THE MICROFACIES ARCHITECTURE AND ZONING OF DELTA FRONT IN DEPRESSED-TYPE LACUSTRINE BASIN——TAKING THE YANCHANG FORMATION OF TRIASSIC IN ORDOS BASIN AS AN EXAMPLE

Taking the Yanchang formation of Triassic in Ordos basin as an example,this paper research in detail the microfacies architecture and association types of depressed-type lacustrine delta fronts.The microfacies of delta fronts include truncated distributary channels and integrated distributary channels,river mouth bars,distal bars,front sheet sandstones and interdistributary bay.According to association type the microfacies may be divided four types of sandstones that consist of nine subtypes including superimposed distributary channel,superimposed channel mouth bars,subaqueous distributary channels and channel mouth bar associations and channel mouth bar,distal bar and front sheet association.The authors discusses in detail distribution microfacies association of delta fronts.The depressed-type lacustrine delta fronts can be divided into three parts: platform delta front,sloping delta front and basin delta front by the bounds of slope break.Every part has unique microfacies association type.