Tectonic Evolution And The Control On Coal-measure Source Rock Of The East China Sea And The Northern South China Sea Basins

The formation and tectonic evolution of one basin control the sedimentary filling and sequence stratigraphic framework, which establishes the source rocks’ formation and development. Basins have a structural and sedimentary response in the regional tectonic setting, and different source rocks’ generated pattern and distribution response to different tectonic evolution stage. Based on the previous research on the formation mechanics of the East C hina Sea and the northern South C hina Sea, this paper selected the typical coal-bearing depressions, which contains Xihu sag in the East China Sea Basin, the northern depression zone of Q iongdongnan Basin and Baiyun sag of Zhujiangkou Basin in the northern South China Sea, with the activities of syndepositional faults analyzed and the balanced cross-section technique applied, investigated the structural and tectonic evolution features under different forming mechanisms, and in accordance with the study of syndepositional structural slope-break zone controlling the deposition space, finally indicated the coal source rocks’ forming model as well as the response to the basin forming mechanism. The main research results and knowledge are listed as follows:1. The formation and evolution of the East China Sea and the northern South China Sea basins were discussed. Both as the Cenozoic extensional faulted basins, the East China Sea and the northern South C hina Sea got the structural system of former rift and latter depression. The East C hina Sea kept away from India, Europe and Asia collision region, with the Pacific subduction retreating, experiencing residual back-arc rifting, depressing by neighborhood basin expansion and regionally thermally subsiding. While the northern South China Sea was in the principal stress releasing area of India, Europe and Asia collision region, with the complex mechanism, experiencing rifting initially by the mantle dragging and the former south C hina sea diving, toughly settling with the South C hina Sea expansion, depressing with the transition of seafloor spreading ridge and finally thermally setting with the end of seafloor spreading.2. The formation and structural control of coal-bearing source rocks were explored. Coal-bearing source rocks were formed in the late period of basin rifting, corresponding to the regionally tectonic transition. The syndepositional structural slope-break zone played a decisive control on coal deposition distribution. Pinghu formation of Xihu sag was formed when the Pacific subduction steering from NNW to NWW, to which the fractures on the south-east side were sensitive. By the control of syndepositional faults, the coal deposition was restricted to the gentle slope break belt on the north-west side. Yacheng formation of the northern Qiongdongnan basin was formed in the early period of seafloor spreading. With the weak activity of syndepositional faults, the steep and gentle slope break belt all had the coal deposition, which were discontinuity from each break belt. Affected by the position and base structure, Baiyun sag was overla id by one tough settlement, which resulted that the overall sag presented a gentle slope break belt for extensive coal deposition accumulation.3. According to the settlement mechanism of coal accumulating, the coal forming model of stretched coal accumulating and fault-depression coal accumulating was advanced. The former was characterized by the episodic faulting and extended block of tilt, the coal deposition would be migrated to the gentle slope break belt and the steep slope break belt accumulated without coal deposition, for which the stretched coal accumulating model might be further divided into the single gentle slope accumulating for Xihu sag as typical and the double side slope belt accumulating for northern of Q iondongnan basin. Due to the tough settlement, Baiyun sag was belonged to the type of fault-depression coal accumulating model.