Abstract: Effects of Submarine Diagenesis on Primary Porosity in Caprinid Reefs and Associated Sediments

T. J. Petta

Pervasive submarine diagenesis severely reduced the primary porosity of a lower Glen Rose rudist reef now exposed in central Texas. Primary reef framework cavities are filled with multiple generations of fibrous sparry calcite cement and internal sediment consisting of pellets, calcareous silt, and marine fossils. Besides lining cavities, the fibrous cement also intercalates layers of internal sediment and coats pellets. Organic borings resembling clionid sponge borings from modern coralgal reefs transect skeletal framework and cavity fillings. The Cretaceous borings, commonly superimposed, are filled with micrite, pellets, and fibrous cement.

The pellet and calcareous silt fraction that comprises most of the internal sediment probably was produced by boring sponges in the submarine phreatic environment. Characteristics of the silt and pellets are very similar to internal sediment in modern coralgal reefs and to meteoric vadose sediment reported from Permian carbonate mounds.

Framework degradation by sponges may be more effective in modern reefs than in the caprinid reef studied. Lower volume infestation possibly coupled with lower calcium carbonate generation rates resulted in reduced internal sediment production in the Cretaceous reef. In turn, more pore space was cemented in the marine environment. Near-reef carbonate sand bodies composed of rudist debris were not affected by submarine diagenesis and retain primary intergranular porosity.

Results of this study suggest that Cretaceous reefs that were not subjected to early meteoric or later fresh formation waters should have very low porosity. Potential targets for future exploitation of subsurface rudist reefs in South Texas are near-reef carbonate sand bodies.

AAPG Search and Discovery Article #90961©1978 AAPG Annual Convention and Exhibition, Oklahoma City, Oklahoma