The residual Bouguer gravity anomaly map of the Bahamas shows that density contrasts below the level of the floors of intraplatform straits and basins correlate

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remarkably well with present platform topography. The straits and basins invariably are areas of negative anomaly whereas the platforms generally are areas of positive anomaly. Seismic measurements show that refractors with similar transmission velocities are displaced downward in Florida Straits and Northwest Providence Channel. Limited subsurface control, utilizing paleontologic data and velocity surveys, indicates the deepest and highest velocity refractor traceable in and adjacent to the Bahamas to be at the top of Lower Cretaceous rocks.

The best explanation for the observations available is that some combination of post-Late Cretaceous downfolding or downfaulting occurred in the areas of the present intraplatform straits and basins. Erosion of adjacent highs and accompanying infilling of lows with relatively light material subdued the topography directly reflecting structure and at the same time created the conditions necessary to explain the residual gravity anomalies. Perpetuation of the structural high areas by continuation of shallow-water carbonate sedimentation, during the Cenozoic, gave rise to the relief of the Bahama Banks. It follows that, although the plan distribution of Bahamian platforms versus strait and basin areas is structurally controlled, the great relief of the platforms is a function of local up uilding by carbonate sedimentation in step with regional subsidence.

Tectonic control of the limits of shallow-water carbonate sedimentation is also indicated for various other modern and ancient platforms. The Blake, Campeche, and Great Barrier Reef platform edges all coincide with regional structure features, i.e., zones of transition of continental to oceanic crust. The Devonian Leduc-Rimbey trend of Alberta, Canada, follows the structural grain of Precambrian basement rocks. The relationship of the Permian Central Basin platform to a structurally high block in its foundation is well known from both geophysical and drilling information.

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