The discovery of Late Bathonian-Early Callovian ammonites in the middle of northern Qiangtang basin, Tibetan Plateau

Recently, a number of ammonites were discovered in the Bandao Lake area of northern Qiangtang basin, ranging in age from the Late Bathonian to Early Callovian (Middle Jurassic). The ammonite assemblages sampled in the Changhong River area are of good preservation and have obvious features, thus providing a solid basis for comparison with the Western Tethys ammonoid zones. According to the comparison with regional ammonoid assemblages, the fossiliferous strata are probably Xiali Formation instead of the lower part of the Suowa Formation. The generation of Macrocephalites in the Early Callovian time was closely connected with the global rising sea level, and the endemic Macrocephalites sequences often indicate a regional transgression event. The existence of Early Callovian Macrocephalites in the middle Qiangtang basin implies that a regional transgression took place in the Qiangtang basin during the Early Callivian period, and the Bandao Lake area in the middle of the northern Qiangtang basin was in a relatively deep carbonate platform environment. The regression event in Middle Jurassic occurred later than Early Callovian instead of in Callovian. The discovery of the Late Bathonian-Early Callovian ammonoid assemblages is of great importance for the understanding of sedimentary-tectonic evolution of the Qiangtang basin.