Microbial Sand Chips in Transgressive Sandstones of the Precambrian: An Example from the Late Paleoproterozoic Dahongyu Formation at the Huyu Section of Nankou Town, Beijing, China

Abstract

It is often difficult to identify the primary structures induced by microbes in terrigenous clastic rocks because the direct evidence indicative of the microbe is generally absent. Both sandstone strata of the transgressive system tract and sandy-muddy dolomite strata with stromatolite of the high-stand system tract make up the regular succession of sedimentary facies for the Late Paleoproterozoic Dahongyu Formation at the Huyu Section of Nankou Town in the northwestern suburb of Beijing, China. This characteristic has become the main criterion to divide the Dahongyu Formation into three third-order sequences. There is an odd type of sand chips in sandstone beds of the transgressive system tract of the Dahongyu Formation. The peculiar sand chip, similar to the carbonate intraclast, may indicate the development of microbial mats on the depositional surface of the Precambrian terrigenous clastic deposits. The chips together with palimpsest ripples and wrinkle structures form series of matground structures. The special configuration and features of these structures is a typical indication of the development of microbial mat in the Precambrian sandstones. Therefore, the discovery of the microbial sand chip and related matground structures is beneficial for the further study of similar sedimentary structures, particularly for those generally known as the oldest trace fossils of metazoa in the Precambrian. It also shows that there is some evidence indicative of microbial activity in terrigenous clastic rocks, the matground structures (the primary sedimentary structure of a fifth category) in the Precambrian, in addition to microbial sedimentary structures in carbonates such as the stromatolite. These unique sedimentary structures are important for reconstruction of the Precambrian sedimentary environments. The diversity of microbial metabolism results in the production of an immense amount of biomass, which has a global widespread distribution. Before the biological radiation that occurred during the Middle Ordovician, the microbes in the Precambrian to the Early Ordovician formed a particular habitat, matground. Similar habitat has frequently occurred in the Middle Ordovician to the present in desolate environment of seafloor after the biological extinction event (Fang, 2004). The microbe and its mucilaginous extracellular polymeric substances, like adhesive organic envelops, defined as “microbiofilms”, often cover and enclose sedimentary grains. And these microbiofilms further grow under favorable ecological conditions to form thick and distinct organic layers, termed as microbial mats (Decho, 2000; Stoltz, 2000). Although stromatolite is the most typical sedimentary structure related to microbes, lack of direct evidence indicative of its building organism has led to the continuous dispute about its origin, one of the “Precambrian enigmas” (Riding, 2000). To identify microbial mats in terrigenous clastics is even more challenging (Schieber, 1999). The present study on the matground and structures that developed above matground (Seilacher, 1999; Noffke, et al., 2001) (the primary sedimentary structure of a fifth category (Pflüger, 1999)) is, therefore, meaningful for understanding why and how those abnormal sedimentary environment appeared in geological history and is also helpful for the reconstruction of the sedimentary environments in the Precambrian.