GRAVITY- FLOW- SUBMERGED- FAN DEPOSITS OF LOWER CRATACEOUS XIAGOU FORMATION, JIUXI BASIN, GANSU

The strata of Lower Crataceous Xiagou Formation, Jiuxi basin, were deposited in the fault subsidence of nomarine.The sedimentation of gravity-now-submerged-fan is studied in this paper.The authors defined the fan-shaped bodies of sand conglomerates interlayer with lacustrine mudstone and not changed to the river deposits, as Igravity-flow-submerged-fan according to the investigating more than one hundred layers of sandy gravel rock and eighty-four pieces of thin section.The submerged-fan consist of four lithofacies ( conglomerate facies, conglomeratic mudstone facies, sandstone facies and silt / mudstone), including fifteen sublithofacies.The analysis of lithofacies show that conglomerate facies were mainly deposited by subaqueous debris flow, sandston facies by high-density turbidite and silt/ mudstone by low-density turbidity currents and lacustrine sedimentation.Conglomeratic mudstone facies is polygenetic, that is , a genetic spectrum was not formed by them. Having analyzed spacial relationship of lithofacies for the genesis, it is proposed thai the events of catasstrophic flood (subaqueous llandslip) , high-density turbidity currents and low-density turbidity currents are the genetic sequence of hydraulic development in the little sized fault-sidence basin it is discussed that the spacial relationship of lithofacies, control factor of tectonic and the environment evolution in the sedimentary context model of the gravity-flow-submerged-fan at last.To make the sedimentological study of suberged-fan is favourable for developing auto-genetic tnd auto-stored oil-reservoir.