Sedimentary characteristics of slump turbidite fans in the upper third member of the Hetaoyuan Formation in the Biyang Depression

Analyses of lithology assemblage,grain size and seismic reflection characteristics of the upper third member of Hetaoyuan Formation reveal that the provenance of slump turbidite fan in the deep lake came from Houzhuang braided river delta in the northeast.The slump turbidite fans are characterized by grey medium-fine sized sandstones,interbedding with dark grey lacustrine mudstone in sharp contact,coexistence of lens and progradation reflection on the seismic profile,as well as low slope and one segment of grain-size probability curve. The drop of lake level was favorable for the formation of slump turbidite fan in delta front without external triggering mechanism.As water in northeast delta front was shallow,when the lake level dropped,part of the delta front exposed and lost stability due to the lost of buoyancy,resulting in local slumping.Wave action could strongly corrode and modify delta front sediments near the wave base and resulted in the formation of turbidite sandbody due to wave back flow.All the slump turbidite fans were developed in the lake mudstones between braised river delta and nearshore subaqueous fan.When the delta pushed forward rapidly and the lobes protruded long forward,it was easy to form slump turbidite fans large in both number and area.The number of slump turbidite fans increases and their area enlarges near the delta lobes.