Geomorphology of the Bushveld Complex

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

Geomorfología del Complejo de Bushveld

LAGEAT,Y.

This paper deals wich che differenc relacion becween scruccure and lichology in che geomorphology of che Bushveld Complexo The final resules demonscrace chac, even so differenc scale ofsize, widerfor che epirogenic-cecconic movements and smaller for che lichology, che cwo faccors need co be considered for a beccer underscanding ofche landscape evolucion ofche area.

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

LAGEAT, Y. (Universicé Blaise Pascaland UPRES-A 6042 (CNRS). Clermont-Ferrand (France) 210 Lageat, Y. CAD. LAB. XEOL. LAXE 22 (997)

INTRODUCTION subdued topography. So, being concerned with the relationships between scenery and In spiteofthestrong emphasisonclimatic structure, the study wil1 consider two topics in French geomorphology since the different but complementary topics: fifties, research dealing with structural differential erosion and regional evolution landforms developed in crystalline shields ofa shield area (Y. LAGEAT, 1989). pioneered by the late Professor P. Birot has been upheld. In numerous regional monographs, widely distributed between GEOLOGICAL POTENTIAL theArcticCircleand theTropicofCapricorn, special attention has been devoted to the A presentation of the geology of the relationships between landforms and Bushveld Complex is essential to any geological structure, with the purpose to understanding of its surface morphology. distinguish between the direct control of Ranging in composition from ultrabasic ro recent faulting and the response of acid, it outcrops largely within a region contrasting rock units todifferential erosiono covered bya characteristicvegetation termed This ambiguity may be easily overcome in «Bushveld». Beneath its acid roof, the mafic the Bushveld Igneous Complexin theSourh sequence houses the world's largest reserves African interior. of platinum, chromium and vanadium The Bushveld Complex is the largest (G. von GRUENEWALDT, 1979). exposed plutonic intrusion in the world, This intrusion was emplaced circa 2050 covering sorne 67 000 km2 inthecentralpart M.y. ago in the Precambrian Kaapvaal craton in the central Transvaal. Elliptical in plan, which consists of an archaean crystalline with a latitudinal long axis of 460 km, it basement locally overlain by remnants of consists ofagranitic core ringed byexposures theTransvaal Supergroup (mainlyquartzites of basic and ultrabasic rocks which at the andshales) ofEarlyProterozoic age. Contrary eastern and western margins extend over toearlier interpretations it is now considered, more than 12 000 km2 (fig. 1). While most (i) that the overall structure of the investigations in shield areas are concerned Complex is not lopolithic but rather that with acid pluronic rocks, the Bushveld several cone sheets, not necessarily connected Complex offers an opporrunity to examine at depth, have been intruded; landforms deriving from lithologies which (ii) that differentiation did not take pla are rarely encountered at outcrop. ce in a single huge chamber but rather that If, originally, the objectives ofthe field several discrete magmatic pulses occurred, work were well defined, since the purpose as shown by variations in mineral was to establish a scale ofrelative resistance compositionandawell establishedSr-isotope to weathering and erosion for the eastern stratigraphy. rim of the Complex, which exhibits a well Strucrure, which embraces both the defined scarp-and-vale scenery, the lithological nature of rock types and the investigation was subsequently extended to volumetric arrangement of rock units, is the western rim where the same exposed essential to any understanding of surface lithostratigraphic units only give rise to morphology in the Bushveld Complexo The n > / ..... ~ /0-0 -.,..., t-' / .... ~ i > j J . al ~ "./.,~ tJ:j "'" ¡~.- ¡-." \.! o i ..1 -.-) ~ / r-' t-' ._0'" ," TranSfl8a/ / ...... ,I > !.. .J'''' .",...... -... ¡. ~ :.'_' .,' I B I tJ:j ,---·~~-:r- N N South Africa ~ \D ./-'--". \D r I ;::;! \. ¡_J .... o SOO In km J -- lIT] Acid rool recks

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Fig. l. The eastern Bushveld Complex :geological sketch ofthe layered suite and generalized cross-section. N...... 212 Lageat, Y. CAD. LAB. XEOL. LAXE 22 (997)

41500m gOOOm

.5~"'""o o~ 4000m ... B1500m • ..9- :cj .2 ~ e"

31500m BOOOm •e .... o i i~~~o" t;a~ 3000m 71500m

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1- Anorthosite. 2- Gabbro-norite. 3-Pyroxetholite. 4- Feldspathic pyroxenolite. 5- Harzburgite. 6-Dunite. 7- Diorite. 8- Magnetite Gabbro. 9- Chromitite and magnetitite. Fig. 2. Generalized columnar section through the eastern Bushveld Complexo CAD. LAB. XEOL. LAXE 22 (1997) Geomorphology o/the Bushveld Complex 213 distribution ofvarious crystalline rocks has Since they associate two classes of ma been explained in terms of fractional terials these magmatic «sediments» may be crystallisation and segregation of different analysed in the same way as clastic sedimen mineral aggregates from a basic magma, tary rocks as : involving either the appearance and - the crystals that settled, known as the disappearance ofliquidus mineralphases, or cumulus grains, and through variations in the chemical - the intercumulus liquid which compositions of these minerals. They have cristallized in situ cementing the detrital led to the establishment of a stratigraphic grains. succession comprising fout distinct zones The consolidation of this interstitial (fig. 2). From bottom to top, the thicknesses magma produces rocks which are named typical of the eastern rim are: cumulates. Atleast three importantprocesses - 1,600 m for the Lower Zone, are involved in the cementation ofcumulus - 1,000 m for the Critical Zone, crystals: - 4,000 m for the Main Zone, and - simple space filling by minerals - 1,500 m for the Upper Zone. different from those the cumulus phase (phoro 3); These zones consist ofsuperposed layers - partial replacement, as shown by the characterised by lateral continuity but also resorption ofroundedolivinegrains enclosed byvariations in thickness. The best example in large orthopyroxenes produced by the of this layering, analogous ro bedding in crystallization of the trapped liquid (pho sedimentary rocks, is provided by the strong to 4); contrast in colour between black chromite - overgrowth ofthe cumulus crystals by layers (chromitites) and white plagioclase material ofthe same composition, a process layers (anorthosites) in the Dwars River bed which can produce completely monomi in the eastern Bushveld (photo 1). However, neralic rocks (phoro 5). beside these thin layers, othersmaybe several hundred meters thick, according to whether The basic and ultrabasic layers can, to all the crystallization rates are rapid or slow. intents and purposes, be regarded as sedi Thus dome-like forms are sometimes obser mentary formations dipping rowards the vable in homogeneous piles by contrast centre ofthe Complex at angles between 10 with the prevailing homoclinal pattern. and 30°. However, despite havingseemingly The layered rocks of the Bushveld identical structures, the morphologies of Complex are believed to be the result of the eastern and western regions of the crystals settling out of a cooling magma. Bushveld differ substantially, for the former This peculiar arrangement reflects the is characterised by a distinct scarp-and-vale decisive influence ofgravity, butotherfactors topography, while the latter is a region of have also been involved in the process of low relief. Showing the same asymmetry as magmatic sedimentation : convective observed in the sedimentary Paris basin, the circulations have to be evoked inaddition to eastern section, though in crystalline rocks, the simple sinking ofcrystals, as evidenced exhibits an unusual cuesta-like morphology by fluidal planes (phoro 2). (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. Poikilitic 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. CAD. LAB. XEOL. LAXE 22 (1997)

LITHOLOGICAL CONTROL can be readily established. The various rock textures can be correlated with weak and By contrast with other shield areas the resistant layers, which in turn express the issue of differential erosion is fairly simple rhythmic macrolayering in the crystalline in the Bushveld Complex, with its sequence. Figure 3 illustrates the relative superposition of differentially weatherable weatherability of the various rocks in the layers. However, just as in other crystalline studyarea. rock suites, there is no direct relationship At the intersection between a row and a between lithology and relative resistance to column, the relative resistance between any erosiono two geologically adjacent layers is indicated The morphologyofthe eastern Bushveld by a plus or minus signo Ofcourse, if there can be summarised by a section running is no indication ofrelative weatherability at between the Olifants and Steelport rivers, any one intersection this implies that there and along which the greatest variety of is no contact between the two rock types in landforms is displayed. These include from question. Blank spaces indicate that direct southwest ro northeast (fig. 1): observation regarding relative weathe - a granitic cuestawhich limits a plateau rability cannot be made. From this figure it where remnants of a culminant erosion is possible ro establish a crude hierarchy of surface - the «Highveld surface» - are well weathering with respect to the main rock preserved at a mean elevation of 1,500 m types. (phoro 7); Mineral composition has little influence - a depression formed in ferrogabbros on surface expression except for the major and ferrodiorites ofthe UpperZone between contrast between acid roof rocks and the 1,200 and 1,000 m; upper part of the layered basic complexo - the Leoloberge Range, culminating at Otherwise differential weathering and . 2,000 m, which coincides with the Main erosion in the mafic sequence is almost Zone gabbros : its western margin is everywhere independent ofmineralogy. For controlled by a system of faults, while the example, monomineralic anorthosites or eastern edge exhibits a 400 to 600 m high bronzites, which, consisting as they do of cuesta (photo 8); such highly susceptible minerals (at least, - at the foot ofthe lattera marginal plain according to theGoldich stabilitysequence), extending across the Critical and Lower as plagioclase andpyroxene, could reasonably zones at an elevation of800 m, with ridges be expected to suffer deep weathering and ofanorthosite and bronzitite (phoro 9). erosiono Yet layers of such materials count Differential weathering and erosion of amongst the most resistant to be found and the layered basic andultrabasic rocks exposed are usually associated with ridges and other in the eastern Bushveld Complex is readily upland features. demonstrated thoughthecontrollingfactors The most resistant rock types are are not simple.The scarp-and-vale adcumulates inwhichthe constituentgrains topography reflects the relative resistance of are cemented, or which are densely packed individual magmatic layers, and a hierarchy with closely interlocking grains. Thus the of weatherability (and hence erodibility) gabbros ofthe 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 composed especially in retms of srtenghr of rhe links of inrerlocking orrhopyroxene, clino berween minetals, as the ceystal faces evolve pyroxene and plagiodase. There is only one ro minimum eneegy configurarions in exceprion among rhese adcumulates: rhe adcumulates. Thisfacror has been underlined peridorires which are largely or even by porosay measurements and complerely converred into serpenrines compressibiLityresrs, bur irdoes nor however exhibiog rypical mesh textures. provide a sarisfaetory account of relative By COntrast wirh rhese densely packed weatherability in all rock types. texrures, weaker members of rhe layered There sriU remains a morphologieal sequence are characrerised by the presence enigma, regarding rhe suscepribiliry ofrhe of inrerstitial or poikiliric minerals more ferrogabbros and ferrodiorites ofthe Uppet pranetoaltetarion rhan rhe cumuluscryStals. Zone, rhefabrieofwhich is unable coexplain An analogy may usefully be drawn with a rbeir wearherabiliry, rhe only exceprion quarrzite consisring of quarrz grains being magnetire monomineealic layees. cemented by silica, and, say, a calcareous These rocks are obviously subjecr ro more sandsrone wirb quarrz fragmenrs held rapid disinregrarion. Their eompararively cogether by a calcire cemento grearersuscepribiliry may bedue rochemical Thus rexture appears ro be rhe major environmenrsand reacrions peculiar ro basic factOr explaning differenr¡al weathering, rocks. Laborarory experimenrs on the 220 Lageat, Y. CAD. LAB. XEOL. LAXE 22 (1997) hydrolysis offragments ofthese rocks using REGIONAL MORPHOLOGICAL distilled water have shown that CONTRASTS clinopyroxenes decompose more rapidlythan Overall the landscape ofthe eastern part do feldspars, a feature which is confirmed by of the Complex is characterised by the the respective ratios of these minerals in rejuvenation of an initial surface, the sand fractions in the field (fig. 4). «Highveld Surface», the remnants ofwhich The release ofmagnesium and calcium is are well preserved on resistant acid roof favoured by a higher hydrogen ion rocks (felsites and granophyres). The concentrations in thesolutions so thatwaters structural relief of the region, with its a in contaet with magnetite-bearing rocks, distinct scarp-and-vale morphology, has like the ferrograbbros and ferrodiorites, are been developed during two subsequent more acid than those in contact with other stages offluvial incision and lateralplanation. rock types. The sulphurcontentofthe rocks By contrast, despite a seemingly shared enhances their rapid breakdown.Sulphur is geological structure, the western region of released during the oxidation of sulphides the Bushveld Complex is a region of low in the Upper Zone. This causes decrease in relief known as the the «Bushveld Basin» pH which in turn enhances the alteration of which lies between 1,000 and 1,200 m. the iron rich ferromagnesian minerals, as This planation surface is only punctuated shown by the chemical analysis of water by residuals belonging ro the Main Zone of samples (all units mg/l except pH): the mafic sequence. There is only ane

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

pH at 25° C 7,98 7,74

Ca 32,3 58,4

MG 20,5 28,3

Na 10,0 18,4

K <1,0 <1,0

CI 5,1 17,4

S04 7,5 48,3

Total alkalinity as 159,0 191,0 CAD. LAB. XEOL. LAXE 22 (1997) Geomorphology 01 the Bushveld Complex 221

13 ;z 12 ~l/( 11 ~ 10

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20 J / 10

O II I T 111 n 0,08 0,2 0,5 1,25 3',15 mm 0,05 0,125 0,315 0,8 2,Omm :> IJ ¡;: lJj Plagioclase FERROGABBROS OF E:3 GABBROS OF @ THE UPPER lONE THE MAIN lONE O,. [TI Pyroxenes ¡;: @ N rs:sJ Magnetite N 'D 'D- Fig. 4. Mean eomposition ofthe sandy fraetion of regoliths. ;:;' CAD. LAB. XEOL. LAXE 22 (1997) Geomorphology o/the Bushveld Complex 223

1 133 ...

1255 , ,

\ ... \ 1232 1 127': •

20km I

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Fig. 5. Geomorphological sketch of the Rustenburg area (Southwestern Bushveld). 1- Acid toofrocks. 2- Granitic cuesta. 3- Basic and ultrabasic rocks. 4- Main Magnerite Seam. 5- Main Zone gabbtoic residuals. 6- Syenitic dykes. 7- Merensky Reef. 8- lower conract of the layered suite. 9- Sedimenrs of the Pretoria Group. 10- Quartzite homoclinallandforms. -,':" ,,\/ :/ _.... ,\_\_,._~,,:_1-~_-_ ...:,J'_-; ti " ,¿',' .. ,'~;\ .... N 1\ , ·1,. ... , - ... \ " 1 \ -1 N _ ' __ .,1/1 .... ''''- ' ~ ;\/ "_, .... ,1'"...... , .... - l_-j,."""'/I . ....' 1 _', \ _1 \_ ....,\ ' ...... _ _ 1 -.. _, _\ l' \- 1 \ ,. I , ...... 1 , - 1 .... 1 ..... - \ / I, I \ ~ , 1/ '_ 1_' " "'"t-< .-.... ': .... ::.' ~:. -,-, -:-.... -; \-;. ;-~' :-<' ~ ---;- .:, ~ ~/ , ...... \ -," ... \ . ~ :: ~ .:\ ~ ,_ ,/, I- 1 l' I- , .... , -;. , " .•; l' .... ', ~ -, ' ~ ~ .,. ,"" - \ \: \ •\ ,.' _ .... I \_ \_ -\ _1 _, -; I _, - -,1-::' -' - - ,-1'" \ '\ ... , ,'" _,"\-"", .... ,..,1 -, ,,1"'1' ,-\ ,I ,. , ,-/ _ '1 " ...... -, ...... -,-,_" •. 1, -_ ,_ ...... ,-, ..... , ..... ,,...... ~\-,\' :< \', - -, ..... -, ,-,.... "\/' \- ... , ~ ~ \I-,.\I'\/-J',~_';I. I_I_:'~I:I- t ,'- I , ....._, '- _ 1 , ,- -,. ,,- ,- / - ¡ .... \ _... ,- ,- ,,' " '::" ':., \' ': _\ -, \_'" 1... ',_, .... , ...".. ::'-'-,,-_1 ~\,'"-\-.. ,-...'-,,_..,. {Sc1'''--" /~ ~ -. -'\111\ I /' ,_'-;-' ....._/_1-_ 1 >I, ... ,\ ... ,':,, "\~\,\'";~~- ~'~ " _.... _I- I ' J - '1 +t;lfl' \- " .... / ....."/ I'/~;-''''~' t.I ,l' , ...... _ \"J 1'1 .... , '_T' _,,1 \ \ - -.!!" -'..'f l'\ "I_:"\,,\... ).:~ L...-~:','1,1 ' ,. '¡;,')'..,-\I '-'\ .... -'1- f _>_, \...... \ I 1'-' ", \ "',.... I~";.\ .... /~ ...... ,() " ::: I :. I \ 4!(:. J ,- I 1 ~ '\ :-',-,: 'J-'~"'- '" " --I- , . '~ - , ,. , \ ..... , _\ ..... '\ \-1 . / .... '" " ' ' \- " i 'U-, .... ,-;, I..... ~I _\- ""-'1l' '/,_'_'''1',,'" .... ,,,\,, .... '''_\~.... """.,,/-:-~'" __ ,,1-,,1"',1 ... -' -, _1\-\ , .... , '-J'".',-,. ' ....:., ....._\',-'''-~_, "'''\-\''_J~, ... ~:. "'-',',;' ,..., ~'': / _ ... " ...... ; '.... ,1,,,'-'-' ~ 1__ \/. -t7_'/\~,"""''''''~\_ \- ..... 1\1 \ , '-- \ - \...... '_1 " ~ \ ... I , ...... ~ ( I -,', I- .... - 1'" -;- l' ,-'.: _ I - 1 1 ~ / ... -..... , ~ " -rJ~73 -f?-" -Á .. ~ '- '¡ - -- -,------...... - - + + ~ 1039 ~ l:Jj r,.; ~ + + ~ ~ , .. ~ . tT1 N N 1- Upper Zone and magnetite layers. 2- Main Zone. 3- Acid roof rocks. 4- Gabbroic inselbergs 'oD 'oD- Fig. 6. Geomorphological sketch ofthe Northam area (Northwestern Buskveld). ;::;! () > ~ ENE wsw ~ tJj A ~ :~¡~¡~~ ¡!,!~!~j ~

5 10 1, 20 25 50 75 100 120 sw § N N

~ B \D "'Og ., \D :~1R .::J e ....f· ... '::: :~~~i~~~~~~~~~~~~~·:·::: ·.~i1j~:\fmJ~lliill§lli§ÜD;,:;~~:::»»<»)j

NW SE D ~~@¡¡m¡llliM@,:;{.~'.;i;~»>< r ':::i:::;:;:;:;:;:::;::::8"...... '... 0G .• ,•• ~'" ...... ~""'" <..,; l-7;-;:¡, ~2 ~, ~4 ~ ~~ ~ ~ fITill 5 EIJ 6 ~ tl::I 1- Archaean basemenl. 2- Transvaal Supergroup 3- Lower and Critical Zones. 4- Main Zone 5- Upper Zone. 6- Roof rocks (I =Highveld surface) ~ ~ ;;:: Fig.7. Generalized sections across the Bushveld Complexo (j A. & B. Eastern Bushveld. ~'" C. South-western Bushveld. D. North-wester Bushveld. lf t0 t0 V\ 226 Lageat, Y. CAD. LAB. XEOL. LAXE 22 (1997) exception, the prominent Pilanesberg (1985) who demonstrated that sorne of the Complex, an inselgebirge developed on a diamondiferous alluvial gravels in the ring-complex mass of alkaline and Lichtenburg area in the southwestern hyperalkaline rocks, which is exposed as a Transvaal at an altitude of 1,500 m derived result of the stripping of the Waterberg from a northern source region which now Sandsrones into which the crystallines were lies more than 400 m lower down. originally intruded sorne 1,300 M.y. ago. Any analysis of the morphological EIsewere the expression of differential evolution ofthe Bushveld Complex should erosion is quite discreteo Thus, in the involve a consideration of the epeirogenic southwestern sector the only manifestation deformations which have affected, albeit of structural control in the landscape is unequally, the whole region. These are associated with gabbros belonging to the apparent in the contrasted morphological central portion of the Main Zone : these expression ofspecific stratigraphic horizons. outcrops occur as ridges (known as the For example, the central part of the Main "Pyramids") in a depression which has been Zone forms the backbone of a prominent excavated below the level of a culminant mountain chain in the eastern Transvaal, surface preserved on the roofofthe intrusion theLeoloberge which culminates over 1,900 (fig. 5). On the other hand, in the m, the "Pyramids" north of Pretoria at an northwestern sector, the acid roofrocks and altitude of1,400 m, and a few inselbergs in basic intrusives are truncated by the same the northwestern Transvaal under 1,100 m. planation surface, with just a few gabbroic TheMainZonedoes notfind anypronounced inselbergs standing aboye the generallevel surface expression in the western Bushveld of the high plain, the «Bushveld Surface», basin because of persisting subsidence and which merges westwards with the constant regradation ofthe initial Highveld depositional surface of the Kalahari Basin surface which has been asymmetrically (fig. 6). deformed. On the other hand upwarping of Iris suggested that this contrast is due to theeasternmarginhas led to repeatedphases recent tectonic events that have affected the ofdifferential erosion through polycyclical rims ofthe «Bushveld BasiD». Thesouthern development with a marked structural rim, which is also the divide between the imprint of the two latter planation cYcles Orange and Limpopo drainage systems, co (fig. 7). incides with an asymmetrical updoming, The planation surfaces of the eastern whereas the northern rim is affected by Bushveld are readily correlated with those vertical displacements associated with a recorded along the Great Escarpment ofthe major fault system known as the «Palala Transvaal. Together with the intervening ShearZone». As theeastern rim corresponds scarps theyform a steppedsequence thought to a steepening of a marginal swell, this ro be due to pauses in the uplift of the implies that this «basiD» was produced by a marginal swell. lfwe accept the traditional «saggingprocess». This hypothesis has been denudationmodelestablished byL.e. KING thoroughly discussed by A L. DU TOIT (1972): African, post-African l, and post (1933) and given more recent backing byT. African II cycles, uncertainties remain STRATTEN (1979) and J.)' MAYER concerning the chronology of this