湖北省宜昌县震旦系南沱组成因的新认识

在峡东王丰岗南沱组剖面上有一些杂砾岩透镜体和一些水道砂岩透镜体。这些是冰水环境的产物。在南沱组与莲沱组的界线上,见到了几个骑线砾石。这是冰陆环境的有力证据。由此可见,早期主要是冰陆环境,冰川刨蚀和剥蚀作用占主导,沉积作用是次要的。随着气温的回升,冰盖逐渐后退。当冰盖前沿退到研究区内,或刚退出本区时,就形成了本区冰期的主要沉积。因此,可以认为,本区南沱组杂砾岩主要是近源冰海相沉积。

THE ORIGIN OF THE SINIAN NANTUO FORMATION IN YICHANG COUNTY,HUBEI PROVINCE

The term Nantuo Tillite had been proposed since the first decade of this century. From 1924 it has become a formal lithostratigraphic unit of Sinian, and has been extensively used as an effective index layer for stratigraphic classification and correlation. The origin of the formation was exclusively thought to be continental glaciogenic, i. e. lodgement tillite and/or hasal tillite. However, no typical continental glaciogenic structures, e. g. gravel pavement, striated surfaces, and glacier movement induced deformation have been reported from the area. Our study in the region discovered a glacial origin structure and several subaqueous structures, therefore, the sedimentary environment of Nantuo Fm seems to be proximal glaciomarine.Firstly, the c. 60m thick massive diamictite is much thicker than the average thickness of a single layer of continental origin glacial diamictite reported from Varanger, Northern Norway. The monotonous lithology also suggests a marine origin.From the lower part of the unit lenses of diamictites, sandstones, conglomerate,as well as claystone indicate that sorting or winnowing had taken place in a subaqueous environment. The argillaceous or carbonate coating of clasts had long been suggested as an index of continental glacial origin; however, it has been found that the carbonate coating not only occurred on the surface of clasts, but also at the cleavages of sandstones and claystone implying that the origin of the coating can not be related to the origin of the diamictites.Along the boundary between Liantuo and Nantuo Fms several clasts are found riding on the boundary with a larger portion below the boundary, leaving a smaller part in Nantuo Fm. This structure shows that continental glacier once happened in this area, which probably occurred before the above mentioned submerged stage.Based on the above criterion a model of environmental evolution for the area studied is proposed. During the main glacial epoch ploughing and erosion caused by the ice sheet was predominant. With the warming of the climate, when the front of the retreating ice sheet was near to the studied area, the ice sheet would occasionally be floating and grounding, resulted from the effect of climate fluctuation.However, the main period of sedimentation took place when the ice sheet in the area had totally buoyant or retreated.