内蒙千里山地区中、上渐新统的发现及其意义

在这篇文章中,根据所发现的脊椎动物化石和岩性,将内蒙伊克昭盟千里山地区陆相第三纪地层分为四部分。并建立了两个组:上部上渐新统伊克布拉格组和中部中渐新统乌兰布拉格组。后者被分为上、下两段。在亚洲,一直未能发现完整的,具有丰富脊椎动物化石的中、晚渐新世地层的剖面。千里山地区渐新统连续剖面的发现,对亚洲中、晚渐新世地层的层序的建立和对比提供了可靠的依据。

STRATIGRAPHY OF THE UPPER AND MIDDLE OLIGOCENE OF QIANLISHAN DISTRICT,NEI MONGOL (INNER MONGOLIA)

Qianlishan hills represent the northern part of Zhouzishan, which stretches along the eastern bank of the Yellow River, and occupy the western bound of Yih Ju Meng. Nei Mongol. The exposure of a series of thick continental sediments ("red bed") of over 300 m. extends for about 13 square Km. in this region. They formed monoclinal strata dipping NW-NWW and cut by a number of faults.They had not been subdivided and no determinable fossil had been found there until 1978, when the Fourth Regional Syrveying Brigade of Ningxia Huizu Zizhiqu discovered some mammalian fossils. During 1978 and 1979 a field party of IVPP was sent to cooperate with the Brigade three times for further investgation. As a result, a considerble amount of additional fossils were collected and a new geological mapping was made.1. The Tertiary sediments of the region can be divided into four units:(1) The basal unit: Early Oligocene (?) in age, overlying uncomformably on Cretaceous, Early Palaeozoic and even older bed-rocks. Thickness about 25 m.(2) Wulanbulage Formation: Middle Oligocene, covering uncomformably on the basal unit. Thickness over 204 m.(3) Yikebulage Formation: Upper Oligocene, lying conformably on the Wulanbulage Formation. Thickness over 58 m.(4) The upper unit: Early Miocene (?) in age, overlying uncomformably on the Yikebulage Formation, Thickness over 77 m.2. The age of the Wulanbulage Formation:Among the rich mammalian fossils collected from the Formation, 10 species representing 9 genera were found in Hsanda Gol Formation earlier, they are: Desmatolagus gobiensis, Karakoromys decessus, Plesiosminthus tangingoli, Cricetops dormitor, Cyclomylus lohensis, C. minutus, Tataromys sigmodon, Schizotherium sp. Eumeryx sp., Tsaganomys sp. etc., of which Desmatolagus gobiensis, Cyclomylus lohensis, C. minutus, Cricetops dormitor are usually considered as characteristic of Hsanda Gol Formation. The above list shows great similarity between the Hsanda Gol and the wulanbulage faunas. As the Middle Oligocene age of the Hsanda Gol Formation is affirmed radiometrically, the Wulanbulage Formation may well be of Middle Oligocenc as well. In China depodits of Middle Oligocene can also be found at Qingshiuyin (Tsingshiyin) of Ningxia and Shuidonggo (Chouitong-kou), Sanshenggong (Saint-Jacques) and Houljin of Nei Mongol.Judging from the sedimentary characters and the contents of the fauna, the Wulanbulage Formation may be further subdivided into two members:The lower member is made of dark red-purple mudstone and clayish siltstone with orange sandstone intercalation. The upper part of the member is definitely of Middle Oligocene in age. Apart from Desmatolagus gobiensis, a quite characteristic fossil of Middle Oligocene of Central Asia, from the layers 11 and 12 we have found also Cadurcodon ardynensis and Lophiomeryx gobiae, which have been met so far merely in Early Oligocene Ardyn Obo Formation, At present it is rather difficult to give an exact geological dating. The part of the sediments under the layer 10 may be of an earlier stage of the Middle Oligocene, or even Early Oligocene.The upper member consists of greyish orange sandstone with a few gravels. Thickness about 72 m. The mammalian fossils contain representatives of Middle Oligocene and some of Late Oligocene or even later ages as well, such as Plesiosminthus asiae-centralis, Paraceratherium and Aprotodon. Though the species of latter two genera are different from that of genera known before. There is every reason to suppose that the upper member may represent a later stage of the Middle Oligocene.3. The age of Yikebulage FormationThe Formation consists of brownish red sandy and silty mudstone, light orange sandstone, sandy gravel intercalated with layers of brownish red mudstone. Thickness over 58 m.Except Tachyoryctoides obrutchewi and Tataromys deflexus found in Middle Oligocene and Tachyoryctoides kokonerensis and Tataromys suni found in the Early Miocene Xie-Jia Fauna (Li and Qui, 1980), most of mammalian fossils collected from the Yikebulage Formation were also found in Taben buluk and Shih-ehiang-tzu-ku, such as Amphechinus kansuensis, Sinolagomys kansuensis, S. major, S. gracilis, Tachyoryctoides obrutchewi, Plesiosminthus tangingoli, P. parvulus etc. The faunal simitarity shows that the Yikebulage Formation may be of Late Oligocene. The fact that the Wulanbulage Formation precedes the Yikebulage Formation stratigraphically demostrated clearly that the age of Taben buluk and Shih-chiang-tzu-ku may be later than that of Hsanda Gol Formation.The point expressed by Flerov and al. (1974) that there wouldn't be Upper Oligocene in China and Mongolian Republic at all and all the Oligocene localities, (even including a Middle Eocene locality ?!) would be referred to Middle Oligocene seems to us groundless.