Geotectonic Evolution and Subsidence of Bahama Platform

Abstract

The thick section (greater than 5 km) of flat-lying Cretaceous and younger carbonates capping the Bahama platform implies an unusual geotectonic history, characterized by great subsidence. We suggest that this platform is underlain neither by sial nor by a volcanic foundation creating a “mega-atoll.” Instead we propose a basement of oceanic crust about 11 km down which has undergone slow subsidence. Triassic rifting in the Atlantic probably initially created a closed small ocean basin, or mediterranean, in the Bahama region. This was accomplished by a rotational movement of the North American plate away from North Africa accompanied by shearing across the top of the South American plate which remained stationary and attached to Africa. A wedge-shaped sphenochasm resulted which became a sediment trap within Pangaea and quickly filled to sea level with turbidites. With renewed continental drift, the Bahama platform became a subsiding marginal plateau attached to the North American craton upon which algal-coral growth explosively flourished under holo-oceanic conditions, providing sufficient upbuilding to offset subsidence and maintain a sea level freeboard. This interpretation obviates the overlap of the Bahama salient onto the African craton when the Atlantic Ocean is closed under continental drift reconstruction. It accounts for the long history of subsidence without calling on “oceanization.” It also explains the inference, based on some Cuban-Soviet geophysical evidence, that there is a thick sedimentary section beneath the Bahamian carbonates. An underlying clastic sequence could also include the source of the probable salt domes at the bottom of Exuma Sound.