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Contribution to book (peer-reviewed)

Book Proceedings of the 14th Symposium on the Geology of the Bahamas and other carbonate regions
Editor , J.B. Martin and F.D. Siewers
Publisher Gerace Research Center, San Salvador Bahamas
Page(s) 90 - 106
ISBN 0-935909-91-5
Title of proceedings Proceedings of the 14th Symposium on the Geology of the Bahamas and other carbonate regions

Abstract

Fossil lacustrine sediments were found at about 15 m above sea level, directly above beach deposits dating from Marine Isotope Stage (MIS) 11 on a tectonically stable coastline in Eleuthera (Bahamas). This new discovery brings further support to the hypothesis suggesting that a major collapse of polar ice sheets occurred during this interglacial period. Two stratigraphic sections (GW1/GW2), including several carbonate units separated by paleosols, were logged near Glass Window (northern Eleuthera). Both sections comprise a basal unit made of cross-bedded, bio-peloidal limestone, including an early generation of fibrous rim cement of marine origin and a late phase of drusy calcite mosaic. A second unit displays well-defined planar bedding at both sites, whereas faint trough cross-stratifications are also visible at GW1. It consists of a bio-peloidal grainstone containing Halimeda fragments and lithoclasts derived from the underlying unit, and is further characterized by an early generation of isopachous fibrous cement. The upper part of this second unit shows one thick laminated crust with desiccation polygons and numerous spherulites of cyanobacterial origin, and is capped by a paleosol. The top of the GW2 section is represented by a karstified oolitic/peloidal grainstone, not found at GW 1, whereas the upper part of GW1 includes one bioclastic and one oolitic limestone unit separated by a paleosol. Based on sedimentary structures, petrographic composition, and amino-acid ratios borrowed from previous authors, the basal and upper units exposed at both sections can be identified as eolianites dating from MIS 11, 9, 7 and 5e, respectively. Occurring between +12 and +14.5 m, the basal part of the second unit can be interpreted as beach deposit, whereas its upper part, at +15 m, likely corresponds to lacustrine or pond sediments, both dating from MIS 11. Considering the subsidence rate affecting Eleuthera, these beach and pond deposits, and the phreatic cements in the basal eolian units unavoidably imply a sea-level highstand at ca. 20 m during MIS 11. This exceptional event was probably caused by the collapse and melting of polar ice-sheets during this very warm climatic period. In the context of present-day global warming, a similar scenario could be repeated in the near future.
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