川西高原的上白垩统及第三系

UPPER CRETACEOUS AND TERTIARY OF WESTERN SICHUAN PLATEAU

川西高原的上白垩统及第三系属于山间盆地型陆相沉积 ,红层发育 ,化石稀少 ,地层划分、对比困难。本地区有化石证据的白垩纪地层仅见于雀儿山南麓的德格县 ,错阿组为一套红色含砾砂岩和粉、细砂岩 ,上部夹灰黄、灰黑色钙质粉砂岩及含铜砂岩 ,深灰色含铜砂岩中产有以希指蕨孢居统治地位的孢粉植物群 ,地质时代为晚白垩世中期。本区古近系红层发育 ,理塘县热鲁村附近的热鲁群下部为红色岩系 ,中上部灰黄、灰色粉砂岩内产以 Pali-binia和桃金娘科为主的植物群 ,时代为早—中始新世。目前可以肯定的中新统仅见于松潘县红土坡一带 ,为一套含煤的灰、灰黄色碎屑岩系 ,其中所含的叶化石和孢子花粉表明其时代为中新世。上新统至更新统分布较广 ,北部以白玉县昌台地区较为典型 ,称昌台群。无论从化石或岩性看昌台群均可分为上、下两套 :下部以杂色碎屑岩为主 ,夹砾岩和玄武岩 ,其中的孢粉组合以麻黄及榆科、藜科花粉丰富为特征 ;上部以含褐煤的暗色碎屑岩为主 ,除丰富的植物化石外 ,孢子花粉为松科、菊科、杜鹃科等居优势的组合 ,昌台群的地质时代为上新世到到早更新世。南部的上新统—更新统以昔格达组为代表 ,主要是河湖相暗色岩系 ,局部地区有褐煤夹层 ,产植物叶化石和孢子花粉 ,时代主要属于更新世 。

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.