Stratigraphy of the Neoproterozoic Damara Sequence in northwest Namibia: Slope to basin sub-marine mass-transport deposits and olistolith fields

Abstract The Neoproterozoic Damara Sequence (>1000 m thick) is composed of siliciclastic and carbonate rocks that crop out in the Damara Belt, Namibia. In Damaraland (including the Vrede, Bethanis, Austerlitz and Toekoms farms), these rocks were deformed and metamorphosed under greenschist facies (biotite zone) conditions during the Damara Orogeny. The stratigraphy and paleoenvironments of the Damara Sequence rocks are debated by the scientific community. We use field data, including detailed 1:25,000 geological mapping, elaboration of stratigraphic profiles and observation of preserved primary structures, textures and composition, to identify lithofacies and lithofacies associations, and to interpret lateral stratigraphic relations. The Nosib and Otavi/Swakop groups of the Damara Sequence in the study area are herein interpreted as gravitational deposits, including up to 150 m thick slide blocks, related to the opening of the Outjo Basin. Debrites of the Nabis Formation (Nosib Group) are basement derived, and were developed in alluvial fan setting. Siliciclastic, carbonate and mixed successions of the Otavi/Swakop groups reflect slope to basin deposits and can be grouped in two tectonic sequences. The lower sequence consists mainly of slope and proximal submarine fan successions; medium to distal fan deposits characterize the upper sequence. A U–Pb SHRIMP zircon dating of a dacite layer yielded a 757 ± 5 Ma age for the base of the lower sequence. In the study area these sequences are limited by an unconformity. This unconformity records basin margin onlap and was later partially reactivated as a thrust fault during Pan-African compression. The Chuos and Ghaub formations, considered by several authors as Sturtian and Marinoan glaciogenic deposits, are interpreted alternatively as gravitational subaqueous mass flows. Rudstone beds similar to those of the Ghaub Formation occur in the Berg Aukas and Karibib formations and reinforce this interpretation. Large blocks, locally containing stromatolites are interpreted as olistoliths and isolated blocks (pebble to boulder size), are interpreted as sieved from gravitational flows, leaving a non glacial interpretation for these formations as a distinct possibility.