Storm deposits prior to the Triassic – Jurassic boundary in terrestrial Sichuan Basin, east Tethyan margin, China

Abstract:

The coal-bearing Xujiahe Formation is the upmost Triassic lithological units in Sichuan Basin, Southwest China, where located in east Neo-Tethyan margin during the late Triassic. More than 30 years core drill and outcrop investigation of Xujiahe Formation shows the storm deposits were widely distributed in Sichuan Basin, characterized by the irregular muddy gravel associated with hummocky/swaley cross stratification. In particular, the irregular muddy gravel commonly found in upmost Xujiahe Formation, just below the Triassic – Jurassic lithological boundary.

Recent two Xujiahe Formation outcrop were investigated from section Zilanba and section Xindianzi were further provided the new evidence of storm  in both north and south Sichuan Basin during the latest Triassic. In north Sichuan Basin Zilanba section, the in-situ wood trunks from paleosol surface (28.7m below Tr-J lithological boundary) at 5 member of Xujiahe Formation shows 6 of 9 trunk fossils lodging direction (NWW) is opposite to the paleocurrent direction (SSE)(data from gravel in 4th member of Xujiahe Formation). It is suggested that a strong southeasterlies prevail in the northern Sichuan Basin at that time. In south Sichuan Basin Xindianzi section, massive muddy gravels were found under the lithological Triassic – Jurassic boundary. These muddy gravels were poorly rounded or shaped with plastic deformation, shows no evidence of transport, similar with storm retention deposit.

The sedimentary interpretation of Xujiahe Formation is mainly composed of braided delta and lacustrine facies. However, the formation of a storm theoretically requires a water depth more than 60m and the temperature above 26.5℃, therefore, compare with the ocean environment, inland lakes such as Sichuan Basin are less likely to form tempestite due to the limited width and surface temperature during the latest Triassic.

Although terrestrial storm deposition is not well theorized. But on a global scale, the distinctive paleogeographic pattern of Pangea gave rise to a global scale monsoon system, the “megamonsoon”, with seasonal reversal of circulation and large-scale migration of the ITCZ (Inter-Tropical Convergence Zone, ITCZ) over the Tethys Ocean at a maximum latitude of 60º N/S in winter and summer. In fact, tempestites also occurred in year-round migration range of ITCZ during the Triassic – Jurassic transition, such as England, East Greenland, Italy, the United Arab Emirates, and South Tibet. Similar with above locates, storms in terrestrial Sichuan Basin should interpreted in the context of global surface wind background, i.e., ITCZ year-round migrate belt.

Meanwhile, in geological past, tempestites also occurred in T-OAE (Toarcian Oceanic Anoxic Event, T-OAE). Consider that There was no significant change in global paleogeographic pattern during the TJB (Triassic – Jurassic boundary) and T-OAE. In addition, by comparing the storm-deposition records of T-OAE associate with the present climate simulation, we propose that the storm-deposition records at the upmost of the Xujiahe Formation, which just below the Triassic Jurassic boundary in the Sichuan Basin, were probably forced by atmospheric CO₂ concentration arising.

Acknowledgements

This study is financially co-supported by the National Nature Science Foundation of China (41972120; 42172129) and the State Key Laboratory of Palaeobiology and Stratigraphy (Nanjing Institute of Geology and Palaeontology, CAS) (No. 173131).