中元古代叠层石—非叠层石碳酸盐岩层序地层序列及其沉积特征——以北京延庆千沟剖面高于庄组为例

北京延庆千沟剖面中元古代高于庄组为一套发育在海侵砂岩上的叠层石—非叠层石碳酸盐岩沉积序列,厚度千余米,包括4段:第一段(或称为官地亚组)主要为海侵砂岩地层;第二段(或称为桑树鞍亚组)为发育少量叠层石的含锰白云岩和灰质白云岩地层;第三段又称为张家峪垭组,为一套以均一石灰岩为主的地层序列,发育席底构造且贫乏叠层石;第四段(环秀寺亚组)以叠层石礁白云岩为特征。与蓟县剖面相比较,该剖面的高于庄组具有以下特点:(1)第一段厚度百余米的海侵砂岩地层不但表明了与下伏大红峪组之间存在较为明显的区域不整合面,而且与蓟县剖面厚数百米的叠层石白云岩地层形成明显的区别;(2)在第三段的非叠层石碳酸盐岩沉积序列中,未发现臼齿状构造,但发育丰富而典型的席底构造,而且以均一石灰岩为特征;(3)第四段的厚度为500余m,由罕见的叠层石礁序列所组成。这些特征表明,千沟剖面的高于庄组组成了一个复杂而有序的叠层石—非叠层石碳酸盐岩沉积序列,成为依据沉积相序列进行前寒武纪三级层序划分和二级层序合理归并的典型实例。更为重要的是,第四段中的叠层石生物礁和第三段的均一石灰岩序列的存在,反映了一些值得进一步研究的问题:如前寒武纪叠层石生物礁的沉积模式,非叠层石灰岩序列是否代表了一次叠层石衰减事件等等。

Sequence-Stratigraphic Succession and Its Sedimentary Features for the Stromatolitic-Nonstromatolitic Carbonates of the Mesoproterozoic: An Example from Gaoyuzhuang Formation at Qiangou Section in Yanqing County, Beijing

The Mesoproterozoic Gaoyuzhuang Fm.at the Qiangou section in Yanqing County of the northwestern suburb of Beijing is a sedimentary succession of stromatolitic-nonstromatolitic carbonates with the thickness of more than 1000 m and with a base of transgressive sandstones.The Gaoyuzhuang Fm.at the Qiangou section can be subdivided into four parts: The first member(or the Guandi Subformation)is made up of a set of transgressive sandstones;The second member(or the Sangshuan Subformation)is constituted by a set of manganese dolomites and lime dolomites with stromatolites;The third member(or the Zhangjiayu Subformation)is marked by a succession of the leiolite limestones richening with matground structures and lacking of stromatolites;The fourth member is a particular sedimentary succession of the stromatolitic reef.Comparing to the Gaoyuzhuang Fm.at the Jixian section,there are three characteristics for the Gaoyuzhuang Fm.at the Qiangou section.First,a set of transgressive sandstones of the first member with the thickness of more than 100 m,not only indicates an obvious regional unconformity between the Gaoyuzhuang Fm.and the underlying Dahongyu Fm.but also makes a clear contrasting of the stromatolitic dolomite succession with the thickness of several hundreds meters at the Jixian section.Second,in a set of leiolite limestones lacking stromatolites and making up the third member,there are no molar-tooth structures that widely develop in leiolite limestones of the upper part of the third member at the Jixian section but there are plentiful of matground structures.Third,a particular sedimentary succession of stromatolite reef with the larger thickness of more than 500 m constitutes the fourth member.These features show that the Gaoyuzhuang Fm.at the Qiangou section forms a complex but regular sedimentary succession of stromatolitic nonstromatolitic carbonates,which is a representative of both the division of the third-order sequence and the reasonable grouping of the second-order sequence of the Precambrian in terms of the sedimentary-facies succession.More importantly,the stromatolitic reef of the fourth member and the succession of leiolite limestones of the third member express some meaningful problems that need to further research,such as the depositional model of the stromatolitic reef of the Precambrian,a stromatolitic decline event reflected by the nonstromatolitic succession of leiolite limestones and so on.