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Abstract

The Kalahari Desert contains extensive networks of ephemeral and fossil drainage which are potential indicators of past and present neotectonic activity and climate-driven environmental change. An absence of topographic data has hindered our understanding of their development. We present long-profile information for twenty-nine valley networks derived from Shuttle Radar Topographic Mission (SRTM) digital elevation data. In total, 8354 km of valley talweg was measured for x, y and z information. Most valleys exhibit concave-up profiles. Fifty-five previously unknown knickpoints were identified. The majority coincide with lithological boundaries or fractures, but many developed in response to Neogene uplift and/or downwarping or occur where valleys cross palaeolake shorelines. The headwaters of four valleys cross the Kalahari-Limpopo drainage divide and predate the presumed Miocene uplift of the Kalahari-Zimbabwe axis, suggesting that they are of considerable antiquity.

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... The modern 945 m contour (dashed line), and shoreline ridge point elevations were obtained using SRTM. The river knickpoints (solid triangles), by Nash and Eckardt (2015) were extracted from river long profiles using SRTM also. SRTM only depicts dune ridge topography, the flat dune vegetation stripes depicted as dashed EeW lines are seen in Landsat 7 imagery only. ...
... Topologies of the endorheic MRZ drainage channels are aligned parallel to these faults, in contrast to the Okavango River, which drains perpendicularly across the ORZ faults (Fig. 1). Recent work by Nash and Eckardt (2015) has shown that some of the knickpoints associated with the MRZ drainage appear to be controlled by the faults identified here. The mapped faults and associated forms clearly depict neotectonic processes in the MRZ. ...
... Magnetic data for Zimbabwe not available and not included in the rose diagram. The location of knickpoints from Nash and Eckardt (2015) are indicated by solid triangles. Present day pans are shown by the grey shading. ...
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The Makgadikgadi basin and wider Middle Kalahari region of Botswana and beyond host landforms which have been attributed to Quaternary environmental change, including palaeolake level fluctuations and aeolian activity. Tectonic processes and landforms on the other hand, have mostly been linked to the Okavango graben and associated rift zone (ORZ) to the west of the Makgadikgadi. In this paper we establish the extent of tectonic surface expression associated with the Makgadikgadi Rift Zone (MRZ). We identify a series of parallel, NNE-SSW, normal faults and scarps, expressing horst and graben structures linked to seven major blocks in the northern Makgadikgadi basin, using both Shuttle Radar Topographic Mission (SRTM) and geomagnetic data. Subtle expression of rifting has controlled endorheic drainage topology, replicated regional dune-field patterns and displaced the 945 m palaoelake contour since lake desiccation. These observations underscore the role of neotectonic " piano key " block movement in shaping surface landforms across a large expanse of the Kalahari region. This paper provides the first detailed map and introduction to the Makgadikgadi Rift Zone (MRZ) and its geomorphology.
... According to previous studies, historically, such analyses have primarily been carried out from river history examinations, in addition to regional tectonic background, paleochannel, and alluvial fan surveys at the mouth of the river. Nash et al. [3] used digital elevation models (DEMS) from the Shuttle Radar Topographic Mission (SRTM) to map the profile of the Kalahari Desert river network, finding that the network development was very sensitive to tectonic movements, from where most of the river valley inflections were derived. ...
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The Qinghai-Tibetan Plateau, known as the world’s “third pole”, is home to several large rivers in Asia. Its geomorphology is exceptionally vulnerable to climate change, which has had a significant impact on historical riverbed development through runoff and sedimentation processes. However, there is limited research combining climate change, sedimentology, and chronology with river dynamics to investigate riverbed evolution patterns in geological-historical time scales and their changes in overland flow capacity. In the current study, the evolution of a representative portion of the river channel in the Nangqian basin in the Lancang River headwaters was investigated to explore the reaction of the riverbed to climatic change during the geological period via field surveys, riverbed drilling, optically stimulated luminescence (OSL) dating and bankfull channel geometry parameters. The generalized channel section of the historical period was obtained by linking sedimentary layers of the same age on the distribution map of borehole sections, and the bankfull area of the river was computed accordingly. The restored bankfull areas can effectively reflect the ability of historical river channels to transport water and sediment, thus reflecting the climate change at that time. The findings showed that river morphology in the mounded river section could be successfully reconstructed using OSL dating and sedimentary records and that the conceptual sections of the historical warm periods at 2000 years (2 ka) and 0.7 ka can be recovered. Based on the reconstruction, the calculated bankfull areas during the two warm events were larger than present by factors of 1.28 and 1.9, respectively, indicating a stronger capacity for transporting water and sediments. This is the first trial in the Lancang headwaters to investigate the response of river morphology to climate change on a geological time scale.
... The subtle, yet profound, control of the OKZ Axis over the regional drainage has been demonstrated by atypical profiles of ephemeral and fossil drainage networks across the southern Kalahari Basin. The concave-up long profiles of the majority of these arid river channels (revealed by precise mapping) are attributed to epeirogenic flexure (Nash and Eckardt 2016). ...
Chapter
The Zambezi rises with considerable modesty in north‐west Zambia from a small spring on the gentle upland of the Southern Equatorial Divide – the watershed that separates the river from north‐west‐flowing tributaries of the Congo. The evolution of the Zambezi River has repeatedly modified the distribution of riverine plant and animal species. The hydrology of the Zambezi is further influenced by water exploitation by different users, along its main channel and tributaries. The dams have had severe ecological impacts on the major floodplains, as a result of the reduction of the supply of water and sediment. The major Early Cretaceous Zambezi‐Limpopo River system entered the Mozambique coastal plain via a line of crustal weakness that was exploited by a major west–north–west trending dyke swarm. Drainage evolution of the Palaeo‐Chambeshi system has been invoked as the primary cause of the recent evolution of the molerats.
... Tooth, 2018) of different fluvial forms in response to long-term landscape evolution, or changing climate and land management practices (e.g. Larkin et al., 2020;Nash and Eckardt, 2016;Wilcock and Iverson, 2003). ...
Article
A mix of catchment-scale controls operate to determine geomorphic river diversity in a catchment. These are contextualised here as imposed and flux controls. Imposed controls do not adjust over geomorphic timeframes and produce the environmental setting in which a river functions. Flux controls are dynamic interactions between flow and sediment that heavily influence geomorphic river diversity. We measure and statistically analyse the network scale mix of imposed and flux controls occurring along longitudinal profiles of various shape. We use this to explain the diversity, pattern and sequence of river types at different positions in a catchment. Using the Richmond River catchment, New South Wales, Australia, as our case study we analyse the imposed controls of slope and valley bottom width and the flux controls of bed material size and gross stream power for five river types, ranging from confined continuous rivers in headwaters to laterally unconfined discontinuous rivers in lowland plains. We find that slope and gross stream power are strong, positively correlated controls on all river types, but act most strongly on rivers with continuous channels. In contrast, bed material size is a dominant control on rivers with discontinuous channels. Slope and gross stream power are also critical for determining the downstream pattern of river types along longitudinal profiles. However, the relationship between these controls is complex as not all rivers are influenced by these controls in the same way. Understanding the mix and patterns of controls operating along longitudinal profiles can be used to explain the variability and pattern of river types we see in the landscape.
... Lake Ngami water levels have varied considerably during the Holocene and late Pleistocene (Burrough et al., 2007;Huntsman-Mapila et al., 2006;Murphy et al., 1998;Robbins et al., 2008Robbins et al., , 2009Shaw et al., 2003) and in historical times Endfield and Nash, 2002;Hamandawana and Chanda, 2013;Shaw, 1985b). These variations have generally been related to local and regional climatic influences, as well as the influx of water from Angola through the Okavango Delta (Burrough et al., 2007;McCarthy, 2013;Shaw, 1985b;Shaw et al., 2003;Wilson, 1973), although tectonic adjustments are also thought to have played a role in the amounts of water feeding into the lake (Moore et al., 2012;Nash and Eckardt, 2016) (Fig. 2). ...
Article
Pollen, spores, and microscopic charcoal from a sediment core from Lake Ngami, in the Middle Kalahari, reflect paleovegetation and paleoclimatic conditions over the last 16,600 cal years BP. The location of Lake Ngami allows for the receipt of moisture sourced from the Indian and/or Atlantic oceans, which may have influenced local rainfall or long distance water transport via the Okavango system. We interpret results of statistical analyses of the pollen data as showing a complex, dynamic system wherein variability in tropical convective systems and local forcing mechanisms influence hydrological changes. Our reconstructions show three primary phases in the regional precipitation regime: 1) an early period of high but fluctuating summer rainfall under relatively cool conditions from ∼16,600–12,500 cal BP, with reduced tree to herb and shrub ratio; 2) an episode of significantly reduced rainfall centered around c. 11,400 cal BP, characterized by an increase in xeric Asteraceae pollen, but persistent aquatic elements, suggesting less rainfall but cool conditions and lower evaporation that maintained water in the basin; and 3) a longer phase of high, but fluctuating rainfall from ∼9000 cal BP to present with more woody savanna vegetation (Vachellia (Acacia) and Combretaceae). We propose a model to relate these changes to increased Indian Ocean-sourced moisture in the late Pleistocene due to a southerly position of the African rain belt, a northerly contraction of tropical systems that immediately followed the Younger Dryas, and a subsequent dominance of local insolation forcing, modulated by changes in the SE Atlantic basin.
... The areas of interest include the Zambezi River catchment and central Mozambique Channel, an area that represents the complete source to sink of the channel system. Southern Africa has experienced three significant periods of uplift over the past 30 Ma; mid-Oligocene, mid-Miocene, and upperPliocene (Siesser and Dingle, 1981;Dingle et al., 1983;Partridge and Maud, 2000;Castelino et al., 2016;Nash and Eckardt, 2016). These periods of hinterland uplift are associated with increases in sediment delivery to the adjacent Atlantic and Indian Oceans. ...
Article
Submarine channels are not stand-alone systems. They are long-lived systems modified by imperceptibly slow processes and rapid gravity flows, in some part controlled by hinterland dynamics. The submarine Zambezi Channel, within the Mozambique Channel, receives sediment from the Zambezi River catchment which has a dynamic tectonic and morphological history. Using recently collected multibeam bathymetry and PARASOUND data we discuss the geomorphology of the Zambezi Channel. Results show this system to be distinct in geomorphologic character when compared to other low-latitude submarine channels, sharing similarities with high-latitude systems. We propose a new, source-to-sink, hypothesis for the evolution of the Zambezi Channel, taking in to consideration hinterland tectonics, palaeo-lake development, river capture and rapid gravity flows. This hypothesis accounts for the unique present-day anatomy of the Zambezi Channel within the dynamic framework of the systems regional setting.
... The timing and origin of southern and eastern Africa's unusually high-elevation topography has been the focus of considerable attention for many years (e.g., King, 1963;Meadows, 2001;Moore and Larkin, 2001;Goudie, 2005;Grab et al., 2015;Nash and Eckardt, 2015). Researchers have sought to identify the connections between surface uplift and topography, and their link to fluvial (drainage) transport and sediment dispersion (e.g., Moore and Larkin, 2001). ...
Article
The structural depression that occupies the Okavango Basin in southern Africa comprises a depo-centre within the intracratonic Kalahari Basin where sediments of the Cenozoic Kalahari Group have accumulated. The Okavango Basin has been formed due to stretching and subsidence at an area of diffused deformation, southwestwards to the main East African Rift System (EARS). Sediments from two full Kalahari Group sequences, located on opposite sides of the Gumare Fault that forms a major fault within the Okavango Basin, were studied to determine their provenance and chronology. Terrestrial Cosmogenic Nuclide (TCN) 26 Al/ 10 Be burial dating was used to constrain a chronostratigraphical framework, and Pb, Sr, and Nd isotopic ratios combined with geochemical and sedimentological analyses were applied to track the source areas of the sediments.Results indicate the following sequence of basin filling: (a) Accumulation between ca. 4-3 Ma during which the currently down-thrown (southern) block received a mixture of sediments mostly from the Choma-Kalomo, Ghanzi-Chobe, and Damara terranes, and possibly from the Lufilian Belt and/or Karoo basalts during earlier stages of deposition. Simultaneously, the up-thrown (northern) block received sediments from more distant Archean sources in the Zimbabwe and/or Kasai cratons, (b) Hiatus in sedimentation occurred at both sites between ca. 3-2 Ma, (c) Sediments on both sides of the Gumare Fault share a similar source (Angolan Shield) with minor distinct contributions to the downthrown block from the Kasai Craton and local sources input to the upthrown block, and (d) Regional distribution of aeolian sand since at least 1 Ma. The change in source areas is attributed to rearrangements of the drainage systems that were probably linked to vertical crustal movements on the margins of the Okavango Basin. The tectonically induced morphodynamics controlled the landscape evolution of the endorheic basin where vast lakes, wetlands and salt pans have developed through time. K E Y W O R D S cosmogenic nuclides dating (26 Al/ 10 Be), incipient rifting, intracratonic morphodynamics, Okavango Basin, provenance analyses (Pb, Sr, Nd isotopes)
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The drainage system in south-central Africa has undergone major reorganisations since the disruption of Gondwana. Isopachs of the Kalahari sequence and a variety of geomorphological features can be used to pinpoint abandoned drainage lines. Continental fluvial sediments of Mesozoic-Cenozoic age reflect river systems which existed prior to and immediately following continental break-up. The east coast sedimentary sequence documents changes in the location of major supplies of terrigenous sediments, and provides a framework for establishing the timing of changes in drainage configuration. This evidence indicates that during the upper Jurassic to Cretaceous, the Okavango, Cuando and Zambezi-Luangwa rivers formed the headwaters of the proto-Limpopo. The lower-Zambezi-Shire formed a separate graben-bound river system with a discharge point into the Indian Ocean in the vicinity of mouth of the present-day Zambezi. A third major drainage entered the Indian in the vicinity of the modern Save mouth. End Cretaceous uplift along the Okavango-Kalahari-Zimbabwe Axis severed the links between the Limpopo and the Okavango, Cuando and Zambezi-Luangwa. This resulted in a senile endoreic drainage system which supplied sediment to the Kalahari basin. However, the uplift rejuvenated the lower Zambezi, initiating headward erosion and progressive capture of the Luangwa, upper Zambezi and Kafue. Predatory headward extension of the Zambezi is still active, and this river will eventually capture the Okavango. The model developed for drainage reorganisation provides a framework for interpreting kimberlitic heavy mineral dispersion patterns. It also forms the basis for explaining fish and plant dispersion patterns, and understanding recent water level fluctuations in the Makgadigadi pans system in Botswana.
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Major drainage divides in southern Africa are interpreted to reflect lines of epeirogenic flexuring of the subcontinent associated with the formation of co-related basins. The Great Escarpment, which separates coastal and inland drainage systems, marks the locus of the Escarpment axis. It was initiated by Early Cretaceous rift flank uplift associated with the break-up Gondwana. Geophysical studies suggest that subsequent erosion, coupled with sedimentation on the continental shelf, would have resulted in progressive inland migration of this flexure. The divide between the Orange-Vaal River system and the Limpopo and Molopo-Nossib-Auob drainage basins is designated the Etosha-Griqualand-Transvaal (EGT) axis. Upper Cretaceous flexuring along this axis disrupted old drainage lines, and initiated deposition of the Kalahari formation. The end-Cretaceous Ovamboland-Kalahari-Zimbabwe (OKZ) axis forms the watershed between the Zambezi and Limpopo Rivers in Zimbabwe, and separates the latter river system from fossil endoreic drainage lines in the Kalahari, which originally emptied into the Makgadigadi Pans system. In the south of Botswana, this axis is defined by the Kalahari Schwelle, which separates the fossil Kalahari drainages from the Molopo-Nossib River system. Processes responsible for initiating the EGT and OKZ flexures are poorly understood. However, the inferred ages of both these two axes and the Escarpment axis correspond with episodes of alkaline volcanism in southern Africa. This argues for a link between continental flexuring and volcanic activity. Major Pliocene uplift occurred along a line intermediate between the Great Escarpment and the present coastline in the east of the country (the Ciskei-Swaziland axis). More subdued Plio-Pleistocene flexuring along a southwest-northeast axis (designated the Bushmanland-Harts axis) traversing the interior of South Africa was responsible for the formation of major pans ('floors') in Bushmanland and the Orange Free State. There are a number of subordinate lines of uplift (the Khomas, Otavi, and Zoutpansberg axes) which are parallel to the Bushmanland-Harts axis. They are presumably related to the same stress field, and thus probably of similar age. These latter axes are all sub-parallel to active faults in northern Botswana which are interpreted to reflect southwestwards migration of the east African rift system, following lines of structural weakness. Sequential uplift along the axes which have been identified provides a framework for interpreting the evolution of drainages and erosion surfaces on the sub-continent.
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Drainage network evolution of the Trascău Mts, including the formation of the famous gorges (e.g. Cheile Turzii) is constrained by the transitional position between the high central Apuseni Mts and the Mureş Valley, further on by the ~N-S oriented geologic settings. The aim of this study is to use digital terrain analysis tools in order to better understand this evolution. Elevation, slope and aspect distributions, topographic swath profiles, stream profiles and doline morphometry were analyzed based on the SRTM dataset, topographic maps, and partially on field GPS measurements. Elevation and slope histograms according to rock groups quantitavely supported differences in rock resistance. It is demonstrated that Cenozoic rocks can be well distinguished from Mesozoic and older rocks based solely on morphometric parameters (slope and standard deviation). Swath analysis highlighted a characteristic W-E change in the slope of the envelope surface that is attributed to tectonic movements. Swaths profiles also helped the recognition of water gaps and wind gaps, which are very important remnant landforms of the post-Cretaceous drainage network. Stream profiles of the study area can be modelled mostly by exponential and linear functions, and a large number of identified knickpoints are in relation with rock boundaries. The denudation blocking effect of the main limestone ridge is clearly seen on stream profiles. It is demonstrated how the original Post-Cretaceous radial drainage pattern evolved to a trellis pattern. It is argued that superposition (with antecedence) played the most important role in the formation of water and wind gaps. All analysis highlighted the differences between the areas north and south of Arieş river. These areas had similar landform evolution, but are at different stages. The Post-Sarmatian evolution of the northern part copies the Post-Cretaceous evolution of the southern part. Differences in doline density between the northern and southern parts are attributed to different duration of subaerial karstification.
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Discovering the connection between processes observed to occur at the surface of the Earth and its internal dynamics remains an essential goal in the Earth sciences. Deep mantle structure, as inferred from seismic tomography or subduction history, has been shown to account well for the observed surface gravity fieldand motions of tectonic plates. But the origin of certain large-scale features, such as the anomalous elevation of the southern and eastern African plateaux, has remained controversial. Whereas the average elevation of most cratons is between 400 and 500m, the southern African plateau stands more than 1km above sea level, with the surrounding oceans possessing a residual bathymetry in excess of 500m (ref. 4). Global seismic tomography studies have persistently indicated the existence of a large-scale low-velocity anomaly beneath the African plate and here we show that mantle flow induced by the density variations inferred from these velocity anomalies can dynamically support the excess elevation of the African `superswell'. We also find that this upwelling mantle flow-which is most intense near the core-mantle boundary-constitutes a significant driving force for tectonic plates in the region.
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The 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 stands, 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 magnitude 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 14C 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.
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The physiography of southern Africa comprises a narrow coastal plain, separated from an inland plateau by a horseshoe-shaped escarpment. The interior of the inland plateau is a sedimentary basin. The drainage network of southern Africa is characterized by three river divides, broadly parallel to the coastline. These features contrast strongly with the broad dome and radial drainage patterns predicted by models which ascribe the physiography of southern Africa to uplift over a deep mantle plume. The drainage divides are interpreted as axes of epeirogenic uplift. The ages of these axes, which young from the margin to the interior, correlate closely with major reorganizations of spreading regimes in the oceanic ridges surrounding southern Africa, suggesting an origin from stresses related to plate motion. Successive epeirogenic uplifts of southern Africa on the axes, forming the major river divides, initiated cyclic episodes of denudation, which are coeval with erosion surfaces recognized elsewhere across Africa.
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A distinct series of beach ridges marking the former shorelines of large inter-connected lacustrine basins in the Kalahari can be clearly identified from Landsat imagery and Shuttle Radar Topography Mission (SRTM) data. These basins, which form the terminal sump of the Okavango system in northern Botswana, are now almost completely dry. During the Quaternary they were intermittently occupied by large stable lake bodies and are thought to have periodically filled to a point of coalescence inundating an area that, at its largest extent, encompassed 66,000 km2. Poor chronological control has previously limited the utility of this important palaeo-archive. As part of a region-wide lake palaeo-shoreline research programme, a systematic optically stimulated luminescence (OSL) dating programme has utilised a lightweight hydraulic auger to take samples at depth from relict shoreline features. Twenty drill-sites have generated 140 samples for dating, establishing a firm chronology for multiple lake full phases in all three component basins (Ngami, Mababe and Makgadikgadi) of this mega-lake. This paper presents the final set of ages in the programme, derived from four cores from the western and north-eastern shorelines of Makgadikgadi, and uses these ages to establish a chronology of mega-lake high-stands during the last ∼300 ka providing a rare directly dated, long terrestrial record of positive hydrological excursions within the southern hemisphere.
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The Kalahari region has become a major source of Quaternary palaeoenvironmental data derived primarily from the analysis of geomorphological proxies of environmental change. One suite of data, from palaeolacustrine landforms, has recently provided a new record of major hydrological changes in the last 150 ka [Burrough, S. L., Thomas, D. S. G., Bailey, R. M., 2009. Mega-Lake in the Kalahari: A Late Pleistocene record of the Palaeolake Makgadikgadi system. Quaternary Science Reviews, in press.]. Here we present an improved analysis of the drivers and feedbacks of lake level change, utilising information from three main sources: data from the lake system itself, from analyses of other late Quaternary records within the region and from climate modelling. Simulations using the Hadley Centre coupled climate model, HadCM3, suggest that once triggered, the lake body was large enough to potentially affect both local and regional climates. Surface waters and their interactions with the climate are therefore an important component of environmental dynamics during the late Quaternary. Through its capacity to couple Middle Kalahari environments to distant forcing mechanisms and to itself force environmental change, we demonstrate that the existence or absence of megalake Makgadikgadi adds a new level of complexity to the interpretations of environmental proxy records in southern Africa's summer rainfall zone.
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The present paper is focusing on improving the rainfall-runoff modeling in a large basin, at a daily scale, using the distributed hydrological model CEQUEAU and the GIS IDRISI. A hydrogeomatic module was implemented using a supervised process to provide the input data required by the hydrological model. SRTM (Shuttle Radar Topography Mission, USGS) images were used, with a spatial resolution of 30" (∼ 1 km), for the purpose of defining watershed divides, which eliminates significant sources of uncertainty and reduces processing times. On the other hand, the discharge of the Senegal River has been gauged at the Bakel hydrometric station since the beginning of the 20th century until today, so a relatively long time series of data is now available. Various hydrologic studies about this basin have been performed, reporting a watershed area of roughly 289 x 103 km2, which is greatly underestimated according to the present study. The basin contains very diverse climatic conditions, with high variability in total annual precipitation, from 2 000 mm in the south to 50 mm in the north. Physiographic parameters have been computed taking into account the extensive area of the basin located in Mauritania, which had been neglected as part of this watershed by previous studies. Since the simulations of daily volumes for the period 1970-2000 produced good results (Nash coefficients generally above 0.80), it is concluded that simulations are more suitable when using the new hydrogeomatic module and the CEQUEAU and represent a solid basis for water resources management in the area.
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The findings of a decade of field research on the caves and associated phenomena in the Kwihabe hills, and the Makgadikgadi lake basin are summarized. A chronological scheme for the evolution of these features is presented, and climatic inferences are drawn. -from Author
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The western part of the Makgadikgadi basin in northern Botswana displays a variety of lacustrine, fluvial and aeolian landforms in close juxtaposition. The area is divided into a number of distinct geomorphological units, whose form, relationships and evolution are described. A sequence of stages in the evolution of the landscape is put forward, and a chronology suggested on the basis of 20 new 14C dates on samples from widely separated parts of the whole basin. Correlation with adjacent parts of southern Africa is examined, and some palaeoclimatic inferences drawn.-Authors
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Unpublished documentary materials written by missionaries working for the London Missionary Soclety in the Kalahari Desert of central southern Africa during the nineteenth century have been used to identify historical hydrological events in drainage systems of the region. Analysis and interpretation of missionary correspondence has identified previously unrecognized flood events in the Kuruman (1817/18) and Molopo (1871) rivers in South Africa, which represent the earliest recorded floods during the historical period in these systems. Documentary evidence also suggests that the 1894 Kuruman flood may have been the most extensive on record, with flow occurring along the entire length of the Kuruman and lower Molopo valley before linking with the Orange River. Floods or near-surface water are also recognized in a number of present-day 'fossil' Kalahari valleys. These include a flood event in the Letlhakane valley in 1851, only the second documented flood in an endoreic Kalahari drainage system. The presence of water is also identified in the fossil Xaudum and Ncamasere valleys in 1879. Documentary evidence has additionally been used to identify that the flow of water in the Thaoge River system, one of the key inflows to Lake Ngami, ceased in late 1879 to 1880, shortly before the desiccation of the lake in 1881. The possible causes of these hydrological events are discussed in the context of global and regional environmental changes.
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Lying on the continental margin of South Africa are seven major depocentres containing variable thicknesses of Cretaceous drift succession, which dates in age from Early Barremian to Late Maastrichtian, or approximately from 127 to 65 million years ago (Myr). Accumulation of these sedimentary piles was episodic, and the succession is subdivided by major and minor unconformities. A foraminifera-based biostratigraphy is presented for this Cretaceous drift succession, reliant on integrated studies of 215 oil exploration borehole sections from all seven basins, as well as three onshore outcrop areas (Wanderfeld IV, Mzamba and Needs Camp/Igoda), and 772 Middle Albian to Late Santonian sea-floor samples from off the west coast of southern Africa. The biostratigraphic and chronostratigraphic successions of each basin are compared, and sea-level curves established where possible. It can be seen that sedimentation patterns during the Cretaceous drift succession were controlled almost exclusively by the relative availability of accommodation space, caused by sea-floor subsidence, still-stand or uplift, of different parts of the southern African continental margin. From Early Barremian to Early Cenomanian times (approximately 127 to 97.2 Myr) most of the basins subsided and uplifted in unison. From Late Cenomanian to Late Maastrichtian times (approximately 97.2 to 65 Myr) the southern African continental mass appears to have repeatedly suffered mild east-west rolling motions, which have resulted in distinctly different stratigraphic successions especially on the Atlantic and Indian margins. Thus, the Atlantic and Southern margins subsided particularly in the Early Turonian, the Late Coniacian and the Late Santonian. In contrast, the Indian Margin subsided particularly in the Late Cenomanian, the Early Santonian, and the Late Campanian. The boundary between the two regions must lie in the vicinity of East London. Each basin fill is sufficiently dissimilar from the others for these successions to have accumulated purely as a response to the tectonic regime at any given time. Except possibly for the Early Turanian Horizon '11' shelf-wide forced regressive sandstone episode, no recognized events are sufficiently consistent across the seven basins for them to be attributable to global sea-level rises or falls. The global sea-level change record in the seven basins must thus be represented by relatively minor base-level changes that are entirely concealed behind the tectonically driven movements controlling the stratigraphic record described here.
Article
A topographic map of the Okavango Delta and environs has been constructed using a combination of elevation data including trigonometric beacons and spot heights from the government of Botswana, surveys of the navigable channels, U. S. Department of Defense data and measurements made during a geophysical survey of the region. The topography provides insight into the local tectonic and sedimentary history. Local tectonics are dominated by uplift and horst formation associated with the Ghanzi Ridge, and an arch to the north of the Panhandle, which appear to represent the tips of incipient rifts which are propagating from the northeast. The Delta has formed in the resulting depression between these arches. The Panhandle has developed along a fault, and may be largely an erosional feature incised into the northern uplift zone. The Delta itself is an alluvial fan of remarkably uniform gradient. There is no evidence of regional tilting of the fan surface. Local highs and lows are developed on the fan, but channel location is relatively insensitive to this local topography. Moreover, marked elevation differences exist between adjacent channels, creating hydrologically unstable conditions. These unusual features of the local hydrology arise because of the confining effect of channel-flanking vegetation. Sedimentation in the Delta appears to be causing crustal sagging of the central Delta, which has: tilted the major palaeo-shoreline of the Mababe Depression to the west; formed a local depression within the Ghanzi Ridge facing the Delta; and detached a sliver of the ridge along the Thamalakane fault. It is suggested that local seismicity also results mainly from sediment loading. The Selinda spillway occupies a marked local depression, which is a graben between the Gumare fault and an extension of the Linyanti fault. It is probable that southwesterly propagation of the uplift zone associated with the incipient rift will ultimately deflect the Okavango River into the Chobe-Zambezi river system via this graben.
Article
Dry 'fossil valleys (mekgacha) are an important landscape feature in the semi-arid to arid Kalahari Desert of central southern Africa. The majority of valleys do not contain flow, except in their headwater regions, but may contain pools of water on a seasonal basis. As a result, they are frequently mentioned in historical accounts of the region recorded by European travellers, and from these reports it is possible to assemble a picture of hydrological change. This paper describes the evidence for such change, for the period c.1750 until c.1910. Discussion mainly concerns the most frequently observed valleys, the Molopo and Kuruman, although Middle Kalahari systems are also considered. The picture which emerges is one of little change in Middle Kalahari systems. A decline in flow in the Kuruman since c.1750 is tentatively identified, pre-dating the period of permanent human settlement in this region. The river appears to have established its present regime after 1800. A previously unrecognized flood event in the Kuruman in 1820 is also identified. These hydrological changes are placed within the context of historical climate evidence from the southern Cape of South Africa and show close correlation. This suggests that documentary evidence may be used to extend the climate record established for the southern African summer rainfall zone into the Kalahari.
Article
The recent development of digital representation has stimulated the development of automatic extraction of topographic and hydrologic information from digital elevation model input, using geographic information system (GIS) and hydrologic models that integrate multiple databases within a minimal time. The objective of this investigation is to compare the drainage extracted from Shuttle Radar Topography Mission (SRTM) data with the drainage digitized from topographic data (1:50,000) and also to draw attention to the functions of an add-on tool in ArcGIS 9.2 (Arc Hydro v.2) of Kuttiyadi River basin. The analysis reveals that the watershed extracted from the SRTM digital elevation model (DEM) (90 m resolution) is having an area of 668 km2 and that from toposheet is 676 km2. The river mouth in the drainage network from the SRTM DEM is found to be shifted to the northern side from where it actually exists. The drainage network from SRTM DEM at stream threshold 15 (0.0002 % of maximum flow accumulation) is delivering best results than the other threshold value in comparison with the drainage pattern derived from toposheets. The study reveals the importance, reliability, and quaintness of drainage network and watershed derived from the SRTM using the Arc Hydro Tool, an extension for Environmental Systems Research Institute ArcGIS. The advantage of the Arc Hydro Tool is that it would help a novice with little GIS knowledge to run the model to obtain watershed and drainage network.
Article
The degraded linear dunefield of north west Ngamiland, Botswana occurs in a seismically active area, lying to the northwest of the Okavango graben, widely considered to be the tail end of the East African Rift system. To assess the effects of neotectonism on the dunes, an area was selected for examination close to the Gumare fault, which bounds the graben on the northwest side. Digital SRTM data were converted to light shaded representation of the topography. it was found that dune forms only occur along the margin of the rift and on both sides of incising valleys, graded to the graben floor. A selection of long profiles showed dune crests standing some 25 in above the straats at the edge of the rift. The relative relief pinches Out away from the fault, towards interfluves that do not depict dune morphology. Landsat satellite imagery shows linear features continuous across the flat interfluves, inviting the incorrect inference from the vegetation that the linear dune forms occur there. A model is proposed that an original linear dune field formed under arid conditions, was entirely flattened under wetter conditions, crest material being washed into the straats, thus obliterating the original dune morphology. Upon rifting and steam incision, these forms ire being replicated, an example of equifinality. Replication is suggested to be by the action of infiltrating water, controlled by the groundwater gradients. These observations Suggest that the active dunefield significantly predates the tectonic processes. With the additional time required for degradation and replication it would Suggest that the dunes are of considerable antiquity. it has already been recognised that the dunefield has a complex history of construction and destruction, to which must now he added a process of base level-controlled replication.
Article
Southern Africa's high Kalahari plateau and its flanking mountain ranges formed over an extended period of similar to 200 million years through vertical tectonic processes different from those at convergent plate boundaries. We refer to this as the Kalahari epeirogeny. Episodic uplift and erosion during the Kalahari epeirogeny is inferred from thereto-chronology and from stratigraphy of sediment accumulated around its continental margin. A total thickness of 2 to 7 km of rock (of which basalt was a major component) was eroded from the subcontinent's surface during two punctuated episodes of exhumation, in the early-Cretaceous and in the mid-Cretaceous. Major exhumation was over by the end of the Cretaceous, and the rate of erosion decreased by more than an order of magnitude to remove less than 1 km thickness of rock during the Cenozoic. Cosmogenic dating shows that erosion rates today are almost an order of magnitude less still. The cause for the Kalahari epeirogeny remains elusive, but three striking observations stand out: (i) major exhumation occurred during the late Mesozoic break-up of Gondwana; (ii) large scale basaltic magmatism closely match two accelerated episodes of exhumation: first along the west coast (similar to 132 Ma, Etendeka large igneous province [LIP], associated with vast amounts of underplating along this entire passive margin), and the second flanking the sheared south coast margin (similar to 90 Ma, Agulhas oceanic LIP). Yet exhumation that might have accompanied the vast Karoo LIP (similar to 180 Ma) is not readily detected; (iii) the two main regional episodes of accelerated exhumation and coastal sediment accumulation are near synchronous with two regional 'spikes' of kimberlite intrusions: > 450 kimberlites at similar to 90 Ma, and > 200 kimberlites at similar to 720 Ma. Southern Africa is underlain by an anomalous warm region in the lowermost similar to 1500 km of the mantle that is linked to core to mantle heat loss. This warm region was inherited from a large Cretaceous thermo-chemical anomaly in the mantle created during long-lived subduction beneath Gondwana. Associated Mesozoic kimberlite genesis and basaltic magmatism may have been sufficient to cause volatile/heat-induced density changes in the lithospheric mantle of southern Africa to sustain the Kalahari plateau. In addition, such changes may have been induced by tectonic uplift and decompressional melting resulting front far-field collision processes between Africa and Europe, and/or final continental lithosphere decoupling between the Falkland plateau and southern Africa. CO(2) released into the ocean-atmosphere system during the Kalahari epeirogeny contributed to the global mid-Cretaceous hot-house conditions. However, because the CO(2)-consumption rate associated with basalt weathering is about eight times that of granite, atmospheric CO(2) was also efficiently sequestrated during the rapid erosion of Karoo basalt. Thus, the Kalahari epeirogeny also may have catalysed the onset of long-terns global Cenozoic cooling.
Article
This paper is based on the concept that the drainage system of c. 7.5-4.6 Ma is still reflected in the geomorphology of northeastern Chad and adjacent areas of Libya. During the Messinian and early Pliocene a large lake was present in the Chad Basin that is termed Neogene Lake Chad. It fluctuated in size in response to the precessional cycle and at times overflowed to the east, NE or north, giving rise to the Sahabi rivers. The Eosahabi flowed during the drawdown of the Mediterranean (late Messinian) and eroded the Erdi and part of the East Tibesti Valley. The post-rift Miocene deposits of the Chad Basin, some several hundred metres in thickness, record a transgression over an irregular and faulted terrain with the deposition of coarse and fine elastic material. Fluviatile and lacustrine environments are represented. At least part of the Miocene succession belongs to a phase of late Miocene lake development. Fluctuating climate during the Messinian and early Pliocene led to repeated changes in the environment of the Chad and Eosahabi Basins with over 100 climatic cycles developed. This would favour the development of animal species with high adaptability, for example to littoral, riparian, woodland and savanna habitats.
Article
Tectonic activity is commonly held responsible for abrupt coarsening of alluvial fan sequences and the role of climate is often relegated. The Pleistocene fanso f the Dead Sea rift indicate clearly the problems of differentiating the sedimentary signatures of climatic and tectonic adjustments in a closed lake basin. Exposures are excellent. The rivers are currently incising in response to the low lake levels of the Holocene, producing cliffs >50m high. These reveal an interdigitation of lake deposits (dominantly fine grained carbonates) and the coarse river gravels which characterize the fan deposits. The bounds of each facies are sharp: river sediments are superimposed on lake sediments with little evidence of either reworking or loading; in turn, aragonite crusts are draped over alluvial gravels indicating the sudden return of still-water conditions. Neither can be explained by gradual adjustment of the hinterland river system after tectonic shock. Instead, it is clear that the coarse alluvium was deposited by discrete high magnitude floods that occurred only sporadically because of the infrequent nature of rainfall in this arid environment. In interpreting basin development in ancient settings, it is always tempting to assume that coarse deposits are indicative of tectonism. However, the Dead Sea fans demonstrate that climate is just as likely to be responsible for the conglomerates in a sedimentary sequence.
Article
Many dryland rivers undergo marked downstream changes owing to factors such as infrequent floods, flow transmission losses, and typically few tributary inflows beyond the headwaters. Along the Sandover, Bundey (Sandover–Bundey) and Woodforde Rivers on the Northern Plains of arid central Australia, downstream channel changes are broadly similar. In upland zones, small, rocky channels transporting sand and gravel gradually increase in size before entering piedmont zones, where channels and narrow floodplains are confined by bedrock, alluvial terraces, or aeolian dunes. In lower gradient lowland zones, channels and floodplains remain confined and, in the absence of tributary inflows, channel cross-sectional areas and discharges decrease downstream. Confining landforms are not present in floodout zones, which results in splay formation, increased floodplain widths, and marked overall downstream decreases in cross-sectional areas. Eventually, channelised flow and bedload transport terminate, although occasional large floods continue across extensive unchanneled alluvial surfaces termed “floodouts”. These broad similarities apart, downstream changes along the three rivers differ in detail. The Sandover is largely a single-thread channel, whereas many reaches of the Sandover–Bundey and Woodforde are anabranched. On the small Woodforde River, downstream decreases in parameters such as cross-sectional area and width are roughly linear. On the larger Sandover and Sandover–Bundey, downstream changes are more irregular, particularly through the floodout zones where there are marked fluctuations in widths, depths and bed slopes. The irregular downstream changes typical of the lower reaches of these large rivers may be due to the reduced influence of vegetation on bankline stability and width adjustment relative to that of smaller rivers. On the Northern Plains, as in other drylands, complex interactions between discharge, sediment transport, slope, patterns of tributary drainage, bank sediment type and vegetation result in variable patterns of downstream channel change.
Article
The cover sands in the Republic of Botswana are of four major types: (a) Type 1 is a pure quartz sand found in the northwestern corner of the country. It is derived from a series of ancient longitudinal dunes. (b) Type 2 sand is composed of two major components: probable eolian material with a dominant size around 2.5 o, and a feldspar rich component, largely around 3.0 o in size, that derives from the arkosic Ghanzi Sandstone. (c) Sand type 3 is a pure quartz sand which is statistically similar to sand type 2, but lacks the characteristic feldspathic component. It is assumed that this sand evolved from the breakdown of underlying Karroo Supergroup Sandstones. (d) Type 4 is a fluvially deposited sand found in the eastern part of the country where bedrock outcrop is common and the cover sand thickness is less. There are several sub-varieties of this sand type, each defined by the dominant bedrock source in the area of occurrence. Where the fine-grained "Kalahari" sands (types 1, 2, and 3) lie thickly there is little chance for recharge of underground water supplies, a fact of critical importance in the vast arid areas of Botswana.
Article
The northern South Mozambique Graben was assessed for hydrocarbon potential by reconstructing its structural, burial and thermal histories using a seismic line and an array of basin modelling techniques. The structural and depositional history was determined by simultaneously palinspastically restoring and backstripping a depth converted cross-section. The thermal history was predicted from extensional models which relate palaeoheat flow to basement subsidence. Maturation potential cross-sections were derived by applying a generation model to each of the pseudo-wells, assuming each layer to be a potential source rock. Considering the range of uncertainty when using these models, this analysis suggested that regardless of the extension model type used, and within the maximum and minimum limits of the palaeoheat flow, the eastern side of the northern South Mozambique Graben is more hydrocarbon prone than the west. In addition the timing of hydrocarbon generation, regardless of the palaeoheat flow possibilities modelled, is favourably after source and seal rock deposition, and after the development of trapping structures. It is apparent from this study that a stratigraphic well with down-hole thermal indicator measurements and bottom hole temperature information would greatly enhance the understanding of the hydrocarbon potential of this basin.
Article
The current 14C-based chronology for the Quaternary palaeo-lakes of the Middle Kalahari has inherent inaccuracies arising from the dating method and materials used. The application of the optical dating technique to a 250-cm diatom bed at Moremaoto on the Boteti River provides a more secure dating framework with potential for use on fluvio-lacustrine sediments, whilst analysis of the diatom flora indicates lake-marginal energy conditions and water chemistry during two high lake level phases between 32 000 and 27 000 years BP. These ages are older than, and comparable to, existing 14C dates on terrace calcretes in the Boteti, which represent minimum ages for the formation of these pedogenic carbonates.
Article
The recently released Shuttle Radar Topography Mission (SRTM) 3-arc second digital elevation data set provides a complete global coverage of the Earth's land surface. In this paper we examine the SRTM data for three catchments in Australia over a range of climates, geology and resultant geomorphology. To test this new data set the SRTM data are compared with high resolution digital elevation models. We use basic hydrological and geomorphological statistics and descriptors such as the area–slope relationship, cumulative area distribution and hypsometric curve, along with Strahler and networking statistics. The above measures describe the surface morphology of a catchment, therefore integrating catchment geology, climate and vegetation. The SRTM data were also assessed as input into the SIBERIA landscape evolution and soil erosion model as were runoff properties, using a wetness index. The results demonstrate that the 90 m SRTM data provide a poor catchment representation. Hillslopes appear as a linked set of facets and display little of the complex curvature that is observed in high resolution data. While catchment area–slope and area–elevation (hypsometry) properties are largely correct, catchment area, relief and shape (as measured by the width function) are poorly captured by the SRTM data. Catchment networking statistics are also variable. The large grid size of the SRTM data also results in incorrect drainage network patterns and different runoff properties. Consequently, care must be used for quantitative assessment of catchment hydrology and geomorphology, as in all cases SRTM-derived catchment area is incorrect and smaller digital elevation grid sizes are required for accurate catchment-wide assessment. While only a limited number of catchments have been examined, we believe our findings are applicable to other areas. © Crown Copyright 2006. Reproduced with the permission of the Controller of HMSO. Published by John Wiley & Sons, Ltd.
Article
Kalahari dry ‘fossil’ valleys (mekgacha) have been little used in palaeoenvironmental reconstruction, partly because the origins and functions of the valleys are a subject of debate, and partly because they contain few datable sediments. In the Middle Kalahari the valleys are endoreic, and, following a decline in ground-water tables owing to human interference over the past 150 years, rarely contain water. Evidence from three sites suggests that the valleys contained standing water between 15000 and 12000 yr BP, and that the main Okwa-Mmone system flowed into palaeolake Makgadikgadi at 920 m a.s.l. during this period. The Southern Kalahari contains the exoreic Molopo network which is hydrologically more active, being prone to surface flows in response to extreme rainfall events. The spring—fed Kuruman valley has two terraces, the lower composed of a series of flood deposits of late Holocene age. The flood of February 1988 provides a useful analogue for the conditions under which these deposits were formed, and of the temporal regime of the Kuruman River. Comparison with adjacent sites suggests that evidence put forward to indicate cycles of climatic amelioration in the last 4000 yr may, in part, represents the effect of extreme precipitation events. Historical floods show a strong correlation to Southern Oscillation high-phase (cold event) episodes, suggesting possibilities for extension of the Southern Oscillation record through palaeoflood studies.
Article
Remote sensing is a particularly invaluable tool that has helped the detection of paleomorphologies produced by river dislocation in a variety of landscapes, which has contributed in reconstructing the geological evolution of many fluvial systems. This technique might provide useful information to discuss the evolution of large fluvial systems, in special those located in areas of difficult access where the acquisition of field data is difficult. Application of remote sensing for paleodrainage characterization in densely vegetated tropical areas is scarce in the literature. This work records processing of the Digital Elevation Model (DEM) derived from the Shuttle Radar Topography Mission (SRTM), which succeeded in revealing an ancient drainage complex of the Madeira River, one of the main Amazonas tributaries, where other remote sensing products failed the detection. Analysis of this paleodrainage and of its modern counterpart within the geological framework available for this region leads to propose that activity along pre-existent faults during the latest Quaternary would have promoted the southeastward dislocation of a nearly 200 km long segment of the Madeira River. During this process, an impressive paleodrainage network was left behind, which was only able to be detected using the DEM-SRTM. Application of this technique might be of great help to the detection of paleodrainage morphologies in densely vegetated areas similar to the Amazonas lowland. The dynamics of channel migration in this and many other large scale tropical river systems might benefit from the investigation based on data derived from DEM-SRTM.
Article
Abstract The Minshan Mountain and adjacent region are the major continental escarpments along the eastern Tibetan Plateau. The Minjiang drainage basin is located within the plateau margin adjacent to the Sichuan Basin. Based on the analysis of the digital elevation model (DEM) acquired by the Shuttle Radar Topography Mission (SRTM), we know that the Minjiang drainage basin has distinct geomorphic characteristics. The regular increasing of local topographic relief from north to south is a result of the Quaternary sediment deposition within the plateau and the holistic uplift of the eastern margin of the Tibetan Plateau versus the Sichuan Basin. Results from DEM-determined Minjiang drainage sub-basins and channel profiles show that the tributaries on the opposite sides are asymmetric. Lower perimeter and area of drainage sub-basins, total channel length and bifurcation ratio within eastern flank along the Minjiang mainstream are the result of the Quaternary differential uplift of the Minshan Mountain region. Shorter stream lengths and lower bifurcation ratio might be the indications of the undergrowth and newborn features of these eastern streams, which are also representative for the eastern uplift of the Minshan Mountain.
Article
We describe and apply a method for estimating uplift rate histories from longitudinal river profiles. Our strategy is divided into three parts. First, we develop a forward model, which calculates river profiles from uplift rate histories. Height variation along a river profile is controlled by uplift rate and moderated by the erosional process. We assume that the erosional process can be represented by a combination of advection and diffusion, which are parameterized using four erosional constants. Second, we have posed and solved the geologically more interesting inverse problem: which uplift rate history minimizes the misfit between calculated and observed river profiles? The inverse algorithm has been tested on synthetic river profiles, which demonstrates that uplift rate histories can be reliably retrieved. Our tests show that the erosional process is dominated by advection (i.e., knickpoint retreat) and that changes in lithology and discharge play a secondary role in determining the transient form of a river profile. Finally, we have inverted river profiles from a series of African topographic swells, namely the Bie, South African, Namibian, Hoggar, and Tibesti domes. Fits between calculated and observed river profiles are excellent. Calculated uplift rate histories suggest that these domes grew rapidly during the last 30-40 million years. Uplift rate histories vary significantly from dome to dome but cumulative uplift histories agree closely with independent geologic estimates.
Article
Dry land areas cover large parts of the land masses of Earth. Of these a large portion is mantled by aeolian (wind-blown) deposits and subjected to aeolian process, i.e., the transport of sand or dust by wind. One of the most prominent morphologies created by wind-driven deposits are dunes and in their larger form draas or mega dunes. These mega dunes tend to be widely spaced; very large; often forming a base on which smaller dunes form, producing a compound dune pattern.In February 2000 the Shuttle Radar Topography Mission, onboard the space shuttle Endeavour flew an interferometric radar system whose objective was to create a DEM of nearly all Earth land masses. In this paper DEM data from the C-band 90 m data set and the X-band 30 m data set were used to map and characterize the height and spacing of the large sand seas (Ergs) on Earth. Only the larger forms of dunes could be mapped reliably. The dune spacing and heights extracted from the SRTM DEMs tended to be in overall agreement with those reported in the literature. In a comparison between the X-band and the C-band data it was found that the X-band data are more sensitive to the smaller scale undulations on the compound dunes and better revealed the full height of the dunes.
Article
The Kalahari basin formed as a response to down-warp of the interior of the southern Africa, probably in the Late Cretaceous. The down-warp, along with possible uplift along epeirogenic axes, back-tilted rivers into the newly formed Kalahari basin and deposition of the Kalahari Group sediments began. Initial deposition of basal gravels occurred in the channels of the Cretaceous rivers, with other unsorted gravel beds deposited at the base of scree slopes along the edges of valleys and fault-bounded structures. The accumulation of gravels continued as the down-warp of the basin progressed with interbedding of the gravel layers with sand and finer sediment carried by the rivers. Thick clay beds accumulated in the lakes that formed as a result of the back-tilting of rivers, with sandstone being deposited in braided streams interfingering with the clays and covering them in some areas as the shallow lakes filled up with sediment.
Article
The longitudinal profile of the Zambezi River forms two concave-upwards sections, with their boundary at the Victoria Falls. This form has been ascribed to the process of pediplantation and the Victoria Falls identified as one of several knick points that have traversed the river since the breakup of Gondwanaland. An alternative model explains the river's long profile by suggesting that the Upper and Middle Zambezi evolved as entirely separate river systems, which only joined together in comparatively recent times.The alluvial sequence of the Middle Zambezi is described and interpreted in terms of the latter hypothesis. The river capture event caused a change in grade and is marked by a deposit believed to record a cataclysmic flood. Capture is inferred to have resulted from overtopping of the lake that formed the end sink of the proto-Upper Zambezi, rather than from headward erosion of the proto-Middle Zambezi. This event is dated, from archaeological evidence, to the peak of the last interglacial, at the end of the Middle Pleistocene. Subsequent drainage diversions between the Middle Zambezi and the Kalahari are interpreted as the product of rifting of the Chobe Graben and aggradation of the Chobe Swamps.River capture by overtopping implies high rainfall over Central Southern Africa at the peak of the last interglacial, which is contrary to predictions that Africa's rain belts then lay north of their modern mean positions. This anomaly is resolved by postulating a southern polar warming episode at that time and agrees with suggestions that the last interglacial was marked by large scale ablation of the West Antarctic Ice Sheet.
Article
Aeromagnetic and gravity data collected across the Okavango rift zone, northwest Botswana are used to map the distribution of faults, provide insights into the two-dimensional shallow subsurface geometry of the rift, and evaluate models for basin formation as well as the role of pre-existing basement fabric on the development of this nascent continental rift. The structural fabric (fold axes and foliation) of the Proterozoic basement terrane is clearly imaged on both gravity and magnetic maps. The strike of rift-related faults (030–050° in the north and 060–070° in the south) parallels fold axes and the prominent foliation directions of the basement rocks. These pre-existing fabrics and structures represent a significant strength anisotropy that controlled the orientation of younger brittle faults within the stress regime present during initiation of this rift. Northwest dipping faults consistently exhibit greater displacements than southeast dipping faults, suggesting a developing half-graben geometry for this rift zone. However, the absence of fully developed half-grabens along this rift zone suggests that the border fault system is not fully developed consistent with the infancy of rifting. Three en-echelon northeast trending depocenters coincide with structural grabens that define the Okavango rift zone. Along the southeastern boundary of the rift, developing border faults define a 50 km wide zone of subsidence within a larger 150 km wide zone of extension forming a rift-in-rift structure. We infer from this observation that the localization of strain resulting from extension is occurring mostly along the southeastern boundary where the border fault system is being initiated, underscoring the important role of border faults in accommodating strain even during this early stage of rift development. We conclude that incipient rift zones may provide critical insights into the development of rift basins during the earliest stages of continental rifting.
Article
A systematic drilling and optical dating programme on Middle Kalahari beach ridge (relict shoreline) sediments has enabled the identification of multiple episodes of lake high stands of an extensive palaeolake system at the terminus of the Okavango Delta, northern Botswana. This paper presents 23 ages from the Mababe Depression and establishes four shoreline construction phases in the late Quaternary coeval with other sub-basin lake high stands (Lake Ngami). These synchronous lake phases result from a coalescence of the sub-basins into a unified palaeolake, Lake Thamalakane, covering an area of ∼ 32,000 km2. Six additional ages are also presented from the Chobe enclave to the north of the basin where shoreline ridges were emplaced at the same time as Lake Thamalakane phases. This suggests that increased flow in the Chobe and Zambezi system significantly contributed to the Middle Kalahari lake phases in both the post-glacial and Holocene periods. The integration of these new data and their compatibility with other regional and tropical palaeo-archives is discussed in the light of understanding Quaternary climate drivers within the Kalahari.
Article
The development of the Zambezi drainage system is discussed within the framework of the post-Gondwana tectonic evolution of southern Africa. An internal drainage system, including teh proto-Upper Zambezi, has been progressively captured during the late Cainozoic by a more agressive coastwise system. Supporting geomorphic evidence is presented from the eastern Kalahari rim. Drainage alignments and gradients, and terrace sequences are discussed. Lacustrine features found on the present watershed between the Middle Zambezi and internal systems are described and explained as remnants of the former proto-Upper Zambezi tributary system. Their interaction with linear dune activity is also examined. Despite the problems of dating the drainage changes described, it is concluded that their elucidation is important in understanding sedimentation and landform development in the eastern part of the Kalahari basin.
Article
Much of the interior of central southern Africa is a sand sea, within which aeolian and lacustrine landforms and sediments of local and regional extent are preserved. Closed cave sites are restricted to very few locations, while fluvial systems traverse the margins of the interior. Until the early 1990s, chronologies of late Quaternary environmental and climatic changes developed for this region were based on only a limited number of proxy data sets, derived largely from lacustrine deposits and precipitates. In particular, directly determined ages from aeolian deposits, the most extensive suite of features in the region, were absent. The application of luminescence-dating techniques to dune sediments, and the development of further detailed chronologies from cave precipitates, is now providing a more comprehensive record for the last 50 ka, with some chronologies extending back a further 100 ka. We present and review these data, assess their contribution to enhanced understanding of late Quaternary environmental changes in the region, and for the first time assess them against corrected radiocarbon ages from lacustrine sites. It is concluded that there is now an enhanced understanding of the spatial and temporal complexity of climate changes affecting the region in the last glacial cycle, including a complex record of punctuated aridity, but that many issues, including data-set integration and forcing mechanism controls, are imperfectly understood.
Article
We calculate the solid sediment load history of the Zambezi River for the last 120 Ma (i.e. Middle Cretaceous–Recent times). Our starting point is a detailed grid of two-dimensional seismic reflection profiles, calibrated by well-log information. After conversion from two-way travel time to depth, we made simple assumptions about the compaction history in order to construct isochore maps of solid sediment load and yield as a function of geological time. Our results show that solid sediment load has varied by ∼1 order of magnitude over the last 120 Ma. There have been three periods of elevated flux. The first period occurred in Late Cretaceous times (90–65 Ma) and was synchronous with the rapid denudation of southern Africa recognised by, for example, apatite fission track modelling. The second period occurred in Oligocene times (34–24 Ma) during proposed rapid regional uplift of Southern Africa. The youngest phase of elevated flux started in Late Miocene times (10 Ma) and has continued to increase rapidly through to the present day. A large proportion of this increase can be attributed to a doubling of the size of the Zambezi catchment during the Pliocene. At other times, we suggest that load variations can be used to bound the uplift history of the region, which encompasses the catchment. Finally, changes in the shape of the Zambezi Delta through time suggest that the Mozambique current, which sweeps through the Mozambique Channel from northeast to southwest, initiated at the start of the Miocene.
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The study of abrupt changes in longitudinal river profiles, or knickpoints, is currently approached through an empirical power law: the slope–area relationship. Results based on digital elevation model (DEM) analyses and stream extractions are generally intended to determine crustal uplift rates and identify transient landscape conditions. In this article, we present an alternative geomorphometric method for locating knickpoints and knickzones based on local slope gradient and curvature attributes. Intended as a rapid, regional scale, automated knickpoint detection technique, the accuracy of this slope–curvature method is tested on two digital elevation grids, NASA's SRTM (ground resolution of 90 m, resampled here to 75 m) and the ASTER DEM (15 m) in the Sierra Nacimiento (New Mexico, USA), a basement-cored mountain range recently exhumed by waves of headward drainage integration in response to remote tectonic deformation in the adjacent Rio Grande rift. Out of every 10 gradient anomalies detected by the SRTM-derived numeric routine, up to 8 are certifiable knickpoints recognized among a population of georeferenced occurrences surveyed in the field. An independent comparison with the slope–area method provided a further accuracy test, which was particularly useful at sites that could not be validated in the field for practical reasons. Given the low tectonic activity of the study area, the majority of knickpoints was also found to coincide with lithologic boundaries, making it difficult without further geomorphological data to single out dynamic knickpoints directly caused by the upstream propagation of channel instabilities relating to base level change.
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A topographic map of the Okavango Delta and environs has been constructed using a combination of elevation data including trigonometric beacons and spot heights from the government of Botswana, surveys of the navigable channels, U. S. Department of Defense data and measurements made during a geophysical survey of the region. The topography provides insight into the local tectonic and sedimentary history. Local tectonics are dominated by uplift and horst formation associated with the Ghanzi Ridge, and an arch to the north of the Panhandle, which appear to represent the tips of incipient rifts which are propagating from the northeast. The Delta has formed in the resulting depression between these arches. The Panhandle has developed along a fault, and may be largely an erosional feature incised into the northern uplift zone. The Delta itself is an alluvial fan of remarkably uniform gradient. There is no evidence of regional tilting of the fan surface. Local highs and lows are developed on the fan, but channel location is relatively insensitive to this local topography. Moreover, marked elevation differences exist between adjacent channels, creating hydrologically unstable conditions. These unusual features of the local hydrology arise because of the confining effect of channel-flanking vegetation. Sedimentation in the Delta appears to be causing crustal sagging of the central Delta, which has: tilted the major palaeo-shoreline of the Mababe Depression to the west; formed a local depression within the Ghanzi Ridge facing the Delta; and detached a sliver of the ridge along the Thamalakane fault. It is suggested that local seismicity also results mainly from sediment loading. The Selinda spillway occupies a marked local depression, which is a graben between the Gumare fault and an extension of the Linyanti fault. It is probable that southwesterly propagation of the uplift zone associated with the incipient rift will ultimately deflect the Okavango River into the Chobe-Zambezi river system via this graben.