Molar Tooth Structure: a Contribution from the Mesoproterozoic Gaoyuzhuang Formation, Tianjin City, North China

摘要

Molar-tooth (MT) structure is an enigmatic sedimentary structure consisting of variously-shaped cracks and voids filled with a characteristically uniform, equant calcite microspar. It is globally distributed but temporally restricted to rocks from Neoarchean to Neoproterozoic age. The origin of MT structures has been debated for more than a century and the topic continues to be highly contentious. Some features of MT structure occurring in micritic limestones of the Mesoproterozoic Gaoyuzhuang Formation (ca. 1500 Ma to ca. 1400 Ma), Jixian section, Tianjin City, North China show that: 1) there is a definite interface or lining, rich in organic material and pyrite, between the MT crack-filling calcite microspar and the micritic host rock, which is also rich in organic matter; 2) the micritic host rocks are notable for the absence of stromatolites and microbial laminites; 3) distinctive conglomeratic lag deposits made up of intraclasts of MT microspar result from storm reworking of the MT structures; 4) the MT structure is associated with possible algal megafossils such as Chuaria; 5) the MT microspar is made up of the larger calcite crystal and the MT crack is marked by the diversity of configurations; 6) both the TOC content and the carbon-isotopic value (δ13C_PDB) among the host rock, the MT microspar and the possible algae fossil are obviously different. For the forming mechanism of the Gaoyuzhuang MT structure, these features can still indicate that: A) the MT microspar was formed by rapid precipitation and lithification; B) the MT microspar precipitated directly within the cracks; C) the decomposition of organic matter within the host micrite might be the chief mechanism producing gas bubbles; D) microscale gas-sediment interaction led to the generation of the MT cracks and the precipitation of microspar therein; E) the MT cracks might represent the track of migration and expansion of gas bubbles, and that the recrystallization of host micrites cannot be eliminated during forming process of the MT microspar; F) the MT structure is occurred in early diagenetic period; and G) the formation of MT microspars is a complex diagenetic process. Therefore, model of the microbially-induced gas-bubble expansion and migration is the best interpretation for the formation of the MT structure. Effectively, MT structures are a type of sedimentary structure that is formed in the early diagenetic period and is related to microbial activities and organic matter degradation.