Detailed Stratigraphy across the Permo-Triassic Boundary at Chaotian in Northern Sichuan, China

Abstract

To reveal environmental changes across the Permo-Triassic boundary (PTB), detailed lithostratigraphy was analyzed of the PTB interval at Chaotian in northern Sichuan, China. The study interval is composed mainly of shelf carbonates that are lithologically divided into 7 units, i.e., Units A to G in ascending order. Units A-E (8.5 m) correspond to the uppermost Permian, and Units F-G (3.5 m) to the lowermost Triassic. On the basis of a field study and microscopic observations of more than 200 thin sections, the main extinction horizon is recognized at the Unit DIE boundary. Various fossils, such as ammonoid, brachiopod, bivalve, radiolaria, and conodont occur abundantly in Units A-D, while they are absent (or nearly absent) in Units E-G. Radiolarians in particular, show a clear contrasting mode of occurrence : abundant in Unit D to its top, while barren in Unit E and above. The Unit D/E boundary is thus identified as the event PTB horizon with major extinction. The PTB extinction terminated various Late Permian organisms, particularly radiolarians, which are representative planktons in open ocean, suggesting a sharp decline in total marine productivity. On the other hand, the Griesbachian (Early Triassic) index conodont Hindeodus parvus first appears at the base of Unit F, thus the Unit E/F boundary is recognized as the biostratigraphically-defined PTB horizon. The marl of Unit E represents the interval of strong environmental stress that appeared around the PTB. The population of the latest Permian radiolarians decreased remarkably across the Unit B/C boundary, while their mean shell size stayed constant, and even increased rapidly within Unit D. As large-shelled radiolarians often predominate in the cool waters of modern oceans, the radiolarian size increase in Unit D may indicate that the depositional site may have been invaded by a cooler water mass at the end of Permian period.