青白口群地层学研究

青白口群地层学的研究,涉及我国震旦系的范围和界线。无产阶级文化大革命以来,由于全国区域地质调查工作的丰硕成果和对震旦系的深入研究,目前有更好的条件重新讨论青白口群的层序、界面、内部分组、区域分布、对比及其在地层柱中的位置。 一、青白口群岩石地层单位的讨论 表1概括地表示青白口群及其地层单位划分和时代界线的研究史略。

INVESTIGATION ON STRATIGRAPHY OF THE QINGBAIKOU GROUP OF THE YENSHAN MOUNTAINS, NORTH CHINA

The Qingbaikou group of the Yenshan Mountains may be divided into three formations, Xiamaling, Longshan and Jingeryu, in ascending order.The Xiamaling formation rests on the underlying Tieling formation with a remarkable, though low-angle, unconformity. The basal part of the Xiamaling formation displays a clear section of typical residual deposits of a lateritic type. For this widespread epeirogenic uplifting at the end of the deposition of the Jixian group and the consequent continental erosional gap thereon, the name "Qinyu movement" is here proposed.The Qingbaikou group, which lies between the unconformity marking the Qinyu movement below and that reflecting the Jixian movement above, is an entity representing a complete sequence of marine transgression. The foundation of the Qingbaikou sedimentary basin is composed of geological formations of different ages. In its central part is distributed the Tieling formation, while near both the eastern and the western ends of the basin occurs the still lower Wumishan formation. Further outwards, along the margin of the basin, are old crystalline rocks. Resulting from -such a framework of the foundation, the Longshan formation, which occupies a higher position in the Qingbaikou group, usually overlaps either the old karst topography of the Wumishan formation or the erosional surface formed on more ancient crystalline rocks. It is therefore evident that the Qinyu movement tectonically demarcated two marine transgression cycles ’iof deposition belonging to different ages.The microflora recovered from the Qingbaikou group bears distinctive characteristics of its own, and is rather different from that of the underlying Jixian and Changcheng groups. A broad correlation based upon the microflora as well as the isotopie ages of these rocks indicates that the unconformity below the Qingbaikou group as resulted from the Qinyu movement is roughly contemporaneous with that below the Sinian of South China, which, resulted from the Jinning movement,Evidences in various lines-stratigraphical, sedimentational, epeirogenic andpaleontological-all indicate that the Qingbaikou group is chronologically equivalentto the Lower Sinian of South China, which comprises the strata above the unconformity connected with the Jinning movement and below the unconformity caused by the Chengjiang movement. These two sequences form the genuine platform blanket of East China.The "Sinian" of Jixian, the type section of the "Sinian" of North China, consists of three groups, in ascending order: the Changcheng, the Jixian and the Qingbaikou. They represent several independent stratigraphical sequences, among which the Qingbaikou group is an outstanding one, distinct from the others.Further investigation of this remarkably well-developed section would make it possible to establish more than one chronostratigraphic units of the system rank in the Late Proterozoic time scale.