ABSTRACT: The Rise and Demise of Jurassic Carbonate Platforms on the U.S. Atlantic Margin

C. Wylie Poag

A 30,000-km grid of intersecting multichannel seismic reflection profiles correlated to 150 boreholes reveals the structure, stratigraphy, lithofacies, and depositional history of the U.S. Atlantic margin. The Upper Jurassic sequence (Kimmeridgian-Tithonian) is characterized by a thick (1-2 km) carbonate platform which stretches 2500 km from south Florida to Georges Bank. The Late Jurassic continental shelf was nearly 900 km wide across the South Florida and Bahamas basins and was covered mainly by carbonate deposits. To the north, however, a greater influx of siliciclastic sediments restricted carbonate accumulation to a much narrower (5-25 km wide) rimmed platform along the outer shelf. As carbonate deposition culminated near the end of Late Jurassic time, a discontinuo s, pinnacled, barrier reef rimmed Baltimore Canyon Trough and parts of Georges Bank Basin. Maximum reef-to-basin relief was about 3 km. Several fanlike sedimentary prisms originated at gaps in the reef system and extended 300-500 km across the lower continental rise.

During the Early Cretaceous (Berriasian-Valanginian), a paleoclimatic change accompanied by sea level fall, shelf exposure, and subsequent development of hypernutrient shelf waters (as sea level rose again) proved a fatal combination for carbonate-secreting communities of the shelf edge. The demise of these communities allowed several large siliciclastic deltas to bury the carbonate rim and extend into deep water; contiguous submarine fan complexes reached 500-600 km into the Hatteras Basin. The northern shelves never regained their extensive reef-supporting

carbonate environments, whereas carbonate deposition and reef building have continued to the present in the South Florida and Bahamas basins.

AAPG Search and Discovery Article #91003©1990 AAPG Annual Convention, San Francisco, California, June 3-6, 1990