Abstract The Rasthof Formation is a mid-Cryogenian cap carbonate succession deposited in Namibia following the Sturtian glaciation. It includes a microbial member, typically >100 m thick. This member exhibits contorted intervals, and is divisible into two informally defined units. The lower unit (microbial member 1: MM1) comprises thickly laminated microbialites (1–6 mm); the upper unit (MM2) is characterized by thinly laminated microbialites (sub-millimetre layering). Contortion of the microbialite deposits – a recurrent feature of this succession – is interpreted to result from soft-sediment deformation. Deformed intervals and styles range from metre- to decimetre-scale chaotic folds in MM1 to a few centimetre-scale, localized roll-up structures in MM2. Study of the microfacies of MM1 and MM2 reveals two essentially different architectures. In MM1 the microfacies is dominated by an alternation of thin micritic laminae with thicker cemented intervals; this probably gave less rigidity to the sediments than in MM2 where the laminated fabric is also present but connected vertically as well, forming a continuous framework. We suggest that the continuity of this framework limited the frequency and scale of soft-sediment deformation. In the Rasthof Formation, the microarchitecture is thus suggested to translate into different degrees of rigidity of the macrofacies.

Abstract The lower cap carbonate (Rasthof Formation) overlies Neoproterozoic glacial deposits (Chuos Formation) and is exposed in the Khowarib-Warmquelle area in Northern Namibia. The basal 14.2 m part of the Rasthof Formation (total about 220 m) consists of the carbonate rhythmite. The rhythmite part of the Rasthof Formation contains 1 m cycles of dark- and light-coloured rhythmites. Alizarin-red staining of thin sections and elemental mapping of polished samples indicate that the dark-coloured parts are rich in calcite, whereas the light-coloured parts are dolomite-rich. On a 1 cm scale, a reddish clay layer is intercalated in each calcite rich dark-coloured rhythmite part. These cycles of reddish clays as well as some associated major turbidites can be well correlated between columns up to about 20 km distance. Furthermore, at one locality (K4), rip-up clasts occur in a turbidite bed. Their lithology consists of dark- and light-coloured rhythmite and a reddish clay layer and can be judged to have been derived from underlying horizons. Because the clasts are elastically deformed, it is strongly suggested that the difference in lithology observed within the basal part of the Rasthof Formation existed when clasts were ripped-up shortly after sedimentation. This suggests that the cycle involving dolomite is synsedimentary, and not a diagenetic feature. Direct precipitation of dolomite does not occur in present day open marine seawater. Hurtgen et al. (2002) suggest that seawater was depleted in sulphate after Neoproterozoic glaciation. It is proposed here that some possible depositional models of cycles that consist of calcite-rich and dolomite-rich parts as well as reddish clay beds in rhythmites of the Rasthof cap carbonate.