The origin of bedding-parallel fibrous calcite veins in the Lower Permian Chihsia Formation in western Hubei Province, South China

Bedding-parallel fibrous veins occurring as lenticular to flattened intercalations were found in the organic-rich marlstone/calcareous shale of the upper Lower Permian Chihsia Formation in western Hubei Province, South China. They dominantly consist of fibrous calcite crystals with smooth and tight boundaries, forming fence-like inward, syntaxial growth clusters toward the vein center along which a median suture line generally occurs. Petrographic evidence indicates that these veins may form at relatively shallow burial depth, where fluid overpressures would have incrementally created the bed-parallel vein space, resulting in displacive growth of fibrous calcite. On the other hand, the C, O and S isotopic data across the vein reveal slightly depleted δ13Ccarb values (−3.32‰ to +0.19‰ VPDB) and moderately depleted δ18Ocarb values (−9.6‰ to −7.3‰ VPDB) with respect to those of coeval seawaters and slightly heavier δ34Spyrite values (−7.88% CDT) with respect to those of ambient rocks. Stable isotope evidence consistently suggests significant contribution of bacterial sulfate reduction (BSR) to the formation of the fibrous calcite cements in the vein. The BSR could have been intensive with the availabilities of residual sulfate and abundant organic matters in the Chihsia sediments during shallow burial, increasing the alkalinity of pore waters and further promoting carbonate precipitation. Thus, the bedding-parallel fibrous calcite vein in the upper Lower Permian Chihsia Formation is an important time-specific petrographic capsule, providing clues for understanding the diagenetic process in organic-rich sediments.