Muddy roll-up structures in siliciclastic interdune beds of the c. 1.8 Ga Waterberg Group, South Africa

Concentrically rolled-up silty mudrock laminae, 1-2 mm thick, are found in the uppermost of four, 40-110 cm thick interdune beds within a thick aeolia n succession of the c. 1.8 Ga Makgabeng Formation, Waterberg Group, South A frica. These curved laminae are analogous to previously described "roll-ups ," biogenic structures generally ascribed to soft-sediment deformation or d esiccation of microbial mats overlying either carbonate or siliciclastic se diments, within shallow- to deep-marine paleoenvironments. The Makgabeng ro ll-ups are thought to reflect desiccation of a microbial mat, followed by r esedimentation of cohesive, discrete, curled mat fragments. Their alignment is considered to be the result of an extreme precipitation event. The sign ificance of the South African example of roll-ups is that they appear to ha ve formed within a fully terrestrial paleoenvironment, in one of the oldest known Precambrian deserts. Thus, they represent the oldest evidence for mi crobial colonisation of a terrestrial setting.