Classification and genesis of leopard skin(spot)carbonate rock: a case study of the Member 4 of Majiagou Formation in Ordos Basin
WEI Liubin1,2, WANG Zongyan3, LI Yi3, PANG Zhichao3, WANG Qianping1,2, BAO Hongping1,2, YANG Qiqi3, WANG Zhen3, SU Zhongtang4, ZHANG Lijun3
1 Research Institute of Exploration and Development of PetroChina Changqing Oilfield Company,Xi'an 710018,China; 2 National Engineering Laboratory for Exploration and Development of Low-Permeability Oil & Gas Fields,Xi'an 710018,China; 3 School of Resources and Environment of Henan Polytechnic University,International Joint Key Laboratory for Biogenic Trace and Sedimentary Minerals of Henan Province,Henan Jiaozuo 454003,China; 4 Institute of Sedimentary Geology,Chengdu University of Technology,Chengdu 610059,China
Abstract：Leopard skin (spot) limestone, a type of marine carbonate rock with unique morphology, is widely distributed in the Cambrian-Ordovician strata of North China. It has attracted significant attention due to its distinct characteristics and formation mechanism. Abundant leopard-skin (spot) carbonate rocks developed in Member 4 of the Middle Ordovician Majiagou Formation in the Ordos Basin. Eight species of 7 genera have been identified, including Thalssinoides network, Thalassinoides boxwork, Planolites isp., Palaeophycus tubularis, Phycodes isp., Chondrites isp., Skolithos isp.,?Zoophycos isp. Two morphologic types of Thalassinoides (horizontal enigmatic network and three-dimensional boxwork) are predominant, belonging to the Glossifungites ichnofacies under firm ground condition. Based on detailed macroscopic and microscopic characteristics of outcrops and drilling cores, this paper systematically summarizes the types of carbonate rocks caused by bioturbation. By examining the morphological characteristics of bioturbation and burrow, as well as the calcite/dolomite content, this paper proposes a classification for leopard-skin (spot) carbonate rocks formed by bioturbation. These rocks are categorized into leopard-skin limestone and leopard-spot dolomite, corresponding to the Thalassinoides network (two-dimensional horizontal mystery palace) and the Thalassinoides boxwork (three-dimensional box-like structure), respectively. The leopard-skin (spot) carbonate rocks in Member 4 of the Majiagou Formation in the Ordos Basin are primarily controlled by the interaction of bioturbation and early diagenesis during high-frequency sea level fluctuations. The bioturbated burrows in the firm substrates creat pathways for Mg2+ rich brines, promoting dolomitization within the burrows. Two distinct types of leopard-skin (spot) carbonate rocks, induced by bioturbation, were generated under varying bioturbation types, parasyngenesis, and buried dolomitization.
WEI Liubin,WANG Zongyan,LI Yi et al. Classification and genesis of leopard skin(spot)carbonate rock: a case study of the Member 4 of Majiagou Formation in Ordos Basin[J]. JOPC, 2024, 26(4): 880-894.
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