关于中国元古宙地质年代划分几个问题的讨论

本文简略回顾了我国元古宙划分的进展和问题。在我国地质文献中,元古宙通常以2.5Ga、1．8Ga、1.0Ga．和0．57Ga为年代界线划分为早、中和晚元古代。本文建议以古、中和新元古代代替早、中和晚元古代的命名。古元古代介于2．5Ga至1．8Ga之间,可包含三个纪,内部年代界线置于2．3Ga和2．05Ga。文中未对三个纪的名称和代表性地层单元提出明确的建议。中元古代通常包含长城纪和蓟县纪,纪的界线置于1.4Ga,而该代的顶部时限置于1．0Ga。然而,中元古代内位于1．6Ga、1．4Ga和1．ZGa均有明确的地层界线,所以有可能进一步划分为四个纪。新元古代包含青白口纪和震旦纪,以0．8Ga作为它们的分界,但对于震旦纪的时限存在着明显的分歧,其底界年龄有置于0．9Ga、0．85Ga和O．8Ga等不同意见。有些地质学家建议震旦纪可再分为二个纪,亦有以冰碛层的底或顶为界的不同划分方法,因而内部界线分别置于0．7Ga或0．65Ga。本文作者倾向以国际上建议的0．545Ga代替我国现行使用的0．57Ga,作为震旦纪与寒武纪的年代界线。

A REVIEW ON SUBDIVISION OF PROTEROZOIC EON IN CHINA

Aims of this paper are to review some advances and problems on subdivision ofProterozoic Eon in China. In Chinese literatures the Proterozoic is generally subdivided into theEarly, Middle and Late Proterozoic Eras, and 2. 5 Ga, 1. 8 Ga, 1. O Ga and O. 57 Ga are selectedas boundary times between eras. A new time subdivision of Proterozoic of China is proposed in thepaper. The Proterozoic Eon is subdivided into the Paleoproterozoic, Mesoproterozoic andNeoproterozoic Eras in stead of the Early, Middle and Late Proterozoic ones. ThePaleoproterozoic ranges from 2. 5 Ga to 1. 8 Ga in geochronology. It contains three periods, and2. 3 and 2. O5 Ga as time boundaries between them. Neither period names or typical stratigraphicnames, i. e. "Group" and "Foarmtion", are suggested. In generally the Mesoproterozoic includesthe Changcheng and Jixian Periods, and their time houndary is placed at 1. 4 Ga. The top timeboundary of the Mesoproterozoic is 1. O Ga. However there are clearly stratigraphic boundaries in1. 6 Ga, 1. 4 Ga and 1. 2 Ga, so the Mesoproterozoic may be able to subdivide into four Periods.The Ne Neproterozoic can be divided into Qingbaikou and Sinian Periods in current literatures, and0. 8 Ga is selected as their time boundary. However there are different points in Chinese geologistson the time range of the Sinian Peried. The lowermost time is selected as O. 9 Ga, 0. 85 Ga andO. 8 Ga, respectively. Some geologists suggest the Sinian can be subdivided into two periods, thebottom or top of the tillite beds, that is O. 7 Ga or O. 65 Ga is as the boundary time between thetwo new periods. The author of this paper prefers 0. 545 Ga rather than O. 57 Ga as the boundarytime between Sinian and Cambrian Periods.