TIMING OF LOWER TRIASSIC CARBONATE BANK BUILDUP AND BIOTIC RECOVERY FOLLOWING THE END-PERMIAN EXTINCTION ACROSS THE NANPANJIANG BASIN, SOUTH CHINA

The end-Permian extinction and subsequent Early Triassic recovery are exceptionally well-recorded in the carbonate platforms and basins of south China. Lower Triassic sections in south China contain 4-5 silicic volcanic tuff horizons that vary in thickness from 0.1 to 5.0m. These ash beds offer the potential to determine the distribution of time during carbonate deposition using high-precision U-Pb zircon geochronology. Several of these tuffs have been regionally correlated and used as time lines independent of their radiometric ages. A highly resolved chronostratigraphy allows detailed stratigraphic correlations across the basin and a detailed understanding of the dynamics of carbonate platform development including absolute accumulation rates. Areal patterns of accumulation rates are essential information in distinguishing between tectonic and eustatic influences in the platform evolution. In addition, high-precision geochronological constraints allow us to determine the duration and rate of biological recovery following the end-Permian extinction, which is essential for a better understanding of the extinction mechanisms.

The architecture of isolated carbonate platforms in the Nanpanjiang basin is generally similar. These platforms had ramp profiles in the Early Triassic with oolite shoals at the margin, shallow-subtidal and peritital deposits in the interior, and pelagic mudstones, debris-flow deposits, and turbidites on gentle basin-margin slopes. Basin-wide, typically the first rock unit in the interior of isolated platforms are Renalcis biostromes followed by lime mudstone, oolite, and cyclic peritidal limestone. Preliminary U-Pb zircon analyses from ash beds and conodont biostratigraphy suggest that biostrome growth began immediately following the end-Permian extinction ca. 251 Ma and may have lasted as little as 1 Ma. U-Pb zircon analyses from ash beds from the biostratigraphically controlled Scythian-Anisian boundary on the Yangtze platform and the isolated platforms indicate an age of ca. 247 Ma implying a duration of approximately 4 Ma for the Early Triassic, thus shortening the timeframe for biotic recovery following the end-Permian extinction.