叠层石衰减事件及臼齿构造作用幕:形成机制与研究进展

Stromatolite Declines and Molar-Tooth Carbonates:Origin and Advancement

叠层石和臼齿构造碳酸岩的兴衰是沉积学界倍受关注的研究主题。地史中叠层石经证实发生了5次比较重要的衰减事件,分别在2000Ma、1250Ma、1000Ma、675Ma、450Ma。最近,根据燕山地区非叠层石碳酸盐岩序列,认为在1450Ma又发生过叠层石衰减,所以存在6次叠层石衰减事件。现今叠层石的丰度已经非常稀少。究其原因主要在于:叠层石的形成与微生物钙化作用有关,而其钙化作用又受周围海水化学变化所影响。显生宙后生动物及海水饱和态共同影响了叠层石的形成。著名的"前寒武纪谜"表明,海水化学条件及微生物的进化则是叠层石发展的主要控制因素。臼齿构造(MT)仅出现在前寒武地层中,具有一定的时代限制,其形成及消失受海洋中CaCO3沉淀的影响,而且似乎主要发育在叠层石衰减事件之后,在空间上臼齿构造多产出于比叠层石更深的水体环境。所以古海洋化学的变化影响着地球表面生物演化及生物沉积的历史,上述研究进展对深入了解海洋化学在地质历史时期的长期变化具有重要意义,而且也有利于解开地质历史上的种种谜团。

Stromatolite and Molar-tooth structure are of five major declines occurring at 2000 Ma, 1250Ma, 1000 interest for geologists. Stromatolites underwent Ma, 675Ma and 450 Ma. Recently, according to a non-stromatolitic sedimentary succession in Yanshan area, there might be another stromatolite decline occurring at 1450Ma. Thus, there are six stromatolite declines during geological time and they are comparatively scarce at the present day. The success of stromatolites depends upon synsedimentary lithification and calcification, which are essential for the accretion and preservation to occur. Calcification reflects environmental constraints especially seawater chemistry. Both metazoan influence and seawater saturation state have combined to determine the abundance of stromatolites throughout Phanerozoic. In contrast, for the major part of the Precambrian that seawater saturation state, together with microbial evolution, was the principal factor determining stromatolites development. And ＂the Precambrian enigma＂ is so interesting and confusing, but has not yet been well understood. Molar-tooth structure appears to be limited time and space, only in fine-grained carbonates of Precambrian age, and it also reflects the saturation state of CaC03. In the secular Precambrian, after most stromatolite decline are frequently developed molar-tooth deposits and MT occurred in the deeper water than stromatolite. Both the molar-tooth structtire and the stromatolite can provide meaningful clues for understanding of the Precambrian carbonate world. And the fluctuations in seawater chemistry are also important for us to understand the geological history.