Mapping a 300-million-year old glacier bed: The Noordoewer section, Orange River, Namibia

Abstract

Glacial geomorphological structures produced during the Late Palaeozoic Ice Age (LPIA) of Namibia provide exceptional insight into the flow direction, flow behaviour, and plumbing of subglacial drainage systems in deep time. At Noordoewer (southern Namibia), along the Orange River, a unique and largely undocumented ∼300 Ma assemblage of subglacial structures within the Dwyka Group is investigated. A detailed geological map documents a complex array of both hard- and soft-bed features shaped by the combined action of subglacial abrasion and meltwater. Structures trend parallel to the Orange River, supporting the view that part of its course originated as a Late Carboniferous valley. Key glacial indicators in a hard bedrock area developed on the Nama Group (Ediacaran metasediments) include (i) striated pavements, (ii) multiple roches moutonnées, and (iii) meltwater channels / p-forms. In areas comprising either a veneer or sheet of sandy diamictite, additional soft subglacial bed features are recognised, including (iv) a flute field, (v) soft-sediment striated surfaces (SSSS) and (vi) glaciotectonic structures including deformation bands. Collectively, these six sets of features support a NNW-directed palaeoflow, in direct contrast to previous interpretations of a southward or westward glacial flow down the axis of the Karasburg Basin. Thus, the new findings have broad implications for reconstructing past ice sheet dynamics and interpreting the geological record of glacial landscapes in the Late Palaeozoic record.