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Geomorphology of the Bushveld Complex

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Geomorphology of the Bushveld Complex Cademo Lab. Xeo16xico de Laxe Coruña. 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 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 1:-:';':-1 Upper lene b,'.j Main lone o 100km I ~ f;;;;;:::;~Lewer & Critical lenes ~ ~ ;:,... SMetasedimentary recks ~ ~ ~ Le%be,ge s. Ch,om/te HlIIs '" A t!:I B ~ ;¡; ¡:;:; Cl ;1;t.;i0!it@'lfl!&'liigf;¡;{!if.J:f~f~~ ~ ~ ro 3'0 S'O ~ 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 wz o N ..J -< 21500m 7000m ~ OC u i:.c.. 2000m 81500m of &.. .l;~ .a • el.. " 8ronzllll. d" S.rokolo 11500m llOOOm !~.- UJ z :>" 2 ~ ~ Ir; -"; ..•.•....:.; w :J:" IOOOm ~----_._---_\; 151500m ..J 1500m 5000m -'~r-"" ~lsu lb -3 ••.. MI' OIÑA[-t:O~ !'~f om 41100m (Norll. d. Sh.ller)' ~,~",:¡ ~s 2 c=J 3 ffiHil] 4 ~ ~ 7b;;da ~9 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
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