ABSTRACT

Caicos Bank is a shallow water, sub-circular carbonate platform about 100 km across. The northern shore is fringed with islands that shelter interior tidal flats. Seaward of the tidal flats, within the bank interior is an ooid-grapestone shoal complex 20 km long and 10 km wide. Individual shoals are 300 to 2000 m wide, up to 4 m thick, and are separated by straight tidal channels up to 800 m wide and 2.5 m deep. Both shoals and tidal channels are asymmetrical in cross-section. Parts of the shoals are emergent and form small, low relief islands.

Twenty-five vibracores recovered from the shoals and adjacent sediments show a general coarsening-upward sequence characterized by increasing grain size and grapestone content, and decreasing proportions of ooids, pellets and mud. The sequence is interpreted as a shallowing-upward cycle that began as a series of ooid shoals flanked by pelleted packstone and evolved into the present grapestone shoal complex. Sedimentation was initiated during Holocene flooding of a gently south sloping ramp. Sediment accumulation rates exceeded sea level rise so that the shoals lengthened and emerged with time.

Shoal growth is promoted by: 1) lateral accretion of longshore derived beach sets; 2) elongation at down drift shoal terminations; 3) storm and hurricane generated surges; 4) tidal channel abandonment and shoal coalescence.

Synsedimentary diagenesis plays an important role in shoal stabilization. Sediments exposed on emergent islands have been cemented by equant meniscate calcite of vadose origin. Irregular, biologically stabilized, cemented patches up to 1 m across and 15 cm thick occur on the surface and within submerged shoals. Cements are micritic, peloidal and isopachous fibrous aragonite formed in submarine phreatic environments.

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