Early Triassic (Griesbachian) Deposits of High-Energy Events at the Northwestern Upper Yangtze Region, China

Abstract

Following the latest Permian mass extinction (LPME), the ocean experienced turbulent conditions due to elevated temperatures during the Griesbachian (Early Triassic). These adverse conditions resulted in a prolonged delay in biotic recovery. However, the current understanding of the sedimentological responses of the ocean at that time to these adverse conditions remains incomplete. The Griesbachian harsh ocean conditions led to the formation of abnormal carbonate rocks (known as anachronistic facies) all around the world. These facies were influenced by a combination of environmental conditions, biotic factors, and actualistic sedimentological processes (e.g., waves and currents, sedimentation). However, the role of actualistic sedimentological processes in forming anachronistic facies during the Griesbachian has been underemphasized in the existing studies. In this research, we examine calcirudite beds, such as flat-pebble conglomerates, from Member 1 of the Feixianguan Formation across multiple sections, including Shangsi, Yudongzi, Dagouli, and Jianfeng in the Upper Yangtze Region. Our analysis is grounded in field investigations, thin-section observations, and the study of triggering mechanisms and formation processes. We identified and described five calcirudite beds (S1–S5) in the Shangsi section, two beds (D1–D2) in the Dagouli section, one at Jianfeng (J1), and one (Y1) in the Yudongzi section. Bed S1 features matrix-supported flat pebbles with a normal grading, interpreted as the result of a debris flow mass-transport process. Beds S2, S3, D1, and J1 contain large mud rip-up clasts, ooids, and bioclasts, suggesting a potential link with tsunami backwash. Beds S4, D2, and Y1 display hummocky cross-stratification and ‘chrysanthemum-shaped’ flat pebbles, indicating storm influence. The presence of vermicular limestones in Bed S5 suggests harsh marine environmental conditions. Collectively, this evidence suggests that high-energy oceanic conditions, including tsunamis and frequent storms, potentially influenced Early Triassic biotic recovery.