End-Triassic storm deposits in the lacustrine Sichuan Basin and their driving mechanisms

Abstract

Storm deposits or tempestites are event sequences formed by storms, requiring at least a water temperature of 26.5°C. While inland lakes are unlikely to form storm deposits because of their limited width and water temperature. The Upper Triassic Xujiahe Formation in the Sichuan Basin is a set of coal-bearing, clastic sequences with dominant sedimentary facies varying from braided river delta to lacustrine settings, with storm deposits widely reported. In the Zilanba of Guanyuan area, in situ tree trunks on a palaeosol surface in Member V of the Xujiahe Formation provide new evidence of a storm event. Six fallen-down directions of nine in situ tree trunks were predominant in the NW direction, contrary to the palaeocurrent direction of the underlying strata, suggesting that the southeasterlies prevailed during the end-Triassic in the northern Sichuan Basin. Massive mud clasts were frequently recorded in sandstones of the Xujiahe Formation, as well as in the Xindianzi section. These mud clasts showed a rip-up or a plastic deformation with upside-down V-shapes, were capped on an erosional surface, showed no transport traces and were therefore interpreted as a storm lag deposit. The megamonsoonal climate prevailed during the Late Triassic, although the megamonsoons themselves could not generate a storm deposition in the Xujiahe Formation due to its low maximum surface wind speed. The driving mechanism for generating storm deposits in the Xujiahe Formation is suggested to be tropical cyclones over the Tethys Ocean moving eastward, further landfalling on the western margin of the Sichuan Basin. Statistics of storm events in the circum-Tethys region show a widespread storm surge in low latitudes during the end-Triassic. The storm deposits at the top of the Xujiahe Formation represent a sedimentary response to the end-Triassic hyperthermal event.