THE REMARKABLE SIMILARITY IN THE ARCHITECTURE OF ISOLATED TROPICAL AND COOL SUBTROPICAL CARBONATE PLATFORMS

The tropical platform of Great Bahama Bank (GBB) and the cool subtropical platforms on the Marion Plateau (Australia) show a remarkable geometric similarity despite their differences in the faunal assemblages. Both platforms are asymmetric with a steep aggrading margin and lower-angle prograding margin. On GBB, the asymmetric platform architecture is the result of dominant easterly wind direction whereas, on the Marion Plateau, ocean currents produce a similar geometry with a steep, up-current and a low-angle, down-current margin.

GBB is an edifice of tropical platforms with abundant coral reefs, calcareous green algae and precipitation of aragonite mud and ooids on the platform top. Slope sediments consist mostly of aragonite mud and silt. Easterly trade winds export the sediment predominantly to the west creating a prograding leeward margin. In the Neogene, leeward progradation has advanced the margin an impressive 25 km into the Straits of Florida and closed a seaway in the middle of the bank. The southern Marion Platform initiated during the earliest Miocene and was drowned in the latest Miocene. Cores retrieved during ODP Leg 194 document a cool subtropical faunal assemblage consisting primarily of red algae, bryozoans and large foraminifers. These calcite-dominated sediments are often silt- to fine-sand size and have a lower diagenetic potential than the muddy aragonite of GBB. Currents from the northeast inhibit deposition in the up-current direction and form low-angle clinoforms in the down-current position.

A hierarchy of sea level changes controls sediment production in both platforms in which bank flooding with concomitant shedding to the slope is alternating with periods of bank exposure with reduced carbonate production and largely pelagic sedimentation in the basin. High-frequency sea-level changes are recorded as shallow-water packages capped by exposure horizons on the banks and as marl/limestone alternations in the basin. Longer-term sea- level changes coincide with progradation pulses that are seismically imaged as depositional sequences.

The fact that the cool subtropical fauna assemblage produces platform geometries that are reminiscent to tropical carbonates indicates that physical parameters may be more important for platform architecture than the biological ones.