论震旦系划分与对比问题

1922年葛利普根据我国早期地质工作者的资料和要求,把“震旦系”作为一个时间地层单位古生界的第一个系提出来,而其含义过于笼统而广泛,可以装进大量的、时间范围很长的地层。1924年李四光在长江峡东区确定了一个厚不过800米,时间范围较小的“震旦系”。1934年高振西等在河北蓟县又确定了一个厚达10000米,时间长达10多亿年的大“震旦系”。

PROBLEMS OF CLASSIFICATION AND CORRELATION OF THE SINIAN SYSTEM

There exist a number of problems in the Sinian system research, especially the stratigraphic classification and correlation, which need to be inquired and discussed deeply and repeatedly. The present paper lays emphasis on the argument concerning the scope, the diverse facies, the principle of classification, and the classification and correlation of some representative lithostratigraphic units of the Sinian system.In southern China, Sinian and Cambrian rocks are used to connect closely and transitional sections of which are well presented in several areas. The Sinian system is defined by a sharp unconformity left by the Chinning orogeny at the base. There occurs in it the glacigenous sediments of worldwide significance. And its time range is estimated as ca. 300 m.y. So it is favorable for establishing a ehronostratigraphic unit of system rank in the uppermost Preeambrian.Different lithofacies types of platform, geosyncline, and the intermediate transition are recognized from the Sinian system, which were controlled by the paleotectonic and paleogeographic background thus exhibit different contents and features. For the time being, based upon the analysis of the extensive unconformity resulted from crustal movement or tectono-magmatic cycle cooperating with isotopic age determination the subdivision of the Precambrian major ehronostratigraphic units would be the only workable measure. In determining the Sinian-Cambrian boundary, some visible marks of lithological change should be taken into account in addition to the prevailing method basing upon the first appeared shelly fossil zone. And in setting up the lower boundary on the basis of unconformity produced from the Chinning movement, one should strictly distinguish the regional unconformity from those of local existence which are liable to fade away in a short distance, because of not only a single break occurred near the basal horizon of the Sinian sequence. According to some new isotopic dating data, the age of the lower limit of the Sinian system is inferred at about 900 m.y.It is quite appropriate to subdivide the Sinian system into two series on the basis of the unconformity caused by the Chengkiang movement and to set the division boundary at the bottom of the glacigenous beds. A three-fold classification of series, i.e.singling out the glacigenous and the related rock association——commencing with atillite sequence, followed by an interglacial succession and terminated by a recurrence of tillite——as an independent middle series would be another scheme worthy of consideration. It is demonstrated that in the Sinian system there exists a large-scale late Precambrian ice age, named Nanhua, which includes two major sub-ages, Changan the former and Nanto the later, with an interglaeial age named Daitungpo/Fulu between. The glacigenous accumulations are composed of diverse types of terrestrial, fluvio-glacial and glacial-marine facies. The correct handling of a series of problems, i.e. the demarcation of the glaciation periods, the classification of the glacial deposits, the understanding of the characteristics of the glacial and interglacial sediments and the correlation of the two’ kinds of sediments, would be a key to solve lots of correlation problems of the Sinian system.The Luoquan formation in western Henan is composed of two members, the lower one is equivalent to Nantou tillite and the upper may be correlated with Doushantou formation of the eastern Yangzi gorge. In vast northern China, Upper Sinian rocks are sparsely distributed along a wide belt from western Henan-southern Shansi, further northwestward to the Shaansi, Gansu, Ningxia, Qinghai and Xinjiang. Whether the Sinian rocks are persisted in some other places at the extensive east part of northern China remains to be settled through further investigation.The so-called Xuefeng movement of Hunan is identified with Chengkiang movement, while the Wuling or Dongan disturbance is recognized as equivalence of the Chenning tectonism. The Banxi group of western Hunan is known to be actually a contemporanous.