A Successful Test of the “Standard” Depositional Sequence Model at a Lacustrine-Fluvial Setting and Application to Paleogeographic Reconstruction of 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 were successfully placed in a depositional sequence model for passive-margin marine siliciclastic deposits. 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 100×90 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-dominated deltaic deposits and profundal shale, forming a fining-upward retrogradational pattern. Deltaic and shorezone environments migrated northward outside the study area. 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 is composed of 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 m 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 lake basin, whereas lake level changes determined accommodation space and repetitive migration of depositional systems. Post-depositional erosion determined the preservation of HST. Sediment supply was copious throughout the sequence. 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 critical to interpreting history of fluvial-lacustrine basin filling.
AAPG Datapages/Search and Discovery Article #90291 ©2017 AAPG Annual Convention and Exhibition, Houston, Texas, April 2-5, 2017