Sedimentary facies and evolution of the second member of the Palaeogene Dainan Formation in the deep area of eastern Gaoyou depression,North Jiangsu Basin

The present paper gives a detailed description,on the basis of cores,well logs and regional geological background,of the sedimentary facies and evolution of the second member of the Palaeogene Dainan Formation in the deep area of eastern Gaoyou depression,North Jiangsu Basin.The second member of the Dainan Formation consists of the fan delta facies,delta facies and lacustrine facies.The fan delta facies appears in the southern steep slope zone;the delta facies develops in the northern ramp zone,and the lacustrine facies is organized around the fan delta and delta facies and in the central zone of the deep area.The sedimentary framework is generally controlled by the faults and palaeotopography.From the margin to the centre of the depression,the sedimentary facies pass from the fan delta or delta facies to the littoral-shallow lacustrine facies.Vertically,there are coarse-fine-coarse gradations in the grain sizes of the sediments from bottom to top.Five stages can be distinguished for the sedimentary evolution during the deposition of the second member of the Dainan Formation.On the whole,the water bodies displayed the shallow-deep-shallow variations during these periods.The retrogradation occurred first in the fan delta and delta facies,followed by the progradation.At the latest stage,the lake basin was subjected to the uplift and erosion,which resulted in the unconformable contact with the overlying strata.