A UNIQUE MARL UNIT ACROSS THE PERMO-TRIASSIC BOUNDARY IN NORTHERN SICHUAN, CHINA

Detailed litho- and bio-stratigraphy was analyzed for the fossiliferous Upper Permian and Lower Triassic sequence at Chaotian in northern Sichuan (China). The study section consists of fine-grained limestone and mudstone of deep basinal facies, retaining the Permo-Triassic boundary (PTB) horizon clearly defined by conodonts. A sharp biodiversity decline is detected for ammonoids, conodonts, and radiolarians across the PTB. The boundary interval is characterized by a unique marl unit (1.4 m thick) bounded by 1) the extinction horizon of major Permian fauna (event horizon) at the bottom, and 2) the first appearance datum (FAD) of Hindeodus parvus at the top. This boundary marl unit is free from burrowing and barren of mega-/micro-fossils except tiny ammonoids of the Permian holdover taxa. The marl unit represents a period of the strongest environmental stress appeared at the PTB. The uppermost 3.5 m of the Permian including the unique marl contains 12 beds of cm-scale, fine-grained tuff with euhedral quartz, plagioclase, apatite, and zircon. These observations suggest that violent rhyo-dacitic volcanisms may have triggered the PTB environmental turmoil and extinction. The successful survival of the Permian radiolarians into the Early Triassic lately confirmed in New Zealand positively denies the putative link between the Bedout (NW Australia) impact and the PTB extinction.