Pliensbachian–Toarcian Ammonite Biostratigraphy and Sedimentary Condensation in Southern Qiangtang, North Tibet

Recent study on early Jurassic ammonites, collected bed by bed from the Ganbeixiama section allows the establishment of the biostratigraphical scheme in the southern Qiangtang basin for the first time. Three beds in the section are characterized by the enrichment of mineralized ammonites, a mixture of the different ammonite zones ranging from the Pliensbachian to Toarcian, and small thickness, identified as condensed sequences. The thickness of these beds is between 0.5 to 2.5 meters, containing most of the Pliensbachian and Toarcian biochron zones with gaps and hiatuses. These condensed beds appear repeatedly in the lower, middle, and upper parts, separated by less-fossiliferous beds between 30-80 meters thick in the section. Based on ammonite preservation and rhythmic bedding sequence, a bathymetric transect from offshore up to continental slope environments can be considered, and the sea level rise during the late Toarcian (Speciosum Zone) likely reached a maximum. This stratigraphical relocation and repetition in the section are interpreted in terms of mass movement resulting in sliding down in continental margin environments, which was frequently triggered by earthquake events.