Paleogene structural evolution and distribution characteristics of sedimentary facies in the Fangzheng fault depression of the Yishu graben

Abstract：The Fangzheng fault depression can be divided into five second-order structural units from south to north; they are the southern slope, southern subdepression, central rise, northern subdepression and northern slope. The Paleogene System was the main phase of formation, development and evolution of the Fangzheng fault depression. The period of deposition of the Wuyun and Xin'ancun formations was an intense fault downfauting period. The period of deposition of the Dalianhe Formation was a continued downfaulting period. The period of deposition of the First Member of the Baoquanling Formation was a period of transformation for downfaulting to downwarping, and the period of deposition of the Second Member of the Baoquanling Formation was a period of shrinking of the fault depression. In the period of deposition of the Wuyun-Xinancun Formation and the Second Member of the Baoquanling Formation, a fan delta-lakeshore and shallow lake deposition system developed by the drainage system in the basin. In the period of deposition of the Dalianhe Formation and the First Member of the Baoquanling Formation, in the basin there occurred a fan delta to semi-deep lake deposition system, in which a horizontal water system was developed. From the Wuyun-Xinancun Formation to the Second Member of the Baoquanling Formation, the depocenter migrated from north to south and then again to the north, and simultaneously the large lake area became small and again became large and the shallow lake became deep and then again became shallow.