Anatomy of a subglacial “deforming bed” beneath a Late Palaeozoic glacier in Namibia

Namibia is remarkable in terms of its network of 300-million-year-old fjords, cut by Gondwanan glaciers at high palaeolatitudes during the Late Palaeozoic Ice Age. A classic suite of structures is preserved on Proterozoic bedrock, including striations, roches moutonnées and other subglacial features within many of these palaeovalleys. Some palaeovalleys are plastered with comparatively thin diamictites (a few metres) of presumed subglacial origin, in turn capped by paraglacial to postglacial delta successions (tens of metres). Close examination of deposits in the Otjihaa valley reveals an architecture that shows departure from this trend. There, boulder-bearing diamictites pass laterally into highly contorted heterolithic successions comprising folded and faulted sandstones, siltstones and shales. Aerial imagery from UAVs in tandem with traditional field observations permits mapping of (i) highly asymmetric fold structures (tens of metre-scale), (ii) ball and pillow structures, (iii) the orientation of the long-axis of boulders together with (iv) striation orientations, (v) pervasive lineations and (vi) shear bands in sandstone. Collectively, this assemblage of structures points towards subglacial deformation, and hence an origin as a “deforming bed” beneath a glacier. In this paper, we present a synoptic profile along the palaeo-fjord axis charting the architecture of these deposits. The lateral and vertical distribution of different deformation products, together with their orientations, is argued to allow subglacial stresses to be reconstructed, and hence ice flow dynamics to be better understood. In studies of deep time glaciations, the discovery of a significant “deforming bed” cradled within a bedrock palaeovalley is a new development, interrupting the classic deglaciation-transgression sequence established by previous work.