Abstract
Detailed stratigraphic, sedimentologic, paleocurrent, and subsidence analyses were conducted on Mesozoic nonmarine sedimentary sections of the south Junggar, north Tarim, and Turpan basins, Xinjiang Uygur Autonomous Region, northwest China. These three basins have been foreland basins throughout the Mesozoic and Cenozoic eras, as demonstrated by asymmetrically distributed basinwide sediment accumulations, foreland-style subsidence profiles, and a variety of outcrop and subsurface facies data. Mesozoic paleocurrent indicators measured in the south Junggar and north Tarim basins, as well as Mesozoic sandstone compositions from both basins, indicate that the intervening Tian Shan has existed as a positive physiographic feature partitioning the two basins throughout Mesozoic and Cenozoic time. Paleocurrent, facies, and subsurface isopach data suggest that the Turpan basin was established as a discrete feature by the Early Jurassic period.
The timing and style of depositional systems within the north Tarim Mesozoic depocenter, the south Junggar Mesozoic depocenter, and the central Turpan basin are remarkably similar. Upper Triassic strata of each basin consist of alluvial conglomerate and associated braided-fluvial sandstone and siltstone which fine upward into Lower through Middle Jurassic, locally organic-rich, meandering-fluvial, and lacustrine strata. Upper Jurassic braided-fluvial red beds in each basin are overlain by a distinct pulse of uppermost Jurassic alluvial conglomerate. Lower Cretaceous exposures consist of fine-grained red beds in north Tarim and Turpan and interbedded red and gray shale with local silty carbonates in south Junggar. Upper Cretaceous strata of the north Tarim and south Junggar basins are composed of alluvial conglomerate with associated braided-fluvial sandstone and siltstone.
Subsidence curves constructed for all three basins are remarkably consistent. Each contains distinctly concave-down segments associated with the alluvial, coarse, clastic pulses in the latest Triassic, latest Jurassic, and, in the north Tarim basin, the Late Cretaceous periods. Flexurally driven Mesozoic subsidence and deposition of alluvial conglomerate in these basins occurred in response to periodic deformation of the Tian Shan and associated reactivation of basin-bounding thrust and reverse faults. These three main Mesozoic deformational episodes in the Tian Shan are interpreted to have been driven by tectonic accretion onto the south Asian margin of the Qiantang Block in the Late Triassic, the Lhasa Block in the latest Jurassic, and the Kohistan-Dras arc complex in the Late Cretaceous periods.
The Mesozoic nonmarine stratigraphic record of central Xinjiang supports the hypothesis that a strong monsoonal circulation was present during Early and Middle Jurassic time, but it waned by Late Jurassic in response to the breakup of Pangea. During the Early Cretaceous period, rejuvenation of the Tian Shan markedly influenced the climate of central Xinjiang. As southward-directed paleo-winds crossed the Tian Shan, it cast a paleo-rain shadow in the north Tarim basin, whereas regionally extensive, well-oxygenated lakes developed in the south Junggar basin.
