Astronomical Calibrated Magnetostratigraphy Of Mid-Carnian (Late Triassic), South China

During the middle of the Carnian Age (-230 Ma) of the early Late Triassic, there was a global disruption of Earth’s climate-ocean-biological system. This mid-Carnian episode, the "most distinctive climate change within the Triassic", has been given various names depending upon the local characteristics, such as "Carnian Pluvial Event" in northern Europe, "Carnian Wet Intermezzo" in the Germanic Basin, "Reingraben turnover" in the Austrian Alps, or "Raibl Event" in the Italian Dolomites. This mid-Carnian episode was initially recognized within the Germanic Basin by an influx of fluvial to brackish-water sands into the arid facies of the Keuper and within the Alpine region by the termination of the prograding reefs of the earlier Carnian accompanied by a negative carbon-isotope excursion. These fluvial sands of the "Wet Intermezzo" in the Germanic Basin are correlated by conchostracan and megaspore assemblages to marine deposits of the upper part of the Austrotrachyceras austriacum ammonoid zone of the uppermost Julian substage. The end to this "wet intermezzo" interval in the Germanic-Alpine region coincides with the substage boundary between Lower (Julian) and Upper (Tuvalian) substages of the Carnian. The trigger for the onset of this mid-Carnian episode might be the eruption of the immense Wrangellia flood basalt province at ca.230 Ma, but uncertainties both in the age model for the Late Triassic biozones and in the stratigraphic age of the main eruptions of the Wrangellia volcanic province preclude a reliable correlation.The Late Triassic currently lacks a high-precision integrated time scale. Indeed, estimates for the span of the Carnian stage 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 for its upper substage and upon other factors. A reference magnetic polarity time scale spanning 20 myr of the Late Triassic was developed from eccentricity-cycle-tuned scaling of the magnetostratigraphy from the fluvial-lacustrine Newark Supergroup of eastern North America. Those Newark deposits reflect major variations in the seasonal strength of monsoonal precipitation as it was modulated by orbital-climate cycles at eccentricity and precession frequencies. Theoretically, as with the cycle-scaled C-sequence of polarity chrons that underpins the Cenozoic time scale, this Newark series should enable a precise Late Triassic time scale. However, calibrations of the Newark polarity scale to the independent magnetic stratigraphies derived from a partial assembly of many Late Triassic ammonoid-or conodont-zoned outcrops have many possible matches. This ambiguity is also caused by the absence of magnetostratigraphy integrated with cyclostratigraphy from those outcrop facies and also no verified correlations of marine biozones to the succession of Newark terrestrial deposits are available. The lack of a verified Late Triassic age model and of a cycle-scaled bio-magnetostratigraphy scale hinders our ability to deduce the relative timing of events in different basins and to test the synchroneity and postulated causal factors for this proposed global wanning pulse.During the Triassic, the South China Block was situated between the mega-Pacific Ocean ("Panthalassa"’, meaning "all seas") to the east and the seas and the seaways of the Paleo- and Meso-Tethys to the west and the south. The South China Block progressively drifted to the north during the Triassic, and was located ca.30°N during the Late Triassic. After the mid-Triassic, the South China Block sutured with the North China Block to the north and with the Indochina and Siamo blocks to the south and west. The Yangtze Platform of South China, considered to be one of the Earth’s largest regions of shallow-water deposition, covered most of present Guizhou Province and parts of neighboring Sichuan, Yunnan, Guangxi, and Hunan provinces. Thick successions of shallow-water carbonates span much of the late Proterozoic through the mid-Triassic. This Yangtze Platform and many other Triassic carbonate platforms were terminated in the Carnian, but unlike the Dolomites and Alpine regions, carbonate deposition on this Yangtze Platform never resumed and its overlying Norian deposits are typically braided river systems emptying into coastal swamps.Thick successions from the margins of the former 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. We sampled two representative sections in Guizhou province near the Yangtze Platform southern margin (Geopark Wayao= WY, Laishike= LSK), and one typical section of its western margin in the present Sichuan province (Hanwang= HW). Magnetostratigraphy combined with cyclostratigraphy derived from spectral gamma-ray (SGR) intensity curves was obtained from two sections in Guizhou Province and the Hanwang section in Sichuan Province, in addition to obtaining a cyclostratigraphy from the previously published detailed magnetostratigraphy from a different Wayao transect in Guizhou Province, aiming to construct a regional astronomically-calibrated geomagnetic time scale utilizing the combination of cyclostratigraphy and magnetostratigraphy. Our combined magnetostratigraphy and cyclostratigraphy study of expanded sections of middle Carnian shelfal to basinal facies near the margins of the Yangtze Platform has three main objectives:(1) Compile an integrated scale of magnetostratigraphy, biostratigraphy and cyclostratigraphy to test whether the termination of the Yangtze Platform shallow-water carbonates is approximately synchronous between its southern (Guizhou) and northwest (Sichuan) margins. (2) Correlate to the outcrop-based magnetostratigraphies from fossiliferous European sections to determine if final phase of termination of the Yangtze Platform carbonates as marked by the influx of clastic-rich facies was during the latest Julian and therefore coeval with the "Wet Intermezzo" or "Carnian Pluvial Event". (3) Apply the cycle-scaling of the durations of major polarity zones through the middle Carnian of South China to determine which portions of the pattern, if any, have corresponding cycle-scaled polarity chrons in the Newark geomagnetic time polarity scale (GPTS), therefore placing constraints on the entire Late Triassic time scale.The cyclostratigraphy from all the Guizhou sections indicate 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 are consistent with the cyclicity and characteristic SGR features, thereby enabling a cycle-tuned magnetic polarity time scale spanning ～2.4 myr. The main feature is a 1.3-myr interval of reversed polarity zone 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 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 probably 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 appears to support 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. From this exciting pilot study, I am planning to verify and extend the results in other sections within Guizhou and Sichuan provinces in my future PhD research in Purdue University (USA). In addition to establishing a precise Early Carnian time scale, we will extend this South China calibrated-magnetostratigraphy to Europe and North America to unravel the rates and responses of different sedimentary successions to this major volcanic-induced greenhouse episode in global climate.