The genesis of conglomerate in Lower Triassic Liujiagou Formation of western He'nan Province

There are several layers of conglomerates in the Lower Triassic Liujiagou Formation in western He'nan Province.The con-glomerates consist of flat sand or mud gravels and round sand gravels, with some of the sand gravels having ring structures just like ooids in limestone.This obvious characteristic has become one of the important marker beds in dividing and correlating strata in the study area, though its cause remains a mystery and should be further ascertained.Based on the study of lithofacies of outcrops and lab experiments, the authors hold that the rings in gravels are microbially mediated structures, and the formation of conglomerates was probably associated with mats substrate after the Mass Extinction.Mat living caused the surface of sediments to have some viscosity and toughness, some destroying mat fragments caused curling, rolling and the development of "sand ball" with ring structure in vibrat-ing water, and those mat fragments and sand balls were transported into deeper water by lake gravity flow later, forming the obvious conglomerates in Liujiagou Formation of Lower Triassic in western He'nan Province.