Structural And Stratigraphic Evolution And Petroleum Potential Of The Blake Plateau

Abstract

ABSTRACT Correlation of marine seismic refraction and reflection profiles with deep wells on land reveal that the Blake Plateau is underlain by 7 to 11 km of Triassic and younger carbonates and evaporites, with some terrigenous and volcaniclastic sediment. Rock dredges and deep-sea drilling project results show the area is bordered by a continuous reefal-carbonate bank complex of Cretaceous and earlier age that extends from the Blake Escarpment through the Bahamas and Cuba to the West Florida Escarpment. Joides drilling on the Blake Plateau indicates an abbreviated Tertiary sedimentary section due to erosional sweeping by the ancient Gulf Stream. Deep sea drilling project results and piston coring in the Blake-Bahama Basin reveal a complex Cenozoic history of erosion by contour currents and deposition of large sediment drifts as the Blake Outer Ridge. The origin of the Blake Plateau basement is still unknown. Gravity and magnetic anomalies indicate a basement depth of about 10 km with a basement of intermediate density and seismic velocity. Analogous crusts are observed under the Red Sea, and the Blake Plateau crust could be interpreted to have formed in a similar manner in the Triassic, as North America and Africa rifted apart. Basement faults associated with the rifting and subsequent plate rotation of North America controlled the subsidence of the geosynclinal basin under the Blake Plateau. Deep Cretaceous porous dolomite horizons and. cavernous limestones have been sampled in the area. Regional dipping of beds caused by differential subsidence or compaction, faults affecting Cretaceous and older strata and possible salt doming might offer entrapment situations that could affect these excellent reservoirs. Flushing by sea water of the Cretaceous marginal reef complex along the Blake Escarpment makes it non prospective. Back from the escarpment Golden Lane analogies and Smackover continuations might be possible in the Early Cretaceous and Jurassic limestones. Terrigenous elastics interfingering with the carbonates, and Triassic volcaniclastics beneath Jurassic limestones offer other targets. INTRODUCTION The flat topography of the broad intermediate depth Blake Plateau, with the narrow East Florida shelf, and bordering steep Blake Escarpment (Fig. 1), were significantly different physiographic features for the Atlantic continental margin to attract early scientific study. Similarly, the Blake Outer Ridge and Blake-Bahama Basin were atypical features when compared with the continental rise to the north. The general subsurface structure of the Blake Bahama area was determined by seismic-refraction studies18,31 (Fig. 2). These refraction data were integrated with later refraction information on the West Florida shelf.1 Also, geologic interpretations were based on extrapolation of deep well data on land2,3,21,25 and were inferred from coring and dredging on: the escarpment. 17,33,34 Ocean drilling for scientific purposes was carried out successfully first under Joides direction on the Blake Plateau,20 and subsequent deep-sea drilling project holes on the Blake Outer Ridge15 and on the escarpment and deeper basin4 provide critical data on the Jurassic to Holocene geology.