Sedimentary Facies In The Second Member Of Dainan Formation, The South Slope, Gaoyou Sag

Gaoyou Sag is a typical continental rift basin. Being controlled by the boundary fault, the south slope of the sag represents a higher gradient than the north slope. The second member, which was deposited on the south slope of Gaoyou Sag, contains an extensive record of braid-delta deposits. The intended layer is characterized by coarse- to fine-grained sandstone sediments, and some well-rounded, pebbly to cobbly conglomeratic deposits. Sedimentary structures are dominated by trough cross-bedding, parallel lamination and scour surface. Based on the petrographic analysis, log interpretation and the ichnography combined with the previous research, we can reach a conclusion: The braid delta of Dainan Formation in the second member was deposited in a shore-shallow lake with not high relief, the sources of clasts are from the southern uplift (Tongyang Uplift and Wubao Uplift). The delta is mostly characterized by subaqueous shifing channels. Delta front is the most common in the study region and can be further subdivided into distributary channel, front bar, sand sheet and interdistributary. The distributary channels (Facies A), which stretched for vast extent longitudinally and laterally, constructed the sandbody framework of the delta front. front. Because of the bifurcating and diverting braided streams, the mouth bar deposits at each of the lakeward end of the channels often merged and formed a near-continuous sand strip around the entire periphery of the delta, therefore is called front bar. Front bar deposits (Facies B) display a thinner successive, a finer grain size and low depositional dips. Sand sheet (Facies C) deposits are mainly found in front of Facies B, gradationally contacting with the prodelta. Being controlled by sediment supply, both progradational sequences and retrograding sequences occur within the second member of Gaoyou Formation.