燕山地区中元古代高于庄组非叠层石碳酸盐岩序列的沉积特征及其重要意义

在燕山地区中元古代高于庄组一套厚度千余米的碳酸盐沉积序列中，第三段组成一个特别的非叠层石碳酸盐岩序列。两个剖面(天津蓟县剖面和北京延庆千沟剖面）的详尽观察与研究结果表明：高于庄组第三段的非叠层石碳酸盐岩沉积序列以灰岩序列为特征，厚度为300 m至600 m不等，在蓟县剖面发育有存在成因争论的臼齿状构造，在延庆千沟剖面发育奇形怪状的席底构造。该非叠层石碳酸盐岩沉积序列表明，在前寒武纪除了3次叠层石衰减事件(分别发生在2 000 Ma、1 000 Ma和675 Ma）外，在1 450 Ma左右还可能发生过一次叠层石衰减事件。再者，高于庄组第三段构成的非叠层石碳酸盐岩沉积序列，可以大致与北美地区的Belt超群中的Helena组非叠层石碳酸盐岩沉积序列相对比，从而表明了1 450 Ma左右的叠层石衰减事件因具有全球性而有重要意义。在漫长的前寒武纪，臼齿状构造常常集中发育在叠层石衰减事件之后，因此，尽管这两种沉积构造的成因还不完全清楚却成为了解前寒武纪世界的重要线索。1 450 Ma左右的叠层石衰减事件，正好处于叠层石发育的黄金时段(2 800 Ma至1 000 Ma），对该次事件的识别，有助于正确认识漫长而复杂多变的前寒武纪碳酸盐世界。

Sedimentary Features and Their Implication for the Depositional Succession of Non-stromatolitic Carbonates, Mesoproterozoic Gaoyuzhuang Formation in Yanshan Area of North China

Within the Mesoproterozoic Gaoyuzhuang Fm.in Yanshan area there is a set of more than 1,000 m-thick carbonate strata,the third member makes up a particular non-stromatolitic carbonate succession.The detail research at two sections,i.e.the Jixian section in Tianjin and the Qiangou section of Yanqing County in Beijing,indicates that this non-stromatolitic succession is a set of limestones of leiolite and laminite with the thickness ranging from 300 m to 600 m,in which lots of grotesque matground structures(wrinkle structures and palimpsest ripples) are developed in beds of leiolite limestone at the Qiangou section and lots of molar-tooth structures are developed in beds of leiolite limestone at the Jixian section.This non-stromatolitic sedimentary succession might reflect a stromatolite decline event occurring at ca.1,450 Ma of the Proterozoic besides other three events respectively occurring at ca.2,000 Ma,ca.1,000 Ma and ca.675 Ma.And this non-stromatolitic sedimentary succession can be generally correlative to 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 endows the non-stromatolitic sedimentary succession of the third member of the Gaoyuzhuang Formation in the Yanshan area with important significance.In the secular Precambrian,after most stromatolite declines are frequently developed molar-tooth deposits,thus both the molar-tooth structure and the stromatolite can provide meaningful clues for the understanding of the Precambrian world,although their origin and forming mechanism remain uncertain and is highly contentious.Therefore,like other three stromatolitic declines respectively occurring at ca.675 Ma,ca.1,000 Ma and 2,000 Ma,the identification of the stromatolite decline occurring at ca.1,450 Ma during the Golden Age of stromatolites (2,800 Ma to 1,000 Ma) has important meaning for the further understanding of the Precambrian evolving carbonate world.