Role of Cenozoic Progradation in Evolution of Great Bahama Bank
Gregor P. Eberli, Robert N. Ginsburg
The Bahama Banks have long been though to have grown upward with only minor changes in the position of their margins. Instead, new multichannel seismic profiles over the top of northwestern Great Bahama Bank reveal that progradation of some margins during the Cenozoic nearly doubled the size of smaller, nuclear platforms of the Late Cretaceous.
The smaller Late Cretaceous platforms were separated by seaways. Since then, these platforms have grown vertically about 1,500 m, but some of their margins have prograded as much as 25 km! This progradation, combined with high rates of accumulation in the seaways, resulted in the coalescence of the small platforms into the present-day configuration of the bank.
The prograding systems display different seismic characteristics, suggesting various modes of bank margin migration. Rapid prograding systems are characterized by a series of complex sigmoid-oblique sequences with toplap within the individual sequences, steep slopes, and reflection-free spots at the edge of some of the sequences, which are interpreted as reef buildups. In shallow (- 500 m) closed seaways, lower energy systems occur characterized by sigmoidal prograding sequences. Along deep closed seaways, parallel reflectors indicate the accretion along relatively steep slopes. Stationary margins are recognized on east-facing windward margins. On Northwestern Great Bahama Bank progradation occurred preferentially on west-facing, leeward margins. However, in profiles from Southern Gre t Bahama Bank, the direction of progradation varies and may be influenced more by circulation in the adjacent seaway than by the direction of the winds.
AAPG Search and Discovery Article #91038©1987 AAPG Annual Convention, Los Angeles, California, June 7-10, 1987.