The sedimentary features of Proterozoic microspar (Molar-tooth) carbonates in China and their significance

Abstract

On the North China platform Molar-tooth (MT) carbonates were formed during 1600-650Ma (i.e. the Middle to Late Proterozoic), within various supercyclic sequences, such as the Changcheng, Jixian, Qingbaikou and Sinian Systems. Detailed field observations on the MT carbonate sections and microscopic studies show that MT carbonates may be divided into two major groups-autochthonous and allochthonous and 6 subgroups according to their depositional features. The autochthonous MT may be subdivided into four types, i.e. filamentous MT, curved banded MT, complicated sheet-like MT and dotted-tumorous MT, while the allochthonous MT may be subdivided into two types, i.e. mimicry-intraclastic MT and intraclastic MT. Based on sequence statistic and analysis of megascopic microfabrics, a sedimentary model of MT carbonates is proposed. MT structures occur within a range of facies from the subtidal to the basin margin zone. Various megascopic microfabrics may serve as useful environmental markers. MT structures might be a link to the revolutionary changes in the biosphere, which heralded the appearance of complex multi-cellular metazoan in the Late Precambrian. It is proposed that MT carbonates occupied the interval between Archean Times without life (but with an interval of inorganic aragonite, and tuff precipitation) and the Phanerozoic with skeletal carbonate precipitation. After a detailed discussion on many hypotheses for the origin of MT, and five evidences against their seismic origin it was concluded that the structures of MT carbonates are various and distributed so randomly that they may be related to the geological complexity. But the components of microsparite show MT's common characteristics, so the key to solve the origin of MT carbonates lies in the research on the origin of the microsparite. Hence, the most important factor for the origin of MT may be traced from the biological and geological evolutionary events of the Pre-Cambrian ocean. Most likely the formation of MT structure and microspar carbonates were terminated due to the obvious changes of particular organism and geochemistry in the paleo-oceanic environment of the Proterozoic in Phanerozoic era. A comprehensive study of MT carbonates will greatly enhance our understanding of the history of Proterozoic marine carbonate deposition as well as the history of early organic evolution. This forms the main objective of IGCP project 447.