LOWER TRIASSIC CALCIMICROBIAL FACIES OF EASTERN TETHYS

Distinctive calcimicrobial mounds and biostromes composed of micritic, globular, and tufted fossils similar to Renalcis developed in shallow-marine carbonate platforms across a vast area of the eastern Tethys and Panthalassa immediately following the end-Permian extinction. The calcimicrobial facies occurs within the Hindeodus parvus zone in the Nanpanjiang and Sichuan basins of southwest China and in southern Japan.

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.