Abstract:

The deposits surface with development of a microbial mat or microbial film is called as the matground in imagery.The development microbial mat or microbial film on matground leads to the strong mucosity of sediments that are resistant to reformation of current. Resulted from the action of microbes a series of primary sedimentary structures in grotesque shape such as palimpsest ripples, wrinkle structures, microbial mat chip, spindleshaped or subcircular syneresis cracks and so on are formed both on the bedding plane and within beds；And these primary sedimentary structures are nominated as the matground structures. On the other hand, mixground refers to the deposit surface without a microbial mat or microbial film, where individual sedimentary grain yields to current shear or deformation, forming structures such as normal ripples and so on; And these primary sedimentary structures are nominated as mixground structures that are different from and correspond to matground structures. Thus, concepts about matground and relative to matground structure are used to focus on the result of microbial sedimentation and become very important on sedimentology. According to the analysis of concept evolvement on sedimentology, mixground structures are actually the primary sedimentary structures that are grouped four categories by Pettijohn and Potter(1964); and matground structures are actually mat-induced structures(or matrelated primary structures). Considering the great difference of both the forming mechanism and configuration between matground structures and mixground structures, the matground structure or mat-induced structure(the mat-related primary structure)ought to be placed as its own category in the existing classification of primary sedimentary structures proposed by Pettijohn and Potter(1964), i.e. the primary sedimentary structure of a fifth category. The change of concept from the matground structure to the primary sedimentary structure of a fifth category expresses the importance of microbes- sedimentation and the thorough cognition to this kind of sedimentation. Like stromatolites, these primary sedimentary structures and their forming matground were common through the Precambrian. With the onset of Cambrian agronomic revolution, these primary sedimentary structures and their forming matground were replaced by mixground and its relative mixground structures in all but the most inhospitable habitats due to effective grazing and burrowing of metazoa. In addition to the stromatolite, microbial related primary sedimentary structures developed in matground are highly facies-indicative and can aid in paleoenvironmental reconstruction, which is the chief reason for the terminology as “the primary sedimentary structure of a fifth category”. The widespread distribution in the Precambrian and the sporadically distribution in the Phanerozoic, especially for their mischance flowage in sea floor after the mass extinction event, make this category of sedimentary structures become meaningful and useful to study the evolutionary change of earth surface. Ultimately, the concept change from the matground structure to the primary sedimentary structure of a fifth category is an important change on sedimentology. According to this change of concept, the reasonable cognition for origin of those grotesque sedimentary structures in the Precambrian(e.g., the spindle or subcircular cracks that are frequently thought as trace fossils, the molar-tooth structure)and the special lamina from the Precambrian to the Phanerozoic can be obtained. On the other hand, the study on the microbialinduced primary sedimentary structures should give rise to new questions and perspectives of the complex relationship between the biological change and the sedimentary change.

Key words: matground structure, microbial-induced primary sedimentary structure, the primary sedimentary structures of a fifth category, meaningful evolvement of concept