Volcanosedimentary fill of the early Cretaceous Yingcheng formation and response to the end of continental rifting in the Songliao Basin: Constraints from well SK-2, northern Xujiaweizi fault depression

Abstract

Well SK-2 of the International Continental Scientific Drilling Project (ICDP) obtained continuous core samples with a length of 365.32 m in the Early Cretaceous Yingcheng Formation, showing a stratigraphy of interbedded rhyolites and coarse clastic rocks in the Xujiaweizi fault depression. The Yingcheng Formation in well SK-2 is divided into three sequences. Sequences 1 (S1) and 3 (S3) consist of coarse clastic rocks, and four types of conglomerate lithofacies are identified. The conglomerate lithofacies of S1 is characterized by gravity flows. In comparison, the conglomerate lithofacies of S3 is characterized by a progradational braided channel, reflecting a proximal provenance and a shallowing upward setting. Sequence 2 (S2) rocks are rhyolites, and geochemical analysis shows these to have mainly originated from the middle-upper crust. On the basis of volcanic rocks in wells Shengshen 2–1 and Shengshen 203 and seismic profiles crossing these wells, we identify two types of volcanic edifices: stratovolcanoes in the east and shield volcanoes in the west. The Early Cretaceous strike-slip faults, including the Xuzhong and Xudong faults, play an important role in controlling this area. In comparison with extensive, dark, and fine-grained clastic rocks of the Shahezi Formation, the felsic and coarse clastic rocks of the Yingcheng Formation in well SK-2 mark a sedimentary response to strike-slip tectonics and indicate the ending fault movement of the Xujiaweizi fault depression, and perhaps even the end of the Early Cretaceous Songliao Basin rifting stage.