, SE BAHAMAS
The geology of Mayaguana, a small island in the SE Bahamas, was recently investigated using a multi-method approach including stratigraphic logging, sedimentological and petrographic analyses, and U-Th, amino-acid racemization and 14C dating.
From bottom to top, the NW Point section shows one basal coral framestone unit, with a pedogenized bioclastic grainstone matrix, comprising a rich faunal assemblage (Acropora palmata, A. cervicornis, Pocillopora palmata, Diploria sp.) dated at ca. 117 ka BP. The top of this unit corresponds to a flat erosional surface at about 1.3 m above mean sea level. This surface is capped by a cm-thin, orange, laminar crust that gave an age of 9.5 ka BP. The latter is overlain by a bioclastic grainstone about 2 m thick showing three sedimentary facies: (1) a basal beach characterized by low-angle crossbeds, fenestral porosity, and early phreatic cements; (2) a backshore facies represented by bioturbated beds that downlap on the underlying beach and the crust-capped reef; (3) an eolianite identified by steep, landward-dipping foresets, that grades into unlithifed sands. This bioclastic grainstone gave an age of 2.8 ka BP.
The following sequence of events can be reconstructed from the presented geological features. A coral reef formed during the MIS 5e regression was pedogenized during the ensuing glacial lowstand. During the middle Holocene, a higher than present sea level (ca. + 2 m) removed the capping soil and truncated the Pleistocene reef. Beach-dune sediments prograded over the erosional surface during the falling stage of this mid-Holocene highstand as shown by the downlapping of the backshore facies on the underlying units. Finally, a capillary rise-zone calcrete, now exposed by recent regressive erosion, precipitated at the top of the coral framestone. Its old age (9.5 ka) is due to the incorporation of both Holocene and Upper Pleistocene carbon.