The topography of the Okavango Delta, Botswana, and its tectonic and sedimentological implications

T. GUMBRICHT, T. S. MCCARTHY AND C. L. MERRY

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IntroductionDespite its remoteness, the Okavango Delta of northernBotswana (Figure 1) is remarkably well covered by mapsat a variety of scales, the most detailed being a 1:50 000series. Topographic information for the region is,however, sparse, and is confined to Governmenttrigonometric beacons in the Maun area, along majorroads, and in widely scattered localities in and aroundthe Delta. As a consequence, only very generalizedcontour maps of the Delta have been prepared (e.g.UNDP, 1976; Cooke, 1980). The paucity of topographicelevations within the active Delta prompted Merry et al.(1998) to undertake a detailed survey, using differentialGPS, down the length of the Delta. This survey provideduseful insight into various aspects of the tectonics andsedimentation in the Delta (McCarthy et al., 1997). Thesurvey network has since been extended to include allof the major, navigable channels of the Delta.Nevertheless, the coverage of the Delta as a wholeremains relatively sparse. The reasons for the lack ofelevation data are the difficulty of access in the region,and the very low topographic relief. The latter isillustrated by the observation that the difference inelevation of the water surface between Shakawe andMaun, a distance of 240km, is only about 60m (McCarthyet al., 1997). The lack of regional topographic coverageof the Delta has recently been radically improved as a

result of a regional gravity survey carried out on behalfof the Geological Survey of Botswana, which involvedprecise elevation determination, using differential GPS,on a grid of 7.5km. This survey resulted in the firstdigital elevation model of the Delta region (Modisi et al.,2000). In this paper we integrate the results of thatsurvey with other available topographic information, aswell as satellite-derived information on waterdistribution, to produce a topographic contour map ofthe Okavango region.

The Okavango Delta is an area of activesedimentation, and moreover, is situated within thegreater Kalahari Basin, which is itself an activeintracontinental sedimentary basin. In addition, the areais tectonically active, forming a possible southernextension to the East African Rift System (Scholz et al.,1976). As a consequence, there are very few erosionallandforms in the region. Precise topographic informationis particularly valuable in such settings as it provides insight into sedimentation and tectonicdeformation. Moreover, water distribution in theOkavango constantly shifts (Wilson, 1973; Ellery andMcCarthy, 1994), which has been attributed tosedimentation (McCarthy et al., 1986), to tectonic effects(Wilson, 1973) or to major floods (UNDP, 1976).Regional topographic information could potentiallythrow light on this phenomenon.

The topography of the Okavango Delta, Botswana, and its tectonicand sedimentological implications

T. Gumbricht and T. S. McCarthyDepartment of Geology, School of Geosciences, University of the Witwatersrand, Private Bag 3, Wits 2050.

email: [email protected]

C. L. MerryDepartment of Geomatics, University of Cape Town, Rondebosch 7700.

ABSTRACTA topographic map of the Okavango Delta and environs has been constructed using a combination of elevation dataincluding trigonometric beacons and spot heights from the government of Botswana, surveys of the navigablechannels, U. S. Department of Defense data and measurements made during a geophysical survey of the region. Thetopography provides insight into the local tectonic and sedimentary history. Local tectonics are dominated by upliftand horst formation associated with the Ghanzi Ridge, and an arch to the north of the Panhandle, which appear torepresent the tips of incipient rifts which are propagating from the northeast. The Delta has formed in the resultingdepression between these arches. The Panhandle has developed along a fault, and may be largely an erosionalfeature incised into the northern uplift zone. The Delta itself is an alluvial fan of remarkably uniform gradient. Thereis no evidence of regional tilting of the fan surface. Local highs and lows are developed on the fan, but channellocation is relatively insensitive to this local topography. Moreover, marked elevation differences exist betweenadjacent channels, creating hydrologically unstable conditions. These unusual features of the local hydrology arisebecause of the confining effect of channel-flanking vegetation. Sedimentation in the Delta appears to be causingcrustal sagging of the central Delta, which has: tilted the major palaeo-shoreline of the Mababe Depression to thewest; formed a local depression within the Ghanzi Ridge facing the Delta; and detached a sliver of the ridge alongthe Thamalakane fault. It is suggested that local seismicity also results mainly from sediment loading. The Selindaspillway 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 riftwill ultimately deflect the Okavango River into the Chobe-Zambezi river system via this graben.

Methodology and Results

Processing of the GPS dataThe position and heights of the gravity stations weredetermined using carrier phase differential GPS,employing a system of local GPS base stations. Thesebase stations were referenced to two permanent sites ofthe International GPS Service, located at Sutherland andHartebeeshoek in South Africa. The positions andheights of these two base stations are precisely knownin the WGS84 reference system. As a result, the heightsof the gravity stations were originally determined withrespect to the WGS84 system. Specifically, the heightswere heights above the WGS84 ellipsoid (Poseidon,1999).

The Botswana Geological Survey required the data tobe referenced to the Cape datum, which uses themodified Clarke 1880 ellipsoid. Consequently, the final

data set consists of positions and heights referenced tothis ellipsoid. The transformation from WGS84 to Capedatum used a simple three-dimensional translationbetween datum origins (Poseidon, 1999).

We carried out two operations to convert theseheights (above the Clarke 1880 ellipsoid) to orthometricheights (above the geoid or mean sea level):(a) The positions and heights were transformed back tothe WGS84 datum, using the same transformationparameters (with opposite sign) as were originally usedfor the transformation from the WGS84 to Cape datum,and;(b) The WGS84 ellipsoidal heights were converted toorthometric heights using the EGM96 (Earth GravityModel 1996) geoid model (Lemoine et al., 1996). Thismodel of the separation between the geoid and theWGS84 ellipsoid is currently the most complete andaccurate global geoid model available. Nevertheless, the

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Figure 1. Map showing the distribution of fluvial sediments in and around the Okavango-Linyanti system (Modified from Shaw and

Thomas, 1992).



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estimated accuracy of this model is in the order 50 to80cm (Lemoine et al., 1996), and is the limiting factor inthe accuracies of the point heights used in this study.

Geostatistical interpolation of the DigitalElevatiom Model (DEM)A digital elevation model was constructed from severaldifferent datasets (Figure 2). The primary dataset wascomposed of a subset of elevation points from theregional gravity survey. This dataset has a goodcoverage of the central Delta and the Panhandle, whereit represents the land surface (as opposed to watersurface) elevation. The dataset does not cover theLinyanti area outside Botswana, nor the region east ofLake Ngami and south of the Kunyere fault (Figure 2).Initial analysis of this dataset revealed that mostsurveyed points represented lows in the landscape,leaving out apparent heighs such as Tsodilo Hills. Thisprimary dataset was hence supplemented with otherdata sources. Trigonometrically determined beaconsincluded in the Botswana survey dataset were selectively

added to represent highs in the landscape, and to coverthe area east of Lake Ngami. Elevation points given onthe U.S. Defense Mapping Agency OperationalNavigation Chart were used to close the data gap aroundthe Linyanti. Water elevation data and supplementarydata for land surface in the Delta were taken from thedifferential GPS surveys reported by Merry et al. (1998)and their unpublished data.

The major fault lines in the study area (parallel to thePanhandle, and along the Kunyere and Thamalakanerivers) are clearly reflected in the surface topography. Toaccount for the fault lines the study area was dividedinto four regions (Figure 2). The DEM of each regionwas generated separately, using overlapping data whereno faults interrupt the elevation (Figure 2). The Linyantifault in the northeast was treated as a fault line inside itsregion. Each region was interpolated by using kriging,with manually fitted variogram models. Except for thecentral delta, all variogram models where linear, withoutany nugget effect. For the central delta an exponentialvariogram model was fitted. The kriging interpolation

Figure 2. Elevation point data, and the division of the study area used for generating the digital elevation model (see text for details;

various areas indicated by different shadings)

was set to use 4 sectors with a maximum of 6 valuesfrom each sector. Search radius was set to 50km, exceptfor the southeastern region, where a larger radius had tobe set because of the relatively sparse data coverage.Kriging standard deviation was calculated along with theinterpolated surface.

The four DEM regions were combined, and a 7x7convolution filter with double weight for the centralpixel applied to smooth the transition between thecentral Delta (i.e. excluding the Panhandle) and thenorthwest region. Contour lines at 5m intervals werederived from the DEM and superimposed on a LandsatTM derived image of the study area (Figure 3).

A DEM covering the western shoreline of theMakgadikgadi pan was also created, using all availabledata points. This DEM is not shown, but was used toassist in the construction of topographic sections acrossthe Delta. This DEM was also created using variogramfitted kriging.

The standard deviation of the non-flooded areassurrounding the Delta is comparatively high (Figure 4).To improve the confidence of the paleo-shoreline studies

in these areas (see below), three separate interpolationswere done for Mababe Depression, Lake Ngami andMoremaoto ridge. These krigings were also done usingmanually fitted variogram models. Only data points fromthe primary dataset related to the local topographyaffecting the baseline of the shorelines were used.

Elevation data for water surfaces were restricted totransects along the major channels and included datafrom four campaigns carried out at different times (1994, 1995, 1996 and 1998). As the most extensivemeasurements were from the 1995 survey, the otherheights were transferred to the 1995 water level by usingoverlapping points and applying an inverse distanceweighting with an exponential distance decay function.A water surface was interpolated, again by usingvariogram fitted kriging, with an arbitrary fault lineintroduced to separate the parallel but offset channels ofNqoga and Jao.

Assessment of the error in the DEMThe standard deviation of the interpolated surface variesbetween the regions (Figure 4). The Delta surface,



Figure 3. Landsat TM image and elevation contours of the Okavango region.



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including the area between the Kunyere andThamalakane faults, has a standard deviation notexceeding 2m. These areas have no natural highs orlows in elevation, and hence the variogram models fitthe data points extremely well. The two other krigingregions are characterized by more elevation variation,and hence were less well fitted in the variogrammodeling. The smaller area to the southeast ofThamalakane fault incorporates both lows and highs,with the Boteti river and its valley running across it. Datais sparse in a large part of this region, which called forthe inclusion of the few existing trigonometric beacondata points (see Figure 2). Consequently the standarddeviation in this data poor region is high. The remainingregion also has a high standard deviation. The largekriging region surrounding the Delta and to thenorthwest, including Lake Ngami and the MababeDepression, is of intermediate accuracy. Apart from theNgami and Mababe lows, the region contains the relictdune fields west of the Delta, as well as Tsodilo Hills.This prevents the construction of a well-fitted variogram

model. The apparently evenly distributed error over thewhole region is hence an artifact. As the kriging used 4 sectors with a maximum of 6 values from each, and a50km search radius, the error outside the anomalousregions is probably smaller than estimated.

Fitting a planar conical surface to the Delta The genesis and development of a fan like theOkavango Delta potentially creates a very planar,conical surface over the geoid. A perfectly planar surfaceover the Okavango Delta was constructed by fitting apoint of origin to the arcs formed by the topographiccontours of the Delta (Figure 5). Independent fitting inthis manner put the origin of the fan 4km downstreamof the bifurcation of the Okavango into the Nqoga andThaoge channels. Using the fan origin as starting point aperfectly planar surface was fitted so that its distal endon average corresponds with the DEM elevations justupstream the Kunyere fault line. The difference betweenthe perfectly planar surface and the actual DEM of theDelta was calculated and the residuals analyzed for

Figure 4. Standard error of the generated DEM.

tracking sedimentological and tectonic patterns. Thepoint identified as the fan origin was also used forcomparing elevation of channels and land surfaces atvarious distances down the fan.

DiscussionRegional setting of the Okavango DeltaBefore discussing the topography of the Delta, it isimportant to examine its regional setting, as thisprovides a framework for the interpretation of thetopography. The investigation of the seismicity in thearea by Scholz et al. (1976) gave rise to the notion thatthe Delta has developed in a graben structure, whichthey regarded as an extension of the East African Riftsystem. This view has been largely accepted bysubsequent workers (Hutchins et al., 1976; Thomas andShaw, 1991; McCarthy et al., 1993; Modisi et al., 2000).However, the Delta is an integral part of the KalahariBasin, and it is necessary to review the settings of bothin order to provide a context for evaluating theimplications of Okavango topography.

1. The African SuperswellThe gross geomorphology of southern and eastern

Africa is dominated by the African Superswell (Nybladeand Robinson, 1994), a region of relatively elevatedterrain which manifests itself both on the continent andon the ocean floor to the southwest. It commences inthe Afar region of the Red Sea and extends southward,incorporating the eastern and western branches of theEast African Rift. South of Lake Victoria, the Superswellwidens to include almost the entire sub-continent,where it is clearly delineated by the 1000m contour(Figure 6). This broad southern section is asymmetrical,with the eastern flank being of higher elevation. Theterrain within the Superswell lies at an average elevationat least 500m higher than equivalent crustal rockselsewhere on the world’s continents (Nyblade andRobinson, 1994). The origin of the Superswell is linkedto dynamic topography, caused by the presence of hot,low-density mantle near the core-mantle boundarybeneath southern Africa, and to somewhat shallower hotmantle beneath the East African Rift (Lithgow-Bertelloniand Silver, 1998).

Burke (1996) suggested that the Superswell began todevelop about 30 Ma ago, when the African Plate ceasedmovement relative to the hot-spot reference frame. Incontrast, Partridge (1998) has proposed a multi-stage



Figure 5. Satellite image showing the smooth conical surface fitted to the fan, as well as the location of topographic section lines.



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uplift history for the Superswell. During the break-up ofGondwana, uplift along the rift zones resulted thedevelopment of a marginal escarpment around the sub-continent. Subsequently, two main periods of upliftoccurred, during the Miocene and Pliocene. Uplift wasasymetrical, and Partridge (1998) estimates total uplift of1200m along the eastern escarpment and 450m in thewest. However, the separation of the Falklands Plateauoccurred via transform faulting along the AgulhasFracture Zone, and no rift developed along the southernAfrican coastline from as far north as northernKwazulu/Natal. Along this section of the Africancontinent, which includes the most elevated section ofthe escarpment, post-break-up uplift must thereforeaccount for all of the present elevation, suggesting thatPartridge’s (1998) estimates are too low, and that muchof the present elevation is due to Superswell uplift.

2. The Kalahari BasinThe African Plate is also characterized by large, basin-

like depressions bounded by elongated regions of localrelative uplift which Burke (1996) termed “sub-swells”,and which he believes reflect zones of local mantleupwelling. These sub-swells were first recognized insouthern Africa by du Toit (1910, 1933) and more widelyon the continent by Holmes (1944). The basins definedby the sub-swells accommodate Cenozoic sedimentarymaterial. Du Toit (1933) established that these sub-swellsplay an important role in delimiting drainage basins insouthern Africa, indicating a youthful origin orprolonged and continuing uplift, observations whichhave been further emphasised by later workers (e.g.Mayer, 1973; McCarthy, 1982; Moore, 1999; Moore andLarkin, 2001).

One of these depressions, the Kalahari Basin, issituated within the Superswell (Figure 7; Haddon, 1999).It is displaced to the western side of the sub-continentby the higher terrain of the eastern margin of theSuperswell. The Basin is divided into a northern andsouthern sub-basins by the Ghanzi Ridge, along which

Figure 6. The distribution of the African Superswell on the continent.

basement is exposed. Isopachs of the Kalahari Groupindicate that the southern basin has only a singledepocentre, while the northern basin has three, one ofwhich lies beneath the Okavango Delta.

The stratigraphy of the Kalahari Group is poorlyknown, and it evidently displays considerable lateralvariation. A basal conglomerate is widely developed,overlain by fluvial sand, marl, aeolian sand and calcrete(Thomas and Shaw, 1991). It has been documented indetail in a limited portion of the eastern section of thesouthern basin (du Plessis, 1994). The lower portion ofthe sequence is characterized by fluvial and lacustrinesediments, which are overlain by a residual gravel lag(the “Letlakane Stone Line”). This in turn, is overlain byaeolian sands, silcretes and calcretes which include themobile surficial sands of the Gordonia Formation (the“Kalahari sands”). Partridge (1998) suggests that thelowermost units of the Kalahari Group are probablyupper Cretaceous in age, as these overlie Cretaceouskimberlites in which crater facies are still preserved.

3. Southern African drainagesThe drainage patterns of southern Africa are stronglyinfluenced by the Superswell, its associated rifting andby the smaller sub-swells. The Eastern and Western Riftsare characterized by highly disrupted, commonlyinternal drainages and numerous lakes (Figure 8). Thenorthern and northwestern slopes of the Superswelldelimit the Congo basin to the northwest. Uplift of theSuperswell has accentuated the marginal escarpment ofsouthern Africa, which is characterized by short, steeprivers, particularly along the southern coastline. Themarginal escarpment has been breached by three majorrivers: the Zambezi, the Limpopo and the Orange, andthese rivers drain much of the interior of the Superswell.The watersheds which separate these basins are in partdetermined by sub-swells (du Toit, 1933). Within theSuperswell in central southern Africa, two internaldrainage basins are developed: the Etosha Basin and theOkavango-Makgadikgadi Basin. These are partly respon-sible for on-going sedimentation in the Kalahari Basin.



Figure 7. Thickness distribution of the Kalahari Group (based on Haddon, 1999)



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The evolution of the major drainages of centralsouthern Africa has been partly established, althoughcertain aspects remain controversial. It is believed thatthe Kafue, upper Zambezi and possibly the Kwando andOkavango Rivers once formed a single drainage system(Thomas and Shaw, 1988; Moore and Larkin, 2001),which may have flowed southward into the ancestralOrange River (Lister, 1979; McCarthy, 1982). Uplift alongthe sub-swell now defined by the southern margin of the Kalahari Basin (Transvaal-Griqualand Axis of duToit, 1933) disrupted this link and an internal drainagebasin developed, which later diverted into the Limpopo.Uplift along the Kalahari-Zimbabwe axis isolated theZambezi-Okavango system from the Limpopo and re-established the internal drainage system, giving rise topalaeo-Lake Makgadikgadi, possibly during the Pliocene(Thomas and Shaw, 1988). Meanwhile, headwarderosion by the lower Zambezi, as well as local tectonism(see below), led to progressive capture and eastwarddiversion of the Kafue, the upper Zambezi and theKwando systems by the lower Zambezi during the late Pliocene to mid-Pleistocene (Thomas and Shaw,1991; Shaw and Thomas, 1992; Moore and Larkin, 2001).Local tectonism also disrupted the Okavango River.Palaeo-lake Makgadikgadi was thus deprived of most of its inflow, creating the ephemeral pan of today.The formation of the Okavango Delta therefore mostlikely began during the late Pliocene or earlyPleistocene, and post-dates the formation of the KalahariBasin.

4. Seismicity on the sub-continentSeismicity on the sub-continent is mainly concentratedin the higher terrain along the eastern side of theSuperswell (Figure 9; Graham and Brandt, 2000). TheEastern Branch of the Rift Valley has little seismicity,although the off-shore extension of the rift along theDavey Ridge in the Mozambique Channel (Mougenot et al., 1986) is relatively active. In contrast, there isconsiderable activity along the Western Branch. Thisextends eastwards through Lake Malawi and along the eastern escarpment where it branches towards thecoastline in the Urema Rift near the mouth of the Zambezi. Seismicity is diffusely developed on theinland branch along the eastern escarpment as far as30°S, where it turns abruptly to the west, extending in abelt across the continent (Hartnady, 1990). Theseismicity related to Witwatersrand mining forms an arcto the north of this. Diffuse siesmicity extendsnorthwards along the western escarpment. Apart fromthe Okavango region, the Kalahari Basin is virtuallydevoid of seismic activity.

A diffuse zone of seismicity is developed to the westof the arc defined by the Western Rift and its southerlyextension along the western escarpment. Within thisdiffuse zone, several southwesterly-trending belts ofenhanced seismicity can be decerned, two of which areparticulary prominent and each representing incipientrift zones. The first of these belts branches from the

southern end of Lake Tanganyika and includes Lakes Mweru and Tshangalele and the Kabompo River(Figure 8). The second extends from the northern end ofLake Malawi along the Luangwa River valley and LakeKariba to the southwest. In their northeasterly reaches,these rift zones are characterized by central depressionswith marginal uplifts, and Lake Bangweulu and theLukanga swamps lie in the intervening depression(Figure 8). The Okavango region is also seismicallyactive, but this activity is located northwest of theLuangwa-Kariba seismic axis (Figure 9). Focal mec-hanism studies by Scholz et al. (1976) indicated dip-slip motion on the faults in the Okavango area.

Although Burke (1996) suggested that the interior ofthe southern portion of the African Plate may be undercompression due to ridge-push effects, Coblentz andSandiford (1994) and Andreoli et al. (1996) suggest thatthe northern portion of the Superswell is experiencingnorthwest-southeast directed tension, while in the south,stress orientation changes to northeast-southwest. Theseintra-plate stresses may be related to the excesstopography of the region (Coblentz and Sandiford,1994). The location of active and incipient rifts whichhave developed on the continent in response to thesestresses is closely related to pre-existing structures, andrift zones usually follow old but suitably orientatedcontinental sutures (Fairhead and Stuart, 1982). Inparticular, Pan-African metamorphic belts are commonsites of reactivation under the present stress regime. Thisis especially true of the Luangwa-Kariba rift (Sebagenzi,1997) and the Okavango region (McCarthy et al., 1993;Modisi et al., 2000).

Topographic subdivision of the DeltaThe most striking feature evident on the contour map inFigure 3 is the very low topographic relief of the region.Apart from the isolated pre-Kalahari inselbergsnorthwest of the Mababe Depression, the elevationvaries from a maximum of 1025m at Mohembo to a minimum of 920m in the Mababe Depression, a total topographic range of only 105m over an area of 72 500km2. There is an overall regional tilt on the landsurface from northwest to southeast, but the pattern ofcontours allows the area to be subdivided into threebroad domains, within each of which local structure isevident. These domains form northeasterly trendingbelts across the area. They are: (1) the area in thenorthwest, including the Panhandle (the Panhandledomain), which is the most topographically elevateddomain; (2) the area to the southeast of theThamalakane River, which includes the Boteti River (the Boteti domain), which is of intermediate elevation;and (3) the central portion of the area, which includesthe Delta, Lake Ngami and the Mababe Depression (the Delta domain), in which the lowest elevationsoccur. The relationships of the three domains is furtherillustrated by three northwest-southeast topographicsections to the northeast and southwest of thePanhandle (Figure 10).

1. The Panhandle DomainThis domain is characterized by a generally south-easterly slope, except in the west, where the groundrises towards the Tsodilo Hills. It terminates in the southalong a northeasterly trending zone which is coincidentwith the expansion of swampland at the southeasternextremity of the Panhandle and which is considered tobe marked by the Gumare fault (Figure 1). The Pan-handle lies parallel to the regional slope.

The terrain to the east and west of the Panhandle ischaracterized by large, linear dunes, now heavilydegraded (Jacobberger and Hooper, 1991), whichcontrast markedly with the floodplain terrain of thePanhandle, characterized by numerous fluvial featuressuch as oxbows and scroll bars. The dune terrain slopesto the southeast, with a gradient of about 1:2000. Incontrast, the gradient on the floodplain of the Panhandleis 1:5600 (McCarthy et al., 1997), and the floodplainbecomes progressively more deeply incised into thedune terrain from southeast to northwest (Figure 10).This has evidently resulted in some lateral erosion of the

flanks of the Panhandle in its upper reaches, asindicated by the gentle swing of the contours towardsthe Panhandle in this area, in contrast to the abruptswing of the contours in the middle reaches of thePanhandle. The shoulders of the Panhandle are mostabrupt in this middle reach, but decrease in height to thesoutheast, due to the steeper slope on the dune terrain(Figure 10). The floodplain widens slightly in thissouthern area, suggesting that floodplain sediments maylocally overlap the flanks of the Panhandle. This isparticularly evident on the west bank, which lies at aslightly lower elevation that the east bank (see below).A nick point is developed at the head of the Panhandleat Popa Falls (Figure 10, section B).

The origin of the Panhandle has been ascribed toboth erosional incision (Stanistreet and McCarthy, 1993)and to graben faulting (McCarthy et al., 1993). Thetopography can provide some new insight into thisproblem. The contours on either side of the Panhandletrend northeast. However, there is a distinct offset of contours across the floodplain, with contours of



Figure 8. Map showing the major drainages of southern Africa.



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equivalent elevation to the northeast beingsystematically displaced to the southeast relative to thoseto the southwest of the Panhandle by about 10km. Theoffset appears to be consistent from the 1000m contourto the end of the Panhandle, but it is not clear if an offsetis present in the northwestern portion of the Panhandlebecause of curvature of the contours in this region. Thisoffset could be accounted for by a fault within at leastthe southeastern portion of the Panhandle with a 5mdownthrow to the southwest, suggesting that thePanhandle has developed along a fault. Faulting in thisorientation is common in the region (Sinding-Larsen et al., 1991). The presence of a nick point at its apex aswell as progressive deepening towards the apex suggesterosion has been important in formation of thePanhandle, and it is probably not a graben structure ashas been suggested previously. In addition, gradients ofthe floodplain and the terrain forming the left(northeast) and right (southwest) banks are perturbed(Figure 10), suggesting that the Panhandle is transectedby a northeasterly striking fault. Such faulting was also

detected during a detailed topographic survey in thePanhandle (Smith et al., 1997). This fault appears todownthrow to the southeast.

The Panhandle domain therefore consists of threemajor structural sub-domains: the floodplain of thePanhandle itself, which is fault controlled, an elevatedblock to the northeast of the Panhandle, and a slightlylower block to the southwest. These three sub-domainsare transected by a northeasterly striking fault whichdownthrows to the southeast.

2. The Boteti DomainThis domain lies in the southeastern portion of the map,and its northwestern boundary is defined by theThamalakane fault, along which the Thamalakane riverflows, and the southeastern margins of Lake Ngami andthe Mababe Depression,which are defined by theKunyere fault (Figure 1). The domain forms part of theGhanzi Ridge which separates the Okavango Delta fromthe Makgadikgadi Depression. The domain ischaracterized by a central, basin-like depression across

Figure 9. Map showing the distribution of seismic activity in southern Africa (after Graham and Brandt, 2000)



which the Boteti river flows, here termed theMakalamabedi depression (Figures 3; 10, section B). Tothe southeast, the depression is bordered by low ridgewhich is about 15 to 20km wide, here termed theMoremaoto Ridge (Figures 1, 11). The Boteti river cutsthrough this ridge in the Moremaoto Gorge (Shaw et al.,1988). The Moremaoto Ridge rises to an elevation above955m, and expands to the northeast and southwest,merging with the high ground to the south and east ofthe Delta, confining the Makalamabedi depression(Figure 11).

Immediatetly east of, and in part amalgamated with,the Moremaoto Ridge lies the Gidikwe Ridge, which isdeveloped between the 935 and 940 contours, andrepresents an ancient shoreline of palaeo-lakeMakgadikgadi. The Gidikwe Ridge is spectacularlyevident on satellite images of the area. On these images,it is comprised by a distinct band, which represents theeast (windward) facing slope, and a more diffusewesterly fringe, characterized by westerly trendinglineations, probably representing linear sand dunes,blown from the crest of the ridge.

The relationship between the Moremaoto Ridge andthe Gidikwe Ridge is unclear. Cooke and Verstappen(1984) and Shaw et al. (1988) did not distinguish betwenthese features, and regarded the Moremaoto Ridge aspart of the Gidikwe shoreline. However, the localtopography suggests a clear distinction, and implies thatthe Moremaoto Ridge is part of a regional feature,unrelated to the shoreline. Within the limited areacovered by the contour map, the Moremaoto Ridge canbe seen to rise to above 955m, and no ridges ofequivalent height and size occur around Lake Ngami orthe Mababe Depression, whereas ridges equivalent tothe Gidikwe Ridge, lying at a lower elevation than theMoremaoto Ridge, are evident along both the MababeDepression and Lake Ngami. Moreover, the ridge is upto 20km wide, far greater than the width of the shorelinevisible on satellite images. We therefore favour a non-lacustrine origin for the Moremaoto Ridge.

The Boteti river abruptly transects the GidikweRidge, indicating that the present course of the river issuperimposed on the ridge. At the time of formation ofthe Gidikwe Ridge, the Boteti River entered palaeo-lake

Makgadikgadi further to the north, as indicated by the embayment in the ridge some 50km northeast of theMoremaoto gorge. Although only a limited portion of the Gidikwe Ridge is covered by the topographic map,the survey results suggest that the ridge rises in elevationfrom south to north by about five metres (Figures 12a,b),probably as a result of neo-tectonic activity in theregion.

The Makalamabedi Depression is confined by higherground to the northeast and southwest, and by theMoremaoto Ridge to the southeast (Figure 11). Curvi-linear features which appear on satellite images on bothbanks of the Boteti river within the depression at around940m elevation have been interpretted as old shorelinesby Shaw et al. (1988) and indicate that a small lakeoccupied this depression in the past. The Boteti riverflows across the centre of the depression, and is fairlydeeply incised along much of its course. The Boteti isunderfit, reflecting greater discharges in the past (Shawet al., 1988). The basin shallows gently towards theThamalakane fault to the northeast, and a slight rise isdeveloped along the southeastern side of theThamalakane fault facing this depression (Figures 10,section B; 11). The Boteti River does not cross the riseat its lowest point.

The Makalamabedi Depression is not an erosionalfeature, and is underlain by Kalahari Group sediments(Figure 6). Although the evidence is equivocal, seemsmore likely to be tectonic in origin, and is possibly fault-bounded on its southeastern as well as northwesternmargins. The proposed geometry of these faults isillustrated in Figure 11. The Kunyere fault is the majorfault bounding the southeastern margin of the Delta(Modisi et al., 2000). The Thamalakane fault is a curvi-linear structure along the segment facing theMakalamabedi Depression. The upthrown, southeasternside appears to have a slight lip similar to that describedby Mackey and Bridge (1995), which we believe isresponsible for the slight ridge along the Thamalakanefault (Figures 3; 10, section B). We suggest a second,curved bounding fault, the Makalamabedi fault, alongthe southeastern margin of the depression.

3. The Delta DomainThis domain extends diagonally across the centre of the map from southwest to northeast, and is bounded inthe northwest by the Gumare fault and in the southeastby the Thamalakane river and the southeastern marginsof Lake Ngami and the Mababe Depression. Thisboundary is defined by a system of faults including theThamalakane and Kunyere faults (Figure 1). The domainextends to the northeast and includes the Linyantiswamp. It has been previously recognized as theOkavango graben, and is considered to represent aseries of grabens bounded by northeasterly strikingfaults (Shaw and Thomas, 1992). The dominant featuresof this domain evident on the topographic map are thearcuate contours of the Delta (Figure 3) and its twoflanking depressions. For convenience it is here divided

Figure 10. Northwest-southeast topographic sections across the

Delta. See Figure 5 for locations.



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into three sub-domains: the Central Delta sub-domain,which consists of the Delta itself; the northeasternLinyanti-Mababe sub-domain; and the southwesternLake Ngami sub-domain.

The Central Delta Sub-domainThe Delta itself is characterized by semi-circularcontours of remarkable uniformity, except in the areaaround and to the north of the Maunachira channel,where a few pronounced deviations are developed(Figure 1; 3). The contours are centred on a point nearthe southern end of the Panhandle. Chief’s Island, aprominent feature on satellite images of the Delta, is notreflected as a topographic high in the contours, and ingeneral there is no obvious relationship betweencontours and inundated areas. It appears that thedistribution of water is determined by very subtletopography, which is to a large extent not resolved atthe scale of topographic resolution used here. This isdiscussed in more detail later.

On the whole, the gradient on the Delta surface isremarkably uniform, averaging 1:3550. The uniformsurface is the product of sedimentation, which isprimarily controlled by the distribution of water on theDelta. Two kinds of sedimentation are involved; clasticand chemical (McCarthy and Ellery, 1998). Clasticsedimentation dominates the upper Delta, and primarilyinvolves fine sand with lesser silt and clay, whilechemical sedimentation in the form of silcrete andcalcrete dominates the more distal portions of the Delta.The uniform gradients observed on the fan surfaceindicate that these two types of sedimentation maintaina delicate balance. Excessive chemical sedimentation inthe distal portions or excessive clastic sedimentation inthe proximal reaches would result in deviations from auniform gradient.

Details of the fan gradient are illustrated in a seriesof radial sections (Figure 13: see Figure 5 for locations),superimposed on a reference gradient of 1:3550. TheSelinda and Thaoge profiles cross the southern edge of

Figure 11. A contour map of portion of the Boteti Domain, showing the Makalamabedi depression and its relationship with the

Thamalakane fault and the proposed Makalamabedi fault.

the Panhandle (Figure 5), which reflects as spikes in theprofiles. Downslope from the spike in the Selinda profileis a pronounced depression in which the Selindaspillway flows. The remaining profiles all show a slightconvex shape, with maximum bulge in the upper mid-fan region. In the case of the Maunachira section, thebulge is notably elongated.

The residual elevation map (Figure 14) showsradially distributed regions of positive and negativeelevation deviation from a theoretical, planar conicalsurface fitted to the Delta surface. On the whole, thispattern contrasts markedly with the generallynortheasterly strike of major faults in the area, indicatinga sedimentary origin. Comparison of the distribution ofmajor distributory channels with the topographicresiduals (Figure 14) reveals both expected and alsosome unexpected features. The Thaoge channelfollowed a region of generally higher residualtopography down the western side of the Delta. TheBoro system is confined to pronounced low region,

which also includes much of Chief’s Island. Only thenorthern tip of Chief’s Island forms a topographic high.The Nqoga - Manachira system flows across a relativetopographic high almost along its entire length, whilethe Mboroga channel, which diverges from theMaunachira, flows down the eastern flank of the Chief’sIsland low. The Kunyere system also appears to followa relative topographic high. The Selinda flows along apronounced relative topographic low.

These relationships are further illustrated by a seriesof topographic profiles along arcs at increasing distancesfrom the point of origin of the fan (Figure 15: see Figure 5 for locations). These profiles clearly show thatthe channels do not necessarily follow relativetopographic lows on the fan surface.

There is clearly no simple relationship between localtopography and the distribution of water on the Deltasurface. The critical area of water distribution across theDelta is the apex at the southern end of the Panhandle.Minor topographic differences and possibly vegetationgrowth, especially of papyrus, in this region evidentlyhave a profound effect on water distribution throughoutthe Delta. Perhaps the most obvious is the apparenttopographic high which restricts flow to the Selindaspillway, reducing outflow to a narrow defile, whichthen widens to the east - virtually a fan within a fan. Thetopographic highs of the Duba Island cluster andnorthern tip of Chief’s Island limit flow to the Nqoga-Maunachira channel system. Because of theserestrictions to easterly water dispersal, the major waterleakage occurs to the southwest, and it is in this regionthat the major seasonal swamps are developed(McCarthy et al., 1998; Gumbricht et al., 2000). The lowtopographic gradient over the Delta means that minor,local topographic disturbances influence local waterdispersal. Small fault scarps, which appear to bewidespread across the Delta, direct flow (McCarthy et al., 1993). The “Sandveld Tongues”, which separatethe major distributaries, do not form pronouncedfeatures in the residual topography, suggesting that theyare transient, of no long term significance in the overallstructure of the Delta.



Figure 12b. Elevation profile along the western shoreline of

palaeo-lake Makgadikgadi. The bold line shows the estimated

elevation, while the two enveloping lines represent the standard

deviation. Breaks occur in the lines where the palaeo-shoreline

cannot be identified on the satellite image.

Figure 12a. Enlargement of the western shore of palaeo-lake

Makgadikgadi, with elevation contours and palaeo-shoreline (the

Gidikwe Ridge) indicated.



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The fan surface has a slightly convex shape (Figure 13), and the distribution of the bulge coincidesapproximately with the distribution of permanentswamp. Although chemical sedimentation exceedsclastic sedimentation in the Delta by a factor of two(McCarthy and Ellery, 1998), clastic sedimentation isconfined to the Panhandle and the upper fan, whichoccupy a far smaller area than that prone to seasonalflooding and chemical sedimentation. This may accountfor the slight bulge towards the apex of the fan. In effect,the Panhandle has prograded about 35km downstreamof the centre point of the fan.

The pronounced relative topographic low in thedistal portions of the Selinda spillway appears to berelated to the Linyanti and Gumare faults. The Linyantifault downthrows to the north, and the Linyanti swampterminates against the scarp. The fault strikes southwest,and the depression in the topographic residual map lies to the northwest of a projection of the fault trace(Figure 14), suggesting that this fault is propagatingsouthwestwards into the Delta. The depression isbounded in the northwest by the Gumare fault, whichdownthrows to the southeast. The resulting localizedgraben structure is occupied by the Selinda spillway.

Changes in water distribution in the Delta have beenascribed to tectonic effects, one of which is regionaltilting. Such tilting would be reflected in radial gradientsaround the fan. Radial gradients are, on the whole, veryuniform (Figure 13). For example, along an arc 125kmfrom the fan centre point, the elevation of the landsurface around the Delta varies by only 2.5m, the lowestbeing on the Selinda profile, and the highest on theMaunachira profile (Figure 15). Residual topographicelevations (Figure 14) also indicate a relativelyhomogenous spread of positive and negativetopography. The uniformity of the gradient on the fanindicates that the Delta has not experienced any largescale regional tilting. Moreover, it is evident that channellocation is relatively insensitive to local topography(Figure 15), and only major regional tilting (in the orderof tens of metres) could have any effect on channel, andhence water distribution. Tectonic tilting of the fansurface must therefore be discounted as the cause of thechanges in water distribution on the fan which areknown to have occurred over the last 150 years (Elleryand McCarthy, 1994).

The southeastern termination of the Delta is definedby the scarps of the Kunyere and particularly theThamalakane faults. The scarp of the Kunyere fault haslimited expression opposite the Panhandle (Figure 10,section B) as it is overlapped by sediments of the Delta.To the southwest, the scarp becomes more prominentdue to the curvature on the Delta surface, and here theKunyere channel is deflected by the scarp and flowssouthwestwards to Lake Ngami. In contrast, to thenortheast the Kunyere fault does not appear to exhibitscarp development. However, the extension of this faultdefines the southeastern margin of the Mababedepression, indicating increased scarp development.

This variability in expression of the Kunyere fault mayreflect variable displacement, but we are of the opinionthat it is probably the result of sedimentation which haslocally overlapped the scarp.

The scarp of the Thamalakane fault is considerablymore prominent than that of the Kunyere fault (Figure 10), although the strike length is shorter, and thevertical displacement appears to be smaller (Modisi etal., 2000). This fault forms the ultimate limit to surfaceflooding on the Delta. Discharge crossing the Kunyerefault due to sediment overlap of its scarp enters theThamalakane River, the course of which is determinedby the Thamalakane fault scarp. Because of theoverlapping of the Kunyere fault scarp by the curvedsediment cone of the Delta, a flow bifurcation has beencreated, and water from the Delta reaching this region ofthe Thamalakane flows both northeast towards theMababe Depression, and southwest towards Lake Ngami(Du Toit, 1925; Wilson and Dincer, 1976).

The natural drainage of the Thamalakane Riversouthwest of the flow bifurcation is to the southwestalong the fault scarp towards Lake Ngami. The scarphas, however, been breached to form the Boteti River.The contours indicate that the breach occurs above the945m contour. Incision by the Boteti evidently made thispossible. The local gradients in this area are such thatflow divides at this junction (Wilson and Dincer, 1976),with some water flowing southwest via the Nhabe Riverto Lake Ngami and some along the Boteti across thedepression discussed above. The Nhabe River couldwell be an erosional feature, developed as a result of the lower elevation of Lake Ngami relative to the

Figure 13. Radial topographic profiles from the point of origin of

the reference surface fitted to the fan. A reference gradient of

1:3550 is superimposed on each of the profiles. See Figure 5 for

locations of the profiles. The elevation of the origin (left hand side

of each section) is 998 m.a.m.s.l, while the elevation of the right

hand end of the reference line is 936 m.a.m.s.l.

Thamalakane River. The Boteti River does not cross the scarp at its lowest point, which suggests that erosionby the river has kept pace with fault displacement. Rivergradients in this region are small, and some of theThamalakane flow is diverted to the southwest towardsLake Ngami.

Detailed elevation surveys of the water surface alongthe major channels were carried out using differentialGPS over several years (Merry et al., 1998). These datahave been corrected to allow for seasonal fluctuationsusing overlapping points measured during eachcampaign. The results are shown in the form of acomposite longitudinal section, with distances measuredfrom the fan origin at the southern end of the Panhandle(Figure 16). In general, the Jao/Boro channel system isdistinctly lower in elevation than the easterly drainagessurveyed. In the region where the Jao and Nqogachannels overlap (the so-called “filter region”, Wilson,

1973), the Nqoga is significantly more elevated (by morethan a metre). The Nqoga channel has beenexperiencing progressive blockage and abandonmentfrom its distal end since the 1920s (Wilson, 1973), andnow extends only as far as Hamoga Island (McCarthyand Ellery, 1995). A narrow and shallow, artificiallymaintained channel connects the Nqoga to theMaunachira, but most water is transferred through aregion of overlap involving both the Maunachira and the Khiandiandavhu (McCarthy et al., 1992). TheKhiandiandavhu lies at a significantly lower elevationthan the Nqoga in the overlap region (Figure 16).Similarly, the Mboroga lies below the Maunachira overits entire length. These relationships reflect potentiallyunstable situations, and one might expect rapid channelcapture to be a feature of the Okavango. This is not thecase, and hydrologically unstable geometries seem to beable to persist for considerable lengths of time. The



Figure 14. A topographic contour map showing the deviations of the actual topography from the fitted reference surface.



259

reason for this is probably a combination of factors,most important of which are the generally low gradientsand the mechanical strength of the channel flankingvegetation, particularly papyrus and Miscanthusjunceus. The relative elevations do suggest, however,that the Nqoga is likely to be superceded by theJao/Boro system as the principal distributary channelsystem.

The Linyanti-Mababe sub-domainIn the northeast, the curved contours of the Delta makean abrupt turn to a more the northeasterly orientation inthe Linyanti-Mababe sub-domain. The northeasterlycontour orientation may reflect an ancient alluvial fansystem related to the Kwando river (Shaw and Thomas,1992; Figure 1). This fan has been disrupted by theLinyanti fault which has diverted flow to the ZambeziRiver in the east. The Linyanti swamp remains as aremnant of the formerly larger system. Further disruptionof the Kwando fan has also been caused by the Chobefault, which upthrows to the southeast (Figure 1). TheMababe Depression is developed as a triangular featurebetween the curve of the Delta, the upthrown block tothe southeast of the Chobe fault and the Kunyere fault,which defines its southeastern margin. Small, pre-Kalahari inselbergs, similar to Tsodilo Hills, aredeveloped to the north of the Mababe Depression onthe Chobe horst, forming prominent hills (Gubatsha andGoha hills).

The contour map provides insight into the erraticbehaviour of the Savuti channel, which has flowedintermittently in the past. The Savuti channel linked theLinyanti swamps to the Savuti marsh at the northern endof the Mababe Depression. Historical records compiledby Shaw (1984) reveal that this channel flowedfrequently until the late 1880’s, but then remained dryuntil 1958, when it again flowed to the MababeDepression. It failed in the early 1980’s, and remains dry.The contour map indicates that the source of the Savutichannel in the Linyanti swamp currently lies at anelevation below 945m, and its course to the southeastcrosses a gentle rise, possibly a lip on the Linyanti faultsimilar to that on the Thamalakane fault describedabove. Under present conditions, flow proceeds only afew kilometres down the channel. However, in wetterperiods, a higher water level is sustained in the Linyantiswamps, which creates sufficient hydraulic head toenable the water in the Savuti channel to cross the riseand reach the Mababe Depression. Higher and lowerrainfall in the catchment of the Kwando appear toalternate under the influence of an 80 year oscillation(McCarthy et al., 2000 ), which entered a low phase inabout 1980.

The Mababe Depression is an old lake which hasbeen permanently dry throughout recorded history. Ithas in the past received water from the Savuti channeland the Khwai channel in the southwest. Flow fromthese sources has been insufficient to fill the lake, due

Figure 15. Topographic profiles along arcs at increasing distances from the point of origin of the reference surface fittedto the fan. See Fig. 5 for locations. The positions of the major channels on the sections are also shown. Note that thehorizontal scale is different for each section (lengths of the sections are shown on the right of each).

to the local high water deficit, and these two channelshave only produced marshy areas at their dischargepoints into the old lake. However, geomorphologicalstudies have revealed that this has been a substantiallake on several occasions in the past (Shaw, 1985; 1988).Ancient shorelines are preserved in the form of raisedbeaches and beach ridges, particularly on the leeward,western shore. These are visible on satellite imagery,and the Magagikwe ridge in particular is very prominent.The lake is bounded on its southeastern side by Kunyerefault, and hence is steeper, and old shorelines are lessprominent. The topographic contours clearly outline thepalaeo-lake, and the lowest point in the mapped arealies within the lake bed.

An enlargement of the contour map of the Mababearea is shown in Figure 17a, and a north-south profilealong the ridge in Figure 17b. The latter indicates thatthe Magagikwe ridge rises in elevation from south tonorth by 15m, from a low of 930m in the south to 954mat the northern end of the palaeo-lake. This tilting of thepalaeo-shoreline is undoubtedly related to neo-tectonicactivity in the Delta and continuing upthrow of the horstblock to the southeast of the Chobe fault

The Lake Ngami sub-domainIn the southwest, the contours of the Delta make anabrupt turn to a south-southeasterly orientation with risein elevation to the southwest. This change in slope isdue to the coalescence of the Delta alluvial fan with asecond fan originating in the west, the Groot Laagtesystem (Figure 1; Shaw and Thomas, 1992). Althoughthe area of coverage is small, it appears that the slopeon the Groot Laagte fan is considerably steeper than thaton the Delta. Lake Ngami is situated in the wedge-shaped depression between the scarp of the Kunyerefault, and the coalescing Okavango Delta and GrootLaagte alluvial fans. The palaeo-shorelines of LakeNgami are difficult to correlate reliably. The highestshoreline, which would correspond with the Gigikweridge of the Makgadikgai Depression and Magagikweridge of the Mababe Depression, lies at an elevationbetween 935 and 940m, and the highest palaeo-

shoreline on the eastern side of the palaeo-lake lies at asimilar elevation. This suggests that the lake bed has notexperienced significant tectonic tilting since formation ofthe highest shoreline. However, the shorelines havebeen extensively disrupted, and conclusive informationon their elevations is lacking.

Structural analysis of the Okavango regionThe Okavango Delta lies within a fault-bounded trough(Figure 1), and the four major faults in the area(Kunyere, Thamalakane, Linyanti and Chobe)downthrow to the northwest. The Thamalakane andKunyere faults dip to the northwest (Scholz et al., 1976).In contrast, the Gumare fault downthrows to thesoutheast and appears to be antithetic to the Kunyereand Thamalakane faults. The overall style of thedepository is therefore a half graben, with the Kunyerefault representing the principal detachment. Maximumthickness of the sediment fill, indicated by drilling, isabout 300m. In addition to the Okavango Delta, the halfgraben also hosts remnants of the Kwando delta, nowbeheaded by the Linyanti and Chobe faults, and theinactive Groot Laagte Delta in the west (Figure 1; Shaw and Thomas, 1992). This half grabenstyle of faulting has been well documented in the EastAfrican Rift (e.g. Rosendahl et al., 1986) and has led tothe notion that the Okavango lies within an incipient rift(McCarthy et al., 1993; Modisi et al., 2000). However,analysis of the regional environment in which the Deltaoccurs, described above, as well as the localtopography, raise certain difficulties with a rift basinmodel for the Okavango. Amongst these are: (1)thinning of the Kalahari Group to the north and south ofthe Delta; (2) the greater elevation of the northern endof the Panhandle compared to the Ghanzi Ridge; (3) thesoutherly tilt of the land surface in the Panhandledomain; (4) displacement of both the Delta and itsseismicity to the northwest of the Luangua-Kariba riftaxis; and (5) the symmetrical disposition of both theThamalakane fault and the Makalamabedi Depressionwith respect to the Delta.

Isopachs of the Kalahari Group (Figure 7) indicatethat the major tectonic style is one of uparching andsuccessive upthrow to the southeast along northeasterlystriking faults, resulting in the Ghanzi Ridge which haspenetrated and divided the Kalahari Basin into two sub-basins. To the northeast, the ridge broadens and is cutby a progressively widening axial rift which accom-modates the Kariba gorge and the Luangwa valley(Figure 8). The depression northwest of this ridge hoststhe Bangweulu and Lukanga swamps. The Okavangoand Linyanti wetlands lie on the southwesterly extensionof this depression (Figure 8). To the northwest of thePanhandle, the land rises, the Kalahari Group thins, andbasement is exposed in the Okavango River at PopaFalls and in the vicinity of Mohembo and Shakawe. Thisrise lies on the projection of the Mweru-Tshangalele-Kabompo rift. Although actively subsiding, these variousrelationships suggest that the Okavango graben may not



Figure 16. Composite diagram showing the water surface slopes

of the major channels of the Delta.



261

be an incipient rift, but a depression between twobasement arches which mark the tips of southwesterlypropagating rifts.

Shaw (1988) has documented the former existence ofextensive lakes in the southern Okavango, which recordtimes when Mababe, Ngami and the Makgadikgadi wereconnected as a single water body. During this timeprominent sand ridges developed on the leeward side ofeach of these water bodies. The most elevated of theseridges forms the Gidikwe Ridge of the MakgadikgadiDepression, the Magagikwe Ridge of the MababeDepression and the ridge to the west of the DautsaRidge of Lake Ngami, and are probably correlatives(Shaw, 1988). The age of these ridges exceeds the limitof 14C dating, and is unknown. Nevertheless, the relativeelevations of the ridges provides information on thecumulative fault displacement and relative movementssince the ridges formed.

The Gidikwi Ridge which bordered palaeo-lakeMakgadikgadi shows evidence of tilting, down towardsthe south (Figure 17b). It lies between 935m in the southand 945m in the north. The ridge to the west of LakeNgami lies between 935 and 940m, suggesting limitedrelative movement between the Mababe Depression andLake Ngami. The Magikwi Ridge to the west of theMababe Depression has been tilted downwards to thesouthwest as discussed above, and lies between the 930and 945m contours. The southern end of the Mababehas therefore been down faulted, while the northern endhas experienced upward displacement. Palaeo-shorelines within the Makalamabedi Depression lie at anelevation between 940m and 945m.

These ridges indicate a complex pattern ofmovement. The elevation of the point where the Boteticrosses the Gidikwe Ridge forms a convenient referencedatum (slightly below 940m). Relative to this datum, the

Figure 17a. Enlargement of the Mababe Depression, with elevation contours and palaeo-shoreline indicated. An arbitrary drawn line was

used as a reference for analysing the elevations along the western palaeo-shoreline (Magagikwe Ridge).

Gidikwe Ridge has been uplifted in the north, while thesouthern portion has subsided. The MakalamabediDepression, in contrast, has experienced a few metres ofuplift relative to the datum. Lake Ngami has probablyexperienced downward displacement of less than 5m,but with limited or no tilting. In contrast, the MababeDepression has experienced uplift of about 5m in thenorth, and downward displacement of between 5m and10m in the south. In times predating the development ofthese shorelines, the Ghanzi Ridge has risen as theBoteti River has eroded into the underlying KalahariGroup, and to the southwest of the Boteti River,basement is exposed along the Ghanzi Ridge. The latterindicates that the ridge has risen by more than 100msince the Okavango began forming.

These relative movements suggest a zone of upliftpropagating from the northeast, which has uplifted the Ghanzi Ridge and tilted the palaeo-shoreline of theMakgadikgadi Depression. This uplift has also tilted the Mababe Depression to the southwest.

In addition, there appears to be a distinct crustal sagwhich has created the Makalamabedi depression in thecentral portion of the Boteti sub-domain. Its symmetricaldisposition with respect to the Delta suggests a linkbetween the two. The strike length of the Thamalakanefault is limited by, and appears to be related to, thisdepression, and possibly formed as a result ofdetachment of a sliver of the rising Ghanzi Ridge duringsubsidence of the central Delta. The Mababe Depressionis tilted towards the region of the sag, suggesting that thecentral Delta sub-domain has also experienced sagging,although this may have involved accommodation byvariable displacement along the Thamalakane-Kunyerefault system. The centre of subsidence appears to lie inthe southern Delta, and it is suggested that it is a productof gravitational loading as a result of sedimentaccumulation in the Delta. Sagging due to crustalloading is well documented in the case of large waterbodies: for example, Lake Bonneville, which was 350m deep, is believed to have induced a 50m sag in thecrust (May et al., 1991). The full extent of the sag is

unknown, but is probably substantially less than the300m of sediment underlying the Delta, as this includesboth pre-Delta Kalahari Group material as well as Deltarelated sediment.

Seismic activity associated with the Delta is displacedfrom the Kariba-Luangua rift axis (Figure 9), and it issuggested that much of this activity may also be relatedto the cumulative effect of sediment loading in the Delta,which amounts to about 6x105 tonnes per annum.Seismic activity is commonly associated with the fillingof large dams (Simpson et al., 1988). In the Okavango,seasonal flooding annually loads the crust with about9x109 tonnes of water, yet McCarthy et al. (1993) failedto find a relationship between seismicity and flooding,and suggested that the seasonal flood was too dispersedto generate sufficient load to trigger seismic activity. Aseasonal increase in activity may be discernable ifsmaller magnitude events are recorded (the data setused by McCarthy et al., 1993, included only eventsgreater than magnitude 2). In contrast, sedimentation inthe Delta creates an aggregating load, which couldgenerate periodic stress release, producing larger events.

The Delta is transected by numerous lineaments(Hutchins et al., 1976; McCarthy et al., 1993). Many ofthese are not aligned with regional structures (McCarthyet al., 1993). It is suggested that they may representaccommodation structures generated by differentialsagging across the central trough which accommodatesthe bulk of the sediment load.

The incision which has given rise to the Panhandleappears to have been induced by uplift along thenorthern portion of this feature, perhaps related to theMweru-Tshangalele-Kabompo rift. This uplift appears tohave tilted the Panhandle Domain to the southeast, andKalahari Group sediments thin in this area, exposingbasement at Popa falls. The Gumare fault is possibly alsorelated to this uplift, forming a horst bounding fault,analogous to the Kunyere fault. Uplift in the northernportion of the Panhandle could also account for theunusual character of the Quito River, which has a wideflood plain that absorbs most of the seasonal flood(Wilson and Dincer, 1976), resulting in a very limitedseasonal stage range (McCarthy et al., 2000). This is alikely consequence of uparching across the northernend of the Panhandle Domain, as this would lower thegradients of rivers to the north and promote flood plainwidening.

ConclusionsThis paper describes the results of the first detailed studyof the topography of the Okavango Delta, madepossible by the advent of the Global Positioning System.The Delta is a region of active sedimentation, but is alsotectonically active, both of which influence topography,and therefore a detailed analysis of the topography can provide insight into both sedimentation andtectonism. In order to place the Delta in its propercontext, the regional setting was also considered in thisanalysis.



Figure 17b. Elevation profile along the western palaeo-shoreline

(Magagikwe Ridge) of the Mababe Depression. The bold line

shows the estimated elevation, while the two enveloping lines

represent the standard deviation. Breaks occur where the palaeo-

shoreline cannot be identified on the satellite image.



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The Okavango depression is an integral part of theKalahari basin, but is genetically unrelated to this largerfeature. Rather, it is related to incipient rifting acrosscentral southern Africa, which is apparently aconsequence of the uplift associated with the AfricanSuperswell. Although closely associated with rifts, theOkavango depression itself does not appear to be a riftgraben. The local tectonic style is rather one of upliftand horst formation along the southern margin of theDelta, which has produced the Ghanzi Ridge. This ridgehas risen by more than 100m, and has penetrated theKalahari Group cover. The Okavango has developed inthe depression flanking the ridge, which is the tip of thepropagating rift.

The Panhandle has developed along a northwerserlystriking fault, and the Okavango River has inciseddeeply along this fault, creating a broad flood plain. Thedepth of incision increases to the north, and a prominentnick point is developed at the northern end of thePanhandle at Popa falls, where basement is exposed. Atthe southern end of the Panhandle, the flood plainwidens across the Gumare fault onto the Delta itself. Thetopography suggests that the incision of the Panhandleis the result of arching along the northern end of this structure, which may be related to the incipientMweru-Tshangalele-Kabompo rift. The Gumare faultmay be a horst bounding structure related to this uplift.The Delta therefore appears to have formed in thedepression between two southwesterly propagating rifts.

The Delta itself is a large alluvial fan. It has a remar-kably uniform conical surface which is slightly convex.Maximum convexity is associated with the permanentswamps, and may be the result of clastic sedimentationnear the apex of the fan. The fan surface is characterizedby localized lows and highs, but major channels are notalways developed in the lows, and in fact some flowalong local topographic highs. This suggests that localtopographic features (e.g. islands) and channel flankingvegetation exert a strong influence on water distribution.Moreover, distributary channels often have a markedlylower water surface than their feeder channels. Thatsuch hydrologically unstable situations can persist forlong periods is also attributed to the confining effects ofvegetation.

Regional tectonic tilting has often been cited as acause for changes in water distribution on the fansurface. The study has failed to detect any such regionaltilting. Furthermore, it has shown that for such tilting tohave any effect on water distribution, it would have tobe very severe, as channel positions tend to beinsensitive to local topography.

The Delta appears to be subsiding, and we attributethis to sediment loading. Central subsidence has tiltedthe Mababe Depression towards the west, and has alsodepressed portion of the Ghanzi Ridge facing the Delta,to form the Makalamabedi Depression. A portion of theridge has become detached along the Thamalakanefault. The seismicity associated with the Delta is

displaced from the axis of the rift, and we also attributethis activity to sediment loading.

The Selinda spillway flows in a pronounceddepression, which appears to be a graben locatedbetween the southwesterly propagating Linyanti faultand the Gumare fault which marks the northern edge ofthe fan. It is likely that the southwesterly propagation ofthe Linyanti fault will ultimately divert the OkavangoRiver into the Linyanti system, and hence into theZambezi River.

AcknowledgementsWe express our gratitude to Anglo American Corporationfor supplying satellite imagery and the GeologicalSurvey of Botswana for the results of the gravity survey.We also thank Luc Antoine and Gerhard Graham forassistance with data acquisition and Lyn Whitfield andDiane du Toit for draughting. Financial support from theUniversity of the Witwatersrand and the Royal SwedishAcademy of Sciences is gratefully acknowledged.

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The topography of the Okavango Delta, Botswana, and its tectonic and sedimentological implications