Microfacies and multi-isotope records of Anisian sequences from the Upper Yangtze Block: possible responses to tectonics and climate-driven relative sea-level change
The Middle Triassic epoch is the key to understanding the environment changes after the Permian–Triassic mass extinction. The Anisian Badong Formation is a carbonate and siliciclastic succession deposited on the Yangtze Platform, Upper Yangtze Block. In this paper, we use facies analysis combined with sequence stratigraphy of the Badong Formation in the Nashuixi section, Yangtze Block, to discuss the Anisian relative sea-level changes. Microfacies analyses were used to define ten microfacies in four depositional facies zones (tidal flat, restricted platform, restricted to open platform, and open platform) indicating an epeiric platform depositional environment. The lowstand, transgressive, and highstand systems tracts and two sequences were identified. Sequence stratigraphy interpretations indicate that relative sea-level at Nashuixi experienced two rises in the Middle Anisian, which is correlated with those in the Yangtze Block, confirming that sea-level changes during the Anisian were of eustatic origin. However, a distinct regression at Nashuixi occurred in the late Bithynian to IIIyrian while the global sea-level rose, which is likely to be caused by interference from regional tectonics during IIIyrian. The carbon and oxygen isotopic shifts in the Nashuixi section exhibit two negative-values stage in the time near end of Aegean and early Pelsonian, which is well correlated with the global typical sections, indicating two middle Anisian warming events. Our study indicated that the relative sea-level at Nashuixi was controlled by global sea-level change and regional tectonics (Indosinian Orogeny), and the C and O isotopes records respond to global climate change.