Evolution of Early Triassic outer platform paleoenvironments in the Nanpanjiang Basin (South China) and their significance for the biotic recovery

Detailed microfacies and paleoenvironmental analyses were conducted through the Early Triassic interval of the outer platform ammonoid-rich series of the Nanpanjiang Basin (Luolou Formation, Guangxi Province, South China). Extensive investigations on outcrops and on thin sections reveal that the widely reported well-diversified latest Permian fauna, dominated by abundant corals, calcareous algae, calcareous sponges, crinoids, brachiopods, gastropods, ostracods and foraminifera, is abruptly replaced by decimated Early Triassic communities essentially composed of low-diversity bivalves, microgastropods, ostracods, and rare foraminifera. Along with this drastic change in faunal assemblages, the recolonization of devastated habitats by calcimicrobial frameworks is observed in the aftermath of the end-Permian mass extinction. Calcimicrobialites of Griesbachian age are followed by mixed, ammonoid- and conodont-rich carbonate-siliciclastic series consisting of dark, suboxic, laminated mudstones intercalated with organic-rich shales of Dienerian to late Smithian age. The carbonate-siliciclastic series are interrupted by a brief carbonate episode during the early-middle Smithian. Massive carbonate production resumes only from the early Spathian onward and persists throughout the Spathian. The initial deposition of both carbonate episodes is coeval with (i) re-establishment of well-oxygenated conditions, (ii) a marked (early Smithian) to extreme (early Spathian) ammonoid/conodont diversification, (iii) a significant increase in the skeletal grain diversity and abundance and, (iv) peak carbon isotope values. A short interval of black, organic-rich shales precedes the onset of these two distinct carbonate events, which are both marked by a positive carbonate carbon isotope excursion known from other Tethyan marine localities. The comparison between the Early Triassic stratigraphic evolution of Tethyan outer platform paleoenvironments and platform/basin margin settings of the Nanpanjiang Basin suggests a causal connection between large fluctuations of the global carbon cycle, climate shifts, sea-level changes, oxygen availability in the oceans and the biological rediversification in the wake of the end-Permian biotic crisis. (C) 2008 Elsevier B.V. All rights reserved.