Bahaman Calcareous Sands

Abstract

ABSTRACT The major part of the Bahama Banks is covered with a mantle of sands composed of calcium carbonate. They rest on a basement of similar sediment which has been consolidated into limestone by the subaerial deposition of calcite cement. Along the extreme edges of the Banks, the sand consists of the debris of neritic organic skeletons; elsewhere it is predominantly non-skeletal, and composed of grains of cryptocrystalline aragonite. These non-skeletal sand grains have hitherto been regarded as a break-down product of the limestones, but the present study, based on the Ragged Island area in the southeast part of the Great Bahama Bank, disproves this, and shows that such derived grains are rare in the recent bottom sediment. The sand grains are instead considered to be primary: they have formed, and are now forming, by the physico-chemical and bio-chemical extraction of aragonite from the sea water, which is saturated or supersaturated with calcium carbonate. The grains develop by the progressive aragonitic cementation of friable aggregates of calcareous silt particles. Further cementation tends to join the grains into lumps, in which an outer dense cryptocrystalline layer commonly allows the slower growth of larger aragonite prisms in the interior parts. The habit and surface texture of the lumps undergo a sequence of changes when traced from the ocean edge toward the interior of the Banks. Corrasion prevents excessive lump growth, and the typical bottom sediment is a well sorted medium-grade sand. The traces of calcareous mud in the sands contain no evidence of “drewite” needles, which are restricted to the protected “shelf lagoons” west of Andros and Abaco Islands. The controlling factor in the formation and distribution of the sands is the tidal currents of cooler oceanic water which sweep onto the Banks with velocities diminishing toward the interior, precipitating calcium carbonate as the water warms and evaporates. Sedimentation is therefore most rapid along the borders of the Banks. Aided by sea-level changes, this has led to the building of lines of elongate cays, parallel, and close to the ocean edge, making each Bank a gigantic atoll. Superficial aragonite ooliths develop on the protected beaches of the cays, and in the track of currents through the inter-cay channels. East of the “Tongue of the Ocean” and elsewhere close to the borders of the Banks, there are extensive areas where the sand is swept into a curvilinear pattern-work of submarine dunes, which occasionally adopt barchan shape. These have not been sampled, but are considered favorable for the formation of true, as opposed to superficial, ooliths. Evidence from the well drilled to a depth of 14,587 feet on Andros Island suggests that calcareous sands, similar to the recent deposits, have played a large part in building the Bahama Banks.