Abstract

The long-running controversy over the origin of the Capitan Formation massive limestone member concerns differing interpretations of the relative importance of its organic vs. inorganic components and whether the organic components were capable of building a rigid reef framework. Answers to the latter question begin with determining the life habit, growth form, and skeletal size and strength of the organisms with greatest reef-building potential and whether they are preserved in life position. Alternatively, the skeletons may be toppled and broken and their fragments deposited as transported debris in shelf margins and/or downslope locations.

Capitan Formation sponges have the greatest reef-building potential because they exceed all other taxa in diversity, especially those with erect growth. In addition, their skeletons are the largest and most rigid, abundant, and widely distributed. On horizontal polished surfaces of 45 randomly distributed quadrats (areas = 200–400 cm2) from two localities (areas = 450 m2 and 5400 m2) near the northeast end of the Capitan outcrop belt, 74% of the sponges are preserved in their erect life positions and 26% are toppled fragments (n = 672). Most sponges are small; fragments of those with the weakest skeletons are randomly arranged, not current aligned. Successive generations of erect sponges built the initial accretionary reef framework. Many sponges were subsequently supported in life position by very weakly skeletonized encrusters and/or syndepositional cement.

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