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The AAPG/Datapages Combined Publications Database
The natural levees of tidal channels on northwestern Andros Island, Bahamas, show how primary sedimentary structures record small-scale variations in elevation and lateral position on a tidal flat. These levees, like stream levees, are highest adjacent to the channels, crest less than 50 cm above mean high water, and slope gradually for 25-300 m into shallow subtidal ponds.
The most pronounced variation in sedimentary structures is between the well-laminated sediments of the levees and the bioturbated sediments of the pond. In the intertidal and subtidal ponds, browsing and burrowing animals completely destroy primary lamination, but the long periods of desiccation on the levees keep burrowers out of the sediments.
Within the levees there are three distinct zones: (1) the levee crest, 30-50 cm above MHW, adjacent to the channel has smooth, parallel laminations 1-5 mm thick; mudcracks, where present, are discontinuous; (2) the central part of the levee, 10-30 cm above MHW, has thinner laminations, mostly less than 1 mm, that are disrupted by shallow mudcracks 2-6 cm apart. Cornflake-size chips and larger clasts are abundant; (3) on the pond side of the levees, 0-10 cm above MHW, thin, crinkled laminations less than 1 mm thick alternate with thicker laminations that have a prismatic structure inherited from surface mats of the blue-green alga Scytonema sp. Small mudcracks (2-8 mm) disrupt the lamination.
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