The Jurassic sediments and their depositional environments in the Yanshiping region of the Qiangtang Basin, northern Xizang

The marine Triassic and Jurassic strata crop out over a wide range of scales in the Qiangtang Basin, northern Xizang. The Jurassic strata composed of alternating marine-continental sediments are characterized by widespread occurrence, laterally persistent strata and abundant fossils. The Middle Jurassic Xiali Formation and Upper Jurassic Suowa Formation are laid down on the southern flank of the Yanshiping geosyncline studied herein. The former contains prevailing clastic rocks including purplish red and/or dark purple mudstones and grayish green siltstones and fine sandstones; the latter is dominated by limestones and contains well-developed beddings and abundant fossils. The integration of field investigation and indoor studies has disclosed that the Xiali Formation is located in a tidal-flat environment whereas the Suowa Formation in a carbonate ramp environment.