The gravel characteristics of Pliocene conglomerates in Zhangxian area on the northern margin of the West Qinling and its geological significance

There are two sets of Cenozoic sedimentary stratigraphic units separated by angular unconformity in Zhangxian area on the northern margin of the West Qinling, namely, the Lower Oligocene-Miocene salt-bearing sedimentary sequence and Pliocene coarse conglomerates.Based on the observation and statistical analysis for sedimentary cycle, gravel size, roundness, arrangement, composition and its vertical changes of Pliocene coarse conglomerates in this area, the authors have summarized the main characteristics of this set of conglomerates and its gravels as follows:①This set of conglomerates is mainly composed of thick, coarse conglomerates with a thickness of 480m, which covers unconformably the underlying Oligocene-Miocene salt-bearing sedimentary sequence and, from the bottom to the top, it can be divided into five lithologic beds, i.e., alluvial, fluvial, alluvial-fluvial, fluvial, alluvial-fluvial conglomerate beds; ②the gravels of the conglomerates are characterized by poor sorting with wide grain size distribution range in which the maximum size is 120-240 mm with the major size being 10-80 mm, gritty cementation, matrix supported texture with a little grain supported texture, being mainly sub-angular and sub-roundness with a little roundness; ③the gravels of the conglomerates mainly are grayish green feldspar sandstones, feldspar quartz sandstone, gray or grayish red limestone and porphyry granite, granodiorite or diorite porphyrite.In addition, the gravel compositions in different lithologic beds are obviously different.Generally, gravels in the alluvial conglomerate section mainly are various granitic rocks and sandstones, while those in fluvial conglomerate section are dominated by limestone gravel with some roundness; ④the gravel southward imbrication orientation indicates the paleocurrent direction from south to north.Based upon the above mentioned conglomerates' and gravels' characteristics, the spatial distribution controlled by south dip F2 thrust faulting, and consistency of the gravel compositions with the rock composition of the West Qinling orogenic belt, it is suggested that the Pliocene conglomerates should be the rapid accumulation, near-source, alluvial fan deposits in front of the mountain with the boundary of the fault on the north margin of the West Qinling with the nature of regenerated foreland coarse molasse basin.Furthermore, a series of conglomerates are similar to the Pliocene Hanjiagou conglomerates in lithic characteristics, age and tectonic setting, such as Ganjia conglomerates in Xunhua basin and Jishishan conglomerates in Linxia Basin on the northeastern margin of the Tibetan Plateau, and hence it is proposed that the development of Pliocene conglomerates in Zhangxian area on the northern margin of the West Qinling was related to the northward growth and expanding of the Tibetan Plateau to the northern margin of the West Qinling Mountain under the India-Europe collision dynamic background.This northward growth and expanding not only triggered the rapid uplift of the Qinling orogenic belt but also led to the reactivation of the northern margin fault and strong northward thrusting, which resulted in the formation of the regeneration foreland molasse basin.