The Evolution and Ages of Makgadikgadi Palaeo-Lakes: Consilient Evidence From Kalahari Drainage Evolution South-Central Africa

Abstract

Tiie Makgadikgadi Pans in northern Botswana are the desiccated relicts of a former major inland lake system, with fossil .shorelines preserved at five distinct elevations (~995 m, 945 m, 936 m, 920 m and 912 m). These lakes persisted in the Makgadikgadi Basin, which evolved in the Okavango-Makgadikgadi Rift Zone: the south-western extension of the East African Rift System (EARS) into northern Botswana. This paper synthesizes cross-disciplinary evidence, which reveals that the antiquity of this lake complex has been widely underestimated. It presents a Regional Drainage Evolution Model that invokes tectonically initiated drainage reorganizations as the underlying control over lake evolution. Lake formation was initiated by rift-flank uplift along the Chobe Fault, across the course of the Zambezi River, which diverted the regional drainage net into the Makgadikgadi Basin. Filling of the basin initiated a major climatic feedback mechanism that locally increased rainfall and lowered evaporation rates. This progressively enhanced water input to the basin, and most likely led to overtopping of the Chobe Horst barrier during the three highest lake stand.s, with outflow into the Zambezi River. During this period, the hydrology of the basin would have been closely analogous to modern, shallow Lake Victoria. Fragmentation of the regional drainage network by successive river captures resulted in sequential contractions of the lake to lower elevation shorelines. In turn, resultant decreases in areas of these successive lakes modulated the magniaide of the feedback mechanism. Thus, loss of the Upper Chambeshi catchment caused the lake to drop from the 990 to the 945 m level. Severance of the former link between the Kafue and Zambezi resulted in a further drop to the 936 m shoreline. Inflow declined further after the impoundment of a major lake (Palaeo-Lake Bulozi) on the Upper Zambezi River, causing contraction to the 920 m shoreline. Continued incision of the Zambezi channel into the Chobe horst barrier ultimately terminated input from this river to the Makgadikgadi depression, causing contraction of the lake below 920 m, sustained by the Cuando and Okavango prior to final desiccation. This Regional Drainage Evolution Model contradicts previous proposals that have invoked Late Pleistocene climatic forcing to explain inferred fluctuations in lake levels. The timeframe developed for the drainage reorganizations requires that the lake was initiated by ~1.40 to 0.51 Ma at the most recent (Early -Mid-Pleistocene), while archaeological evidence shows that it had contracted below the 936 m shoreline before 500 ka. This contrasts with "C and quartz luminescence dates (generally <100 ka), which require that the 945 m lake stage was extant during much of the Upper Pleistocene. The calcareous radiocarbon dates reflect multiple episodes of calcrete formation, while the young luminescence dates are ascribed to the extensive bioturbation of older Kalahari landforms.

Paleo-megalake and paleo-megafan in southern Africa

“…Crustal fl exuring involving continental-scale subsidence in the center and uplift along surrounding swells created the Kalahari Basin (Moore and Larkin, 2001;Burke and Gunnell, 2008;Moore et al, 2012). Changes in erosion and sedimentation rates in major rivers and offshore sedimentary basins and deltas, together with fi ssion-track cooling ages of basement rocks, suggest that crustal fl exuring occurred in the middle Tertiary.…”

“…It has been suggested that the water volume entering PLM gradually reduced as the easterly fl owing Lower Zambezi River progressively captured the Cuando, Upper Zambezi, and Kafue Rivers (Thomas and Shaw, 1992;Moore and Larkin, 2001;Burrough et al, 2009;Moore et al, 2012). There is also evidence that the rivers occasionally or frequently reverted to their former southeasterly courses.…”

“…There is also evidence that the rivers occasionally or frequently reverted to their former southeasterly courses. It has been widely accepted that these drainage-pattern changes were signifi cantly infl uenced by faulting (Mc-Carthy, 2006;Burrough et al, 2009;Moore et al, 2012).…”

Phylogeny and vicariant speciation of the Grey Rhebok, Pelea capreolus

“…This would be consistent with Pickford's (2004) hypothesis that posits a long period of adaptation to arid and semi-arid conditions in relative isolation from similar biotypes elsewhere, and which has been invoked to account for the high levels of endemism encountered in numerous South African plant and vertebrate lineages. Should this hold, the increasingly dry conditions in the late Miocene led to Pelea's adaptation and persistence in exposed habitats, free of standing water (Skinner and Chimimba, 2005), while the lineages giving rise to the extant 'presence-of-permanent-wateradapted' species survived in contracted, water-abundant refugia such as those offered by the Okavango delta and the drainage systems in the southwestern extension of the East African Rift system (Moore et al, 2012). In this regard, it is particularly noteworthy that the vast majority of new reduncine 'species' presented in Groves and Grubb (2011), most likely regional ecotypes and subspecies (Heller et al, 2013;Zachos et al, 2013), are found in these areas.…”

Shaping of modern southern African biomes: Neogene vegetation and climate changes

“…Until c. 3 Ma, Poaceaerich savanna with Mopane trees (Colophospermum mopane) was dominant probably in the eastern hinterland of the Namib Desert. In northern Botswana, which was largely occupied during the Pleistocene by the palaeo-mega lake Makgadikgadi, stretching from the current Makgadikgadi basin up to the Okavango delta (see Figure 1), the late Pliocene Alab dune fields point to rather dry conditions (Moore et al, 2012;Podgorski et al, 2013).…”