Sedimentology zyxwvutsrqpo (1988) zyxwvutsrqp 35,805-819 zyxwvutsrqp Cat Island platform, Bahamas: an incipiently drowned Holocene carbonate shelf zy LAURA L. DOMINGUEZ and HENRY T. MULLINS* Department zyxwvutsr of Geology, Her0.y Geology Laboratory, Syracuse University, Syracuse, New York 13244, USA ALBERT C. HINE Department of Marine Science, University ojSouth Florida, St. Petersburg, Florida 33701, USA ABSTRACT Analyses of high resolution, seismic reflection profiles and surface sediment samples indicate that the Cat Island shelf is presently in an incipiently drowned state. This small carbonate bank is characterized by a thin (<4 m), coarse-grained, relict sediment cover, along with limited reef development, and a relatively deep (20-30 m) margin indicating that it has been unable to ‘keep-up’ with Holocene sea-level rise. Early flooding at relatively high rates of sea-level rise (4 m kyr- ’, 5-8 x lo3 yr BP) in conjunction with small bank size and relatively low elevation, led to a reduced rate of carbonate accumulation and incipient drowning. The shelf edge currently lies beneath the zone of maximum carbonate production and exposes the interior shelf to open marine conditions which may result in permanent drowning if it is unable to ‘catch-up’ with continued sea-level rise. Sediment facies patterns are largely oriented perpendicular or oblique to the shelf edge and appear to be controlled by shelf circulation patterns focused by bank-margin reentrants. In comparison with most of the northern Bahamas, the Cat Island shelf was flooded earlier and at relatively higher rates of Holocene sea-level rise which led to selective drowning, implying that carbonate platforms need not drown synchronously over widespread areas as commonly thought. The potential rock record of this incipient drowning event would be a thin, open-marine sand sheet of highly degraded cryptocrystalline and aggregate grains associated with poorly developed reefs. INTRODUCTION Recent study of small Bahamian bank tops has revealed unexpected patterns of sedimentation not predicted by earlier models derived from the much larger Great Bahama Bank (Dravis, 1977). Because of their size, small platforms are more susceptible to drowning events (Hine & Steinmetz, 1984) which can place them out of the euphotic zone (Kendall & Schlager, 1981; Schlager, 1981) and effectively shut down the subtidal ‘carbonate factory’ (James, 1984). Drowning of carbonate platforms occurs when the rate of relative sea-level rise exceeds vertical accumu- lation and the platform becomes submerged below the euphotic zone (Kendall & Schlager, 1981). Although drowned carbonate platforms are common in the geologic record (Wilson, 1975), potential modern * Please direct all correspondence including requests for reprints to Dr Mullins. analogues are not well known. Overall, drowned platforms present a ‘paradox’ because modern coral growth rates appear to exceed known rates of relative sea-level rise caused by long-term geological processes (Schlager, 1981). According to Schlager (1981), rapid pulses of relative sea-level rise or reduction of benthic growth by deterioration of the environment (Hallock & Schlager, 1986) are, at this time, the only plausible explanations of carbonate platform drowning. Schla- ger (198 1) has also previously pointed out that portions of the modern Bahamas, including the Cat Island platform, appear to be partially drowned. Our investigation utilizes a relatively closely spaced ( zyxwv N 5 km) grid of high-resolution seismic reflection profiles and bottom samples to define the Holocene depositional sequence across the Cat Island shelf. The 3-D picture provided by this grid of seismic lines and 805