Fluvial landscape development in the southwestern Kalahari during the Holocene – Chronology and provenance of fluvial deposits in the Molopo Canyon

Abstract

The southern Kalahari drainage network is in a key position to analyze spatiotemporal changes in the tropical easterly and the temperate westerly circulation over the Southern African subcontinent. However, due to the prevailing aridity, paleoenvironmental archives within the southwestern Kalahari are sparse and often discontinuous. Hence, little is known about Holocene environmental change in this region. This study focuses on reconstructing paleoenvironmental change from the timing and provenance of fluvial deposits located within the Molopo Canyon, which connects the southern Kalahari drainage to the perennial flow regime of the Orange River. To gain insight into temporal aspects of fluvial morphodynamics within the Molopo Canyon, the entire variety of fluvial landforms consisting mainly of slope sediments, alluvial fans and alluvial fills were dated using Optically Stimulated Luminescence (OSL). We additionally applied a provenance analysis on alluvial fill deposits to estimate potential sediment source areas. Source areas were identified by analyzing the elemental and mineralogical composition of tributaries and eolian deposits throughout the course of the lower Molopo. The results allow the first general classification of fluvial landscape development into three temporally distinct deposition phases in the southern Kalahari: (1) A phase of canyon aggradation associated with short lived and spatially restricted flash floods during the early to mid-Holocene; (2) a phase of fan aggradation indicating a decrease in flood intensities during the mid- to late Holocene; and (3) a phase of canyon aggradation caused by the occurrence of supra-regional flood events during the Little Ice Age. We interpret the observed spatiotemporal deposition patterns to latitudinal shifts of the tropical easterly circulation in the early to mid-Holocene and the temperate westerly circulation in the late Holocene. However, despite marked changes in the provenance and timing of fluvial deposits in the Molopo Canyon throughout the Holocene, our analysis did not detect a contribution of sediments originating from the Kalahari interior to the deposition of alluvial fills. These results suggest that the southern Kalahari Drainage remained endorheic and therefore disconnected from the Orange River throughout the Holocene.