The aftermath of the CPE and the Carnian-Norian transition in northwestern Sichuan Basin, South China

The northwestern Sichuan Basin (South China) was a portion of eastern Tethys where, during the Late Triassic, a sharp lithological transition from oolitic-bioclastic limestones and sponge reef mounds to dark grey terrigenous clays, siltstones and sandstones is visible in several localities. The timing and significance of this major facies transition is still unclear. Here we report new biostratigraphic and carbon stable isotope data from Hanwang and Jushui, northwestern Sichuan Basin. Sporomorphs, ammonoids, conodonts and halobiid bivalves show that the lithological change is Late Carnian to Early Norian in age. This amended age determination facilitates recalibration of the magnetostratigraphy in the area allowing correlation between the Late Triassic of the Sichuan Basin and the Astrochronostratigraphic Polarity Time Scale. A carbon stable isotopic perturbation across the Carnian-Norian Boundary (CNB) is missing in our sections, or it is concealed because of the mixed organic matter sources. Our findings pinpoint the position of the CNB to a short stratigraphic interval of c. 12 m thickness in the Sichuan Basin. The studied sections greatly extend the palaeogeographical documentation of the CNB and provide novel information on biostratigraphy and chemostratigraphy that should be considered in defining the best position of the Norian Global Stratotype Section and Point.