Molar tooth Structures and their geological significance

HongweiKuang

Institute of Geology, Chinese Academy of Geosciences, Beijing 100037, China

Molar tooth structure (MTS) is the sedimentary structure made up of series of variously shaped voids and ptygmatical cracks that were filled with unusual, equantmicrosparry calcites from Precambrian.MTSdistributes broadly in the world, but limited in the Meso–and Neoproterozoic with the exception of a few in the Paleoproterozoic to the latest Archean. All of themare made of microsparry calcites (5 –15 μm diameter),but their shapes are complex and diverse. A variety of complicated shape of MTS can’t be reasonably explained by origin theory discussed nowadays, and two-new-type MTSs found recently are very different from ones found before in morphologyormicrofabric. One is cyclic MTS that was discovered in the Zhangqü Formation in Suzhou, Anhui Province, the other is founded in the Helena Formation in the Belt Supergroup in North America. There are up to 10 kinds of original hypothesis that have been proposed since the MTS was initially discovered and studied. The reason of having so many kinds of original hypotheses of MTS is just because their morphology is very complex. Nevertheless, the sedimentary environments, primarily from subtidal to intertidal zone of shallow seaare relatively simple. To discover the truth of formation, analyze the diversity of the types and expose the action of MTS, we compared MTS’ features developed in the world, discussed different types of MTS in globe.Our study further proved the creation of MTS wasn’t a solitary process, but rather was controlled by the physical properties (temperature, solubility of CaCO3 ) and chemical properties (the concentrations of Fe, SO42–, oxygen fugacity, salinity, and the isotopic levels of C, O and Sr) in the paleoocean; and secondly, was also closely related with the environment of atmosphere (PCO2), paleolatitude (that controlled the temperature of seawater), and the biological processes (levels of redox and the circulation between the PCO2 and carbon in the seawater could accelerated the crystallization of CaCO3 as well). Global occurrence of MTS in a limited time interval and specified depositional environment leads us to recognize that it works for the facies indicator and the mark of stratigraphic correlation (the former indicates the shallow water subtidal zone and the latter can be used for Proterozoic stratigraphy correlation that were widely distributed in stable cratons in the world), even though some aspects of the origin of MTS are still controversial, all of the researchers agree these two practical functions of MTS study. The study of MTS has important significance since it could provide a new approach for studying and resolving many important Precambrian questions, such as a better understanding of ocean chemistry, the depositional palaeogeography, stratigraphic correlation, and may even be useful forreconstruction of the supercontinent(?). Microtexture and the geochemical characteristics of MTS to prove the creation and vanishing of MTS not only was controlled by the alteration of chemical properties in paleo–ocean, but they also associated with the joint effort of atmosphere, climate, and organism activity.

Keywords:Molar tooth Structure, microsparry calcite, types of MTS，stratigraphic correlation, sedimentary facies，Precambrian

Acknowledgement:This paper was sponsored by Natural Science Foundation of China(41472082，40772078)

臼齿构造及其地质意义

旷红伟

中国地质科学院地质研究所

臼齿构造是由一系列具泡状或肠状裂隙状的、由等轴或不等轴微亮晶方解石充填的前寒武系沉积构造。臼齿构造虽然在全球碳酸盐岩地层中广泛分布，但其赋存时代仅限于前寒武纪，且主要出现在中-新元古代，仅有极少量出现在晚太古—古元古代。组成臼齿构造的物质成分单一，几乎全是由微米级（约5 –15 μm）的方解石组成，但其形态却复杂多样。多种多样、形态复杂的臼齿构造很难用现有的成因理论进行完整的解释，特别是最近我们新发现的两种新形态，更是用现有的任何成因假说理论都很难做出完全合理的解释：其中一种发现于安徽宿州张渠村新元古界张渠组的环状臼齿构造，另一种见于北美中元古界贝尔特超群海伦娜组。自从臼齿构造被发现以后，至少出现了10种以上的成因假说，其形态的复杂可能正是导致其出现多种成因解释的原因所在。尽管如此，其沉积的环境（从浅潮下——潮间带）却显得相对简单。

为了揭示其真正的成因、分析其复杂多样的形态，发挥其在前寒武纪研究中的作用，我们比较了来自世界各地不同形态的臼齿构造的特征。我们的研究进一步证明：臼齿构造的形成不是一个孤立的过程，它受控于古海洋的物理（温度、CaCO3的溶解度）、化学（Fe, SO42–浓度, 氧逸度、盐度及C, O 和Sr等稳定同位素的水平）条件，其次与PCO2、古纬度及微生物作用过程有关。

臼齿构造发育在特定的时限、明确的沉积环境促使我们认识到其作为沉积相标志及地层对比标志的可能性。尽管其成因解释多种多样，但几乎所有的研究者均认同它这两方面可能发挥的作用。臼齿构造的研究意义在于它提供了对诸如古海洋化学性质、沉积古地理研究及哑地层对比等许多前寒武系重大地质问题的研究方法和途径，甚至有可能为古大陆的再造提供证据（？）。臼齿构造的微观组构及地球化学特征表明其产生与消亡的与古海洋化学性质的变化有关，也与那时的大气圈、古气候及微生物作用相关联。

关键词：臼齿构造微亮晶方解石臼齿构造形态地层对比沉积相前寒武系

致谢：本文由国家自然科学基金（(41472082，40772078)）资助。