Glacio-marine iron formation deposition in a c. 700 Ma glaciated margin: insights from the Chuos Formation, Namibia

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.