碧口群内冰川沉积及冰期的划分与对比

碧口群内前人曾怀疑为冰碛岩的一套岩石,已取得了大量冰川证据,肯定了冰川作用的存在。其时代属早震旦世。据沉积特征、层位及上下接触关系等,将其划为长安、南沱两个冰期,南沱冰期又进一步划分为两个亚冰期,并分别与国内外有关地区作了对比。

DIVISION AND CORRELATION OF GLACIAL DEPOSITS OF BIKOU GROUP AND GLACIAL PERIODS

Bikou Group is distributed on the northwestern margin of Yangtze Plate at the conjunction zone of Shaanxi-Gansu-Sichuan provinces.Previous studies suspected that it is the glacial deposits.The authors recently obtained a lot of glacial evidence,studied the distribution,homzons and depositional characters of the tillites,and makes a division and correlation of the glacial periods. Tillites are mainly distributed on the northern side and periphery of Bikou Group,especially in Gansu Province.Their horizons are located in the upper and lower driftsheets of Yangtianba and Nantuo Groups of Lower Sinian System.A large number of micro-palaeophytes was found in every group of tillitebearing beds of Sinian System.The authors measured the age of overlying pre-Sinian System to be 800—1000 Ma.The overlying beds of Sinian System are Lower Cambrian Series,in which fossils were found.The authors measured its Rb-Sr age to be 561±20 Ma.According to the micro-palaeophytes,isotopic ages and the time of overlying and underlying beds,the horizon of tillites should be Lower Sinian Series. The tillites are roughly of three types:mastive,lamellar and bedded gravel-bearing tillires. Among them massive tillite is dominant,without bedding and sorting of gravels.The gravels nonuniformly scatter in the cements.The gravels and cements change from place to place.In some places the cements have schistositized by the effect of epigenetic processes and changed into the gravel-bearing phyllites.In the tillites basal moraines of continental glacier are dominant,part of them probably is periglacial sea and ice-sea deposits and aquatillites in which laminated tillite has lamellar structures,containing dropstones.The tillite occurs mainly at the top of massive tillite or within it,generally of small thickness.It represents glacial-lake deposits or preglacial fluvioglacial deposits.Bedded tillite represents marine sedimentary rocks containing gravels or rock-fragments and dropstones and is characterized by floating ice sediments or ice rafting sediments.It mainly developed in the lower part of Yangtianba Group,within the sandstones and slates. In the above-mentioned tillites numerous traces of glaciation and ablation were frequently found,such as striated pebbles,compresso-shear stones,drift boulders,dropstones and gravels of various shapes. This set of glacial deposits is extensively distributed in China and has global significance. According to sedimentary characteristcs,metanorphic grade and the relations with overlying and underlying beds,the tillites in Bikou Group formed in Changan and Nantuo glacial periods.The Nantuo glacial period can also distinguished into two stages.On the basis of stratigraphic horizons,micro-paleoplants and sedimentary characteristics the Changan glacial period can be correlated with that in Hunnan-Guizhou-Guangxi region and with Beiyixi glacial period in Xinjiang region,and can also be correlated with Stetrau till sheets of Wongbeilata Group in Adelaide region,Australia.Nantuo glacial period can be correlated with that in North Jiangxi and Hunan-Guizhou-Guangxi region,and with Teruiaiken glacial period in Xingjiang region,and can also be correlated with Walanger Group in Scandinavia of northwest Europe.