北京延庆千沟中元古代高于庄组第三段:一个典型的前寒武纪非叠层石碳酸盐岩沉积序列

在前寒武纪碳酸盐岩中，最为普遍的是叠层石碳酸盐岩沉积序列，而以潮下相沉积为主的非叠层石碳酸盐岩沉积序列与前者形成了较大的差异；因此，与叠层石碳酸盐岩沉积序列一样，非叠层石序列成为了解前寒武纪复杂多变的碳酸盐世界的重要线索。燕山地区中元古代高于庄组为一套厚度千余米的碳酸盐沉积序列，包括四大部分:第一段（或称为官地亚组）为发育在海侵砂岩上的叠层石白云岩地层；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地层；第三段又称为张家峪亚组，为一套以发育灰岩为主的非叠层石碳酸盐岩沉积序列；第四段以叠层石岩礁和叠层石礁白云岩为特征。延庆千沟剖面的高于庄组第三段的非叠层石碳酸盐岩沉积序列，贫乏叠层石是其基本特点，包含3个三级层序，在三级层序的海侵体系域和早期高水位体系域中，层面上发育席底构造的中层均一石灰岩（隐藻泥晶灰岩）和灰黑色薄层泥灰岩组成若干潮下型米级旋回，晚期高水位体系域则以厚层块状纹理石灰质白云岩和白云质灰岩为特征；三级层序的凝缩段单元则以薄层泥灰岩和泥页岩构成的L-M型米级旋回为特征。因此，每一个三级层序均以一个有序的沉积相序列为特征。高于庄组的分布时限大致为200 Ma（1 600～1 400 Ma），考虑到下伏的大红峪组和高于庄组之间的地层间断时限大致为50 Ma至100 Ma，所以推断高于庄组的堆积作用时限为100 Ma（1 500～1 400 Ma）左右，其中高于庄组第一段与第二段的分界线正好处于该组的中部而可以推断该界线的大致为1 450 Ma左右；因此，高于庄组第三段非叠层石碳酸盐岩沉积序列的发育表明，在1 450 Ma左右可能发生过一次可以与前寒武纪其他已经被识别出的三次叠层石衰减事件（2 000 Ma、1 000 Ma和675 Ma）相比拟的又一次叠层石衰减事件。延庆千沟剖面高于庄组第三段的非叠层石碳酸盐岩沉积序列，可以大致与北美地区的起始时限为1 450 Ma左右、Belt超群中的Helena组非叠层石碳酸盐岩沉积序列相对比，表明了1 450 Ma左右的叠层石衰减事件具有全球性，从而赋予高于庄组第三段非叠层石碳酸盐岩沉积序列重要的沉积学意义。延庆千沟剖面高于庄组第三段非叠层石碳酸盐岩沉积序列，特别的岩石类型和沉积构造，成为前寒武纪碳酸盐岩沉积中非叠层石碳酸盐岩沉积序列的典型代表，由其所代表的沉积学特点将有助于深入理解漫长的前寒武纪碳酸盐沉积作用的变化规律而具有重要意义。

Third Member of Mesoproterozoic Gaoyuzhuang Formation at Qiangou Section in Yanqing County of Beijing: a typically non-stromatolitic carbonate succession of the Precambrian

In the long Precambrian, the stromatolitic carbonate succession is very common. But, the non-stromatolitic carbonate succession that is marked by the subtidal deposits forms a strong contrast to the stromatolitic carbonate succession. Both the non-stromatolitic and the stromatolitic carbonate successions are important clues for the further understanding of the evolving carbonate world of the Precambrian. The Mesoproterozoic Gaoyuzhuang Fm. in Yanshan Area is a set of more than 1000m-thick carbonate strata that can be divided into four Members (or Subformations): The first Member (or the Guandi Subformation) is marked by a set of stromatolitic dolomites overlying a set of transgressive sandstones; The second Member (or the Sangshuan Subformation) is a set of manganiferous dolomites with a few stromatolites; The third Member is chiefly made up of limestones and is characterized by a particularly non-stromatolitic carbonate succession of the Precambrian; The fourth Member (or the Huanxiusi Subformation) is composed of a set of dolomites of stromatolitic reefs or lithoherms. The non-stromatolitic carbonate succession making up the third Member of the Gaoyuzhuang Formation at the Qiangou section in Yanqing County of Beijing can further be subdivided into three third-order sequences that are marked by the regular succession of sedimentary facies. In third-order sequences, lots of subtidal carbonate meter-scale cycles made up of the midium-bedded leiolite limestones and the thin-bedded marls constitute their transgressive system tracts (TSTs) and the early high-stand system tracts (EHSTs), lots of meter-scale cycles made up by the thin-bedded limestones and marls constitute their condensed sections (CSs), and thick-bedded to massive dolimitic limestones or lime dolomites make up the late high-stand system tracts (LHSTs). The time scan of the Gaoyuzhuang Formation is deduced as 200 Ma (from 1600 Ma to 1400 Ma). An obvious hiatus between the Gaoyuzhuang Formation and the underlying Dahongyu Formation is deduced as 50 Ma to 100 Ma, so the forming duration of the Gaoyuzhuang Formation is thought as the 100 Ma (1 500 Ma to 1 400 Ma). Furthermore, the subface of the third Member of the Gaoyuzhuang Formation that is just on the mid position of the Gaoyuzhuang Formation and its forming age may be deduced as about 1 450 Ma. According to these materials, the non-stromatolitic sedimentary succession make up the third Member of the Gaoyuzhuang Formation may demonstrates a stromatolite decline event occurring at ca. 1 450 Ma of the Proterozoic besides other three events that respectively occurred at ca. 2 000 Ma, ca. 1 000 Ma and ca. 675 Ma. The forming duration of this non-stromatolitic sedimentary succession of the third Member of the Gaoyuzhuang Formation can be generally correlative that of a similar sedimentary succession in North America, i.e. a non-stromatolitic sedimentary succession of the Helena Formation of the Belt Supergroup, which suggests that the stromatolite decline occurring at ca. 1 450 Ma is a global event. All of these information's endow the non-stromatolitic sedimentary succession making up the third Member of the Gaoyuzhuang Formation at the Qiangou section with important significance. The particularly non-stromatolitic carbonate succession make up the third Member of the Mesoproterozoic Gaoyuzhuang Formation at the Qiangou section might be the representative of the non-stromatolitic carbonate succession of the Precambrian because of its special lithological features and particular sedimentary structures, whose general sedimentary features are helpful and meaningful for the further understanding of changing regularities of the sophisticate and evolving carbonate world of the Precambrian.