248 / 2019-06-30 10:42:38

Sequence structures and sedimentary mechanisms of microbialites in carbonate platforms

Sea level, Sequence structure, Microbialites, Stromatolite, thrombolite

The types, structures and distribution patterns of microbiolites within the space and time system are the primary issues in the study of microbiolite formations. Based on the confine of different microbial evolution phases during the geological time, this paper analyzes the sequence structure and formation mechanism of microbiolite formed in platform enviroment, and proposes three basic types: the shallow-water inner-platform microbiolite; the shallow-water microbial reef; and the deep-water microbial mud mound.
In the inner-platform, the lower part of the TST was characterized by the layered thrombolite. At the top of TST, the subtidal thrombolite transited from layered to dendritic with the rise of sea level, and the intertidal environment mainly developed the LLH-S-type stromatolites. In the bottom of the HST, most of the thrombolites were reticulate, the stromatolites were mainly LLH-C type, and the supratidal was dominated by microbial mats.
On the platform edge, the TST was characterized by the microbial biohermal, and the internal fabric was mostly composed of the thrombolite. In the early HST, the morphology of reef gradually changed from biohermal to biostromal and the internal fabric was mostly composed of columnar stromatolites. Before Ordovician, the microbiolite mainly appeared in the form of individual buildup in the shallow water environment. After that, the buildups were more formed by microbes together with metazoans.
In the slope and basin environment, microbiolites mainly appeared in the form of mud mounds near the maximum flooding surface. The microbial mud mounds were widespread from Neoproterozoic to Jurassic. While after Jurassic, they were gradually replaced by the abundant plankton.
The development of microbiolites in sequence units is controlled by many factors, such as ecological accommodation space, hydrodynamic energy, sedimentation rate, detrital carbonate and nutrient supply, etc., among which the sea-level change is the most prominent. It is concluded that in the shallow water environment, the sea level determines the type and morphology of microbiolites, and high-frequency eustatic change controls the fabric within microbiolites. While for deep water environments, the sea level controls the productivity of mud mound factory, while the high-frequency sea level controls the development pattern of metre-scale parasequences.