TMiddle Permian to Lower Triassic Fluvial-Lacustrine Depositional Systems and Sequence Stratigraphy, Bogda Mountains, Turpan Intermontane Basin, Northwest China

Wan Yang¹, Yiqun Liu², Qiao Feng³, Jinyan Lin², Dingwu Zhou², Dan Wang², and Michel T. Runnion
¹ Wichita State University, Wichita, KS
² Northwestern University, Xian, China
³ Shandong University of Science and Technology, Qingdao, China

Two measured sections of 1,200-m superbly-exposed Mid-Permian-Lower-Triassic rocks and geologic mapping in a 50-km2 area in southern Bogda Mountains, China, delineated rapid facies changes and stratigraphic architecture of fluvial-lacustrine systems and complexity of nonmarine reservoir and source rocks in an intermontane basin. Five types of high-order cycles composed of braided and meandering stream and lacustrine deltaic and mixed siliciclastic and carbonate deposits were identified (total ~800 cycles). Their facies and thickness stacking patterns were used to identify intermediate and low-order cycles (47 and 7, respectively). Cycle correlation, aided with coal, paleosol, and volcanic markers, identified regionally-persistent cycles, which were used to construct a sequence stratigraphic framework that facilitates accurate delineation of facies changes at three levels. Braided stream deposits of Mid-Permian Daheyan sequence overlie Lower-Permian syn-rift volcanoclastics, filling topographic lows. Fluctuating profundal cycles composed of dolomitic shale, shale, sandstone, and limestone of Lucaogou sequence, one of the richest source rock in the world, thin westward into interbedded fluvial conglomerate and palustrine limestone over 6 km. In contrast, fluvial deposits and lacustrine limestone of overlying Hongyanchi sequence thicken westward into dominantly lacustrine limestone. Upper-Permian Quanzijie, Wutonggou, and Guodikeng sequences contain reservoir rocks of thick meandering stream and lacustrine deltaic systems deposited during drift phase, which thicken westward with common local facies changes. The Lower-Triassic Jiucaiyuan-Shaofanggou sequence contains persistent littoral sandstone and shale deposited during tectonic quiescence and semi-arid climate. Our study shows that a multi-order nonmarine sequence stratigraphy can be established through detailed stratigraphic analysis to decipher processes controlling fluvial-lacustrine sedimentation.