Abstract: On the basis of sedimentological research, we study clay minerals by X-ray diffraction and trace element analysis to infer
paleosalinity of the lacustrine environment in which the Xujiahe Formation was deposited in Sichuan Basin. Paleosalinity was
determined using the Couch formula and the paleosalinity values range from 4.8 to 19.7 per thousand, with an average value of 12.8 per
thousand. The paleosalinity of different layers in the Xujiahe Formation changed regularly, and there is a gradual decrease trend from
the second to the sixth member, i.e., from saline to brackish water. The increase in salinity is favorable for the conversion of
montmorillonite to illite, suggesting that clay mineral of the second member of the Xujiahe Formation is mainly illite. The illite content
decreases gradually from the fourth member to the sixth member, indicating thatclay mineral in fourth member and sixth member is
mainly chlorites. Palaeosalinity also has a certain effect on the chlorite composition. with the increase of salinity, Si4+ , Fe2+/Mg2++Fe2+
content decreases and AlIV content increases in the chlorite composition. Combined with tectonic movement, the change in paleosalinity
of the Xujiahe Formation was probably caused by the Indosinian tectonism, the southern section of Longmen mountain uplift during the
third member period, the Sichuan Basin and the sea lost contact and became a separate deposition system. The sedimentary facies of
the second and the third member of the Xujiahe formation should be deposited in a terrestrial environment. However, it was
superimposed by tide. The fourth, fifth and sixth member of the Xujiahe Formation should belong to the normal continental sedimentary
environment.

Key words: Sichuan Basin, Upper Triassic Xujiahe Formation, paleosalinity, clay mineral, Couch formula