Cycle-calibrated magnetostratigraphy of middle Carnian from South China: Implications for Late Triassic time scale and termination of the Yangtze Platform

• Astronomically-tuned magnetostratigraphy scale spans ~ 2.4 myr of middle Carnian.
• 1.3 myr reversed-polarity magnetic zone corresponds to upper Julian (lower Carnian).
• Polarity time scale supports “Short-Carnian/Long-Norian” age model for Late Triassic.
• Newark Basin cycle-magnetostratigraphy reference time scale begins after the Julian.
• Yangtze Platform termination is coeval with European “Carnian Pluvial Event”.

Thick successions from the margins of the Triassic Yangtze Platform of the South China Block record a transition from carbonate-rich facies (Zhuganpo Formation and equivalents) to clastic-rich facies (Xiaowa Formation and equivalents) during the Carnian (early Late Triassic) that mark the final phase of termination of this long-lived platform. Cyclostratigraphy derived from spectral gamma-ray (SGR) intensity curves was combined with magnetostratigraphy of two sections in Guizhou Province (Wayao transect and its upward extension into the Geopark Wayao in Guanling county and the Laishike section in Zhenfeng county) and one location in Sichuan Province (Hanwang section). The cyclostratigraphy from all the Guizhou sections indicates a persistent suite of ca. 34 m, 9 m and 1.8 m cycles, which are consistent with the ratios of orbital-climate oscillations caused by long-eccentricity (405 kyr), short-eccentricity (~ 100 kyr) and precession (20 kyr). The magnetostratigraphy of all sections is consistent with the cyclicity and characteristic SGR features, thereby enabling a cycle-tuned magnetic polarity scale spanning ~ 2.4 myr. The main feature is a 1.3-myr interval of reversed polarity containing brief normal-polarity subchrons, and this reversed-polarity chron appears to correspond to the significant reversed-polarity-dominated interval spanning the upper half of the Trachyceras aonoides through the lower half of the Austrotrachyceras austriacum ammonoid zones of the upper Julian (the lower substage of Carnian). This magnetostratigraphic correlation implies that the termination of the Yangtze Platform is coeval with the beginning of the mid-Carnian episode of climatic disruption in Europe, which is locally called the “Carnian Pluvial Event” or “Wet Intermezzo”, and with the temporary cessation of the platform carbonates in the Dolomites. The cycle-scaled magnetic-polarity time scale supports the “Short-Tuvalian/Long-Norian” age model of the Late Triassic in which the base of the cycle-tuned polarity pattern from the Newark Group of eastern North America is younger than the end of the Julian substage.

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