Formation of the Late Triassic western Sichuan foreland basin of the Qinling Orogenic Belt, SW China: Sedimentary and geochronological constraints from the Xujiahe Formation

Abstract

The Upper Triassic Xujiahe Formation is a succession of continental clastic rocks in the western Sichuan Basin, China. This formation has previously been regarded as deposited in a foreland basin linked to the NE-SW-striking Longmen Shan Thrust Belt. However, this hypothesis is questionable due to the presence of faults within the Longmen Shan Thrust Belt. Focusing on the Late Triassic proto-basin, we obtained new data from both outcrops and boreholes, which provide information on the sedimentary facies and provenance in the basin. The results show that the basin was originally striking E-W and that the depocentre migrated southward during the Late Triassic. Palaeocurrent indicators and detrital zircon age data reveal that the Upper Triassic strata were derived mainly from the Qinling Orogenic Belt, with a minor component of detritus derived from the South China Block. Detrital zircon U-Pb ages of ~228 Ma, from the bottom of the Xujiahe Formation, constrain the timing of onset of clastic deposition. Our results suggest that the Late Triassic western Sichuan Basin was a foreland basin at the southern front of the Qinling Orogenic Belt.