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Paleogeomorphological Controls on the Distribution of Depositional Systems and Sand Bodies in Lower Cretaceous Tanan Lacustrine Syn-Rift Basin, Mongolia


Tectonics is very important to the depositional record preserved in nonmarine sedimentary basins. Tanan Sub-basin is a Lower Cretaceous active-fault bounded basin in Mongolia. Episodic rifting, differential subsidence both along the boundary fault strike and across the basin, resulted in the formation of distinct paleogeomorphology, including various types of transfer zones and fault-break zones. In this study, the control of paleogeomorphology to the spatial distribution and temporal evolution of depositional systems and sand bodies in Tanan Sub-basin were investigated using seismic profiles, cores and well logs. Transfer zones could be subdivided into two types according to the various displacement along the fault strike and the related topographies. They were synthetic approaching transfer zones characterized by the topography of transverse uplifts and synthetic overlapping transfer zones characterized by the topography of relay ramps. Transfer zones controlled the locations of sedimentary provenances, entry points for sediments into the basin, and as a result, the development of depositional systems. The large-scale, coarse grained sandstone/conglomerate of fan deltas are mainly deposited in the transfer zones, in contract, small-scale, fine grained sandstone deposited in other positions along the fault strike. According to fault geometry and the location, four fault-break zones and related depositional systems were identified. They were fault-scarp zones controlling the development of fan-deltas and nearshore sublacustrine, fault-terrace zones controlling the development of fan-deltas and offshore sublacustrine fans, half-graben dip-slope zones controlling the development of braided river and braided deltas, and intra-basinal fault-break zones controlling the development of offshore sublacustrine fans. The fault-break zones mainly influenced the distribution of depositional systems and sand bodies. Areas where the fault-break zone overlapped with transfer zones were sites for major drainage systems and the optimum locations of fan deltas and sublacustrine fans. The sand bodies were mainly accumulated at the lower part of fault-break zones, and the rift-interior sediment dispersal directions are consistent with the strike of the fault-break zone. Due to interbedded with deep lacustrine mudstone, sand bodies deposited here are favorable targets for the prospecting of litho-stratigraphic traps in Tanan sub-basin.