北大巴山区鲁家坪组的厘定

根据鲁家坪组的岩性、岩相和变质程度的变化以及区域分布特征,原定义所包含的岩石内容过于庞大,混淆了岩石地层单位和年代地层单位。通过对鲁家坪组的命名地点和标准剖面所在地——北大巴山南部的陕西紫阳县鲁家坪村的鲁家坪剖面及其他剖面的详细研究,原鲁家坪组按岩性可分为三段,下段以白云岩为主;中段以厚层硅质岩为主,中段下部的硅质岩中夹磷质白云岩和灰岩,其中产小壳化石Archaeooidessp.、Protohertinasp.、Chan-celloriasp.和Hyolithids,中段上部的硅质岩中夹多层毒重石或重晶石矿层以及砂炭(石煤)和火山岩层,即以硅质岩类为主;上段以黑灰色(风化后为浅灰色)含硅炭板岩和泥灰质含炭板岩为主。根据原鲁家坪组各段岩层的岩性、岩相、岩层层序、厚度和分布范围等,原鲁家坪组的下部的厚层状灰质白云岩和硅质白云岩等与原鲁家坪组中部的厚层硅质岩及硅质板岩,以及上部的板岩和千枚岩和页岩的岩性和岩相差别较大,应从原鲁家坪组划出,暂仍称为灯影组,与三峡地区的灯影组可对比;原鲁家坪组的中上部为鲁家坪组的主体,仍称鲁家坪组。厘定后的鲁家坪组与扬子地台的下寒武统的筇竹寺组(云南)、牛蹄塘组(贵州)和宽川铺组(陕南宁强)可以对比,而与后者的主要区别在于后者的下部以黑色页岩为主、硅质岩岩层薄,而鲁家坪组下部的硅质岩特别发育,厚度大,常常夹有毒重石矿层、砂炭和火山岩等。

THE LUJIAPING FORMTION OF NORTHEN DABA MOUNTAIN

Lower Cambrian black shale successions in the Yangtze Platform (e.g., eastern Yunnan, Hunan, Guizhou, Sichuan, and southern Shaanxi provinces) and Lower Cambrian successions in the North China Platform have been studied in much detail. In contrast, much less is known about Lower Cambrian sequence in the important geological unit of China-the northern Dabashan area in the southern Qinglin Mountains.Lower Cambrian black shale successions in the northern Dabashan area are different from those of the Yangtze Platform or the North China Platform. The previously defined upper Proterozoic-Lower Cambrian Lujiaping Formation in the northern Dabashan area consists of three members. The lower member consists of 20-70 m of thick-bedded dolomite and dolomitic limestone, and siliceous dolomite, which overlies late Proterozoic volcanic rocks. The middle member is characterized by thick-bedded siliceous rock. Phosphatic dolomite with the small shelly fossils Archaeooides sp., Protohertina sp., Chancelloria sp., and hyolithids, occurs in the lower part of the middle member. The upper part of the middle member contains several beds of witherite, barite, stone coal, and volcanic tuff. The upper member of the Lujiaping Formation is mainly dark slate, light gray-green (after weathering) sericitized slate, and silt slate. On the basis of litho- and biostratigraphy, we propose that the lower member of dolomite, dolomitic limestone, and siliceous dolomite be removed from the Lujiaping Formation, because this member is lithostratigraphically similar to and can be correlated with the Dengying Formation in the Yangtze Plaftorm. The revised Lujiaping Formation only includes the siliceous, phosphatic, and carbonaceous rocks that are equivalent to the middle and upper members of the previously defined Lujiaping Formation. The revised Lujiaping Formation can be correlated with chert-phosphorite-black shale series of the Lower Cambrian Qiongzhusi Formation in eastern Yunnan Province, Niutitang Formation in Guizhou province, and Kuanchuanpu Formation in southern Shaanxi Province. The major difference between the revised Lujiaping Formation and the Qiongzhusi and Niutitang formations is stratigraphic thickness; the lower part of the revised Loujiaping Formation consists of 20 m of dark cherts and 187 m dark cherts interbeded with the phosphatic carbonates, much thicker than the black chert unit in the lower Qiongzhusi Formation or the lower Niutitang Formation.