SEDIMENTARY CHARACTERISTICS AND DEVELOPING BACKGROUND OF THE SUBLACUSTRINE FAN IN THE LIUSHAGANG FORMATION OF THE FUSHAN DEPRESSION, THE BEIBUWAN BASIN

Some scale of sublacustrine fan deposit was developed in the Liushagang Formation of the Fushan Depression,the Beibuwan Basin.Three main sedimentary facies units can be recognized:turbidity current deposit,underwater debris flow deposit and olistostrome.The sublacustrine fan was developed in the lowstand and transgressive system of the Liushagang Formation.Some bidirectional down lapped reflection structures showed on seismic profiles.The characters of well logging curves are box,bell or funnel type.The sublacustrine fan was composed by three parts on the surface:the proximal,the middle and the distal part.The lower part of the sublacustrine fan vertically developed relatively thick incised valley infilled by gravity flow deposit.Many small turbidite fans accumulated on the vertical sequence.The development of the sublacustrine fan was controlled by the activity of the east and west bilateral faults of the basin,the supply of sediment sources and the change of lake surfaces.The bigger scale sublacustrine fan in the second episode of the Liushagang Formation was related to rapid structural subsidence and sufficient sediment supply.