Depositional characteristics of Early Cretaceous Huixian-Chengxian basin in western Qinling Mountains and their structural significance

The Huixian-Chengxian (Huicheng) basin is a well-preserved pull-apart basin in the western Qinling orogenic belt. There exist various depositional sedimentary lithofacies that show obvious spatial variations. The sedimentary evolution of Huicheng basin can be divided into four stages. The first stage was characterized by minor sediments of Tianjiaba Formation, and the second stage by basin expansion of Zhoujiawan Formation; the third stage of early Jishan Formation was a maximum basin and the fourth stage had features of sedimentary infilling. In the sedimentary period of Tianjiaba Formation, alluvial fan and braided river dominated proximal areas of the southern basin margins; in the sedimentary period of Zhoujiawan Formation, coarse-grained sediments dominated the western basin margin; and in the sedimentary period of early Jishan Formation, alluvial fan and braided river dominated proximal areas of the southern and northern basin margins. Fine-grained lacustrine (prodelta) sediments were prevalent in the basin center. The middle basin was mostly fluvial and delta sandstone and conglomerate facies associations. Sediments were mainly derived from granite and pre-Jurassic rocks in the north and the south, as indicated by restored paleo-current directions and provenance. Tectonic subsidence and sedimentation were primarily controlled by the Huifeng border faults on the northern margin of the basin. The uplift of pre-Jurassic rocks resulted from the tilting of border faults, and this area became the main provenance area.