Types and distribution characteristics of the Caledonian unconformities in Ordos Basin
XIONG Jiabei1,2,3, HE Dengfa1,2,
1 School of Energy Resources,China University of Geosciences(Beijing),Beijing 100083,China; 2 Key Laboratory of Marine Reservoir Evolution and Hydrocarbon Enrichment Mechanism(Ministry of Education), China University of Geosciences(Beijing),Beijing 100083,China; 3 PetroChina Zhejiang Oilfield Company,Hangzhou 311100,China;
Abstract：The Caledonian unconformity is an important interface for transformation of basin types in the Early Paleozoic North China plate. The discovery of Ordovician weathered crust and paleokarsts as the oil and gas reservoirs and aluminous mudstone gas reservoirs in the Benxi and Taiyuan formations in the Ordos Basin has promoted geologists to explore the Caledonian orogeny in the Paleozoic in this area. Based on a number of field outcrops,drilling and seismic data,the following understandings are obtained with respect to the identification marks,geography,structures and time variables of the unconformity formed by the orogeny. (1)The paleogeomorphic formed after the Caledonian orogeny can be roughly divided into the highland area of Etok Banner-Dingbian area,the geomorphic transition area of Shenmu-Jingbian-Fuxian and Wuzhong areas,and the low-lying and gently eastward inclined area in the Middle East. (2)The adjacent strata of the unconformity surface at the top of the Ordovician are in contact with different angles,and have the structure of “truncation(bottom)-integration(top)”,“truncation(bottom)-overlap(top)”,which is characterized by a high-angle unconformity of cutting and folding at the basinal edge,a low-angle unconformity from basinal edge to the basin,widely distributed parallel unconformity in the basin. (3)The unconformity in this area is also the main weathering crust interface formed between the Upper and Lower Paleozoic. The long-term sedimentary discontinuity and weathering and denudation have laid an environmental background for the development of aluminous mudstone in this area. Bauxite is not only an ideal caprock for the Lower Paleozoic gas reservoir,but also a favorable place for large-scale accumulation of natural gas in the Longdong exploration new area,which is worthy of attention.
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