ZIMBABWE GEOLOGICAL SURVEY BULLETIN No. 90 The Erosion Surfaces of Zimbabwe L. A. LISTER, M.Sc., D.Phil. ISSUED BY AUTHORITY HARARE-1987 :-:;: LIBRASÏ ZU - 06.01 Ölungen ïte Cstteilands 72& ZIMBABWE GEOLOGICAL SURVEY BULLETIN No. 90 iSRiC LIBRARY 2.UU §k-°\ Wagentngen, The Netherlands The Erosion Surfaces of Zimbabwe L. A. LISTER, M.Sc, D.Phil. ISSUED BY AUTHORITY HARARE-1987 /<//é8 PREFACE Nearly all of the eighty-nine previous bulletins of the Geological Survey, including Bulletin No. 1 published in 1913, contain a short introductory statement entitled, "Physical Features". The statement typically gives the height of prominent peaks, notes the trend of the topography, which is invariably attributed to the "nature of the under­ lying rocks", records the direction of drainage of larger rivers and, in some instances, inappropriately includes information on vegetation, climate and communications. It is thus a pleasure to now be able to introduce a definitive work on the geomorphology of the whole country. Dr. L. A. Lister is well qualified to rectify our previously scant appreciation of the complicated evolution of the erosion surfaces of Zimbabwe. She has lectured at the University of Zimbabwe since 1963 and this Bulletin is based on her D.Phil thesis, submitted to the University of Rhodesia in 1976. It summarizes the observations of many years of field work and many kilometres of dusty Land Rover travel. During the course of this work, it was realized that the high Central Axis is the result solely of being the divide between the Zambezi and the Limpopo catchments and is not an uparched axis as earlier authors had postulated. Cainozoic tilting in Zimbabwe, which raised the eastern margin of the country relative to the western area, has been along a north- south hinge zone not coincident with the Central Axis. E. R. MORRISON, Director. GEOLOGICAL SURVEY DEPARTMENT, HARARE. 17th July 1986 Scanned from original by ISRIC - World Soil Information, as ICSU World Data Centre for Soils. The purpose is to make a safe depository for endangered documents and to make the accrued information available for consultation, following Fair Use Guidelines. Every effort is taken to respect Copyright of the materials within the archives where the identification of the Copyright holder is clear and, where feasible, to contact the originators. For questions please contact soil.isric(a)wur.nl indicating the item reference number concerned. CONTENTS Page Introduction 1 The processes involved ; 3 Erosion cycles 4 Geology 11 Hydrology 12 Fossil surfaces 14 Pre-Karoo surface 15 Intra-Karoo surface 26 Jurassic surface 26 Cretaceous surface 27 Pre-Kalahari surface 28 The Eastern Highlands 32 Nyanga Mountains 33' Mutare region 40 Vumba Mountains 43 Southern Mountains 44 Chimanimani Mountains 49 The Limpopo-Save Lowlands 51 Central region 52 Gona-re-Zhou region 56 Middle Save Valley 58 Western region 60 The Zambezi Valley 62 Northern region 63 Sanyati-Sengwa basin -64 Binga region 67 The Central Axis 68 Central watershed 70 Great Dyke 76 Harare region 80 Beatrice triangle 86 Centenary-Mount Darwin region 88 North-eastern region 92 Upper Save basin 95 Kyle region 98 Zvishavane region 102 Filabusi region 107 Gwanda region Ill Bulawayo region 115 Hwange region 118 Nkayi region 124 North-western plateaux 127 Midlands region 129 Makonde region 132 Diastrophism 138 Faulting movements 139 Tilting movements 140 Geomorphic history 143 Economic considerations 154 Soils 155 References 156 Index 161 LIST OF ILLUSTRATIONS TEXT FIGURES 1 Relief map of Zimbabwe (1 : 1000 000-scale) (Obtainable from the Department of the Surveyor-General) 2 Geological map of Zimbabwe (1 : 1 000 000-scale) (Obtainable from the Geological Survey Department) 3 Erosion surfaces of Zimbabwe (1 : 2 000 000-scale) In back folder Page 4 Geomorphic provinces of Zimbabwe 2 5a Diagrammatic section showing progressive retreat of scarps relative to successive base-levels 4 5b Diagrammatic section showing the progressive encroachment of the younger cycle and development of the pediplains relative, to that cycle ... 4 6 Types of granite landform 10 7 "Granite topography" south of Bindura 11 8 Hydrological zones of Zimbabwe 12 9 Pre-Karoo landscape 16 10 The Lower Karoo basin of the mid-Zambezi 18 11 Reconstruction of the floor underlying the Karoo System 20 12 Reconstruction of the floor underlying the Kalahari System 31 13 The Nyanga Mountains region 35 14 Mutare-Vumba region 41 15 Erosion surfaces of the Southern Mountains 46 16 The Gona-re-Zhou National Park, Chipinda Pools area 56 17 The small escarpment which marks the nickpoint between the Pliocene and Quaternary erosion surfaces about 12 km south of Binga .... 68 18 The summit planation of the Umvukwes Range is independent of the rock structures of the Great Dyke 77 19 Erosion surfaces in the Harare area 82 20 The retreat of the Victoria Falls 120 21 Looking west from the Munyati valley to the Mafungabusi Escarpment . 128 22 Contours across the African landsurface 141 23 End Jurassic 147 24 Cretaceous 148 25 Oligocène 149 26 End Miocene 151 PLATES Facing Page I Bornhardts near Mutoko ~| II The Gondwana and Post-Gondwana erosion surfaces at the >• 36 Connemara Lakes J III The view westwards from Inyangani towards Juliasdale . \ ,7 IV Jointed and weathered quartzites, Chimanimani Mountains . ƒ V The Mwenezi River is incised into the flat Pliocene pediplain . \ « VI The Mateke Hills J °8 VII The narrow gorge downstream of Victoria Falls \ jo VIII The African planation near Featherstone ƒ IX Gentle incision near Centenary • • 1 X The inselberg Mangurgwe rises above the Pliocene pediplain, y 96 adjacent to the Mazowe River. The margin of the Post-Aanfric J erosion surface can be seen in the background 1 XI The Marehuru Hills >• 97 XII The Mahlalatzi Range J The Erosion Surfaces of Zimbabwe INTRODUCTION Zimbabwe covers 390 759 km2 in south central Africa between latitudes 15.5° and 22.5° south and longitudes 25° and 33° east. National boundaries are partly controlled by natural features such as the Zambezi, Shashe and Limpopo rivers. Altitudes vary from nearly 2600 metres at Inyangani in the Eastern Highlands to less than 250 metres at the confluence of the Runde and Save rivers in the south-eastern corner of the country. More than 75 per cent of the area of Zimbabwe stands above 650 metres altitude and 66 per cent of the country is above 1000 meties altitude. The average elevation of the country is therefore high and accordant with the topo­ graphy in other parts of the African interior. Over most of Zimbabwe the topography shows a broad trend in a south-west to north-east direction. Major units demonstrating this trend are: the Zambezi River along the northern border, the central belt of high land forming the Zambezi-Limpopo divide, and in the south, the ow valley lands of the Limpopo River and the lower valleys of the Runde and Save rivers. Bordering Mozambique, the Eastern Highlands region follows a north to south direction and thereby cuts across the dominant and probably older Zimbabwe geomorphic trend. At numerous localities the geomorphology strongly reflects the under­ lying geology. The most obvious and widespread example of this is the distinctive "granite topography", embracing both exfoliated bornhardts and rectangularly jointed castle koppies and balancing rocks. Certain other resistant strata, such as the pyroxenites of the Great Dyke and banded ironstones of the Bulawayan Group, create high ridges and ranges, e.g. the Umvukwes Range and the Iron Mask Range respectively. The various rock types commonly weather to give characteristic hill-types and slope profiles: banded ironstones exhibiting long and steep talus slopes below relatively small scarps with slope components being separated by sharp nicks, whereas greenstone hills are dome-shaped with summits and upper slopes convex, merging into concave lower slopes. Dolomites and limestones are uncommon in Zimbabwe and elements of karst topography are almost entirely limited to certain outcrops of the Lomagundi Group. The carbonatite ring complexes, e.g. Chishanya in the INTRODUCTION -— A ! s { f ^-1 - t \ 0 ^^y?" ^ —* 'N^ E K a- ƒ /~ 'l B \ I C 1 F ( N \..y—.. ..— " \ lAl / ^^ // v\ } I i \ \ w 1 II /A Sv-— '\ M / J p // D /z" n • V ° \® is 1 N G If wÊÊÊft^ VJo^^ hi >^/" • —*i*{^ _—»•—• ^J V" // l^^^x0 ( i K i \ \J^^® ^>~ s w X] A /^x Seal« of kilomètre» "" ' '^.^^ u 100 0 100 200 Tx—. / / / / 1 EASTERN HIGHLANDS 4 CENTRAL AXIS A Nyanga Mountains A Central Watershed B Mutare Region B Great Dyke * C Vumba Mountains C Harare Region D Southern Mountains D Beatrice Triangle E Chimanimani Mountains E Centenary-Mount Darwin Region F Northeastern Region 2 LIMPOPO-SAVE LOWLANDS G Upper Save Basin A Central Region H Kyle Region B Gona-re -Zhou 1 Zvishavane Region C Middle Save Valley J Filabusi Region D Western Region K Gwanda Region L Bulawayo Region 3 ZAMBEZI VALLEY M Hwange Region , A Northern Region N Nkayi Region B Sanyati-Sengwa Basin 0 Northwestern Plateaux C Binga Region P Midlands Region Q Makonde Region FIG. 4. Geomorphic provinces of Zimbabwe THE PROCESSES INVOLVED 3 Save Valley, characteristically form hills which rise 100 metres or more above the surrounding pediments. Fault scarps occur in the Zambezi Valley and in the Chipinge District, and other less severe movements have been recorded throughout the country. All these topographic variations, and others which will be discussed in the following text, are the result of geological controls. When the geomor- phology of Zimbabwe is expressed in terms of erosion cycles, it is found to be continuous with, and an integral part of, the geomorphology of Africa south of the Sahara. The same six major erosion cycles of the same ages, dating from the Jurassic to the present-day, occur in Zimbabwe as in the other territories of South, Central and East Africa.