Abstract: The Upper Triassic Xiaowa Formation,bearing the Guanling biota,is established and described bed by bed on the basis of previous work and systematic measurements.Study of the petrology and geo-chemistry of the Lower Member of the Xiaowa Formation,as well as the ecological characteristics of the Guanling biota indicates that the Xinpu area of Guanling County,Guizhou Province,was a restricted gulf encircled by land on three sides and connected with the Tethyan Ocean on one side during the early-middle Canian Stage(Late Triassic).The ecological characteristics of a few well persevered plant fossils and lots of wood fossils found in the Guanling biota reveal that this area would have the tropical and subtropi-cal climatic characteristics and plenty of rainfalls during the early Late Triassic.Element geochemical charac-teristics show that seawater in the gulf may have been freshened intermittently under the influence of inter-mittent input of large amount of fresh water,so that the marine environment that organisms relied on for subsistence was intermittently anoxic or dysoxic and then there occurred mass mortality and burial of organ-isms.