A revision and new data on the Jurassic ammonites from the Biluoco area, southern Qiangtang block (North Tibet)

摘要

A study of recently made collections of Jurassic ammonites from the Biluoco area of northern Tibet enables a revision of previous identifications. Two occurrences of ammonite-bearing organic-rich shales are reported, they contain an Oxycerites aspidoides-Bositra buchi assemblage indicative of a late Bajocian age, and a Clydoniceras discuss-Bositra buchi assemblage indicative of a late Bathonian age, respectively. Zigzagiceras (Procerozigzag) cf. pseudoprocerum (Buckman), and Polysphinctites sp. are newly reported from Tibet. Negative excursions of carbon-isotope measurements in the Biluoco section of southern Qiangtang have repeatedly been reported, and interpreted as a response to the Global Toarcian Anoxic event. These results are now subjected to further interpretation as the ammonites of the strata previously placed in the Toarcian, Tenuicostatum and Falciferum zones are revised. There is no evidence of the T-OAE in terms of organic-rich shales in the Qiangtang region (northern Tibet) known at the present time.