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Geomorphology and Sedimentology of Intertidal Environments, Crooked Island, Southeast Bahamas: From Alternative Model for Tidal Flat Development

Berkeley, Andrew 1; Rankey, Gene 1
1 Department of Geology, University of Kansas, Lawrence, KS.

Current understanding of the geomorphology and sedimentology of modern carbonate tidal flat environments is largely based on two well-documented examples: those of Andros Island, Great Bahama Bank and the Caicos platform. The purpose of this study is to describe morphology and evolution of a peritidal system on the Crooked-Acklins Platform in the semi-arid southeast Bahamas. The results reveal a number of similarities and differences from these 'classic' examples.

The tidal flats extend continuously for ~18 km on the southern (platformward) side of Crooked Island, and can reach up to 2 km across. Within the tidal flats, three broad environmental zones occur, which correspond with specific faunal/floral associations and depositional characteristics: (1) supratidal zone (colonised by the mangrove Avicennia germinans and the cyanobacteria Scytonema); (2) intertidal zone (vegetated by the mangrove Rhizophora mangal); and (3) non-vegetated, heavily burrowed subtidal zone (open lagoon, tidal channel). In the absence of a beach ridge at the lagoonward margin and a well-developed tidal creek network, these depositional sub-environments follow shore-parallel belts. Early meteoric diagenesis at the highest supratidal elevations has facilitated development of a wide (up to 500 m) continuously cemented, blackened 'pavement' at the landward margin of the tidal flat.

Coring reveals that the thickness of the mud-dominated peritidal sediment package is generally less than 2 m, with the depth to Pleistocene bedrock shallowing in a landward direction towards an exposed Pleistocene eolianite ridge (3-5 m high). Adjacent to the only well-developed tidal channel, over 3 m of muds have accumulated, suggesting a strong influence of the antecedent topography on channel development and geomorphic patterns in general. Cores include a compacted, organic-rich basal layer at the contact with bedrock that grades upwards into subtidal muds (indicating initial flooding) overlain by a coarsening-upwards trend (reflecting a regressive phase). In the landward parts of the system, the transgression is also delineated in cores by stacked thin (~1 cm) indurated layers. Detailed analysis of benthic foraminiferal assemblages together with 210Pb and radiocarbon dating provide insights into facies formation and post-depositional processes. Collectively, these observations provide an alternative model for patterns and processes in microtidal carbonate tidal flats.


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