震旦系质疑及有关上元古界地层问题

1975年全国震旦系讨论会,根据近年来地层、古生物和同位素年龄的研究成果,认为南方震旦系和北方震旦系是上、下关系;并暂定南方震旦系(三峡层型剖面)称震旦系,北方震旦系(蓟县层型剖面)另建长城系、蓟县系、青白口系,与上述震旦系合称震旦亚界,归入上元古界。1976年出版的中华人民共和国地质图和亚洲地质图的元古代地层图例,即根据此方案编制的。这个方案虽是过渡性的,它对确定我国震旦亚界的地层层序,推动前寒武纪地层的深入研究,还是起了一定的积极作用。 不容讳言,对上述方案,有许多地质工作者曾表示不便反对,也不敢赞同。希望多做些工作,以求在实践中逐步统一认识,解决存在的问题。

A QUESTION ON THE SINIAN SYSTEM AND RELATED PROBLEMS OF UPPER PROTEROZOIC ERATHEM IN CHINA

In 1975,All-China Sinian Stratigraphic Symposium makes it clear that the sinian system (the Yangtze Gorge section) in South China belongs to the up-and-down relationship with the Sinian System (the Chihsien section) in north China,and recommends the term "Sinian Suberathem" to unify and to distinguish the classification of the upper proterozoie erathem,as given in the new Geological Map of China (1: 4,000,000,1977) from then on In most cases,the Sinian System is defined by a sharp unconformity produced from the Chinning Movement at the base (the main tectonic phase of the Yangtze Orogeny about 800 m.y.ago),and is subdivided into two series on the basis of the inferior unconformity caused by the Chengkiang Movement (the end of the Yangtze Orogeny about 700 m.y.ago).It is well known that the Yangtze paraplatform was completely consolidated after the significant Chengking Movement,and the genuine blanket of it began to form with a tillite sequence of worldwide significance (Huang,1978).In other word,the lower boundary of Sinian System is drawn at the bottom of the mollasse formation of the late synorogenic stage,and the subdivision boundary of its upper series is set at the bottom of the glacigenous beds of platform-type sediments.It is found that the classification and subdivision of the Sinian System for the time being are not conformed to the basic principle and concept of Precambrian time,as proposed by Stockwell (1961,1973),based on the major magmatic-tectonic cycles or orogenies.It is stated explicitly that the "boundaries between time units of eon,era &; subera rank are......drawn at the end of orogeny.They are natural boundariesthat set apart the very important events in Precambrian history,&; the units are not defined by isotopic age but are based on rock and geological relationships in type region (Stockwell,1973).In addition,the time range of the Sinian System is estimated as about 300 m.y.at least,which is not favorable for establishing a chronostratigraphic unit of system rank in the uppermost Precambrian,except that the Sinian upper series is retained for the so-called Eocambrian as suggested by Huang (1978).In the contrary,in the past decades,the universally accepted Sinian System or the upper proteroizoic sediments in international literature almost always refers to the Yanshan region with the standard section in Chihsien County near Beijing It comprises sediments ranging in age from 700-1800 m.y.ago to be equal to an acceptable time unit of the rank of era.As stated above,the term "Sinian suberathem" may be here replaced by the "Sinian erathem",and can be received as an equal with the Riphean erathem of SSSE.