摘要
一.引言中国震旦纪普通为矽质石灰岩、板岩及页岩、石英岩等所组成,许多地方下部有冰碛岩。冰碛岩可以做为一个划分老地层很好的标志。也有许多区域震旦纪下部并没有冰碛岩,但仍有很厚的石英岩或有与其相当的碎屑沉积为代表。
The Sinian strata, composed of, in the descending order, siliceous limes-tone, slaty shale, quartzite and the basal tillite, constitute as a whole a goodcriterion to differentiate the same from the even older rocks. In some places,when such tillite is lacking, the base of the Sinian strata can be equallywell demarcated by the thick and massive quartzite, or by the lithologicalsuccession of the Sinian rocks and their clastic members together with otherstratigraphic relationship. The topmost of the Sinian and the basal part of the Cambrian areusually in places represented by some siliceous limestone, but the Cambriansediments are of shallow sea deposition in nature, transgressing on varioushorizons of the underlying Sinian beds. If the basal conglomerate or theoverlapping sedimentation of the lower Cambrian is not found, the stratigra-phical demarcation between the Cambrian and the Sinian is hardly evident. In South China unconformably underlying the Sinian Tillite are thelightly metamorphosed sedimentary beds of several thousand metres in thick-ness, chiefly composed of quartzose sandstones, phyllites, graywacke, and slateswith subordinate amount of schists and marbles. These beds are designatedas the Hsiakiang Series in Kweichow, the Kuenyang Series in Yunnan,the Panchi Series in Hunan, the Nanling Series and the Lungshan Seriesin Kwangsi, the Shuanchiaoshan Series in Anhui, the Shangchi Series inKiangsi etc. They are differently named in different places but were undif-ferentially correlated as of the Sinian Age. As there exists a great angularunconformity between the overlying tillite and this underlying lightly me- tamorphic rocks, the latter would better be assigned to an older age otherthan the Sinian. Below these lightly metamorphosed sediments in the Lushan of Kiangsiare exposed various kinds of schists, denoted as the Lushan Series. In Likiang,northwest Yunnan is revealed as the oldest basement, the Shihku crystallineschist, which is at present considered as the oldest rock exposed in SouthChina. In North China at the Wutai district, the Wutai Series of Willis hasbeen verified to be the metamorphosed equivalent of the Huto Series. Toavoid further stratigraphical entanglement, such designation is tentatively notreferred. At the western slope of the Wutaishan are exposed the metamorphosedsediments, the Green Schist, sometimes named as the Tinghsiang Formation.They are composed of mica-, chlorite-, and amphibole-schists with somequartzite, marble and graywacke When cropped out in the vicinity of igneousintrusions, the metamorphosed phenomena in these rocks are especially promi-nent, while in places without igneous effect, such metamorphism greatlyslackens. The contact of the schists with the granitic gneisses shows a granitizedfeature with no marked boundary. The Sangkan Gneiss of North Shansi, asknown to belong to the Archaen, has no direct contact with the Green Schist. The upper part of the Green Schist has a characteristic banded magnetitebed which can be correlated with the Anshan iron-ore formation, whichlatter is intercalated in the old sediments, definitely lying unconformablybelow the Sinian Tiaoyutai Quartzite and Conglomerate. In the Qurugtaghof Sinkiang below the Sinian tillite are several thousand metres of extrusives,slates and sandstones, constituting a series of complex rocks, the Payi Series.Although this series can be hardly comparable, stratigraphically, with the Hsia-kiang Series of Kweichow or the Green Schist of the Wutaishan, it is in thesame way overlapping on the gneisses initially with a basal conglomerate. With such scattered data of the stratigraphy of the Pre-Cambrian rocks,the writer tentatively divide them into four stratigraphical units or groupsin the descending order as the following: 1. The first group of sediments, ranging from the base of the Cambriandown to the tillite, is assigned to be of Sinian age. 2. The second group lying below the tillite, composed of lightly meta-morphosed sediments, is exemplified by the Kuenyang Series, the HsiakiangSeries, the Payi Series, etc. Pending further study to assign an appropriateage. 3. The third group is a highly metamorphosed schist series, as theLushan Series, the Shihku Series, the Green Schist of the Wutaishan and themetamorphic Series of the Anshan. 4. The fourth group is made up by various gneisses, at present the old-est rocks exposed. These four groups of rocks are mutually unconformably in contac twithone another. Under normal conditions the higher groups of these four are lessmetamorphosed in degree when compared with those below. Under the condi-tion of strong igneous effects, the complex metamorphosed phenomena mightlead to wrongly interpret the true stratigraphical position of the formation,if not tracing the same to some place of less intensive metamorphism. The old correlation of the Taishan Complex as to belong to the Archeanis questionable. The certainty of the Sangkan Gneiss as the basement of theSchist Series is yet to prove. Here the four stratigraphical divisions are sug-gested as a tentative working base.
出处
《地质学报》
EI
CAS
1955年第4期361-369,371-373+448-449,共14页
Acta Geologica Sinica
