摘要
最近,在辽北—吉中下二台群、呼兰群中,发现了奥陶纪昌图动物群。昌图动物群与早寒武世清河镇动物群有密切的演化关系,化石得10属14种(其中3新属、9新种、2未定种)。昌图动物群的发现,对地层时代,奥陶纪生物地层,以及中朝扳块北侧构造地层地体地质演化研究,具有重要意义。
Metamorphosed intermediate-acid volcanic rocks-carbonate rocks named respectively the Xia'ertai and Hulan Groups are developed in northern Liaoning-central Jilin. Their age has been successively referred to asthe Precambrian (Proterozoic), Devonian, Ordovician-Early Silurian, Silurian-Devonian and Cambrian-Ordovician. In 1977, Early Silurian graptolites were found in the black slate from Mengjiagou, Jingjiatai, Yitong county, Jilin province, which was named the Taoshan Formation and since then it has become clear that the ages of the Xiaertai and Hulan Groups might be indirectly settled, but for lack of fossil evidence, no exact dating for the groups can be made, bence searching for fossils is the important task.During 1987—1990, the authors were greatly inspired by finding small shelly fossils from the so-called 'Liache Group-Proterozoic developed in the Qinghe River valley, Kaiyuan county, Liaoning province. Analysing the stratigraphy-tectonics and palaeogeography of the northern Liaomng and cental Jilin region, the authors realized the possibility for the preservation there of the microfossils; then by careful field work small shelly fossils were found from Guanshan, southeast Changtu county. Soon afterward more small shelly fossils (about 500 specimems) were found from 7 localities in Lishu and Panshi counties of Jilin province. All these new finds consitute the Changtu Fauna.The Changtu Fauna is composed of small shells and Gastropoda amounting to 10 genera (3 new), 14 species (9 new) and 2 indetermined species, listed as follows:Cylindricotheca inflata sp. nov., C. bifuraua sp. nov., C. corneola sp. nov., Guanshania annulata gen. et sp. nov., Hyolithellus qingheensis, H. flexa sp. nov., H. voluta sp. nov., Spirellus difformis sp. nov., Changtuella mirabilis gen. et sp. nov., Panshitubulus hulanensis hen. et sp. nov., Cylindrochites daizhuargziensis Archaeooides granulatus, Lophospira sp., Ectomaria sp.The age of the Changtu Fauna is Ordovician based on the following considerations: (1) there are a few common members of gastropod genera (Lophospira and Ectomaria); (2) the Xia'ertai Group lies below the Taoshan Formaticn which contains Early Silurian graptolites; (3) the Xia'ertai and Hulan Groups which contain the Changtu Fauna are composed of a sequence of intermediate-acid volcanic rocks and carbonate rocks but the Qinghezhen Group is of an ophiolite suite; (4) taking the Xifeng-.Kaiyuan fault as the boundary, north of the fault, the strike of the Xia'ertai and Hulan Groups is about NE 45°—50°, but that of the Qinghezhen Group is near-EW or NEE south of the fault; (5) two Rb-Sr ages of the Xiaertai Group are 456±36 Ma and 455.7±10 Ma, while that of the Qinghezhen Group is 850—570 Ma. it shows that the Qinghezhen Group is older than the Xiaertai Group, and the isotopic age of the Xia'ertai also indicates an Ordovician age. Summing up the above-mentioned, the Changtu Fauna is definitely of Ordovician age.The appearance of the Changtu Fauna on the north flank of the Sino-Korean Plate is an important event of biological evolution in the Ordovician, which should indicate a distinctive biogeographical region of the Palaeo-Asian ocean. The peculiar Changtu Fauna appears to be correspondingly related to the Ordovician assemblage containing brachiopoda, trilobites, bryozoans and conodonts from the Da Hinggan Mountains and Xiao Hinggan Mountains, but not symmetrically related to the latter. In the light of the characters and forms of the Changtu small shelly animals, specifically, the way of reproduction and the origin of the shells, they are very much like the smal shells of the Qinghezhen Group. It is believed that the Changtu Fauna might be developed from the Qinghezhen Fauna.The discovery of the Changtu Fauna is of geological significance in that. (1) based on fossil evidence the Xia'ertai and Hulan Groups may be correlated in age; (2) several genera of the Qinghezhen Fauna ranged up to the appearance of the Changtu Fauna with development of some new species; (3) the north flank of the Sino-Korean Plate may be subdivided into three successive metamorphic structural portions arranging from old in the south to young age in the north, namely, the Qinghezhen Group (Z-), marked by the Qinghezhen Fauna lying south, the Xia'ertai and Hulan Groups (O), represented by the Changtu Fauna in the Central, and the Taoshan Formation and Wangjiajie Formation (S-D), containing graptolites, brachiopods and corals in the north.
出处
《地质学报》
EI
CAS
CSCD
北大核心
1992年第2期182-192,T003,共12页
Acta Geologica Sinica
基金
地质矿产部行业基金
关键词
辽北-吉中
奥陶纪
昌图动物群
地层
Northern Liaoning-Central Jilin, Changtu Fauna, Ordovician
