APPLICATION OF THE “STANDARD” DEPOSITIONAL SEQUENCE MODEL AT A LACUSTRINE-FLUVIAL SETTING, UPPER-TRIASSIC ZHANGJIATAN SHALE, ORDOS FORELAND BASIN, CHINA

The basic geometric patterns of aggradation, progradation, and retrogradation and their spatial distributions of fluvial-lacustrine deposits on the ramp margin of Ordos foreland basin are successfully placed in a passive-margin depositional sequence model. Within this framework, detailed lithofacies and depositional systems analyses effectively delineate factors and processes controlling dominantly fine-grained fluvial-lacustrine sedimentation. Thin sections, cores and well-logs of 72 wells, and 4 outcrop sections of Upper Triassic Zhangjiatan (ZJT) Shale in an area of 100x90 km² are studied. The ZJT sequence consists of lowstand, transgressive, and highstand systems tracts (LST, TST, and HST). LST is 5-60 m thick, composed of fluvial deposits in the NE and deltaic in the SW. Isopach and log-facies maps show 3 depositional loci of fluvial valleys and river-dominated deltas. The LST itself forms a complete high-order sequence. TST is 10-65 m thick and composed of wave-dom. deltaic deposits and profundal shale, forming a fining-upward retrogradational pattern. Deltaic and shorezone environments migrated northward. Two depositional loci extend from N to S downslope, then deviate to SE parallel to paleoslope. HST thickens from 10-90 m from NE to SW and contains coarsening-upward deltaic successions. The number of successions increases from 1 to 4 downslope, indicating extensive progradation that forms a highly heterogeneous pattern of multiple depositional loci from NE to SW and severe post-depositional erosion in the east. The entire ZJT sequence thickens from 25 to 160 downslope from NE to SW, then thins to 55 m in the SW, suggesting an elongate NW-SE-oriented lake basin. Paleotopography determined the location and slope of the basin, and lake level changes determined accommodation space and repetitive migration of depositional systems. Sediment supply was copious throughout the sequence. Post-depositional erosion determined the preservation of HST. However, falling-stage ST is not present in the high-order LST and low-order ZJT sequences. Shoreline trajectory indicates that maximum-regressive shoreline may not always be located basinward of the shoreline on the basal sequence boundary. The principles of depositional sequence are useful to interpret fluvial-lacustrine basin filling history.