Reef-Sourced Slope Deposits, Holocene, Bahamas

R. N. Ginsburg, G. P. Eberli, P. M. Harris, R. Slater, P. K. Swart

Observations and sampling to 350 m from a two-person submersible off Chub Cay, Berry Island, Bahamas, support the idea that the Holocene deep reef is a principal source of talus, now cemented, that foots the windward margins of Great Bahama Bank. At the Chub Cay dive site, a wall extends from 30 to 170 m subsea; below is a low-relief fore reef slope, ca. 50°, of limestone veneered with sediment. The upper wall from 30 to 80 m, the deep reef, has a luxuriant growth of corals and a profusion of the calcareous alga Halimeda spp. Below 50 m, living coral decreases, and from 80 to 170 m the wall is highly irregular with discontinuous ledges and blind-end caves.

At depths from 150 to 170 m, the wall gives way to the fore reef slope whose relative smooth surface dips at 50° to 60° and extends to 350 m. The fore reef is limestone, but its topography resembles that of alluvial fans; rounded ridges rise a few meters above the intervening valleys that are tens of meters wide. The limestone surface has a discontinuous veneer of fine sediment and algal plates, and locally loose cobble and boulder-sized blocks of limestone. A sample of the limestone slope is of well-cemented coral clasts and skeletal sediment.

We infer that the deep reef grows outward so rapidly that it caves periodically. The resulting debris bypasses the wall, but some is perched on the steep fore reef slope below where it is soon incorporated into the slope by submarine cementation.

AAPG Search and Discovery Article #91038©1987 AAPG Annual Convention, Los Angeles, California, June 7-10, 1987.