An interglacial on snowball Earth? Dynamic ice behaviour revealed in the Chuos Formation, Namibia



The Sturtian is the oldest (ca 716 Ma) of three pan-global glaciations in the Cryogenian. At Omutirapo, in northern Namibia, a 2 km wide, 400 m deep palaeovalley is filled by glaciogenic strata of the Chuos Formation, which represents the Sturtian glacial record. Sedimentary logging of an exceptionally high-quality exposure permits detailed stratigraphic descriptions and interpretations, allowing two glacial cycles to be identified. At the base of the exposed succession, strong evidence supporting glaciation includes diamictites, ice-rafted dropstones and intensely sheared zones of interpreted subglacial origin. These facies collectively represent ice-proximal to ice-rafted deposits. Upsection, dropstone-free mudstones in the middle of the succession, and the absence of diamictites, imply sedimentation free from glacial influence. However, the reappearance of glacial deposits above indicates a phase of Sturtian glacial re-advance. Comparison with age-equivalent strata in South Australia, where evidence for sea-ice free sedimentation has been established previously, suggests that a Sturtian interglacial may have been extensive, implying global-scale waxing and waning of ice sheets during a Cryogenian glacial event.