Mesozoic Tectonic Evolution of Southern Junggar Basin, Northwestern China: Constraints from Stratigraphic and Sedimentologic Evidences

According to the analyses of depositional facies, seismic and geologic information, the Triassic-Jurassic depocenter was near to Urumqi city, and the Cretaceous depocenter migrated northwards to the central part of the Junggar basin. In southern Junggar basin boundary area, the Triassic, especially the Lower Triassic, was a set of strata of braided stream to braided delta facies, suggesting the Triassic basin boundary was near to that of present day. The Lower Jurassic was alluvial-fan to fan-delta facies in Houxia area (80 km south to Urumuqi) and braided delta facies in Toutun River section. The Middle Jurassic was braided delta facies in Houxia area and meandering river to lacustrine facies in Toutun River section. The Houxia area and Tuntun River sections were formed in the same sedimentary system, which suggests a southward migration of the Jurassic basin boundary toward the Houxia area. Consequently, the Junggar basin couldn't be foreland basin during the Triassic-Jurassic. The Upper Jurassic Kalazha conglomerate in southern Junggar basin and the Cretaceous basal conglomerate recorded rapid uplift and denudation during the Late Jurassic to the earliest Cretaceous. The rapid uplift and denudation might be related to the amalgamation of Lhasa block on the south margin of Eurasia plate. The basin boundary shrank obviously in Cretaceous near to southern Junggar basin boundary of present day. The Mesozoic tectonic evolution of Junggar Basin could be divided into two episodes: the extensional faulted-depression from the Triassic to Jurassic and the compressional intra-continent depression in the Cretaceous.