Abstract

The growth of a reef-knoll in the subsurface Pennsylvanian of the East Side Shelf of the Midland basin in Texas is compared with development of land forms in the Bahamas. Pleistocene lowering of sea-level in the Bahamas permitted the development of great eolian dunes and solution pits on the exposed parts of the undaforms. Adjacent to these, on the seaward edges of the islands, are many examples of barrier reefs. If the sea should drown the present topography, the reef biota would move up on the dunes in order to survive in shallow water. Thus barrier reefing would change to a reef-knoll complex, taking the shape and orientation of the underlying dunes. Nena Lucia is an oil field in just such a Pennsylvanian reef-knoll. Unlike many others, this one has been thoroughly drilled because oil occurs in the growth platform as well as the reef. More than 300 wells penetrate the base and show how survival growth began. The platform was exposed above the sea. Onshore winds developed dunes, and weathering produced solution pits in the surface. Return of the sea brought back a reef-growth biota, and survival occurred in the shallow water on the dunes. The final topographic form of the reef has a bell-shape profile. Inside this simple surface the sedimentary facies are delicately intermeshed. The 3 principal parts of the reef are the eolian dunes, the massive reef facies that buried them, and the final survival peaks with correlative down-slope beds.

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