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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) 75 - 89
ISBN 0-935909-91-5
Title of proceedings Proceedings of the 14th Symposium on the Geology of the Bahamas and other carbonate regions

Abstract

New sedimentological, petrographic and geochronological data obtained from a thin laminar crust and a superimposed reddish, clay-rich conglomerate layer exposed in central Eleuthera, Bahamas, suggest they represent, respectively, an algal-microbial mat and a colluvial deposit of middle Holocene age, which sheds new light on eustatic and climatic events during this time period. Located on the ocean-facing coast of Eleuthera, near Alice Town, the studied exposure comprises one 30 cm-thick reddish layer consisting of a laminated micritic crust, at the base, and a clay-rich conglomerate, at the top, interstratified between two thicker limestone units. Like most similar occurrences in the Bahamas, this composite bed has been interpreted as a Wisconsinian paleosol separating Sangamonian and Holocene peritidal carbonate deposits. Our detailed study of this outcrop yielded the following results. The basal portion of the exposure consists of oolitic-bioclastic calcarenites deposited in a beach setting, locally overlain by rhizolith-rich bioclastic calcarenites accumulated in a foredune environment. Amino-acid ratios and stratigraphic relationships suggest a correlation with Marine Isotope Stage 5e for the beach facies, and with MIS 5a for the eolian facies. At about +2 m, these calcarenites are capped by a 1 to 5 cm-thick, laminated, micritic crust, which includes micron-sized spherulites of cyanobacterial origin and probable remnants of bryophytes. This crust yielded a 14C age of 6’470 + 30 a BP (= 7’375 + 55 cal a BP). The overlying clay-rich conglomerate layer comprises blocks of various sizes and origins, and lacks a horizontal zonation and pedogenic textures. The uppermost unit of the exposure is made of an eolian bioclastic calcarenite, and gave a 14C age of 5’480 + 30 years BP (6’289 + 26 cal a BP). The short time interval (ca. 1’000 years) established for the deposition of the reddish conglomerate layer, as well as the lack of soil horizons and pedogenic textures, suggest it corresponds to a debris-flow deposit resulting from an extreme event (tsunami wave or exceptional rainfall) during the middle Holocene. This case further demonstrates that red brecciated beds interstratified in Bahamian carbonates do not always represent glacial paleosols. The microbial and algal structures found in the studied crust show that laminated calcretes may not all have a pedogenic origin. This particular example, which is very similar to algal-microbial mats forming along the shores of ponds and lakes in the Bahamas today, could indicate that sea-level was about 2 m higher than present during the middle Holocene, confirming earlier reports from Andros and the Exuma Islands.
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