本溪组—岩石地层和年代地层与穿时性

Benxi Formation--Lithohorizon and Chronohorizon with Diachronism

本溪组指中国北方奥陶系灰岩不整合面之上、石炭 -二叠纪含煤岩系之下 ,由碎屑岩夹灰岩或灰岩凸镜体所组成的一套海陆交互相地层。随着研究的深入 ,特别是地层学理论的发展及认识的不同 ,对本溪组岩石地层和年代地层的划分形成了多种争议。本文按地层多重划分的观点和方法认为组是一个岩石地层的自然实体 ,并有区别于相邻自然实体单位的岩石内容和反映其发育的某一特定过程。为此 ,建议以层型剖面 (本溪市新洞沟与蚂蚁村沟间的牛毛岭 )为准 ,将本溪组岩石地层单位定义到《全国地层多重划分对比研究》的成果上来。即奥陶系灰岩顶部不整合面之上、下蚂蚁沟灰岩底界之下的一套碎屑岩层。从该灰岩层(即华北各地下部第一层灰岩 )始 ,到最上部一层灰岩顶界 (如山西太原的东大窑灰岩、河南禹州L8灰岩 )为上覆太原组。该组下部产G、F两层铝土页岩 ,称为湖田段 ;上部为紫色 -杂色富碳岩系 (夹一薄煤层或碳质页岩 )称为新洞沟段。本溪组的地质时代或年代地层 ,依据各地所产化石及上覆太原组第一层灰岩 (即顶界 )所产化石 ,不同地区时限不同 ,在吉南 -辽东地区为大塘阶 -德鸣阶 ;在晋冀鲁为罗苏阶 -滑石板阶 ;而在河南禹州其顶界进入逍遥阶 (或小独山阶 ) -阿瑟尔阶 ;明显地反映出岩石地层单位穿时的普遍性规律。

Benxi Formation is termed with a suit of strata with sea--land alternating facies, consisting of clastic rock and limestone, above the unconformable interface between the Benxi Formation and Ordovician limestone, underlying Permo--Carboniferous coal--bearing strata. Along with stratigraphic study being in progress, there has been dispute in classification and recognition of Benxi Formation in lithohorizon and chronohorizon. According to viewpoints and methods of multiple classification of stratigraphy, the formation is regarded as a natural unit of lithohorizon, reflecting a specified process of its development, which is different from its related other stratigraphic units. Based on the unify knowledge of 'Multiple classification and correlation of the stratigraphy of China', the present authors propose that definition of the Benxi Formation, as a lithohorizon unit, can be unified with the standar of the stratotype of the formation in the Niumaoling of Benxi City. That means the Benxi Formation is a suit of strata of clastic rocks, based by the unconformable interface overlying the Ordovician limestone, and underlying the Mayigou limestone beds. The Taiyuan Formation is composed from the Mayigou limestone (i.e. the first limestone from bottom of the Taiyuan Formation in North China) to top boundary of the uppermost limestone (e.g. the Dongdayao limestone in Taiyuan of Shanxi and the limestone L8 in Yuzhou of Henan). The Benxi Formation consists of two members: the lower one is Hutian Member yielding G--F alum clay shale; the upper is Xindonggou Member yielding purple--variegated carbon--rich rocks. Owing to the fossils from the first limestone of Taiyuan Formation showing different ages, the Benxi Formatian is varied in age also at different places, e.g. the Datangian--Demingian in southern Jilin and eastern Liaoning, the Luosuian--Huashibanian in Shanxi--Hebei--Shandong; and in Yuzhou of Henan, the top part of the Benxi Formation aged in the Xiaoyaoian (or Xiaodushanian)--Aserian. The fact has obviously reflected the characteristics of the lithohorizon with diachronism.