Contemporary intergranular submarine cementation occurs in close association with strongly agitated oolitic sands in the Schooner Cays tidal bar belt, resulting in widespread generation of surficial oolitic hardgrounds. Hardground formation takes place from near the shelf margin bankward up to 15-17 kilometers, within the zone of active oolitic sand, in water depths ranging from 1-II meters. Two forms of lithified oolitic sediment (hardgrounds) are recognized: in situ surficial crusts and unattached clasts (erosional crust fragments). Isopachous fibrous aragonite constitutes the major physicochemical cement; other cement forms of aragonite and high magnesium calcite are widespread but subordinate. Previously undocumented, probable chasmolithic/endolithic algae also effectively bind and cement surficial unconsolidated oolitic sediment by filament calcification. Characteristic decrease in cementation downward from crust surfaces substantiates their formation at or near the sediment-water interface. Patterns of hardground formation apparently result from the interaction of tidal bar surface topography, local hydrography and stabilization by algal scum mats. Radiocarbon dating, field relationships and crust petrography document the rapid and contemporaneous formation of oolitic hardgrounds. Initial crust formation can occur within a few months or less . Hand probing of tidal bars and radiocarbon dating of erosional oolitic clasts suggest that similar patterns of hardground formation occurred during earlier growth stages of the tidal bars.

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