Discovery of caliches at top of the Middle Permian Maokou Formation, eastern Sichuan Basin and their developmental model
Su Chengpeng1,2,4, Tang Hao1,2, Li Hongwei1,3, Chen Hongyu4, Zhao Dongfang4, Wan Weichao4, Tan Xiucheng1,2,3
1 State Key Laboratory of Oil and Gas Reservoir Geology and Exploration,Southwest Petroleum University,Chengdu 610500,Sichuan 2 Sichun Province Key Laboratory of Natural Gas Geology,Southwest Petroleum University,Chengdu 610500,Sichuan 3 The Sedimentary and Accumulation Department of Key Laboratory of Carbonate Reservoirs, PetroChina, Southwest Petroleum University,Chengdu 610500,Sichuan 4 School of Geoscience and Technology,Southwest Petroleum University,Chengdu 610500,Sichuan
Abstract：This paper aims at petrological and mineralogical study on the top of the Middle Permian Maokou Formation where caliches occurred in eastern Chongqing City. It has found that the caliches can be divided into three lithological zones consisting of mother-rock zone,transitional zone and calcrete zone from bottom to top,and that mineral composition and content vary in a regular pattern. Petrological study indicates that,the mother-rock zone consists of medium- to thick-layered grey-white micritic bioclastic limestone,where epigenetic karst is identified by carbonate sands and karstificated breccia filling. The transitional zone is composed of yellowish-white,yellowish-brown and brown-red micritic biolithite limestone,with chert nodules and cherty bandings,while erosional funnels filled by in-situ breccias of calcite cementation are pervasive. The calcrete zone is mainly composed of grey crystalline bioclastic limestone,locally calcretized,but overlaid by khaki porous block calcretes,where representative micro-structures of multi-stage fissures,rhizoliths,alveolar structure and secondary carbonate cements are recorded. Mineralogical study shows that,CaCO3 content inclines to increase gently from the transitional zone to the calcrete zone,from which it infers that the enrichment of secondary calcite interior cementation is the main cause for calcrete developing. The rock assemblage of bauxitic mudstone and bauxite occurred at the top of Maokou Formation in Sichuan Basin is consistent to the palaeogeographic settings during the Middle Permian in Upper Yangtze region,and it suggests that caliche may develop and be preserved afterwards in such a warm-wet climate. Then comparing with the classic caliches,a developmental model of the caliches that occurred in a warm-wet climate has been summarized. The results may not only provide supplements for epidiagenesis produced within Maokou Formation during the Middle Permian in Sichuan Basin,but also offer specific materials for the study of the same types of caliche home and abroad.
. Discovery of caliches at top of the Middle Permian Maokou Formation, eastern Sichuan Basin and their developmental model[J]. JOPC, 2015, 17(2): 229-240.
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