A tale of two microbialites: Stromatolites and microbially induced sedimentary structures

ABSTRACT Microbialites are biosedimentary structures built by microbial mats. Five microbialite groups are distinguished: stromatolites, thrombolites, dendrolites, leiloites and microbially induced sedimentary structures (MISS). This contribution discusses the two most abundant microbialite groups in marine settings, stromatolites and MISS. Microbial interactions with clastic sediment are very similar in both stromatolite and MISS formation. However, in stromatolites, syngenetic carbonate production caused by photoautotrophic and heterotrophic metabolisms and by calcimicrobes also takes place. More so, carbonate may precipitate within the extracellular polymeric substances. Due to the various carbonate forming processes as well as recrystallisation, the microscopic mat fabrics within stromatolite laminae are rarely preserved. Merely the laminae themselves remain visible. In MISS, syngenetic carbonate production commonly does not take place. However, early diagenetic mineral replacement of organic matter by heterotrophic microbes can be observed. Microscopic mat fabric in ancient MISS is well preserved, composed of minerals such as pyrite, silica and others. The early Archean record displays a high number of morphotypes of both stromatolites and MISS, such as those in the 3.48 Ga Dresser Formation, Pilbara, Western Australia. The earlier evolution of these microbialites is not recorded due to the lack of well‐preserved pre‐Archean sedimentary rocks. In course of Earth history, stromatolites developed hundreds of morphologies with their greatest variations in the Proterozoic time, where stromatolites were shown to be useful index fossils for biostratigraphy. MISS, however, include only 17 morphotypes that existed since the early Archean with no morphological change until the modern time. However, MISS morphologies reflect environmental and climatic conditions in great detail because their formation is strongly influenced by physical sedimentary dynamics. Therefore, MISS are valuable facies fossils.