Evolution Features of Depositional and Tectonic Setting from Late Paleozoic to Meso—Cenozoic in the Yong’an Basin

The Yong’an basin developed on the Pre-Devonian basement, locating on the southern margin of the Cathaysian block, distributing along the Zhenghe—Dapu fault zone in NE—SW direction. Making use of the method of sequence stratigraphy and being supplemented by field investigation, writers studied the stratigraphic sequences and rock assemblages of the basin : ① An analysis on the stratigraphic framework and geological column from Late Paleozoic to Early Triassic shows that marine deposit sequence is episodic with the fluctuation of sea level, and different sedimentary facies have disproportion in the eastern part, central part and western part of the basin. The center of deposition and subsidence is approximately in the areas of Longyan and Meixian. ② A research on the cross sections traversing the basin and the intrabasinal auxiliary sections shows that the block faulting has played a main part during Meso—Cenozoic, reformed the Late Paleozoic strata and formed compound fault—fold strata. ③ The layers with organic carbon provide the time limits for the land—sea changes in the basin, and their locations and shapes can demonstrate the dynamic mechanism of the tectonic evolution in the basin. Based on analyzing tectonic events, paleogeography and provenances, deep seated structures, boundary structures, and so on, studying the tectonic setting evolution and its stress mechanism, we can get the conclusions as follows: ① During Late Paleozoic, the speed of incursion and regression of the sea was controlled by the strong and weak alternation of tectonic events in different time, for this reason, the ablation of coastal uplifting areas and weathering denudation of mountain mass with the drafting background provided sedimentary source materials for the basin, and there were two provenance areas in the north and south of the basin. During Meso—Cenozoic, the basin had totally the piedmont and river—lake sedimentary facies on volcanic environment. ② The basin is characterized by devoloping uplift—extension structures, and it has mainly undergone phases of Variscan paralic tremendous thick deposition, Indosinian transition from sustained platform to active continental margin accompanied by compressing and uplifting and Meso—Cenzoic tectonic reworking with the conversion from compressing to pulling after Indosinian. The knowledge from above could provide new fundamental referential information for the weak basic research of the basins in southeast China.