鄂尔多斯中、上奥陶统沉积环境的生物标志

THE MARK ON THE ECOLOGY OF SEDIMENTARIAL ENVIRONMENT IN MIDDLE AND UPPER ORDOVCIAN AT ORDOS BASIN

鄂尔多斯中、上奥陶统的生物带序列划分以与陇县龙门洞的笔石带和耀县桃曲坡牙形石带的对比为基础。多门类化石表明,陇县平凉组顶部C.spiniferus带与北美的相似,已进入卡拉多克末期,为背锅山组定为阿什及期沉积提供了更充分的证据。准确对比了本区7个区带的生物层序,并划分出9个腕足生态群落、5个沉积相和5个遗迹组合。在弄清本区岩相变化的基础上对各岩石地层组作了对比和厘定,并分析了本区海平面升降事件及生物地理区系特征。根据生态群落与海水深度的对应模式,本文尝试作出本区兰维尔期至阿什及期5幅海水等深线图。根据本区南缘从含BA2-3Rhynchotrema-Eospirigerina群落的浅水区-含 BA4-5 Foliome-na群落的半深水区一含Nereites遗迹组合、海绵骨针、放射虫、火山碎屑、滑塌灰岩及深色泥岩的深海区发育良好的斜坡台阶,作出了属于华北地台鄂尔多斯南缘的大陆边缘模式。

Ordos Basin, located in the west part of North China platform, is bounded on the south by Qinling syncline, on the west by Helanshan sycline and on the notht by North syncline, including the eastern part of Gansu Province, the northern and central parts of Shaanxi Province, the mainly part of Ningxia and the south-west part of Inner Mongolia in geographical distribution. There are Mesozoic and Cenozoic strata in the centry and the Paleozoic strata formed the margin around the Ordos Basin. Five intact and important stratigraphical sections are discovered in margins of Ordos basin as follows. The Longmendong section on the western margin in Longxian County, the Gongwusu section at Zhuozishan on northwestern margin, the Dashetai section on north margin in Inner Mongolia, the Taoqupo section on west part of southern margin in Yaoxian County and the Yaoshan section on east part of southern margin in Pucheng County. The Biozones sequence of middle and upper Ordovician in Ordos has been established based on the studing of Longmendong section(graptolites fasies) and Taoqupo section (shell facies ).The Pingliang Formation in Longmendong section can be subdivided into five graptolitic zones according to Fu Lipu (1975, 1979) studied listed in ascending order as follows. 5.Climacograptus spiniferus biozone 4.Climacograptus longxianensis biozone 3.Climacograptus peltifer biozone 2.Nemagraptus gracilis biozone 1.Glyptograptus teretiusculus biozone The Taoqupo section can also be subdivided into four conodont zones according to An Tai-xiang and others(1985, 1990)given in ascending order as follows: Taoqupo Formation Yaoxianognathus yaoxianensis biozone Yaoxianognathus neimongguensis biozone Jinghe Formation Tasmanognathus gracils-T.multidentatus biozone Tasmanognathus shichuanheensis biozone It is interesting to note that the western margin of the Ordos Basin is the area where the Pacific type and Atlantic type biogeographical region meet. The three of the South China-western Europa type conodonts Panderinus anserinus and the North China-Siberia type conodonts Tasmanognathus, Microcoelodus, Erismodus and the Graptolite Nemagraptus gracilis blend together in the Pingliang section. This phenomenon gives a good chance to make a precise correlation between them. The Lithofacies of Sandaogou Formation at Baijiatan, Longxian County bear resemblance to those of the Sandaugou Formation in Liangwucheng, Qianyang County. The two places take 30 km, but the former contains Atlantic type conodonts Pygodus anserinus faunas while the latter contains North China type brachiopodes Eospirigerina jingheensis fauna. This species is more original than the typical Eospirigerina by having only 2 to 3 spiralia in its inner and has been found in Tasmanognathus shichuanheensis biozone of Taoqupo section in Yaoxian County, so it is lower than the horizone of typical Eospirigerina(latest Caradocian to Ashgilian). It is more reasonable to give the interpretation to the phenomenon that it is relation to the temperature of sea water concerning astronomical climatic zones and not to be relation to environmental control and biogeotectonic control. The appearance of brachiopoda Foliomena, conodonts Yaoxianognathus neimengguensis, Protopanderodus insculptus, tabulata Syringopora, Deplopora, graptolites Climacograptus spiniferus in the top of Pingliang Formation in Longxian County indicates that this formation had been steped into the range of Ashgillian or latest Caradocian. This situation resembles to ones in North American that the largely part of C.spiniferus biozone belong to Cincinatin stage of Upper Ordovician(Rive 1976, Mitchell 1991),correspondingly, an age of Beiguoshan Formation overlying Pingliang Formation belonging to Ashgillian is indisputable. Having the discovers of conodonts Spadognathus dolboricus, brachiopoda Ovalospira, Eospirigerina, Schizophorella, Parastrophinella, tabulata Agetolites, Plasmoporella, Neosibirililes from the middle and lower parts of Beiguoshan Formation may be corresponding to Dolbor Formation in Sibiria while the upper part containing Yaoxianognathus yaoxianensis can also make correlation with Taoqupo Formation in Taoqupo section. The upper part of Taoqupo Formation containing Orthograptus fastigatus is equal to the O.fastigatus biozone in Arctic Islands of Canada ( Melchin 1987 ) or Dicellograptus anceps biozone which also contains this species in Britain. On trace fossils, neumerous trace fossils are found in investigation region, they can be divided into five assemblage as follow: 1.Psiloniehnus assemblage. The predominant trace fossils are Psilonichnus which are small, vertical, unbranched, filled tubes with circular in cross section. A few of Thalassinoides, Arenicolites also occur in the assemblage. This assemblage may be equal to Glossifungites facies which occurs in Semiconsolidate but unlithified deposits or in hard ground environments. 2.Taenidlum-Phycodes assemblage: This assemblage is characterized by having abounding Taenidium and acompaning with Chondrites, Volkichnium and Asterichuus, Glockeria, Sublorenzinia, Granularia, Phycodes. 3.This assemblage is characterized by abounding with Volkichium and associated with some Asterichuns, Glockeria, Sublorenzinia, Granularia, Phycodes. All trace fossils in both assemblages are Fodinichinia and lack Domichnia and Pascichnia. Li Ri-hui(1990)concluted that both assemblages represent a quiet water, insufficiently oxogenated environment resembling Zoophycus facies. 4.Circulichnis assemblage, This assemblage consists mainly of Circulichnis montanus, C.sinensis associated with a few Granularia, Volkichnium, Pycodes, Zoophycos, Yangziichnus, Parascalarituba. 5.Squamodictyon assemblage, This assemblage is marked by the occuring in a large number of types of irregular network Squamodictyon. Neorodictyon acompanying with Paleodictyon, Megagrapton, Helminthopsis, Lobichnus. All the trace fossils listed above are the important elements of the Nercites facies occuring commonly in Flysh facies. Formerly, the lithofacies paleogeography map only shown the isoline of sedimentary thickness and the distribution of sea and land, this is not true geography map strictly speaking. The authors made some isoline map from Llanvern to Ashgillian to show the water deepth after studing the sedimentary environments, the changes of water deepth based on the date of the mark on the ecology such as benthic assemblage, biofacies and ichnofacies which distribut on the margin of Ordos basin in a total of 55 places. We believe that it is possible to make a paleogeography map with isoline of water deepth by using the way of ecostratigraphy. Judging from the general aspects of the well developed class on the slope on southern margin of Ordos, of the range of the benthic assemblages strached from shallow water BA2-3 Rhynchotrema-Eospirigerina community to the bathy BA4-5 Foliomena community, of the colluvial breccia, the distribution of the ichnofacies and the radiolarian chert, a continental margin developed on the southern margin of North China platform can be suggested.