Type and evolution of carbonate platforms in Jixian Period Mesoproterozoic: southwestern margin of Ordos Basin

摘要

The Ordos block experienced extension in the Mesoproterozoic, during which the Longshan–Kuanping and Haiyuan rifts developed on its southwest margin. The Jixian Group (1.6–1.4?Ga) is mainly exposed on the rimmed and ramp-type platforms at the Qishan County, southwest of the Ordos block. The deposition associated with the rimmed platform margin consists of platform facies, platform edge facies, and slope facies, which are characterized by small tidal creeks, mud cracks and scour troughs, rockfalls, respectively. The ramp-type platform is dominated by platform edge facies, shelf facies and slope facies and featured by intraclastic dolostones, tempestites and debris flow sediments respectively. The syndepositional normal faults were well developed at the early stage of the rifting. Fault-related cracks and folds present in the adjacent strata. The uplift of the footwall rimmed the platform. The drag structures determine the local sedimentary environment. In the final stage of the rifting, the fault movement stopped. The surrounding rocks were undeformed. The slope became less steep, resulting in the formation of ramp-type platform. The study area in the Jixian period, Mesoproterozoic is located at a palaeo-continental margin, which has already shown good petroleum prospectivity. A clear study on the type of carbonate platforms and the evolution of the platforms can help reduce exploration risks by improve the understanding of the presence of the source rocks.