PALEOENVIRONMENTAL CONTROLS ON EARLIEST TRIASSIC MICROBIALITES IN THE NANPANJIANG BASIN (SOUTH CHINA)

Growth of basal Early Triassic microbialite (BETM) in the Nanpanjiang Basin was directly controlled by block faulting tectonics whose large scale paleotopography was inherited from the Late Permian. Deposition of BETM was restricted to uplifted blocks whereas contemporaneous carbonaceous black shales are confined to adjacent troughs where a stratified water column formed. The BETM bloomed on a large (>12x10³ km²), NW-SE uplifted block centered on NW Guangxi province. The Permian Triassic Boundary (PTB) in these shallow water carbonate platform setting falls within the unconformity that separates the Heshan and Luolou formations. High diversity of skeletal grains, presence of lime mud and lack of hydrodynamic sedimentary structures indicate deposition of the upper part of the Heshan formation in a subtidal open-marine platform with low energy. The last preserved bed of the Heshan Fm. is often represented by a volcanic ash layer whose apatite and zircon geochemical signature and U-Pb ages indicate origination from a single eruption. This ash likely acted as a chemical shield against dissolution. Up to five event surfaces are associated with the unconformity. Two high energy bioclastic packstone-grainstone beds, containing abundant reworked Permian foraminifera indicate short regressive episodes that bracket the lowest and thin microbialite horizon. The contact between the basal calcarenite and this first microbialite episode is a corroded surface. Microfacies analysis supports submarine chemical dissolution along this corroded surface. Carbon isotopes display a 1‰ drop in the basal grainstone, indicating a substantial stratigraphic gap. However, high energy grainstone beds suggest that reworking and submarine dissolution closely alternated. An upward change from tabular to domal microbialite indicates slightly increasing accommodation space. The BETM is capped by a laterally widespread volcanogenic sandstone that occasionally contains clasts of microbialite. This marker bed indicates a main and abrupt tectonic drowning of the BETM superimposed to a longer term transgressive trend. Blanketing by clastic sediments led to the cessation of microbialite growth. A simultaneous change in the water chemistry is unlikely.