The Sligo Formation in Texas is an Early Cretaceous subsurface carbonate sequence representing the upper part of a transgressive cycle. The Sligo carbonates attain a maximum thickness of 305 m at the ancestral shelf edge where the sequence consists of rudistid boundstones and grainstones at depths of 4,570 m.
Porosity and permeability within the Sligo are controlled by the abundance of radiaxial fibrous calcite cement and/or coarse equant-calcite-spar cement. Other cements recognizable in thin section are meniscus calcite and clear euhedral dolomite. The equant calcite spar has an average of δ18O relative to PDB of −1.92 and δ13C of 2.80. The radiaxial fibrous calcite has average δ18O of −1.77 and δ13C of 1.61. The average wholerock values are δ18O of −1.52 and δ13C of 2.61. The similarity between these values suggests isotopic homogenization due to diagenesis; however, the equant-calcite spar has a range in δ18O of −4.98 to 0.29 indicating deposition by meteoric waters. The radiaxial fibrous-calcite cement has a narrow range in δ18O of −2.10 to −1.70 consistent with an origin as an early marine replacement cement.
Intriguing correlations exist between modal abundances of cement types and their δ18O and δ13C values. These data provide important constraints for models predicting porosity and permeability evolution during carbonate diagenesis and have important implications for hydrocarbon exploration strategies.