MICROBIALLY INDUCED SEDIMENTARY STRUCTURES (MISS) AND THE SURVIVOR SPECIES CONCEPT

Most studies have been on stromatolites, microbial build-ups, which mainly occur in carbonate marine environments. In contrast, microbial structures in siliciclastic depositional regimes have been far less investigated. Therefore, this contribution elucidates ‘microbially induced sedimentary structures – MISS’, the siliciclastic counterparts to stromatolites.

The formation of MISS can be observed in modern siliciclastic sedimentary systems that are governed exclusively by physical dynamics. Here, benthic cyanobacteria form soft, unlithified ‘microbial mats’ colonizing quartz-rich fine sands in regions of moderate erosion and low depositional rates. The microbial mats withstand erosion by microbial sediment fixation, known as ‘biostabilization’, or they respond to depositional events by ‘baffling, trapping, and binding’.

Those cyanobacterial activities give rise to the characteristic ‘microbially induced sedimentary structures’ – MISS’. Due to their specific microbial-physical modes of formation, MISS differ in their appearance significantly from stromatolites. Therefore they have been classified as separate category in the classification of primary sedimentary structures.

MISS occur not only in modern marine environments. ‘Wrinkle structures’, ‘multi-directed ripple marks’, ‘erosional pockets and remnants’, and many other MISS are distributed in siliciclastic rock successions from the mid-Archean to the Pleistocene.

Fossil MISS are relatively rare compared to the fossil stromatolites. The study summarizes investigations on MISS conducted in the Mid-Archean Pongola Supergroup, South Africa; the Neoproterozoic Nama Group, Namibia; the Ordovician of the Montagne Noire, France; the Cretaceous Dakota Sandstone at Denver, Colorado; and the Pleistocene of southern Tunisia.

Detailed investigations document that the rare number of specimen is consequence of (i) the fact that microbial mats preferently colonize a specific sedimentary facies characterized by a distinct mineral composition and hydraulic regime, and (ii), the complex taphonomic path, which leads to the preservation of the ancient microbiota. In contrast, modern microbial mats are very abundant. Therefore, the study concludes that MISS-forming microbial mats are survivor species.