Sedimentary patterns and geometries of the Bahamian outer carbonate ramp (Miocene-Lower Pliocene, Great Bahama Bank)

Core, logging and high-resolution seismic data from ODP Leg 166 were used t o analyse deposits of the Neogene (Miocene-Lower Pliocene) Bahamian outer c arbonate ramp. Ramp sediments are cyclic alternations of light- and dark-gr ey wackestones/packstones with interbedded calciturbidite packages and mino r slumps. Cyclicity was driven by high-frequency sea-level changes. Light-g rey layers containing shallow-water bioclasts were formed when the ramp exp orted material, whereas the dark-grey layers are dominantly pelagic. Calcit urbidites are arranged into mounded lobes with feeder channels. Internal be dding of the lobes shows a north-directed shingling as a result of the asym metrical growth of these bodies. Calciturbidite packages occur below and ab ove sequence boundaries, indicating that turbidite shedding occurred during third-order sealevel highstands and lowstands. Highstand turbidites contai n shallow-water components, such as green algal debris and epiphytic forami nifera, whereas lowstand turbidites are dominated by abraded bioclastic det ritus. Gravity flow depocentres shifted from an outer ramp position during the early Miocene to a basin floor setting during the late Miocene to early Pliocene. This change was triggered by an intensification of the strength of bottom currents during the Tortonian, which was also responsible for sha ping the convex morphology of the outer ramp. The Miocene and Lower Pliocen e of the leeward flank of Great Bahama Bank provides an example of the poor ly known depositional setting of the outer part of distally steepened carbo nate ramps. The contrast between its sedimentary patterns and the well-know n Upper Pliocene-Quaternary slope facies associations of the nat-topped Gre at Bahama Bank shows the strong control that the morphology of a carbonate platform exerts on the depositional architecture of the adjacent slope and base-of-slope successions.