Abstract: Does Subaerial Exposure of Shelf Carbonates Lead to Porosity Preservation? An Example from the Late Paleozoic, Central Basin Platform, West Texas

J. A. D. Dickson, A.H. Saller

The Southwest Andrews area of West Texas contains cyclic Pennsylvanian and Lower Permian limestones deposited on the eastern flank of the Central Basin Platform. Most of these limestones were deposited during high stands of sea level during a time of high amplitude and high frequency sea level fluctuations probably caused by repeated waxing and waning of continental glaciers. The Strawn to Wolfcamp Reef interval involves 63 cycles, most capped by subaerial exposure surfaces. A typical cycle includes (1) clayey transgressive packstones (2) glauconitic grainstone-wackestone (3) spiculitic wackestones, and 4) porous grainstones. Thin calcretes, rhizoliths, alveolar structure and mottled textures cap the youngest unit (4), indicating subaerial exposure.

The three most important porosity-modifying diagenetic processes are calcite cementation, aragonite dissolution and compaction. Most limestones underwent substantial early alteration in freshwater during subaerial exposure. The amount of early cement is critical to porosity preservation. Rocks whose primary intergranular porosity was completely cemented had sufficient strength to resist compaction. Aragonite dissolution followed cementation creating unconnected mouldic pores. Rocks whose primary porosity was partially cemented were sufficiently rigid to resist compaction and secondary porosity was connected. Rocks with no early cement developed secondary porosity but all porosity was lost during compaction of the weak rock. Porosity is (1) variable both vertically and horizontally (2) facies selective, occurring in the grainstones 2-5 meters thick and, (3) occurs below certain exposure surfaces.

Stable isotope analyses of bulk rock samples show distinctive patterns. Thicker cycles have ^dgr¹⁸0 values -2 to -5 per mil of PDB and ^dgr¹³C values -4 to +3 per mil PDB. These cycles possess moderate amounts of early cement, retain porosity, and show a distinctive ^dgr¹³C switch from below -4 per mil

below to +3 per mil above subaerial exposure surfaces. They had short periods of exposure when only the uppermost parts accumulated isotopically light, soil-related carbon. The thinner cycles have similar ^dgr¹⁸0 values -3 to -5 per mil, but consistently low ^dgr¹³C values -3.5 to -6 per mil. These cycles were subjected to prolonged subaerial exposure which caused pervasive alteration, a consistently light ^dgr¹³C profile and obliteration of porosity.

AAPG Search and Discovery Article #90986©1994 AAPG Annual Convention, Denver, Colorado, June 12-15, 1994