Provenance and depositional conditions of fluvial conglomerates and sandstones and their controlling processes in a rift setting, mid-Permian lower and upper Quanzijie low order cycles, Bogda Mountains, NW China

This study investigates how provenance lithology, transport distance, and depositional and paleoclimatic conditions had affected the composition and texture of fluvial conglomerates and sandstones in the Middle Permian lower and upper Quanzijie low-order cycles, Bogda Mountains, greater Turpan-Junggar intracontinental rift basin, NW China. Composition and texture of gravels in conglomerates and sandstones define two conglomerate and two sandstone petrofacies, respectively, in Tarlong-Taodonggou area in southern Bogda and Dalongkou area 70 km to the north in northern Bogda. Conglomerate Petrofacies I occurs in Tarlong-Taodonggou and is dominated by basaltic gravels, whereas Conglomerate Petrofacies II occurs mainly in Dalongkou and is dominated by sedimentary gravels. Sandstone Petrofacies I occurs in Tarlong-Taodonggou and contains dominantly basaltic lithics, few sedimentary lithics, and minimal quartz. It is compositionally immature and texturally immature to submature. The compositional maturity increases upsection. Conglomerate Petrofacies I and sandstone Petrofacies I both indicate the young Tian Shan arc to the south as the main sediment source and rift shoulders as a local source. Sandstone Petrofacies II occurs in Dalongkou and contains dominantly sedimentary lithics and abundant and well-developed pedogenic clay coats. The petrofacies in Dalongkou indicate older rift sedimentary rocks exposed at rift shoulders as the main sediment source and distant suture zones as a minor source. In summary, the composition and texture of Quanzijie conglomerates and sandstones are mainly controlled by provenance lithology and, secondarily, depositional and paleoclimatic conditions and transport distance. The results substantiate the rift origin of the Turpan-Junggar basin in Middle Permian, which had many half-grabens and grabens and was bordered by a volcanic arc at the southern basin margin.