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The AAPG/Datapages Combined Publications Database
AAPG Bulletin
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Lithified crusts are common on the Holocene carbonate tidal flats of Florida, the Bahamas, the Caribbean, the Arabian Gulf, and Western Australia. Because these crusts form at the surface by early cementation, they will preserve primary features diagnostic of the environment. One example is the preservation of algal filaments and storm layers in the aragonitic crusts on the tidal flats of northwest Andros Island, Bahamas. These thin (0.01-5.0 cm) surface algal crusts occur at about the same elevation (just above "normal" high-water mark) in three subenvironments of the tidal flats: (1) on the backslope of beach stormridges, (2) on the backslope of natural levees of tidal creeks, and (3) on the inland algal marsh.
The upper surface of the crusts, although essentially flat, is characterized by low knobs and mounds. Internally these knobby crusts show two kinds of structure: (1) overlapping planar to crescentic layers of radiating fibers (100 µ across) or thicker columns (500 µ across), which in plan view have a marked honeycomb structure (the voids may or may not be filled with pelleted mud); and (2) thin (<5 mm) uncemented laminae of loose, soft, ovoid, aragonitic pellets. The fibrous and columnar structures replicate the tufted structure of mats of the filamentous blue-green alga Scytonema (mainly Crustaceum sp.) now living on the tidal flats. The pellet layers are deposited mechanically during winter storms. "Frozen-in" these crusts then is a remarkable record of soft parts of th indigenous algae and of the sedimentation during the annual northwesterly winter gale season.
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