High-resolution Early Triassic ammonoid biostratigraphy of South Tibet, China and implications for global correlations

Abstract

Ammonoids are key fossil indexes for Triassic biochronology, as all Triassic stages and substages were initially defined on ammonoid faunas. In recent decades, the temporal resolution of ammonoid biostratigraphical scales for the Early Triassic has been greatly improved. However, many uncertainties in zones correlation and superpositions remain, mainly due to sampling heterogeneities, preservation biases, and faunal endemicity. In this work, we present the first comprehensive Early Triassic ammonoid zonation from South Tibet, China, a previously poorly investigated region. Ammonoids were sampled from the Kangshare Formation at four sections (Selong, Paizi, Qubu and Xialong), representing a total of 140 species, ranging from the Griesbachian to the Smithian. These new robust data allow the construction of a high-resolution biostratigraphy using the Unitary Association (UA) method. A total of 22 Unitary Association zones (UAZs) were recognized, including two UAZs for the Griesbachian, nine for the Dienerian, and 11 for the Smithian. Then, we integrated data from neighboring basins, i.e., Spiti (India) and the Salt Range (Pakistan), and the new data from South Tibet to construct synthetic, laterally reproducible Dienerian-Smithian ammonoid UAZs, which include 12 UAZs for the Dienerian and 16 UAZs for the Smithian. Based on the newly obtained data and high-resolution biostratigraphic scales, we revised global correlations known for the ammonoid biostratigraphy in the Griesbachian, Dienerian and Smithian. Finally, the high-resolution ammonoid zones are generally in agreement with conodont zones in defining stage/substage boundaries. They also provide a robust and accurate time calibration for Early Triassic carbon isotope trends and temperature changes.