Abstract
Microbialite is a type of organosedimentary desposits where the presence of organic matrix of a microbial biofilm plays predominant role in petrogenesis. This study reviews previous researches on microbial-induced carbonate deposits, microbe–mineral interaction in carbonate precipitation, and sedimentary and petrographic features of these deposits. It also presents modern deposits and some case studies from Hungary. The activity of the bacterial biofilm communities has a significant effect on their environment and can initiate crystal nucleation and growth. The mineral precipation can be both biologically induced and influenced wise, in contrast with biologically-controlled biominealisation that is common in organisms with internal or external calcareous skeletons. The mineralization in the biofilms is related to the increasing alkalinity and the released Ca2+ ions, which elevates the carbonate saturation level of the pore water, or to increasing pH level. Previous studies showed that mineral precipation takes places in several stages. Firstly, there is an increase in local alkalinity in the extracellular polymetric substance (EPS) that favors the formation of amorphous CaCO3 gel. Secondly, nanospheres appear in the matrix that provide substrates for mineral nucleation. Carbonate minerals that form in the realm of diagenesis have a specific petrogaphic features. Clotted micrite as well as the presence of calcimicrobes and fenestral pores are the microscopic components that define the microbialite. In addition to the microscopic fabric, microbialites also have various macroscopic fabric and structures that place them into four categories: laminated stromatolite, blothcy thrombolite, bush-like dendrolite, and structureless leiolite. Biofilm originated crusts can form in cavities of reef frameworks that also belong to the term of microbialite. Microbialites can compose microbial reefs or layered, stratiform sheets that are defined by the shape of the deposits and facies connections.
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