Many Permo-Pennsylvanian reef complexes of the Permian basin, Texas, have flanking carbonate mounds or "reefs." Faunal evidence indicates that these are topographically low relative to equivalent-age sediments in the reef proper. The shallow-water carbonate rocks were deposited either at times of lowered sea level, or they flowed or slid down the slope en masse. It is a real challenge to the subsurface geologist to identify their mode of deposition, particularly in the absence of cores.

Lloyd C. Pray and colleagues, using geopetal fabrics, showed that the so-called Bone Spring "patch reefs" in the Guadalupe Mountains were emplaced by gravity. In Howard and Glasscock Counties, Texas, large reef-dolomite blocks are embedded in a cherty forereef limestone facies of the Wolfcampian Wichita-Albany, and there is no question that they are allochthonous.

On the flanks of the Pennsylvanian and Wolfcampian "Horseshoe atoll" in Howard, Scurry, and Kent Counties, Texas, the problem is less simple. The greatly leached and porous atoll was raised intermittently above sea level, and fringing reefs should have formed on the flanks of islands. However, this does not preclude the other methods of emplacement of shallow-water limestone. Very careful study is required to differentiate the various types of deposits. Differentiation of each type is critical to the exploration geologist because the fringing reefs appear to make prolific petroleum reservoirs whereas the allochthonous or turbidite carbonates commonly host noncommercial accumulations.

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