Buildups are 10 to 130 ft (3 to 40 m) thick, commonly elongate, and several square kilometers in plan. They developed seaward of oolite shoals on paleostructures of three types that created subtle topographic highs on an otherwise ramplike sea floor: (1) basement ridges, (2) faulted basement highs, and (3) upthrown salt-cored fault blocks. Attractive porosities and permeabilities (mean phi of 15%, mean K of 20 md) result from freshwater leaching, dolomitization, or fracturing. The biota varies through the Smackover trend; reefs in southern Arkansas and northern Louisiana are more diverse. A vertical biotic zonation in cores of the reef interval suggests that the reef community of organisms evolved through time.--Modified journal abstract.