LACUSTRINE AND FLUVIAL PALEOGEOGRAPHIC AND PALEOENVIRONMENTAL EVOLUTION OF UPPER-TRIASSIC ZHANGJIATAN SHALE IN ORDOS FORELAND BASIN, CHINA

Detailed sequence stratigraphic and depositional systems analyses can effectively delineate factors and processes controlling deposition of fine-grained lacustrine sediments. Upper Triassic Zhangjiatan (ZJT) Shale in Ordos foreland basin was used as an example. The study area is on the ramp margin, covering 100x90 km². Thin sections, cores and well-logs of 72 wells are used to calibrate log signatures, document lithofacies, and interpret depositional systems in a sequence stratigraphic context. The ZJT sequence consists of lowstand, transgressive, and highstand systems tracts (LST, TST, and HST). LST is 5-60 m thick, composed of fluvial sandstones and shales in the NE and deltaic deposits in the SW. The deltaic deposits consist of upward coarsening shale and sandstone successions and form a high-order sequence. Three depositional loci indicate downslope fluvial valleys and bird-foot river-dominated deltas. TST is 10-65 m thick and composed of deltaic sandstones and profundal shales of a fining upward retrogradational pattern. The isopach map shows NW-SE-oriented shorelines. Two depositional loci originate from N, extend to S, and deviate to SE parallel to paleoslope, suggesting wave-dominated deltaic systems. HST is 10-90 m thick, increasing from NE to SW, and composed of upward coarsening successions of deltaic shales and sandstones. Coarsening upward intervals increases from one updip to 4 downdip, indicating great progradation. Isopach map shows a highly heterogeneous pattern of multiple depositional loci along dip from NE to SW and severe erosion in the eastern part. The entire sequence thickens from 25 m in the NE to 160 m in the center, and thins to 55 m in the SW, suggesting a NW-SE-oriented elongate lake basin. Fluvial environment dominated during deposition of LST, and lake was restricted to the SW corner. Shoreline and deltaic and shorezone environments migrated northward outside the study area during transgression. Deltaic progradation of HST formed a complex mosaic of deltaic and fluvial environments and was extensively eroded. Paleotopography determined the location and slope of the lake. Lake level changes determined accommodation space and depositional systems. Sediment supply had been copious throughout the sequence. Post-deposition erosion determined the preservation of HST.