吕梁山地区新近纪晚期地层研究进展

NEW ADVANCES IN THE STUDY OF LATE NEOGENE STRATIGRAPHY IN THE LLIANG MOUNTAIN

通过对吕梁山西部保德和柳林两个典型剖面的研究,认为山西柳林卫家洼保德组剖面结构与保德剖面相同,完全可以对比。保德组下部的砂砾石确为水成堆积无疑,但是上部的棕红色黏土与棕黄色的钙质结核层越来越被广大学者认为是风尘堆积,中部则为下部水成堆积物与上部红黏土与钙质结核的过渡地段。《中国地层指南及中国地层指南说明书》规定:组的含义在于岩性、岩相和变质程度的一致性,除去变质程度不说,保德组下部水成堆积物和上部风尘堆积根本不是同一相环境所能包含的岩石组合,因此不能用一个组来定义,应予解体。保德组下部的水成堆积物划为芦子沟组,相当于初始定义的"芦子沟系"或"芦子沟砾岩";保德组上部风尘堆积仍称之为保德组,相当于初始定义的保德红土;在这种划分下芦子沟组为不整合于前新生代基岩之上,整合伏于红黏土之下的山前冲积扇相堆积。保德组为整合于芦子沟组之上静乐组之下的所有风尘堆积。

Based on the study of the Liulin and Baode sections in the western Lvliang Mountain, we conclude that the Baode Formation at the Weijiawa section in Liulin, Shanxi province can be correlated with the Baode section. The sandy gravel layer of the lower Baode Formation is aqueous deposits, but the upper sequence of brownish red clay and brownish yellow calcite nodules are considered to be eolian clay layers. The middle part is thought to be the transition zone from the lower aqueous deposits to upper eolian deposits. According to The Stratigraphic Guide of China and Its Explanation, a lithostratigraphic unit （e. g. , formation） should consist of strata of consistent lithology, lithothcies, metamorphic grade. The lower aqueous deposits and the upper aeolian red clay can not be regarded as a rock assemblage that was formed in a uniform environment. Thus,it is inappropriate to use a single formation to define the beds formed in different environments. We suggest that the original definition of the Baode formation be reinstated. The lower sandy gravel deposits should be classified as the Luzigou Formation, roughly equivalent to the ＂Luzigou System＂ or ＂Luzigou Conglomerate＂. The upper eolian deposits belong to the Baode Formation, as it was originally defined. In this definition, the Luzigou Formation represents alluvial fan deposits which unconformably overlies the pre-Cenozoic bedrock but conformably underlies the Baode red clay. The Baode Formation includes all eolian deposits between the underlying Luzigou Formation and the overlying Jingle Formation.