PRELIMINARY RESULTS ON SEDIMENTOLOGY, DEPOSITIONAL ENVIRONMENTS, AND SEQUENCE STRATIGRAPHY OF UPPER-TRIASSIC FLUVIAL AND LACUSTRINE YANCHANG FORMATION, ORDOS BASIN, NORTH-CENTRAL CHINA

Outcrop and well studies of shale and sandstone in 3 members of Upper Triassic Yanchang Fm in Ordos Basin show complex fluvial-lacustrine sedimentation in a cratonic lake basin. On 2 sections, the lower 40 m contains 11 fining-upward sandstone-mudrock cycles of Chang 8 Member with high-relief channel bases. Mudrocks are plant rich with root molds as Argillisols and Gleysols. The cycles are meandering stream deposits. The lower 5 cycles have thick overbank facies, comprising a highstand systems tract (HST); the upper 6 have thin or no such facies, comprising a regressive systems tract (RST) in a small accommodation space. A thin (50 cm) skolithos-rich arenite of Chang 7 Member overlies fluvial deposits. It is overlain by black, well-laminated, thin (70 cm) Zhangjiatan (ZJT) shale with abundant plant remains, bivalves, fish scales, and ostracods. Common normal-graded, sharp-based siltstone and very fine sandstone laminae contain starved ripples and micro-water escaping vents, as distal fine-grained turbidites. Common anhydrite-gypsum and biotite grains suggest a hypersaline-alkaline lake. The arenite and shale form 2 coarsening-upward cycles as transgressive systems tract (TST) and condensed section (CS), respectively. They thin to 0.2 m 4 km to W at the basinward section. Upward 8 coarsening and thickening-upward shale-sandstone cycles are 30 m thick and show km-scale SW-prograding clinoforms, indicating deltaic infilling. The lower 5 deltaic cycles are thick, comprising a HST, whereas the upper 3 are thin, as a RST. Delta-front sandstones thin greatly basinward; are tabular, ripple marked, with extensive but confined convolutions suggesting a seismic origin. Finally, nested fluvial sandstones of Chang 6 Member overlie Chang 7 with a high-relief erosional boundary, as a lowstand systems tract. The TST-CS ZJT shale and HST-RST deltaic deposits of Chang-7 Member thicken to 50 m in 3 wells ~100 km basinward to W. Abundant biotite and common slate, schist, and polycrystalline and volcanic quartz grains in core and outcrop shale and sandstone samples and paleocurrent data suggest a NE mixed volcanic and crystalline provenance. Future detailed sequence-stratigraphic, XRD, and XRF analyses will establish a depositional model for oil and gas exploration of ZJT shale in this and other nonmarine basins.