Introduction
Microbially induced sedimentary structures (MISS) occur in clastic deposits such as sand. The structures are also well known from the fossil record, where they even can be found in sandstones of early Archean age. Most studies focus on MISS in tidal flats, lagoons, shelves, lakes and riverine shorelines. They are also prominent features on costal sabkhas and playas.
In the Treatise of Invertebrate Paleontology, MISS constitute the fifth group of microbialites aside of “stromatolites”, “dendrolites”, “thrombolites”, and “leiolites”. MISS also constitute a separate category of primary sedimentary structures, placed aside “physical sedimentary structures” and “trace fossils”.
MISS result from the interaction of benthic microorganisms with sedimentary dynamics. Microbenthos such as cyanobacteria assemble to biofilms and microbial mats which are basically organic layers covering the sedimentary surface. The only requirement for biofilm colonization of a substrate is that water must be present, even if there were just a few water molecules. Biofilms and microbial mats are the oldest known life form on Earth. In carbonate settings, they may form stromatolites, whereas in clastic (sandy) settings they form MISS.
Biofilms and microbial mats are coherent layers that protect their substrate against erosion by waves and currents. Sediment motion therefore is prohibited. In case of deposition of sediment on top of the organic layers, the individual microorganisms give up their mat fabrics and move upward to reestablish their biofilm on the new surface. This process is called baffling and trapping. The establishment of a biofilm includes interweaving of sediment particles of the sedimentary surface; hence it is called “binding”.
The result of such microbial action is that the morphology of a surface area covered by biofilms and mats looks different than a surface area, where there is only sand. Those different looking surface morphologies are the MISS.
One of the commonly known MISS are wrinkle structures. They are quite frequent in the fossil record, especially in shelf areas. However, such wrinkled surfaces must not be confused with surface textures that result from abiotic causes such as rain drops deforming a sedimentary surface and giving rise to similar features.
Aside wrinkle structures, however, there are many other MISS, most of them quite characteristic. Seventeen MISS-types are distinguished. They are summarized in this graph here:
My students and I love field work. After all, it is the best part of doing geology – right? In the field, we search lithologies that record paleoenvironments, where microbial mats once thrived. Tidal flat settings are usually easy to start with because modern tidal settings are the classical study sites for microbial mat research. In stratigraphic successions recording such paleoenvironments, sandstones that form 2 to 20 cm thick rock beds may include MISS if the upper bedding planes represent the original environmental surface.
The surfaces of each rock bed must represent the original environmental surface. Here we look for MISS. (See “MISS types” for more details). Because microbial mats constructed by photoautotrophic cyanobacteria are best understood, we prefer “clear” (translucent) quartz-rich sandstones.
Once we find a structure, we measure its geometries and dimensions. If we can, we also collect specimen though mostly we carefully remove only a very small piece in order to perform additional petrological analyses in the laboratory. It is important to preserve any fossil in outcrop, because many people may want to see the fossils.
The small rock sample is used for various petrological analyses such as light microscopy of thin-sections, mineralogical analyses with EDX, SEM and many other methods. We work together with many different colleagues in the world, who have such “toys” at their institutions.
Examples for MISS occur in 3.48 billion year old rocks in the Pilbara, Australia, where they constitute some of the oldest biogenic structures in Earth history. Similar structures may be detected also in equivalent rocks on other terrestrial planets such as Mars.
The Subcommission on Precambrian Stratigraphy of the International Stratigraphic Commission works on identifying boundaries for the Archean and Proterozoic eons. The MISS are one indicator group used for the identification of such boundaries.
Microbial mats in coastal settings develop during sea level high-stands. Due to this reason, MISS in a fossil stratigraphic section indicate transgressions. Especially helpful are MISS for stratigraphers analyzing otherwise very homogeneous clastic rock successions with only limited lithological information.
Early Archean life and environments are notoriously difficult to reconstruct. MISS allow, however, concluding on ancient existence of microorganisms that resemble strongly modern cyanobacteria. The microbes most likely were photoautotroph and produced abundant extracellular polymeric substances. Moreso, MISS-morphologies mirror the ancient current systems at the locale of their occurrence.
MISS do not occur at random within a specific area. Their morphology is function of the composition of the mat constituents and of the average sediment-dynamic conditions. The MISS-morphologies are also strongly influenced by the respective meteorological situation. In conclusion, MISS form ‘zones’ within coatal settings, depending on the geomorphology of the colonization site. Fossil MISS record in detail the paleoclimate, whether it was a moderate-humid or a semi-arid tropical setting, etc. One very nice example of a rock succession displaying a climate change experienced by a tidal flat system over time is preserved in the Sinqueni section of the 2.9 Ga Pongola Supergroup, South Africa.