Stratigraphic Framework of Mesozoic-Cenozoic Red Bed in Zhang County, Northern Margin of Western Qinling and Its Geological Significance

The Mesozoic-Cenozoic red beds are distributed in Zhang County, northern margin of Western Qinling, their stratigraphic frameworks and ages have long been controversial. The stratigraphic framework of these red beds has been established in this paper, which is essential for restoring the Mesozoic-Cenozoic tectonic evolution about the northern margin of Western Qinling, and for understanding the remote geological responses to the collision and convergence process of India-Eurasian plate in the northeastern margin of Qinghai-Tibet plateau. After studying the angular unconformities between these red beds and their underlying strata, the angular unconformities among different red beds, analysing the sedimentary sequence and stratigraphic characteristics of these red beds, and discussing the relationships between these red beds and the faults in the northern margin of Western Qinling, the stratigraphic framework of these red beds in Zhang County has been revised through combining the data of pollen analysis. The results confirm that the Mesozoic-Cenozoic red beds in Zhang County can be divided into three different stratigraphic units separated by angular unconformities, which include① Lower Cretaceous purple red sandstone and conglomerate.② Oligocene-Miocene red conglomerate, sandstone and claystone, deep purple-grey mudstone, grey marl and evaporation rocks.③ Pliocene fluvial conglomerate layer, that is, Hanjiagou conglomerate and pluvial conglomerate layer north of F1 fault. Three sedimentary stratigraphic units for these red beds in Zhang County stand for the sedimentary tectonic environment and ancient landform evolution process in the northern margin of Western Qinling, providing the basic geological constraints to understand the Mesozoic-Cenozoic intracontinental tectonic processes of Western Qinling, as well as the uplift, tectonic deformation of Qinghai-Tibet plateau and the remote geological responses to the collision and convergence process of India-Eurasian plate in the northeastern margin of Qinghai-Tibet plateau.