Cenozoic Geology of the Yushe Basin

Abstract

The Yushe Group of Yuncu subbasin constitutes a long series of superposed units that span a large portion of the Late Cenozoic Era from Late Miocene, through Pliocene, to Early Pleistocene time. Hiatuses, some represented by angular unconformities, represent up to a half million years of missing time, but are shorter lower in the section. In Yuncu subbasin, the composite section is 800 m in length and contains four stratigraphic units distinguished by distinctive cycles of sedimentation. The basal Mahui Formation is conglomeratic toward the base, with coarse sands fining upward and with capping marls. It yields Late Miocene age vertebrate fossils. The Gaozhuang Formation contains abundant sands and finer units, with three fining-upward cycles that are recognized as members. The Mazegou Formation, with basal disconformity, contains fine sands and relatively more silts. Paleomagnetic study coinciding with our fieldwork shows that Gaozhuang and Mazegou fossil horizons span much of the Pliocene Epoch. These units dip gently, but are overlain by Early Pleistocene horizontal lake and stream sediments of the Haiyan Formation. Other subbasins contain sediments equivalent to parts of this sequence, and we recognize the Mahui-Gaozhuang contact to the east in Tancun subbasin. The Zhangcun subbasin exhibits a different, more lacustrine mode of deposition, with developed oil shales bearing abundant fish faunas. The Yushe Group is mantled by loess, locally with an older deep red loess likely equivalent in part to the Early Pleistocene Wucheng Loess. More widespread are the younger, thick Lishi and Malan loess blankets. This chapter opens and closes with syntheses of basin stratigraphy, set against the structural and climatic evolution of Yushe Basin. The underlying structure of the Triassic bedrock dominates the tectonics of Yushe Basin. Sediment aggradation along the trunk of the Zhuozhang paleo-river of Yushe Basin began after 7 Ma. Progressive aggradation continued, and the hiatuses in deposition and minor angular unconformities indicate episodes of gentle uplift. As the basin filled, the later Pleistocene loess signaled a new mode of sedimentation.