Sedimentary characteristics and origins of Late Pennsylvanian–Early Permian carbonate mud-mounds at the Shangdan section, Inner Mongolia

Abstract

Late Pennsylvania–Early Permian carbonate mud-mounds exposed in the Shangdan section, Inner Mongolia, are commonly massive, showing depositional relief, and are distinct from the thin- to medium-bedded limestones of intra-mounds. Based on the various proportions of lithofacies, four types of mud-mounds are recognized: MM-A (type A carbonate mud-mound) and B (type B carbonate mud-mound) consist of massive lime mudstone and massive wackestone (Subfacies B), respectively; MM-C (type C carbonate mud-mound) are composed of massive wackestone (Subfacies B) in the lower parts, analogous to MM-B, and massive wackestone (Subfacies A) and boundstone in the upper parts, similar to MM-D (type D carbonate mud-mound). Carbonate mud-mounds are deposited in low-energy settings on a ramp, likely below fair-weather wave base, where the depositional settings of MM-A, B, C and D have a shallowing-upward trend. Carbonate mud-mounds have diverse origins of lime mud: allochthonous lime mud in MM-A; lime mud produced by the disintegration of phylloid algae in MM-B, C and D; lime mud linked to cyanobacterial activity in MM-D and the upper parts of MM-C. Different from MM-A and B, which were formed only by mechanical accumulation, MM-C and D resulted from the combinations of mechanical accumulation and microbial binding and solidifying. MM-A, B, C and D significantly expand the global spectrum of Late Pennsylvania–Early Permian carbonate mounds.