Origin of the northwestern Bahama Platform: Review and reinterpretation

Abstract

The origin of the Bahama Platform has been the subject of debate for more than a century. The major points of disagreement are (1) whether the Bahamian basement is continental or oceanic, and (2) the age and origin of the deep Bahama channels. Geophysical data indicate only that the Bahamian basement is of intermediate density, seismic velocity, and thickness. We propose that this basement was originally pre-Triassic continental material that was pervasively intruded by mafic and ultramafic material during the rifting of North America from Africa and South America in Late Triassic time. To alleviate the Bahama overlap in reconstructions of the North Atlantic we suggest that the Bahama Platform has been rotated approximately 25° to the northeast by the relative impinging motion of the Caribbean plate during Cretaceous and early Tertiary time. By accounting for this postulated rotation, the Bahama Platform forms an excellent fit between Africa and South America. We propose the following model for the origin of the northwestern Bahama Platform: (1) The platform is continental and was originally part of Africa. (2) Rifting of North America from Africa and South America began in the vicinity of the northwestern Bahamas during the Late Triassic, preceded by a large domal uplift of the continents that initiated the deep Bahama Channels as grabens. (3) The Bahama Platform rifted from Africa principally along a large right-lateral shear and thus evolved as a transcurrent-type continental margin. (4) During the incipient development of the deep Bahama channels, Late Triassic continental arkosic rudites and arenites were deposited at the base of these grabens; (5) Initial marine conditions in Early to Middle Jurassic time may have resulted in the deposition of salt and organic-rich shales in the channels due to restricted circulation. (6) With the onset of more open marine conditions 3 to 6 km of Jurassic to Holocene deep-water carbonate and bioclastic turbidite sediments were deposited in the channels. (7) Most of the intraplatform relief of the northwestern Bahamas appears to be the result of the build-up of the banks relative to the channels during regional subsidence.