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Abstract

Lithified stromatolites, attaining 2-m heights and associated with cement-encrusted carbonate-mud beds, are found in many inter-island channels of the southern chain of Exuma islands and cays. They occur within migrating flood-tidal, ooid-sand bars and dunes. Reversing tidal and wind-driven currents, with velocities up to 3 kts (150 cm/s), flow across depositional sites for 3 hrs of each 6-h tidal period. Such a high-energy, bank-margin environment is not usually considered to be the site of stromatolite growth or of mud-size particle deposition; however, mud beds occur in water depths of 4 to 8 m. The normal thickness is usually about 10 cm, but one large bed has a total thickness of 1 m. If exposed to current flow for several weeks, the hard crusts become colonized by a diverse microbial community of algae, diatoms and marine plants similar to that growing and trapping ooids on nearby lithified stromatolites. The white lime muds forming the interior of the crusted-mud beds are cohesive, having the consistency of "tooth paste." At first glance, the interior muds appear to be homogeneous; however, they are pelleted. Uncemented ooid sand occurs above and below the laminated, crusted-mud beds, a relation that shows they occupy the same depositional environment. Their age of less than 4 ka and their depth of occurrence argue against the lenticular-bedded muds being relict deposits associated with the Holocene flooding of the Bahamian platform.

Scanning electric microscope (SEM) images reveal that the muds contain pelletoids of aragonite needles. The ooid grains are held in place and cemented by organic remains of the microbial mat. It is clear from the SEM photomicrographs that the surface crusts on the mud beds are actively undergoing diagenetic changes, whereas the encased uncemented mud is not. Strong currents scour and undermine the crusted beds, forming "rip-up clasts" and "mud chips." Microbial mats colonize many of the larger chips, trapping ooid sand. The chips become part of larger stromatolites as they grow and coalesce. Cross sections of lithified stromatolites show that they contain clasts of mud chips incorporated in their laminated internal structure and are thus contemporaneous with stromatolite growth.

The probable source of the lime-mud needles is precipitation from "whitings" and from resuspended biogenic muds. During tropical storms and hurricanes, fine-grained sediment that blankets the Pleistocene surface of the Bahama carbonate platform forms dense clouds of muddy bank water. The ebb tides and offbank, wind-driven currents carry the mud-charged water mass to the bank edge. There, every 6 hours, the muddy water mass comes in contact with flood-tidal oceanic water and intense mixing takes place. It is below the mixing zone of sediment-laden, slightly hypersaline bank and cooler open-oceanic waters that crusted, laminated-mud beds, interbedded with well-developed ooid-sand bodies, lithified stromatolites, hardgrounds and areas of grapestone, are found.

The occurrence of Bahamian crusted muds within ooid-sand dunes, rip-up clasts and lithified stromatolites, all subjected to strong subtidal currents, represents a newly defined association. If seen in ancient deposits, such associations might be interpreted as supratidal or shoreline deposits. Finding these features in high-energy subtidal channels at the eastern margin of the Great Bahama Bank mandates caution when making such interpretations in the geologic record.

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