Abstract The Chuos Formation of Namibia is the sedimentary product of the Neoproterozoic Sturtian ( c. 720–660 Ma) glaciation and contains massive diamictites intercalated with finely laminated iron formation. Similar Sturtian glacially associated iron formations are found globally. The iron formations are laminated and generally very pure. The diamictites are massive, contain abundant clasts and can be highly ferruginous. These two lithofacies are repeatedly interbedded with no facies transition. The iron formations preserve the rare earth element geochemistry of their contemporaneous seawater and contain rare Ce and Eu anomalies. The geochemistry does not implicate a hydrothermal influence. The Chuos iron formation is interpreted to have been deposited in an ice-proximal glacio-marine setting in a sub-ice shelf environment. Oxygenated fluids, such as sea ice brines and glacial meltwater, are invoked as a mechanism to precipitate iron oxides due to mixing with ferruginous seawater. The iron formation accumulates under an ice shelf with little clastic input. Episodic movement of the grounding line reworks the sediments into ferruginous diamict. Glaciogenic debris flows are intercalated with the iron formations. Palaeobathymetric depressions and accompanying brine pools increased the preservation potential of these iron formations. This model explains the relationship between glaciation and iron formation in the Neoproterozoic. Supplementary material: The full set of geochemical data is available at https://doi.org/10.6084/m9.figshare.c.4031125.v1

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.