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Tectonic Influence on Slope Deposition Along Great Bahama and Cay Sal Bank, Bahamas


The Bahamian archipelago is considered tectonically inactive because a) no seismicity has been recorded in recent times and b) no indication for tectonic control on the facies distribution exists on the shallow platform top. A fold growth analysis on the most distal anticline of the Cuban fold and thrust belt has, however, indicated a continuous shortening between Cuba and the Bahamas throughout the Neogene. New hydro-acoustic data sets, consisting of multibeam echosounder data, multichannel seismic, and sub-bottom profiles, reveal for the first time a tectonic influence on the slope sedimentation of Great Bahama Bank (GBB) and Cay Sal Bank (CSB). Along the southwestern portion of GBB the tectonic influence is recognized as large-scale margin failures and associated mass transport complexes, while lateral displacements of slope canyons and vertical scars on the sea floor document recent tectonic activity along CSB. Large slope failures occur along the entire 450 km of western GBB but towards the south the platform margin is increasingly involved in the collapse. The southernmost margin collapse is bounded on the north side by a vertical scar that is up to 40 m in height and runs from the slope to platform margin, displacing it by 350 m. The scar is interpreted to follow a fault. The fault-related collapse results in a large mass transport complex of nearly 400 square km in aerial extent. Within the complex the largest block is 1.5 x 0.25 km and 60 m high. Several large blocks are 200-300 m long and ∼30-60 m high but smaller debris litter the entire slope and adjacent basin floor. On the eastern side of CSB, multichannel seismic data display deep-rooted faults terminating at various stratigraphic horizons, documenting the ongoing deformation related to the Cuban fold and thrust belt. Some of the faults displace the seafloor on the slope and in the basinal areas. They can be traced over 5 kilometers on the multibeam bathymetry data. Bathymetric maps of the slopes display asymmetric canyons that are oriented oblique to smaller downslope gullies and are in some places displaced laterally. This canyon geometry is interpreted to reflect an underlying fault pattern. Similar to GBB, large debris fields from margin collapse and slope failures are observed on the modern slope of CSB. The seismic data documents that the entire Tertiary slope section is punctuated by large mass wasting events, which in some cases are terminating against buried faults.