LOWER TRIASSIC CALCIMICROBIAL FACIES OF EASTERN TETHYS
In the Nanpanjiang Basin the initial calcimicrobial framestone is 7-15 m thick and conformably overlies Upper Permian skeletal packstone with diverse open-marine fossils, including Palaeofusulina. The framestone contains interbeds of grainstone with thin-shelled bivalves, echinoderms, and brachiopods. The overlying strata are microgastropod packstone followed by thin-bedded lime mudstone. The P/T extinction event is interpreted to occur at the top of the Palaeofusulina packstone, coincident with the abrupt change to calcimicrobial framestone lacking Permian fossils. The biostratigraphic P/T boundary, defined by the first occurrence of Hindoudus parvus, occurs 65 cm higher within the base of the calcimicrobial framestone. The onset of the calcimicrobial framestone coincides with a negative shift of 3-4 in δ13Corg and δ13Ccarb and a drop in TOC. Isotopically depleted carbon persists through the calcimicrobial horizon. In the Nanpanjiang Basin, calcimicrobial facies recurs in the Dinerian and Smithian, again associated with large, negative carbon-isotopic excursions.
The widespread occurrence of the calcimicrobial facies, its exclusive occurrence in shallow-marine environments, and its coincidence with large shifts in carbon isotopes suggest that these calcimicrobial facies were controlled primarily by adverse environmental conditions or seawater chemistry, with a disaster-taxa response (reduced grazing pressure and competition for space) playing only a secondary role.