UPPER CRETACEOUS AND TERTIARY OF WESTERN SICHUAN PLATEAU

In recent years a number of fossils from the Upper Cretaceous and Tertiary of the western Sichuan Plateau and the eastern part of the Qinghai Xizang (Tibet) Plateau have been found during the Comprehensive Scientific Expedition to the Hengduan Mountain area. This discovery makes it possible to date and correlate sedimentary rocks of this area on the basis of fossil evidence. The Cretaceous outcrop is only found in Dege County, south of the Quershan Mountain. It consists mainly of red sandstones intercalated with grayish yellow, dark gray sandstone in its upper part. From the copper bearing sandstone in the upper part a palynoflora dominated by Schizaeosporites with a few pollen grains of Betulaceoipollenites, etc. indicates a Late Cretaceous age. A new name, the Cuo Ah Formation is suggested for the very thick bedded red sandstone and gravel of this area to replace the old name, the Relu Formation which has been proved to be of Early Tertiary age. Because there are much more red unfossili ferous sediments underlying the fossiliferous copper bearing sandstone, it is hard to exclude Early Cretaceous sediments in the lower part of the formation. The Lower Tertiary in this area is named the Relu Formation. Its type section is near Relu, a small village of Litang County, SW Sichuan Province. The section is composed of a series of red siltstone, sandstone at the lower part and gray, grayish yellow sandstone, siltstone at the upper part. Rich leaf fossils from the mid upper part dominated by Myrtophyllum, Palibinia suggest an Early to Middle Eocene age for this formation. The Maladun Formation, composed of gray, grayish yellow clastic rocks with coal beds, represents the Miocene of this area. The Miocene sediments with fossil evidence are restricted to the Hongtupo Basin of Songpan County on the western Sichuan Plateau. Both leaf and pollen fossils show a Middle to Late Miocene age for the Maladun Formation. The Pliocene and Pleistocene sediments crop out more extensively than the Miocene on the plateau. In the northern part of the plateau the Changtai Group represents the Pliocene to Early Pleistocene. Lithologically and palynologically, it consists of two parts: the lower part composed of variegated clastic sediments with abundant Ephedraceae, Ulmaceae and Chenopodiaceae pollen and the upper part consisting of gray, dark gray siltstone with rich Pinaceae, Compositae and Ericaceae pollen. In the southern part of the plateau, the Xigeda Formation is the representative of the Upper Pliocene and Pleistocene. This formation is of fluvial lacustrine sediments yielding varied fossils. The Xigeda Formation is mostly of Pleistocene age based on pollen fossils. In the Yanyuan Basin, the correspondent sediments are named the Yanyuan Group. Based on our pollen evidence the bottom of Yanyuan Group is lower than that of the Xigeda Formation.