Cademo Lab. Xeo16xico de LaxeCoruña. 1997. Vol. 22, pp. 209-227

Geomorphology of the Bushveld Complex

Geomorfología del Complejo de Bushveld

LAGEAT,Y.

This paper deals wich che differenc relacion becween scruccure and lichology inche geomorphology of che Bushveld Complexo The final resules demonscracechac, even so differenc scale ofsize, wider for che epirogenic-cecconic movementsand smaller for che lichology, che cwo faccors need co be considered for a beccerunderscanding of che landscape evolucion of che area.

Key words: ulerabasic and basic rocks, Bushveld Complex, scruccuralgeomorphology, lichological differences.

LAGEAT, Y. (Universicé Blaise Pascaland UPRES-A 6042 (CNRS). Clermont-Ferrand (France)

210 Lageat, Y.

INTRODUCTION

In spite ofthe strong emphasis on climatictopics in French geomorphology since thefifties, research dealing with structurallandforms developed in crystalline shieldspioneered by the late Professor P. Birot hasbeen upheld. In numerous regionalmonographs, widely distributed betweenthe Arctic Circle and the Tropic ofCapricorn,special attention has been devoted to therelationships between landforms andgeological structure, with the purpose todistinguish between the direct control ofrecent faulting and the response ofcontrasting rock units to differential erosionoThis ambiguity may be easily overcome inthe Bushveld Igneous Complex in the SourhAfrican interior.

The Bushveld Complex is the largestexposed plutonic intrusion in the world,covering sorne 67 000 km2 in the central partin the central Transvaal. Elliptical in plan,with a latitudinal long axis of 460 km, itconsists ofagranitic core ringed by exposuresof basic and ultrabasic rocks which at theeastern and western margins extend overmore than 12 000 km2 (fig. 1). While mostinvestigations in shield areas are concernedwith acid pluronic rocks, the BushveldComplex offers an opporrunity to examinelandforms deriving from lithologies whichare rarely encountered at outcrop.

If, originally, the objectives of the fieldwork were well defined, since the purposewas to establish a scale of relative resistanceto weathering and erosion for the easternrim of the Complex, which exhibits a welldefined scarp-and-vale scenery, theinvestigation was subsequently extended tothe western rim where the same exposedlithostratigraphic units only give rise to

CAD. LAB. XEOL. LAXE 22 (997)

subdued topography. So, being concernedwith the relationships between scenery andstructure, the study wil1 consider twodifferent but complementary topics:

differential erosion and regional evolutionof a shield area (Y. LAGEAT, 1989).

GEOLOGICAL POTENTIAL

A presentation of the geology of theBushveld Complex is essential to any

understanding of its surface morphology.Ranging in composition from ultrabasic ro

acid, it outcrops largely within a regioncovered by a characteristic vegetation termed«Bushveld». Beneath its acid roof, the maficsequence houses the world's largest reservesof platinum, chromium and vanadium

(G. von GRUENEWALDT, 1979).This intrusion was emplaced circa 2050

M.y. ago in the Precambrian Kaapvaal cratonwhich consists of an archaean crystalline

basement locally overlain by remnants ofthe Transvaal Supergroup (mainly quartzitesand shales) ofEarly Proterozoic age. Contraryto earlier interpretations it is now considered,

(i) that the overall structure of theComplex is not lopolithic but rather that

several cone sheets, not necessarily connectedat depth, have been intruded;

(ii) that differentiation did not take pla­

ce in a single huge chamber but rather that

several discrete magmatic pulses occurred,as shown by variations in mineralcompositionand awell established Sr-isotope

stratigraphy.Strucrure, which embraces both the

lithological nature of rock types and thevolumetric arrangement of rock units, isessential to any understanding of surface

morphology in the Bushveld Complexo The

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212 Lageat, Y. CAD. LAB. XEOL. LAXE 22 (997)

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Fig. 2. Generalized columnar section through the easternBushveld Complexo

CAD. LAB. XEOL. LAXE 22 (1997)

distribution ofvarious crystalline rocks hasbeen explained in terms of fractionalcrystallisation and segregation of differentmineral aggregates from a basic magma,

involving either the appearance anddisappearance ofliquidus mineral phases, orthrough variations in the chemical

compositions of these minerals. They haveled to the establishment of a stratigraphicsuccession comprising fout distinct zones

(fig. 2). From bottom to top, the thicknessestypical of the eastern rim are:

- 1,600 m for the Lower Zone,- 1,000 m for the Critical Zone,- 4,000 m for the Main Zone, and

- 1,500 m for the Upper Zone.These zones consist of superposed layers

characterised by lateral continuity but alsoby variations in thickness. The best example

of this layering, analogous ro bedding insedimentary rocks, is provided by the strongcontrast in colour between black chromite

layers (chromitites) and white plagioclaselayers (anorthosites) in the Dwars River bed

in the eastern Bushveld (photo 1). However,

beside these thin layers, others may be severalhundred meters thick, according to whetherthe crystallization rates are rapid or slow.Thus dome-like forms are sometimes obser­vable in homogeneous piles by contrast

with the prevailing homoclinal pattern.The layered rocks of the Bushveld

Complex are believed to be the result of

crystals settling out of a cooling magma.This peculiar arrangement reflects thedecisive influence ofgravity, but otherfactors

have also been involved in the process of

magmatic sedimentation : convectivecirculations have to be evoked in addition to

the simple sinking ofcrystals, as evidencedby fluidal planes (phoro 2).

Geomorphology o/the Bushveld Complex 213

Since they associate two classes of ma­terials these magmatic «sediments» may beanalysed in the same way as clastic sedimen­tary rocks as :

- the crystals that settled, known as thecumulus grains, and

- the intercumulus liquid whichcristallized in situ cementing the detritalgrains.

The consolidation of this interstitialmagma produces rocks which are namedcumulates. At least three important processesare involved in the cementation ofcumuluscrystals:

- simple space filling by mineralsdifferent from those the cumulus phase(phoro 3);

- partial replacement, as shown by theresorption ofrounded olivine grains enclosedin large orthopyroxenes produced by thecrystallization of the trapped liquid (pho­to 4);

- overgrowth of the cumulus crystals bymaterial of the same composition, a processwhich can produce completely monomi­neralic rocks (phoro 5).

The basic and ultrabasic layers can, to allintents and purposes, be regarded as sedi­mentary formations dipping rowards thecentre of the Complex at angles between 10and 30°. However, despite having seeminglyidentical structures, the morphologies ofthe eastern and western regions of theBushveld differ substantially, for the formeris characterised by a distinct scarp-and-valetopography, while the latter is a region oflow relief. Showing the same asymmetry asobserved in the sedimentary Paris basin, theeastern section, though in crystalline rocks,exhibits an unusual cuesta-like morphology(photo 6).

214 Lag,at, Y. CAD. LAB. XEOL LAXE 22 (1997)

PI. 1. Rythmic laye["ing (anonhosice and chromitite).

PI. 2. 19neous lamination

CAD. LAB. XEOL. LAXE 22 (1997) GeomorpixJlogy o[¡he Bu,hveld Complex 215

PI. 3. Feldespathic bronzicie (bronl.ite cumulus grains and intersticial plagiodase)

PI. 4. Poikili tic harl.burgite (rounded olivine grains endosed in large bronl.itc crystals).

216 Lag"", 1'. CAD. LAD. XEOL LAXE 22 (997)

PI.;. Monomineral bronzitie (with interlocking graios)

PI. 6. Chromite Hil.15: a cuesta-like scarp in (he northeastern Bushveld

CAD. LAB. XEOL. LAXE 22 (1997) GetJmorphology ollhe Buslweld Complex 217

PI. 7. The roof-rocks scacp aboye [he depression carved out of the Upper zone rocks.

PI. 8. Thc front scarp of [he LeoJobcrge range.

218 Lageat, Y.

LITHOLOGICAL CONTROL

By contrast with other shield areas theissue of differential erosion is fairly simplein the Bushveld Complex, with itssuperposition of differentially weatherablelayers. However, just as in other crystallinerock suites, there is no direct relationshipbetween lithology and relative resistance toerosiono

The morphology ofthe eastern Bushveldcan be summarised by a section runningbetween the Olifants and Steelport rivers,and along which the greatest variety oflandforms is displayed. These include fromsouthwest ro northeast (fig. 1):

- a granitic cuesta which limits a plateauwhere remnants of a culminant erosionsurface - the «Highveld surface» - are wellpreserved at a mean elevation of 1,500 m(phoro 7);

- a depression formed in ferrogabbrosand ferrodiorites ofthe Upper Zone between1,200 and 1,000 m;

- the Leoloberge Range, culminating at. 2,000 m, which coincides with the MainZone gabbros : its western margin iscontrolled by a system of faults, while theeastern edge exhibits a 400 to 600 m highcuesta (photo 8);

- at the foot ofthe latter a marginal plainextending across the Critical and Lowerzones at an elevation of 800 m, with ridgesof anorthosite and bronzitite (phoro 9).

Differential weathering and erosion ofthe layered basic and ultrabasic rocks exposedin the eastern Bushveld Complex is readilydemonstrated though the controlling factorsare not simple.The scarp-and-valetopography reflects the relative resistance ofindividual magmatic layers, and a hierarchyof weatherability (and hence erodibility)

CAD. LAB. XEOL. LAXE 22 (1997)

can be readily established. The various rocktextures can be correlated with weak andresistant layers, which in turn express therhythmic macrolayering in the crystallinesequence. Figure 3 illustrates the relativeweatherability of the various rocks in thestudyarea.

At the intersection between a row and acolumn, the relative resistance between anytwo geologically adjacent layers is indicatedby a plus or minus signo Of course, if thereis no indication ofrelative weatherability atany one intersection this implies that thereis no contact between the two rock types inquestion. Blank spaces indicate that directobservation regarding relative weathe­rability cannot be made. From this figure itis possible ro establish a crude hierarchy ofweathering with respect to the main rock

types.Mineral composition has little influence

on surface expression except for the majorcontrast between acid roof rocks and theupper part of the layered basic complexoOtherwise differential weathering anderosion in the mafic sequence is almosteverywhere independent ofmineralogy. Forexample, monomineralic anorthosites orbronzites, which, consisting as they do ofsuch highly susceptible minerals (at least,according to the Goldich stability sequence),as plagioclase and pyroxene, could reasonablybe expected to suffer deep weathering anderosiono Yet layers of such materials countamongst the most resistant to be found andare usually associated with ridges and otherupland features.

The most resistant rock types areadcumulates in which the constituent grainsare cemented, or which are densely packedwith closely interlocking grains. Thus thegabbros of the central part ofthe Main Zone

CAD. LAn. XEOL. LAXE 22 (1997) Geomorphology 01 ehe B",hveld Complex 219

PI. 9. Oip-slopes of homodinal bronzitite landforms north of the Chromite Hills.

which underlie rhe Leleoberge are composedof inrerlocking orrhopyroxene, clino­pyroxene and plagiodase. There is only oneexceprion among rhese adcumulates: rheperidorires which are largely or evencomplerely converred into serpenrinesexhibiog rypical mesh textures.

By COntrast wirh rhese densely packedtexrures, weaker members of rhe layeredsequence are characrerised by the presenceof inrerstitial or poikiliric minerals moreprane toaltetarion rhan rhe cumulus cryStals.An analogy may usefully be drawn with aquarrzite consisring of quarrz grainscemented by silica, and, say, a calcareoussandsrone wirb quarrz fragmenrs heldcogether by a calcire cemento

Thus rexture appears ro be rhe majorfactOr explaning differenr¡al weathering,

especially in retms of srtenghr of rhe linksberween minetals, as the ceystal faces evolvero minimum eneegy configurarions inadcumulates. This facror has been underlinedby porosay measurements andcompressibiLityresrs, bur irdoes nor howeverprovide a sarisfaetory account of relativeweatherability in all rock types.

There sriU remains a morphologiealenigma, regarding rhe suscepribiliry of rheferrogabbros and ferrodiorites of the UppetZone, rhefabrieofwhich is unable coexplainrbeir wearherabiliry, rhe only exceprionbeing magnetire monomineealic layees.These rocks are obviously subjecr ro morerapid disinregrarion. Their eompararivelygrearersuscepribiliry may bedue rochemicalenvironmenrs and reacrions peculiar ro basicrocks. Laborarory experimenrs on the

220 Lageat, Y.

hydrolysis offragments of these rocks usingdistilled water have shown thatclinopyroxenes decompose more rapidly thando feldspars, a feature which is confirmed bythe respective ratios of these minerals insand fractions in the field (fig. 4).

The release ofmagnesium and calcium isfavoured by a higher hydrogen ionconcentrations in the solutions so that watersin contaet with magnetite-bearing rocks,like the ferrograbbros and ferrodiorites, aremore acid than those in contact with otherrock types. The sulphur content ofthe rocksenhances their rapid breakdown.Sulphur isreleased during the oxidation of sulphidesin the Upper Zone. This causes decrease inpH which in turn enhances the alteration ofthe iron rich ferromagnesian minerals, asshown by the chemical analysis of watersamples (all units mg/l except pH):

CAD. LAB. XEOL. LAXE 22 (1997)

REGIONAL MORPHOLOGICALCONTRASTS

Overall the landscape of the eastern partof the Complex is characterised by therejuvenation of an initial surface, the«Highveld Surface», the remnants ofwhichare well preserved on resistant acid roofrocks (felsites and granophyres). Thestructural relief of the region, with its adistinct scarp-and-vale morphology, hasbeen developed during two subsequentstages offluvial incision and lateralplanation.

By contrast, despite a seemingly sharedgeological structure, the western region ofthe Bushveld Complex is a region of lowrelief known as the the «Bushveld Basin»which lies between 1,000 and 1,200 m.This planation surface is only punctuatedby residuals belonging ro the Main Zone ofthe mafic sequence. There is only ane

Main Zone Upper Zane(8 samples) (8 samples)

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Total alkalinity as 159,0 191,0

CAD. LAB. XEOL. LAXE 22 (1997) Geomorphology 01 the Bushveld Complex 221

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226 Lageat, Y.

exception, the prominent PilanesbergComplex, an inselgebirge developed on aring-complex mass of alkaline andhyperalkaline rocks, which is exposed as aresult of the stripping of the WaterbergSandsrones into which the crystallines wereoriginally intruded sorne 1,300 M.y. ago.

EIsewere the expression of differentialerosion is quite discreteo Thus, in thesouthwestern sector the only manifestationof structural control in the landscape isassociated with gabbros belonging to thecentral portion of the Main Zone : theseoutcrops occur as ridges (known as the"Pyramids") in a depression which has beenexcavated below the level of a culminantsurface preserved on the roofofthe intrusion(fig. 5). On the other hand, in thenorthwestern sector, the acid roof rocks andbasic intrusives are truncated by the sameplanation surface, with just a few gabbroicinselbergs standing aboye the generallevelof the high plain, the «Bushveld Surface»,which merges westwards with thedepositional surface of the Kalahari Basin(fig. 6).

Ir is suggested that this contrast is due torecent tectonic events that have affected therims ofthe «Bushveld BasiD». The southernrim, which is also the divide between theOrange and Limpopo drainage systems, co­incides with an asymmetrical updoming,whereas the northern rim is affected byvertical displacements associated with amajor fault system known as the «PalalaShear Zone». As the eastern rim correspondsto a steepening of a marginal swell, thisimplies that this «basiD» was produced by a«sagging process». This hypothesis has beenthoroughly discussed by A L. DU TOIT(1933) and given more recent backing byT.STRATTEN (1979) and J.)' MAYER

CAD. LAB. XEOL. LAXE 22 (1997)

(1985) who demonstrated that sorne of thediamondiferous alluvial gravels in theLichtenburg area in the southwesternTransvaal at an altitude of 1,500 m derivedfrom a northern source region which nowlies more than 400 m lower down.

Any analysis of the morphologicalevolution of the Bushveld Complex shouldinvolve a consideration of the epeirogenicdeformations which have affected, albeitunequally, the whole region. These areapparent in the contrasted morphologicalexpression ofspecific stratigraphic horizons.For example, the central part of the MainZone forms the backbone of a prominentmountain chain in the eastern Transvaal,the Leoloberge which culminates over 1,900m, the "Pyramids" north of Pretoria at analtitude of 1,400 m, and a few inselbergs inthe northwestern Transvaal under 1,100 m.The Main Zone does not find anypronouncedsurface expression in the western Bushveldbasin because of persisting subsidence andconstant regradation ofthe initial Highveldsurface which has been asymmetricallydeformed. On the other hand upwarping ofthe eastern margin has led to repeated phasesof differential erosion through polycyclicaldevelopment with a marked structuralimprint of the two latter planation cYcles(fig. 7).

The planation surfaces of the easternBushveld are readily correlated with thoserecorded along the Great Escarpment of theTransvaal. Together with the interveningscarps they form a stepped sequence thoughtro be due to pauses in the uplift of themarginal swell. lfwe accept the traditionaldenudationmodel established byL.e. KING(1972): African, post-African l, and post­African II cycles, uncertainties remainconcerning the chronology of this