Sedimentary characteristics of Pliocene Hanjiagou conglomerates in Zhangxian area on the northern margin of the western Qinling and their geological significance

The Pliocene conglomerates, distributed in Zhangxian area of the northern margin of the western Qinling on the north-eastern margin of the Tibetan Plateau, belong to a coarse molasses suite. Their present spatial distribution and sedimentary characteris-tics can provide important constraints on understanding the Late Cenozoic uplift and tectonic evolution of the northeastern margin of the Tibetan Plateau. Based upon the detailed study of the thickness, gravel composition, gravel morphology, grain size distribution, palaeocurrent direction and provenance analyses for the conglomerates, the authors investigated their sedimentary environment and tectonic setting. it is held that the Pliocene conglomerates on the northern margin of the western Qinling represent the Pliocene mo-lasses foreland basin formed in a tectonic setting of the northward thrusting of the western Qinling since Late Cenozoic. Furthermore, the present highest elevation of the conglomerates is similar to the paleo-planation surface of western Qinling Mountain to the south of the north margin fault of western Qinling, which indicates that, after the formation of the conglomerates, they underwent longterm erosion and leveling with west Qinling Mountain and finally formed the unified planation surface at the end of Neogene or in Early Quaternary. Uplift, erosion and disintegration of the planation surface might have been the starting point of crustal uplifting of the northeastern margin of the Tibetan Plateau; in other words, the uplifting of the northeastern margin of the Tibetan Plateau was just a geological event since the end of Neogene or Early Quaternary.