THE ORUTANDA PALEOFJORD IN NAMIBIA: A LATE PALEOZOIC ICE AGE FJORD FILLED ESSENTIALLY BY GRAVITATIONAL RESEDIMENTATION

The stratigraphic architecture of fjords is complex as it is controlled by ice dynamics, sediment supply, relative sea-level fluctuations, and topography. Glacially-transported debris are prone to failure and to be carried downslope to the fjord floor through the entire spectrum of mass movements and density flows, as the submarine landscape moves toward to a more stable state. Paleofjords formed by Gondwanan glaciers during the late Paleozoic Ice Age (362-256 Ma) contain a compelling record for comprehending the imprint of gravitational resedimentation in fjord depositional systems. This work presents the morphology and the depositional history of a glacial-deglacial cycle recorded in the Orutanda paleofjord in the Namibian fjord network. Three stages are defined for the Orutanda fjord evolution: glaciation, early deglaciation, and late deglaciation. During glaciation, the up to 200 m-thick Orutanda glacier eroded a 20 km-long and 3.7 km-wide glacial trough that embodies an overdeepened basin. The progressive retreat of the tidewater glacier, concomitant with marine flooding, increased accommodation in the overdeepening during early deglaciation. During this stage, minimal proglacial sedimentation by iceberg rafting and settling of turbid overflow plumes was outpaced by intense paraglacial resedimentation of previously glacially-transported debris. Successive failures from the fjord walls and downslope resedimentation resulted in coalescing debrite-turbidite lobes on the fjord’s floor. Slumps entirely composed of deformed debrites and turbidites indicate that these resedimented facies were prone to renewed downslope mass wasting. As the Orutanda glacier became land based and lost contact with marine waters during late deglaciation, the fjord experienced the installment and axial progradation of a fjord-head delta with deposits registered only by slumps and turbidites derived from its collapse. The record derived from the Orutanda paleofjord showcases an example of an overdeepened fjord basin fill dominated by subaqueous gravity-driven resedimentation products. This example elucidates the importance of downslope gravitational resedimentation of glacially-transported sediments for an accurate interpretation of glacial poorly-sorted facies in fjord depositional systems.