THE SIGNIFICANCE OF ‘MICROBIALLY INDUCED SEDIMENTARY STRUCTURES – MISS’ FOR THE SEQUENCE STRATIGRAPHY OF SILICICLASTIC DEPOSITS OF THE 2.9 GA OLD WITWATERSRAND SUPERGROUP, SOUTH AFRICA

Today, shallow-marine settings are colonized by a great variety of benthic microorganisms, of which bacteria and cyanobacteria are very abundant. The benthic bacterial communities form ‘microbial mats’, organic, dense layers that cover the depositional surfaces like a tissue. In carbonate environments, the microbial mats induce the formation of stromatolites. In contrast, in siliciclastic sedimentary regimes, the microbenthos creates ‘microbially induced sedimentary structures – MISS’.

The Witwatersrand Supergroup, South Africa, has an age of about 2.9 Ga. It contains the Brixton Formation, which consists of quartzite, silt- and mudstone units. The rock succession records a wave-dominated shallow shelf depositional environment. Because today such marine settings are preferred sites of microbial mat development, we assumed to detect ancient microbial mats in the paleoenvironments preserved in the Brixton Formation as well.

Four fine sand beds contain MISS that record extensive ancient microbial mats covering large areas of the shallow ocean floor. The microbial mats colonized a lagoon, and portions of the inner shelf. The mats are preserved as ‘wrinkle structures’, and ‘erosional remnants and pockets’. In thin-section, the filaments of the ancient (cyano-)bacteria become visible. Facies and sequence analyses show that the microbial mats indicate the turning point between regression and transgression and formed during periods of equilibrium sedimentation in the basin.

It is quite possible that the extensive microbial mats produced large amounts of oxygen through photosynthesis. Little or no organic matter is preserved along the microbially induced sedimentary structures. This could imply that aerobic respiration processes were in place in the Mesoarchean.