Abstract

The northwestern Sichuan Basin (South China) was a portion of eastern Tethys where, during 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, however, are 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 sharp lithological change is Late Carnian to Early Norian in age. This amended age determination allows recalibrating the magnetostratigraphy in the area and therefore proposing a tight correlation between the Late Triassic of the Sichuan Basin and the Astrochronology Polarity Time Scale. A carbon stable isotopic perturbation across the CNB is missing in our sections, or it is concealed because of the mixed organic matter sources. Our findings pinpoint the position of the Carnian/Norian Boundary to a short stratigraphic interval of ∼12 m thick in the Sichuan Basin. The studied sections greatly extend the paleogeographic documentation of the Carnian/Norian Boundary and provide novel information on biostratigraphy and chemostratigraphy that should be considered to define the best position of the Norian GSSP.

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