CYCLE-CALIBRATED MAGNETOSTRATIGRAPHY OF MIDDLE CARNIAN FROM SOUTH CHINA: IMPLICATIONS FOR LATE TRIASSIC TIME SCALE AND TERMINATION OF THE YANGTZE PLATFORM

The Late Triassic currently lacks a high-precision integrated time scale. Indeed, the span of the Carnian stage has been estimated to range from ca. 16 myr to 8.5 myr, depending upon the choice between a “long-Tuvalian” (hence, a short Norian) or a “short-Tuvalian” (long Norian) option. During the middle of the Carnian, the former Yangtze Platform of the South China Block underwent a transition from carbonate-rich facies to clastic-rich facies that mark the final phase of termination of this long-lived platform. We combined magnetostratigraphy with cyclostratigraphy derived from spectral gamma-ray (SGR) intensity curves from two sections in Guizhou Province and one section in Sichuan Province. A persistent suite of ca. 34 m, 9 m and 1.8 m cycles corresponds to the Milankovitch cycles of long-eccentricity (405 kyr), short-eccentricity (~100 kyr) and precession (20 kyr). The magnetostratigraphy of all sections are consistent with the cyclicity and characteristic SGR features; therefore, a cycle-tuned magnetic polarity scale spanning ~2.4 myr was established. A relatively long (1.3 myr) reversed-polarity zone with brief normal-polarity intervals is interpreted to correspond to the significant reversed-polarity-dominated interval spanning the upper half of the T. aonoides through lower half of the A. austriacum ammonoid zones of Early Carnian. This implies that the termination of the Yangtze Platform is coeval with the beginning of the global disruption of Earth’s climate-ocean-biological system known as the “Carnian Pluvial Event” in Europe. The cycle-scaled magnetic-polarity time scale supports the “Short-Tuvalian/Long-Norian” age model of the Late Triassic and implies that 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. We are testing and extending this cycle-calibrated magnetostratigraphy using boreholes in Germany to obtain a verified Late Triassic age model.