Integrated carbon-isotope and calcareous nannofossil stratigraphy in the Middle Jurassic of Qinghai-Xizang (China)

The Jurassic successions of the Qiangtang Basin (Tibet) are w}ell known because they contain petroleum source rocks, as illustrated by the occurrence of organic-rich, black shales. These deposits are very likely the expression of major palaeoceanographic and palaeoclimatic changes. Previous incorrect identification of scarce and poorly preserved Jurassic ammonites, recorded in the interval characterized by organic-rich shales, resulted in the assignment of these black shales to lower Toarcian strata and, therefore, to a tentative identification of the early Toarcian oceanic anoxic event (T-OAE). This age was then challenged because of the record of a diversified calcareous nannofossil assemblage typical for the Bathonian-Callovian interval. The present study focuses on additional samples coming mainly from the base of two previously studied sections (Biluo Co and Amdo 114 station), and confirms the occurrence of the NJ12 nannofossil zone (Bathonian-Callovian), although some intervals could not be dated. A new carbon-isotope curve is also presented and compared to wt% TOC and trace elements for the two sections. The whole data set allows a very effective correlation of the two sections and various intervals can be depicted. For the first time, a prominent negative carbon-isotope excursion of similar to -4 parts per thousand measured on bulk carbonates (NCIE) has been recognized in the lower part of the Biluo Co section (Bathonian p. p.). This feature is accompanied by high ratios of some redoxsensitive elements (such as V/Al, Mo/Al), or productivity-related elements (such as Cu/Al), and is followed by a delta C-13(carb) positive excursion recorded in both sections across the Bathonian p. p. and the Callovian. A peculiar calcareous nannofossil assemblage characterized by the dominance of small-sized Watznaueria specimens also occurred. All these data likely indicate an increasing oceanic productivity during the Bathonianp.p. and Callovian, which is further supported by the accumulation of organic-rich sediments, as observed in the Qiangtang Basin.