A note on two occurrences of inferred microbial mat features preserved in the c. 2.1 Ga Magaliesberg Formation (Pretoria Group, Transvaal Supergroup) sandstones, near Pretoria, South Africa

Abstract

A complex variety of casts of sand cracks occurs at two study areas near Pretoria, within mature, thin sandstone beds of the c. 2.1 Ga Magaliesberg Formation (Pretoria Group, Transvaal Supergroup). This formation is ascribed to deposition within the littoral, sandy portions of a low-energy epeiric sea, then covering much of the Transvaal preservational basin. Reticulate networks of these sand crack casts (which equate to petee ridges) and lesser vermiform geometries, the latter commonly associated with ripple troughs ( cf. Manchuriophycus ), reflect a degree of cohesiveness which is difficult to envisage in sands unless there were either thin muddy interbeds between sandy strata, or the sandy beds were bound by microbial mats growing in their upper portions. In the absence of any observed mudstone interbeds, a genetic role for microbial mats can be supported. Several of these inferred microbially-induced sand cracks appear to have “healed”, presumably having become overgrown by re-establishment of a new mat following partial desiccation of an earlier one. Associated “oncolites”, found at one of the sites, confirm this general model of microbially-mediated “sand-cracking” having taken place. Such microbial mat features appear to be relatively widespread within the Magaliesberg Formation, most likely reflecting the shallow epeiric marine palaeoenvironment which would have favoured their preservation.