Post-Palaeozoic Evolution of Weathered Landsurfaces in Uganda by Tectonically Controlled Deep Weathering and Stripping

Geomorphology 25Ž. 1998 173±192

Post-Palaeozoic evolution of weathered landsurfaces in Uganda by tectonically controlled deep weathering and stripping

R.G. Taylor ), K.W.F. Howard Groundwater Research Group, Department of Geology, UniÕersity of Toronto, 1265 Military Trail, Scarborough, Ontario, Canada M1C 1A4 Received 4 September 1997; revised 18 March 1998; accepted 23 March 1998

Abstract

A model for the evolution of weathered landsurfaces in Uganda is developed using available geotectonic, climatic, sedimentological and chronological data. The model demonstrates the pivotal role of tectonic uplift in inducing cycles of stripping, and tectonic quiescence for cycles of deep weathering. It is able to account for the development of key landforms, such as inselbergs and duricrust-capped plateaux, which previous hypotheses of landscape evolution that are based on climatic or eustatic controls are unable to explain. Development of the Ugandan landscape is traced back to the Permian. Following late Palaeozoic glaciation, a trend towards warmer and more humid climates through the Mesozoic enabled deep weathering of the Jurassicrmid-Cretaceous surface in Uganda during a period of prolonged tectonic quiescence. Uplift associated with the opening South Atlantic Ocean terminated this cycle and instigated a cycle of stripping between the mid-Cretaceous and early Miocene. Deep weathering on the succeeding Miocene to recentŽ. African surface has occurred from Miocene to present but has been interrupted in the areas adjacent to the western rift where development of a new drainage base level has prompted cycles of stripping in the Miocene and Pleistocene. q 1998 Elsevier Science B.V. All rights reserved.

Keywords: landform evolution; tectonics; weathering; erosion; Uganda

1. Introduction lying rock. This in situ alteration of bedrock pro- duces a mantle of unconsolidated weathered rock In those terrains unaffected by Pleistocene glacia- and is known as `deep weathering'. Surface runoff, tion or aeolian erosion, landscapes are transformed aided by gravitational forcesŽ. e.g., mass movement , principally by the action of meteoric water in the removes weathered rock from the surface by `strip- form of groundwater recharge and surface runoff. ping'. Groundwater recharge is made acidic by microbial Whereas deep weathering and stripping operate activity in the soilŽ. i.e., regolith and weathers under- contemporaneously, the cyclical dominance of one process over the other is well-demonstrated by the ) development of landforms such as inselbergs and Corresponding author. Tel.: q1-416-287-7237; Fax: q1-416- 287-7279; E-mail: [email protected]@ duricrust-capped plateaux. InselbergsŽ. `island hills' scar.utoronto.ca are considered to result from a period of deep weath-

0169-555Xr98r$ - see front matter q 1998 Elsevier Science B.V. All rights reserved. PII: S0169-555XŽ. 98 00040-3 174 R.G. Taylor, K.W.F. HowardrGeomorphology 25() 1998 173±192 ering which is followed by strippingŽ Linton, 1955; that deep weathering occurs during long stable peri- Budel,È 1957, 1982; Ollier, 1960; Thomas, 1965; odsŽ 1078 to 10 a. of high sea level with a Doornkamp, 1968; Giardino and Mackel,È 1985. . vegetation-rich, moist climate and low reliefŽ their Bedrock lithologies, which are more resistant to `thalassocratic±biostatic state'. . Stripping of weath- weathering because of variations in structure or min- ered materials during a subsequent, shorter period is eralogyŽ. Pye, 1986 , are less denuded by deep suggested to result from low sea level which pro- weathering. During a subsequent cycle of stripping, duces unstable, polyclimatic conditions and high re- more resistant bedrock is exposed to form an insel- liefŽ. their `epeirocratic±rhexistatic state' . A lack of berg, whereas the surrounding, more deeply weath- evidence for deep weathering prior to the Mesozoic ered and unconsolidated mantle is removed. The constrains field validation of this proposed relation- genesis of duricrustsŽ. e.g., ferricrete, calcrete re- ship between long-term cycles of deep weathering flects prolonged weathering in the absence of signifi- and stripping, and eustatic oscillations. The compara- cant stripping. Rainfall-fed, groundwater recharge tive stability of climates during high sea levels in removes chemically mobile constituents from the relation to low sea levels is also speculative. The weathered mantleŽ. regolith on which the crust is concept that landscape evolution may result from developing, whereas less mobile elements such as long-term, alternating periods of deep weathering iron and aluminum become concentrated towards the and stripping rather than multiple cycles within the top of the mantle and accrete to form a residual crust Quaternary, as in Thomas' `dynamic etchplanation', ŽMcFarlane, 1983, 1991; Nahon, 1986; Nahon and is, however, supported by workers in AustraliaŽ Hill Tardy, 1992. . et al., 1995.Ž and southern Africa Partridge and Maud, 1987.Ž. . In Uganda, Ollier 1960, 1993 has 1.1. PreÕious studies of landscape eÕolution argued that only one cycle of deep weathering during the Mesozoic occurred. A subsequent cycle of strip- Reconciling the development of landforms, such ping is suggested to result from downfaulting of the as inselbergs and duricrusts by cycles of deep weath- westernŽ. Albertine graben in the Miocene. The ering and stripping with a viable model of landscape modern landscape is, thus, considered to be essen- evolution, has historically proved difficult. The the- tially a relict feature reflecting past climatic and ory of pediplanationŽ. King, 1962 , in which landsur- hydrological regimesŽ. Pain and Ollier, 1996 . No facesŽ. pediments are produced by retreating scarps, evidence has been presented, however, to suggest stresses the role of surface runoff in shaping land- that deep weathering or stripping of weathered man- scapes but effectively ignores the function of tles has ceased. groundwater recharge in deep weathering. Contem- poraneous deep weathering and stripping is proposed 1.2. Rationale of this study by BudelÈ Ž. 1957, 1982 in his theory of double-planation surfacesŽ. `doppelten Einebnungsflachen'È . This The uncertainties of landscape evolution include hypothesis, however, along with Thomas' concept of the periodicity of deep weathering and stripping, and `dynamic etchplanation'Ž. Thomas, 1965, 1989a,b , the mechanisms driving these cycles such as climate which involves cycles of deep weathering and strip- and sea level. In this paper, a model of landscape ping that coincide with QuaternaryŽ 104 a. climatic evolution in Uganda is developed from available oscillationsŽ. see Fig. 3 in Thomas, 1989a , cannot geotectonic, sedimentological, climatic and chrono- adequately account for the genesis of thick residual logical evidence which resolves these uncertainties. duricrusts which require protracted periodsŽ 106 a. of Except for a brief overview by OllierŽ. 1993 , no deep weathering to form from crystalline rockŽ e.g., recent discussion of landscape evolution in Uganda Nahon, 1986; McFarlane, 1991. . has occurred since the work of Bishop and Trendall An alternative model of landscape evolution, in- Ž.1967 , de Swardt and Trendall Ž. 1969 and McFar- volving long cycles of deep weathering and strip- laneŽ. 1974 . Following publication of this early work, ping, is put forward by Fairbridge and FinklŽ. 1980 . much progress has been made in understanding of In their theory of a `cratonic regime', they contend ferricrete developmentŽ e.g., McFarlane, 1983, 1991; R.G. Taylor, K.W.F. HowardrGeomorphology 25() 1998 173±192 175

Nahon, 1986; Nahon and Tardy, 1992. , the history 20 yr, a proliferation of research has developed on of sedimentation in the western rift of UgandaŽ Pick- the QuaternaryŽ e.g., Gasse, 1980; Livingstone, 1980; ford et al., 1989, 1992. , as well as associated fossil Hamilton, 1982; Rossignol-Strick, 1983; Taylor, assemblages and the palaeoclimatic settingŽ van 1990.Ž and pre-Quaternary e.g., Pickford, 1992, 1995; Damme and Pickford, 1995. . Furthermore, in the last Jacobs and Deino, 1996. climates in east Africa.

Fig. 1. Bedrock geology in Uganda adapted from Geochemical Atlas of UgandaŽ. Geological Survey and Mines Department, Uganda, 1973 . 176 R.G. Taylor, K.W.F. HowardrGeomorphology 25() 1998 173±192

Synthesis of interrelated geotectonic, geomorphic and Ž.Table 1 are, however, highly silicic Ž Table 2a . and climatic histories in Uganda is now possible. Mor- possess a quartzofeldspathic composition, based upon phoclimatic activity during the late Cenozoic is par- normativeŽ. CIPW calculations Ž Table 2b . , that are ticularly well-constrained by the improved under- consistent with Archaean gneiss observed in northern standing of rift deposits which enable sedimentation and south-central areas. Interestingly, sampled insel- in the western rift to be reconciled with cycles of bergs demonstrate a lower plagioclase to orthoclase weathering and erosion on landsurfaces in Uganda to ratio than discrete bedrock outcropsŽ. Table 2b . This the east. Similar approaches have been successfully is consistent with the observations of PyeŽ. 1986 in applied in southern AfricaŽ Partridge and Maud, Kenya, where relatively lesser denudation of insel- 1987.Ž and Australia Hill et al., 1995 . . bergs was attributed to the greater resistance of orthoclase to weathering. Three suites of supracrustal rocks were produced 2. Geological setting in Uganda during the Proterozoic. Rifting across southern Uganda in the early Proterozoic isolated the The geology of Uganda is not well-understood but Tanzanian craton from the rest of UgandaŽ Goodwin, consists predominantly of Archaean granulites and 1991. . Low- to medium-grade metamorphism of vol- gneisses, as well as metasedimentary rocks of Pro- canic and clastic sedimentary infill, associated with terozoic ageŽ. Fig. 1 . Granulites occur in patches the weathering of adjacent cratons, yielded the across northern and western Uganda and were meta- Buganda±Toro SupergroupŽ. Rwenzori Fold Belt of morphosedŽ. Watian event in the middle Archaean phyllites, quartzites, schists and gneisses. The meta- Ž.Spooner et al., 1970 . Granitic and biotitic gneisses, morphic grade increases roughly from east to west observed in northern and south-central areasŽ Leggo where uplifted schists and gneiss form the Rwenzori et al., 1971, Leggo, 1974; Bjùrlykke, 1975. , derive MountainsŽ. Fig. 1 . The second phase of Proterozoic from the retrograde metamorphism of the granulite tectonism is associated with continental rifting of the facies assemblageŽ. Almond, 1969 during late Ar- Kibaran orogenyŽ. 1800±1400 Ma . Analogous to the chaean tectonismŽ. Aruan event arising from colli- earlier events in Uganda, rifting induced sedimenta- sional events that consolidated Archaean cratonic tion and volcanismŽ. 1400±1200 Ma that was fol- nucleiŽ. Goodwin, 1991 between 2600 and 2550 Ma lowed by low-grade metamorphism marking the con- Ž.Table 1 . The geology of central Uganda has not yet vergence of Kasai and Tanzania cratons. Quartzites, been mappedŽ. Fig. 1 . Discrete Ž ground level . out- phyllites, hematitic sandstones and conglomerates of crops and inselbergs sampled in central Uganda southwestern Uganda derived from this Middle Pro-

Table 1 Observed outcrops and inselbergs of granitic gneiss associated with the Aruan complex Sample Site Number LatitudeŽ. N Longitude Ž. E Bedrock type Age Ž Ma . References XX Bobi outcrop 1 2835 32821 granitic gneiss 2550"10Ž. Leggo et al., 1971 XX Biso quarry 2 1845 31826 banded granulite 2550"10Ž. Leggo, 1974 XX Tank Hill quarry 3 0815 32835 granitic gneiss 2600"50Ž. Leggo, 1974 XX Iganga 4 0829 33827 granitic gneiss 2600"50Ž. Leggo, 1974 XX Mutoga quarry 5 0825 32830 granitic gneiss n.a.Ž. Bjùrlykke, 1975 XX Ibuje inselberg 6 1855 32823 augen gneiss n.a.Ž. this work XX Amii outcrop 7 1857 32824 amphibolite n.a.Ž. this work XX Ngetta inselberg 8 2817 32856 biotite gneiss n.a.Ž. this work XX Bala outcrop 9 2811 32843 granitic gneiss n.a.Ž. this work XX Ayer outcrop 10 2816 32852 biotite gneiss n.a.Ž. this work

Samples 6 to 10 are situated in central Uganda where the geology remains unmapped. For site locations, see Fig. 1. n.a.: Not available. R.G. Taylor, K.W.F. HowardrGeomorphology 25() 1998 173±192 177 a OR r r AN q AN orthoclase q AN OR AB q AB q 223 2 223 225 %% % % AB QOR %% % % % %%%%%%% Ž. Ž. Ž. Ž. Ž .Ž . Ž. Ž . Ž . Ž . Ž. Ž. Ž. Ž. Ž. Ž. Ž. Ž. Ž. Ž. Ž. Ž. LOI: loss on ignition. a Elemental composition of analysed outcrops in central Uganda b Selected normative CIPW mineralogical parameters Table 2 Ž. SampleIbujeAmii SiONgettaBalaAyer Al OMutoga 65.1 72.8 70.3 14.7Ž. 69.6 13.4 13.9Sample 59.6 64.2 CaO 14.1 18.2 17.4Ibuje inselberg 3.0 QuartzAmii MgO outcrop Ž 0.7Ngetta 2.1 20 inselberg Feldspar . Bala outcrop 27 28 1.1Ayer outcrop 1.0 Na 4.0 3.3Mutoga O 69 quarry 0.4 28 0.7 69 65 OrthoclaseCompositions 8 are 18 givena as 0.5 oxides. Plagioclase 2.0 67 3.5 1.7 Plagioclase 4.8 80 70 3.3 K O 29 3.9 Fe O 25 4.2 28 4.2 MnO 28 4.8 40 4.2 TiO 25 20 4.8 44 37 5.0 P 1.7 O 4.6 3.4 4.2 39 3.3 0.1 1.4 LOI 2.9 55 50 0.0 5.0 0.0 4.9 1.8 1.3 Total 1.0 0.0 0.3 0.6 1.4 0.1 0.1 0.3 2.2 2.5 0.1 0.3 0.2 0.7 0.6 0.6 0.4 0.1 0.2 0.2 0.3 99.1 1.6 98.8 0.4 99.5 1.4 98.7 101.4 98.6 178 R.G. Taylor, K.W.F. HowardrGeomorphology 25() 1998 173±192 terozoic metamorphism form the Karagwe±Ankolean tion of vascular plantsŽ. Knoll and James, 1987 . complex. The end of the Precambrian is marked by Erosion of accumulated Palaeozoic sediments on the collision of east GondwanalandŽ Antarctica, Gondwanaland has been attributed to Carboniferous Madagascar, India and Australia. and west Gond- glaciation in west AfricaŽ. Thomas and Thorpe, 1985 wanalandŽ. Africa and South America forming the and AustraliaŽ Fairbridge and Finkl, 1980; Hill et al., Mozambique mobile belt along the eastern boundary 1995. . Evidence of late Palaeozoic glacial deposition of UgandaŽ. Cahen et al., 1984; Goodwin, 1991 . The Ž.e.g., tillites , noted elsewhere in east Africa mobile belt is recognised in Uganda as the Karasuk Ž.Wopfner and Kreuser, 1986 , was inhibited by group of metasedimentary rocks overthrust onto Uganda's high-standingŽ. interbasin position. Dark, Aruan gneissesŽ. Leggo, 1974 . The intensity of the unfossiliferous shalesŽ. Ecca seriesÐFig. 1 , believed Mozambique metamorphism was noted by Harper et to have derived from glacial meltwaters in the early al.Ž. 1972 who recorded Late Proterozoic K±Ar ages PermianŽ. King, 1962 , are the only remaining sup- for the majority of 31 rocks analysed in central and port for Carboniferous Palaeozoic glaciation in northern Uganda. Apparently, this tectonism was Uganda. able to overprint partially or completely the isotopic record of Archaean events. The resulting discordant 3.2. Stage II: Mesozoic deep weathering and the ages make it difficult to define `basement' with any deÕelopment of the Jurassicrmid-Cretaceous sur- certainty. SchluterÈ Ž. 1994 , thus, proposes the term face `granulite±gneissic complex' in place of `basement complex' to describe the granulitic and gneissic ter- Deglaciation at the end of the Palaeozoic is asso- rain of central and northern Uganda. ciated with the northward migration of Gondwana- land to lower latitudesŽ. Caputo and Crowell, 1985 . Gondwana then collided with Laurasia to form 3. Morphoclimatic evolution of Uganda Pangea in the late Triassicrearly JurassicŽ. Fig. 2a , a period characterised by arid, continental climates Close examination of available geotectonic, sedi- Ž.Crowley and North, 1991 during which weathering mentological, climatic, and chronological evidence and denudation are believed to be minimalŽ King, in Uganda provides a model for morphoclimatic 1962. . Subsequent warming related to the low-lati- evolution from the end of the Palaeozoic to the tude position of Gondwana in the JurassicŽ. Fig. 2b present that is characterised by cycles of stripping served to enhance weathering. Prolonged deep and deep weathering. Stripping follows the develop- weathering of Precambrian bedrock surfaces pro- ment of relief from tectonic uplift, whereas deep duced a thick weathered mantle in many areas of weathering is shown to occur on newly formed GondwanalandŽ. Fig. 3 including east Africa surfaces during subsequent periods of tectonic quies- ŽRadwanski and Ollier, 1959; Ollier, 1959, 1960; cence. In the model, six broad stages are recognised. McFarlane et al., 1992; Ollier, 1993. , west Africa 3.1. Stage I: erosion of the Palaeozoic landscape by Ž.Thomas, 1965, 1966; Thomas and Thorpe, 1985 Carboniferous glaciation() presumed southern AfricaŽ. Partridge and Maud, 1987 , South AmericaŽ.Ž Schaefer et al., 1995 , India Demangeot, Development of weathered landsurfaces in Uganda 1975.Ž and Australia Mabbutt, 1965; Fairbridge and during the Palaeozoic was inhibited by earlyŽ Ordo- Finkl, 1980; Hill et al., 1995. . vician.Ž and late Carboniferous . Palaeozoic glacia- KingŽ. 1962 postulated that these weathered sur- tions. These glacial cycles occurred as Uganda faces formed an extensive, almost featureless plain drifted, as part of Gondwana supercontinent, along a Ž.pediplain that covered the Gondwana superconti- convoluted trajectory remaining in a high-latitude nent. Highest level surfaces on continents of the position roughly between 308S and 608S throughout former Gondwana are widely viewed as relics of a the PalaeozoicŽ. Eyles, 1993; Dalziel, 1997 . A pe- `Gondwana surface' that was stripped following riod of warmer, non-glacial conditions during the Gondwana fragmentationŽ e.g., McFarlane et al., Silurian and Devonian was marked by the prolifera- 1992; Ollier, 1993; Thomas, 1994; Schaefer et al., R.G. Taylor, K.W.F. HowardrGeomorphology 25() 1998 173±192 179

Fig. 2. Plate tectonic reconstructions of Gondwana adapted from ScoteseŽ 1991 . . Ž. a Early Jurassic. Ž. b Late Jurassic. Ž. c Early Cretaceous. Ž.d Mid-Cretaceous.

1995. . Modern reconstructions using plate tectonics deep weathering may explain the prominence of Ž.Scotese, 1991 show the hypothesis of superconti- massive vermiform laterites, which form under very nental-wide weathering of KingŽ. 1962 followed by advanced stages of leaching by infiltrating rainfall simultaneous rupture of Gondwana into its con- Ž.McFarlane, 1991 , on the highest level surface in stituent continentsŽ see Table IX in Fig. 20 in King, UgandaŽ. McFarlane, 1974 . 1962. to be invalid. Separation of east and west As summarised in Fig. 4, the late Mesozoic land- Gondwanaland was complete by the early Creta- scape of Uganda consisted of deeply weathered man- ceousŽ. Fig. 2c and preceded the mid-Cretaceous tle produced by the in situ alteration of Precambrian development of the south Atlantic that isolated Africa bedrock primarily from the Jurassic to mid-Creta- from South AmericaŽ. Fig. 2d . Thus, if a deeply ceous. Deep weathering over this period fashioned weathered surface did form across Gondwanaland an irregular bedrock surface due to differential during the Jurassic, this surface cannot constitute the weatheringŽ. Fig. 4a . The formation of a duricrust highest level surface of west Gondwanaland in Africa cap on the weathered mantle reflects accumulation of and South America because this was subjected to less mobile elements through prolonged leaching by further weathering through to the mid-Cretaceous infiltrating rainfallŽ. groundwater recharge . The `thermal optimum'. Indeed, prolonged and intense deeply weathered surface is considered to have ex- 180 R.G. Taylor, K.W.F. HowardrGeomorphology 25() 1998 173±192

Fig. 3. Global distribution of observed Mesozoic deep weathering.Ž. a Schaefer et al. Ž 1995 . . Ž. b Thomas and Thorpe Ž 1985 . . Ž. c Thomas Ž.Ž.Ž.Ž.1966 . d Ollier 1993 . e McFarlane et al. Ž.Ž. 1992 . f Partridge and Maud Ž.Ž. 1987 . g Demangeot Ž.Ž. 1975 . h Finkl and Churchward Ž.Ž.1973 . i Mabbutt Ž.Ž. 1965 . j Hill et al. Ž. 1995 and Pleistocene glaciation Ž modified from John, 1979 . . tended across UgandaŽ. Fig. 4b but, to avoid confu- Ž.Fig. 5a are, on the basis of fossil evidence and sion with the `Gondwana surface' of KingŽ. 1962 , K±Ar dating, of early Miocene age Ž.Žf19 Ma Bi- which is referred to here as the `Jurassicrmid-Creta- shop, 1958, 1965; Bishop and Trendall, 1967. . As a ceous surface'. In the absence of any data, mid- result, the cycle of stripping that extensively re- Cretaceous drainage patterns in Uganda are unknown moved the Jurassicrmid-Cretaceous surface from Ž.Fig. 4 . UgandaŽ. Fig. 5 was necessarily completed by the early Miocene. An earlier Oligocene age for the end 3.3. Stage III: Late Cretaceous to Early Miocene() ? of stripping has been deduced in southern Africa by stripping of the deeply weathered mantle Partridge and MaudŽ. 1987 from an absence of off- shore deposition. Significantly, the tuffs in eastern The development of the South Atlantic Ocean in Uganda predate mid-Miocene initiation of the west- the mid-Cretaceous prompted eastward incision along ernŽ.Ž Albertine graben Pickford et al., 1992 . . Thus, the failed rift that gave rise to the Congo drainage alternative hypotheses which link stripping of the which extended across equatorial Africa to present- Jurassicrmid-Cretaceous surface in Uganda to day Kenya. Incision induced widespread stripping of Miocene riftingŽ Doornkamp and Temple, 1966; van the deeply weathered mantle in UgandaŽ Bishop and Stratten, 1977; Ollier, 1993. , are untenable. Trendall, 1967; de Swardt and Trendall, 1969; Mc- Remnants of the Jurassicrmid-Cretaceous surface Farlane, 1974. where east±west trending drainage in UgandaŽ. Fig. 5 are restricted to ferricrete-capped channels of the Kafu, Katonga and KageraŽ. Fig. 5 mesas underlain by quartzites of the Buganda±Toro formed headwaters of the Congo basinŽ Temple, and Karagwe±Ankolean complexesŽ Doornkamp and 1970. . Volcanic tuffs overlying the stripped Juras- Temple, 1966; de Swardt and Trendall, 1969. . Be- sicrmid-Cretaceous surface in eastern Uganda cause quartzites are more resistant to weathering R.G. Taylor, K.W.F. HowardrGeomorphology 25() 1998 173±192 181

Fig. 4. The mid-Cretaceous landscape of Uganda.Ž. a Hypothetical cross-section modified from Ollier Ž 1960 .Ž. . b Plan-view representation.

than surrounding bedrock typesŽ gneisses, granites, Jurassicrmid-Cretaceous surface is provisionally re- schists and phyllites. , quartzite-supported mesas per- ferred to as the African surface, as suggested by sisted through the cycle of stripping which removed KingŽ. 1962 . the Jurassicrmid-Cretaceous surface from the rest of Uganda. Along the north shore of Lake Victoria and 3.4. Stage IV: Early Miocene deep weathering of the in southwestern Uganda, ferricrete-capped mesas African() stripped Jurassicrmid-Cretaceous surface stand as isolated hills separated by wide valleysŽ Fig. 6Ðphoto. . Indeed, in an area featuring the relict Deep weathering of the African surface in Uganda Jurassicrmid-Cretaceous surfaceŽ. Fig. 5 , McFarlane during the early Miocene is denoted by the formation Ž.1974 found that only 2% of this high-level surface of a duricrust. Ferricrete is not observed beneath remains unstripped. Inselbergs representing the irreg- extrusive rocks deposited on the African surface in ularŽ. i.e., differentially weathered surface of the the early Miocene Ž.Žf19 Ma de Swardt and Tren- bedrock are the other prominent erosional relict of dall, 1969. . In contrast, ferricrete does underlie rift the Jurassicrmid-Cretaceous surface. For the pur- infillŽ. Bishop, 1965; de Swardt and Trendall, 1969 poses of discussion below, the surrounding stripped and so necessarily developed on the African surface 182 R.G. Taylor, K.W.F. HowardrGeomorphology 25() 1998 173±192

Fig. 5. The early Miocene landscape of Uganda.Ž. a Hypothetical cross-section modified from Ollier Ž 1959, 1960 . ; for key, see Fig. 3a. Ž. b Plan-view representation based upon a palaeohydrological reconstruction by TempleŽ. 1970 and physiographic mapping of de Swardt and TrendallŽ. 1969 .

before it was downfaulted to form the westernŽ Al- on pre-weathered rock. Climatic conditions bertine.Ž. graben in the mid-Miocene f13 Ma . Na- favourable for deep weathering, namely that precipi- honŽ. 1986 has shown in Senegal that 6 Ma is tation exceeds evapotranspiration to produce rainfall- sufficient time for ferricrete to form in situ from fed groundwater recharge, are indicated by limited crystalline rock. Because stripping of the palaeoclimatic dataŽ Axelrod and Raven, 1978; Pick- Jurassicrmid-Cretaceous surface in Uganda did not ford, 1995. that show humid conditions capable of proceed down to bedrockŽ. Ollier, 1959 , ferricrete sustaining rainforest cover in east Africa during the development was assisted by the fact that it occurred early Miocene. R.G. Taylor, K.W.F. HowardrGeomorphology 25() 1998 173±192 183

Fig. 6. Photograph showing the Jurassicrmid-Cretaceous surface preserved on a quartzite-supported mesa, taken near Kajansi, 10 km south of Kampala along Entebbe Road, looking eastwardŽ. R. Taylor . The mesa on the left is situated slightly more than a kilometre behind Ž east of. the mesa on the right.

3.5. Stage V: Miocene rifting and localised stripping 1971; Pickford et al., 1989. . Such strata exhibit of the African surface indicators of westward-directed palaeocurrents resulting from continued flow into the Congo basin Commencement of rifting in the mid-Miocene Ž.Bishop, 1965; Bishop and Trendall, 1967 . produced a shallow downwarp and induced local Increased tectonism in the late MioceneŽ. 8 Ma stripping of the African surfaceŽ. Fig. 7a . Ensuing formed a deep graben and led to the establishment of infill initiated a sedimentary sequence which pro- palaeolake ObwerukaŽ. Fig. 7 . This transition was vides a continuous record of subsequent morphocli- marked in the rift by a depositional change from matic activity on landsurfaces draining into the rift coarse, fluvial sediments to more highly weathered trough from the east. Rift sediments at the northern and ferruginised, lacustrine strataŽ Kaiso series; and southern end of Lake AlbertŽ. Fig. 8 have been Oluka, Nyaburogo, Nyakabingo, Katarago and studied in detailŽ. Bishop, 1965; Pickford et al., 1989 Nyabusosi formations. . This shift reflects reduced and a correlation of observed sedimentary assem- downcutting east of the rift and inflow of iron- blages is given in Fig. 9. Basal depositsŽ Kisegi and saturated water to the lake. As such, it indicates a Namsika beds. contain coarse, relatively unweath- transition on landsurfaces east of the rift from strip- ered clasts indicative of downcutting into lower, less ping to deep weathering, a change recorded by the weathered zones of deeply weathered profiles by the development of a ferricrete crust on the Namsika rejuvenation of westward drainageŽ Bishop, 1965, bedsŽ. Fig. 9 during more humid conditions of the 184 R.G. Taylor, K.W.F. HowardrGeomorphology 25() 1998 173±192

Fig. 7. The late MiocenerPliocene landscape of Uganda.Ž. a Hypothetical cross-section; for key, see Fig. 3a. Ž. b Plan-view representation based upon a palaeohydrological reconstruction by TempleŽ. 1970 , proposed palaeolake Obweruka of van Damme and Pickford Ž. 1995 , and physiographic mapping of de Swardt and TrendallŽ. 1969 .

late Miocene and early PlioceneŽ Dechamps et al., The drainage outlet for the western rift from the 1992; Jacobs and Deino, 1996. . The areal extent to time water began to collect in Lake Obweruka in the which the African surface was modified during the late Miocene until evidence of northward flow to the Kisegi stage is unclearŽ. Fig. 7 but mid-Pleistocene Nile in the late PleistoceneŽ. Livingstone, 1980 is the upwarping and renewed strippingŽ. discussed below subject of speculation. Adamson and WilliamsŽ. 1980 are likely to have destroyed evidence of Miocene cited the model of rift development of VeeversŽ. 1977 stripping on landsurfaces to the east of the rift. and argued that westward flow to the Congo basin R.G. Taylor, K.W.F. HowardrGeomorphology 25() 1998 173±192 185

Fig. 8. The late Quaternary landscape of Uganda.Ž. a Hypothetical cross-section; for key, see Fig. 3a. Ž. b Plan-view representation based upon a modified physiographic map of de Swardt and TrendallŽ. 1969 . was blocked by an arch whose crest later down- northward overflow from the western rift during warped to produce the western rift. Deposition of much of the late TertiaryŽ. Adamson et al., 1993 . A claystones in southern Sudan was later attributed to continued westward link to the Congo basin from the 186 R.G. Taylor, K.W.F. HowardrGeomorphology 25() 1998 173±192

Fig. 9. Rift deposition and inferred geomorphology.Ž. a van Damme and Pickford Ž 1995 . . Ž. b Bishop, 1965. Ž. c Pickford et al., 1989, 1992. Ž.d Dechamps et al., 1992. Ž. e Dechamps et al., 1992; Jacobs and Deino, 1996. Ž. f Pickford, 1992. Ž. g Axelrod and Raven, 1978; Pickford, 1995; Jacobs and Deino, 1996.Ž. h Bishop and Trendall, 1967. Dashed lines indicate estimated sediment ages. Shading links rift deposition to cycles of stripping during the Miocene and Pleistocene.

late Miocene to late Pliocene is, however, indicated mid-Pleistocene when fluviolacustrine, drab gray to by the genealogy of molluscan fauna in rift sedi- white sands of Paraa and Wasa beds, truncated highly mentsŽ. van Damme and Pickford, 1995 . Prolifera- weathered, lacustrine strataŽ. Fig. 9 . The transition tion of fresh-water molluscs, studied by van Damme from Kaiso series to Wasa beds marks a period of and PickfordŽ. 1995 , denies the possibility that the uplift followed by stripping where inflow of iron- lake became saline through prolonged closure de- saturated water ceased and downcutting into fresh spite the late Pliocene aridity noted by Dechamps et rock was accelerated by rejuvenated streamflowŽ Bi- al.Ž. 1992 . shop, 1965. . Uplift parallel to, but approximately 30 km east of, the rift escarpment exceeded westward 3.6. Stage VI: Mid-Pleistocene uplift and stripping of incision and reversed river flowsŽ. Fig. 8 . In areas African surface east of the axis of uplift, termed the `inter-arch' basinŽ i.e., between the Albert rift in Uganda and In the late Pliocene, Lake Obweruka was cut in Gregory rift in Kenya.Ž. by Veevers 1977 , a reduc- half by the upthrusting of the Rwenzori horst to form tion in reliefŽ the westward tilt being marginally lakes Albert and EdwardŽ. Fig. 8 . Nevertheless, a reversed. has led to the collection of surface water in distinct shift in rift deposition did not occur until the the wide drainage channels associated with formerly R.G. Taylor, K.W.F. HowardrGeomorphology 25() 1998 173±192 187 westward-flowing headwaters of the Congo. Con- precision but conforms more closely with rejuve- versely, between the axis uplift and the rift escarp- nated drainage into the rift trough than strictly topo- ment, the `intra-arch' basin of VeeversŽ. 1977 in- graphic criteriaŽ. i.e., axes of uplift , suggested by creased relief that has led to incision of narrow earlier physiographic mapsŽ de Swardt and Trendall, drainage channels within the wide valley topography. 1969; Ollier, 1984.Ž. . This phase of intra-arch strip- The impact of this reversal on drainage patterns in ping, resulting from uplift, is recorded by rift deposi- western Uganda, near MasindiŽ. Fig. 1 , is depicted in tion, whereas the reversal of westward drainage in Fig. 10. The extent of intra-arch stripping on the the inter-arch basin is logged in sediments deposited African surfaceŽ. Fig. 8 is difficult to define with along the Kagera river channel at NsongesiŽ. Fig. 8 .

Fig. 10. Surface drainage in the vicinity of Biso, near Masindi in western UgandaŽ. modified from Ollier, 1993 showing broad, swamp-filled drainage channels east of the upwarped drainage divide and narrower, more incised drainage pattern trending west from the divide to the rift trough. 188 R.G. Taylor, K.W.F. HowardrGeomorphology 25() 1998 173±192

At the base of Nsongesi sedimentary sequence Victoria at Jinja initiated northward flow along a Ž.Fig. 9 , early Pleistocene gravels reflect fluvial de- tributary of the Kyoga drainage system and estab- position along a westward-flowing Kagera channel. lished the current drainage network in UgandaŽ Fig. Overlying lacustrine sands, deposited by the end of 8.. mid-Pleistocene times, indicate collection of surface water in the Nsongesi valley because of the reversal of the Kagera river. Significantly, this period of 4. Discussions and conclusions lacustrine deposition coincides with the onset of intra-arch stripping inferred from the change in rift A summary of the post-Palaeozoic evolution of sediments of the Kaiso series to Wasa and Paraa weathered landsurfaces in Uganda is given in Fig. bedsŽ. Fig. 9 . A shift towards drier conditions oc- 12. Cycles of stripping are clearly controlled by the curred around 25 kaŽ. Fig. 11 . This coincides roughly development of relief resulting from episodes of with downwarping along a north±south transectŽ ap- tectonic uplift, whereas cycles of deep weathering proximately longitude 338E. that created the saucer- ensue on newly stripped surfaces during subsequent like depressions which eventually formed lakes periods of tectonic quiescence. Hence, tectonically Kyoga and VictoriaŽ. Fig. 8 . Although relative con- controlled deep weathering and stripping emerge as a tributions are difficult to resolve, lacustrine deposi- viable model for the long-term evolution of weath- tion at Nsongesi was halted by a shift to drier ered landsurfaces by meteoric water. Cycles of deep conditions and downwarping which served to in- weathering and stripping since the Palaeozoic do not crease the surface gradient from the upwarp axis in show a relationship to variations in climate; how- western Uganda to the east. This transition is marked ever, it is conceded that the climate signal through by deposition of paludal clays and silts around 25 ka this period has not been well-defined. A particular Ž.Fig. 9 . The development of wet and warm climatic strength of the tectonic model is that it readily conditions in rapid response to orbital forcing around explains the development of key elements in the 12.5 kaŽ. Fig. 11 filled the Victoria and Kyoga landscape such as inselbergs and duricrust-capped depressions. Downwearing of an outlet for Lake plateaux. Previous models based on climatically or

Fig. 11. Late Pleistocene and Holocene climate in east Africa.Ž. a Rossignol-Strick Ž.Ž. 1983 . b Shackleton Ž.Ž.Ž.Ž. 1987 . c Taylor 1990 . d GasseŽ.Ž. 1980 . e Livingstone Ž.Ž. 1980 . f Kendall Ž.Ž. 1969 . g Gasse et al. Ž. 1989 . Shading links the onset of warm, humid conditions due to orbital forcing to higher lake levels in east Africa. R.G. Taylor, K.W.F. HowardrGeomorphology 25() 1998 173±192 189

Fig. 12. Schematic diagram showing the post-Palaeozoic evolution of weathered landsurfaces in Uganda. Stage refers to tectonically controlled cycles of deep weathering and stripping identified in this study and described in the text.

eustatically controlled episodes of deep weathering to terrestrial geomorphic events in southern Africa and strippingŽ Thomas, 1989a; Fairbridge and Finkl, recorded by off-shore sedimentationŽ Partridge and 1980. do not adequately account for the evolution of Maud, 1987. and, to a lesser degree, the worldwide these landforms. `pediplanation cycles' proposed by KingŽ. 1962 . A The role of tectonic uplift in driving landscape time lag between the separation of east and west renewal by inducing episodes of stripping has been Gondwanaland in the early Cretaceous and the sepa- recognised by other studiesŽ King, 1962; Demangeot, ration of Africa and South America by the mid- 1975; Partridge and Maud, 1987. . Indeed, the Cretaceous, which is recognised in this study, ques- chronology of denudation in Uganda is comparable tions King's hypothesis, commonly adoptedŽ e.g., 190 R.G. Taylor, K.W.F. HowardrGeomorphology 25() 1998 173±192

Ollier, 1993; Schaefer et al., 1995. , that a Gond- parameters were calculated from a PC-based pro- wana-wide peneplain existed, was stripped following gramme supplied by Professor John Westgate of the dismemberment of the supercontinent, and thereby University of Toronto. Finally, during the prepara- constitutes the relict, highest-level surface in Africa tion of this manuscript, the first author benefitted and South America. from the provision of a University of Toronto Open A tectonically-driven model of deep weathering Fellowship and a Graduate Scholarship from the and stripping counters the assertion that these pro- Government of Ontario. cesses are discontinuous and have each occurred over only one intervalŽ Ollier, 1960, 1993; Hill et al., 1995; Pain and Ollier, 1996. . Based upon work in References Uganda, comparatively lesser effects of deep weathering on the continental-weathered landsurfaces dur- Adamson, D., Williams, F., 1980. Structural geology, tectonics ing the Cenozoic, as compared with higher-level and the control of drainage in the Nile basin. In: Williams, Mesozoic surfaces are viewed not as a lack of M.A.J., Faure, H.Ž. Eds. , The Sahara and the Nile. Balkema, `cyclicity' in deep weathering and stripping but as Rotterdam, pp. 225±252. Adamson, D., McEvedy, R., Williams, M.A.J., 1993. Tectonic function of a shorter timescale, and different and inheritance in the Nile basin and adjacent areas. Isr. J. Earth possibly more variable climates. Recognition that Sci. 41, 75±85. weathered landsurfaces are not entirely relict, but Almond, D.C., 1969. Structure and metamorphism of the base- continue to be shaped by contemporary processes, is ment complex of north±east Uganda. Overseas Geol. Miner. important because it implies that the analysis of deep Resour. 10, 146±163. Axelrod, D.I., Raven, D.H., 1978. Late Cretaceous and Tertiary weathering and stripping in the present is relevant to vegetation history of Africa. In: Werger, M.J.A.Ž. Ed. , Bio- long-term landscape evolution. geography and Ecology of Southern Africa. Junk, The Hague, pp. 77±130. Bishop, W.W., 1958. Miocene mammalia from the Napak vol- Acknowledgements canics, Karamoja, Uganda. Nature 182, 1480±1482. Bishop, W.W., 1965. Quaternary geology and geomorphology in the Albertine Rift Valley and Frey Valley, Uganda. In: Wright, This study emanated from hydrogeological inves- Jr., H.E., Frey, D.G.Ž. Eds. , International Studies on the tigations conducted under the Hydrogeology-Uganda Quaternary. pp. 293±321. Phase II project sponsored by the International De- Bishop, W.W., 1971. The Late Cenozoic history of east Africa in velopment Research CentreŽ. IDRC , Canada, with relation to hominoid evolution. In: Turekian, K.K.Ž. Ed. , Late Cenozoic Glacial Ages. pp. 493±527. assistance from the Directorate of Water Develop- Bishop, W.W., Trendall, A.F., 1967. Erosion-surfaces, tectonics mentŽ. DWD , Ministry of Natural Resources, and volcanic activity. Q. J. Geol. Soc., London 122, 385±420. Uganda, and the University of Toronto, Canada. Bjùrlykke, K., 1975. Mineralogical and chemical changes during Development of the presented thesis benefitted sig- weathering of acid and basic rocks in Uganda. Nor. Geol. nificantly from discussions with Professor Nick Eyles Tidsskr. 55, 81±89. Ž. Budel,ÈÈ J., 1957. Die `doppleten Einebnungsflachen' in den University of Toronto , in particular, as well as feuchten Tropen. Z. Geomorphol. N.F. 1, 201±288. Professor Tony PriceŽ. University of Toronto and Dr. Budel,È J., 1982. Climatic geomorphology,Ž Fischer, L., Busche, D. Peter van StraatenŽ. University of Guelph . We are Trans.. Princeton University Press, Princeton, 443 pp. also grateful for helpful correspondence with Profes- Cahen, L., Snelling, N.J., Delhal, J., Vail, J.R., Bonhomme, M., sors Cliff OllierŽ. Australian National University , Ledent, D., 1984. The Geochronology and evolution of Africa. Ž.Clarendon Press, Oxford, 512 pp. Marty McFarlane University of Botswana , Michael Caputo, M.V., Crowell, J.C., 1985. Migration of glacial centers ThomasŽ. University of Stirling , Daniel Livingstone across Gondwanaland during the Paleozoic Era. Geol. Soc. Ž.Duke University , Martin Williams Ž University of Am. Bull. 96, 1020±1036. Adelaide. , as well as Mssrs. Dribidu and Oconga of Crowley, T.J., North, G.R., 1991. Palaeoclimatology. Oxford the DWD. Discussions with Mr. Tindimugaya Callist monographs on geology and geophysics,Ž. 18 339 pp. Dalziel, I.W.D., 1997. Neoproterozoic±Paleozoic geography and of the DWD during field reconnaissance of reversed tectonics: review, hypothesis, environmental speculation. Geol. river flows in western Uganda in November, 1996 Soc. Am. Bull. 109Ž. 1 , 16±42. also proved useful. NormativeŽ. CIPW mineralogical Dechamps, R., Senut, B., Pickford, M., 1992. Fruits fossiles R.G. Taylor, K.W.F. HowardrGeomorphology 25() 1998 173±192 191

pliocenesÁÂÁ amd pleistocenes du Rift Occidental ougandais. Leggo, P.J., 1974. A geochronological study of the basement Signification palaeoenvironmentale.Â Comput. Rend. Acad. Sci., complex of Uganda. J. Geol. Soc., London 130, 263±277. Paris 314Ž. II , 325±331. Leggo, P.J., Aftalion, M., Pidgeon, R.T., 1971. Discordant zircon Demangeot, J., 1975. Recherche geomorphologiquesÂ en Inde du U±Pb ages from the Uganda basement. Nature 231, 81±84. Sud. Z. Geomorphol. N.F. 19, 229±272. Linton, D.L., 1955. The problem of tors. Geogr. J. 121, 470±481. de Swardt, A.M.J., Trendall, A.F., 1969. The physiographic devel- Livingstone, D.A., 1980. Environmental changes in the Nile head- opment of Uganda. Overseas Geol. Miner. Resour. 10, 241± waters. In: Williams, M.A.J., Faure, H.Ž. Eds. , The Sahara and 288. the Nile. Balkema, Rotterdam, pp. 339±359. Doornkamp, J.C., 1968. The role of inselbergs in the geomorphol- Mabbutt, J.A., 1965. The weathered land surface in central Aus- ogy of southern Uganda. Trans. Inst. Br. Geogr. 44, 151±162. tralia. Z. Geomorphol. 9, 82±114. Doornkamp, J.C., Temple, P.H., 1966. Surface, drainage and McFarlane, M.J., 1974. Laterization and landscape development tectonic instability in part of southern Uganda. Geogr. J. 132, in Kyagwe, Uganda. Q. J. Geol. Soc., London 126, 501±539. 238±252. McFarlane, M.J., 1983. Laterites. In: Goudie, A.S., Pye, K.Ž. Eds. , Eyles, N., 1993. Earth's glacial record and its tectonic setting. Chemical Sediments and Geomorphology: Precipitates and Earth Sci. Rev. 35, 1±248. Residua in the Near-Surface Environment, Chap. 2. Academic Fairbridge, R.W., Finkl, C.W. Jr., 1980. Cratonic regime, uncon- Press, New York, pp. 7±58. formities and peneplains. J. Geol. 88, 69±86. McFarlane, M.J., 1991. Some sedimentary aspects of lateritic Finkl, C.W. Jr., Churchward, H.M., 1973. The etched surfaces of weathering profile development in the major bioclimatic zones southwestern Australia. J. Geol. Soc. Aust. 20Ž. 3 , 295±307. of tropical Africa. J. Afr. Earth Sci. 12Ž. 1±2 , 267±282. Gasse, F., 1980. Late Quaternary changes in lake-levels and McFarlane, M.J., Chilton, P.J., Lewis, M.A., 1992. Geomorpho- diatom assemblages on the south-eastern margin of the Sahara. logical controls on borehole yields: a statistical study in an In: van Zinderen Bakker, Sr., Coetzee, J.A.Ž. Eds. , Palaeoecol- area of basement rocks in central Malawi. In: Wright, E.P., ogy of Africa and the Surrounding Islands. Balkema, Rotter- Burgess, W.G.Ž. Eds. , Hydrogeology of Crystalline Basement dam, Vol. 12, pp. 333±350. Aquifers in Africa. Geological Society Special Publication 66 Gasse, F., Ledee,ÂÂ V., Massault, M., Fontes, J.C., 1989. Water-level pp. 131±154. fluctuations of Lake Tanganyika in phase with oceanic changes Nahon, D., 1986. Evolution of iron crusts in tropical landscapes. during the last glaciation and deglaciation. Nature 342, 57±59. In: Colman, S.M., Dethier, D.P.,Ž. Eds. , Rates of Chemical Geological Survey and Mines Department, Uganda, 1973. Geo- Weathering of Rocks and Minerals. Academic Press, New chemical Atlas of Uganda. Government of Uganda, 43 pp. York, pp. 169±191. Goodwin, A.M., 1991. Precambrian Geology. Academic Press, Nahon, D., Tardy, Y., 1992. The ferriginous laterites. In: Butt, New York, 666 pp. C.R.M., Zeegers, H.Ž. Eds. , Regolith Exploration Geochem- Giardino, J.R., Mackel,È R., 1985. Correlative development of istry in Tropical and Sub-tropical Terrains, Handbook of Ex- dambos and dwalas: plateau regions of Zambia. Z. Geomor- ploration GeochemistryŽ. 4 pp. 41±55. phol. N.F. 52, 187±200. Ollier, C.D., 1959. A two-cycle theory of tropical pedology. J. Hamilton, A.C., 1982. Environmental History of East Africa. Soil Sci. 10Ž. 2 , 137±148. Academic Press, London, 328 pp. Ollier, C.D., 1960. The inselbergs of Uganda. Z. Geomorphol. 4, Harper, C.T., Weintraub, G.S., Leggo, P.J., Shackleton, R.M., 43±52. 1972. Potassium±argon retention ages from the basement Ollier, C.D., 1984. Weathering, 2nd edn. Longman, Edinburgh, complex and associated Precambrian metasedimentary rocks 270 pp. of Uganda. Geol. Soc. Am. Bull. 83, 3449±3456. Ollier, C.D., 1993. Age of soils and landforms in Uganda. Isr. J. Hill, S.M., Ollier, C.D., Joyce, E.B., 1995. Mesozoic deep weath- Earth Sci. 41, 227±231. ering and erosion: an example from Wilson's Promontory, Pain, C.F., Ollier, C.D., 1996. Regolith stratigraphy: principles Australia. Z. Geomorphol. N.F. 39, 331±339. and problems. Aust. J. Geol. Geophys. 16Ž. 3 , 198±202. Jacobs, B.F., Deino, A.L., 1996. Test of climate±leaf physiog- Partridge, T.C., Maud, R.R., 1987. Geomorphic evolution of nomy regression models, their application to two Miocene southern Africa since the Mesozoic. S. Afr. J. Geol. 90Ž. 2 , floras from Kenya, and 40Arr39Ar dating of the Late Miocene 179±208. Kapturo site. Palaeogeogr. Palaeoclimatol. Palaeoecol. 123, Pickford, M., 1992. Evidence for an arid climate in Western 259±271. Uganda during the middle Miocene. C.R. Seances Acad. Sci. John, B., 1979. The Winters of the WorldÐEarth Under the Ice 315Ž. 2 , 1419±1424. Ages. David and Charles, London, 256 pp. Pickford, M., 1995. Fossil land snails of East Africa and their Kendall, R.L., 1969. An ecological history of the Lake Victoria palaeoecological significance. J. Afr. Earth Sci. 20Ž. 3±4 , basin. Ecol. Monographs 39Ž. 2 , 121±176. 167±226. King, L.C., 1962. Morphology of the Earth. Oliver and Boyd, Pickford, M., Senut, B., Roche, H., Mein, P., Ndaati, G., Obwona, London, 699 pp. P., Tuhumwire, J., 1989. Uganda Palaeontology Expedition: Knoll, M.A., James, W.C., 1987. Effect of the advent and diversi- resultatsÂÁ de la deuxieme missionŽ. 1987 dans la region Â de fication of vascular land plants on mineral weathering through Kisegi±NyabusosiŽ. Bassin du lac Albert Ouganda . C.R. geologic time. Geology 15, 1099±1102. Seances Acad. Sci. 308Ž. 2 , 1751±1758. 192 R.G. Taylor, K.W.F. HowardrGeomorphology 25() 1998 173±192

Pickford, M., Senut, B., Roche, J.P., Ambrosi, J.P., Dechamps, analysis in tropical environments. In: Whittow, J.B., Wood, R., Faure, M., van Damme, D., Texier, P.J., Baguma, Z., P.D.Ž. Eds. , Essays in Geography for Austin Miller. University Musiime, E., 1992. RevisionÂÂÁ de la biostratigraphie du Neogene of Reading, pp. 118±143. du Rift OccidentalŽ. Ouganda±Zaire . C.R. Seances Acad. Sci. Thomas, M.F., 1966. Some geomorphological implications of 315Ž. 2 , 1289±1292. deep weathering patterns in crystalline rocks in Nigeria. Trans. Pye, K., 1986. Mineralogical and textural controls on the weather- Inst. Br. Geogr. 40, 173±193. ing of granitoid rocks. Catena 13, 47±57. Thomas, M.F., 1989a. The role of etch processes in landform Radwanski, S.A., Ollier, C.D., 1959. A study of an east African development: I. Etching concepts and their applications. Z. catena. J. Soil Sci. 10Ž. 2 , 149±170. Geomorphol. N.F. 33Ž. 2 , 129±142. Rossignol-Strick, M., 1983. African monsoons, an immediate Thomas, M.F., 1989b. The role of etch processes in landform climate response to orbital insolation. Nature 304, 46±49. development: II. Etching and the formation of relief. Z. Geo- Schaefer, C., Dalrymple, V., Dalrymple, J., 1995. Landscape morphol. N.F. 33Ž. 3 , 257±274. evolution in Roraima, North Amazonia: planation, paleosols Thomas, M.F., 1994. Geomorphology in the Tropics. Wiley, and paleoclimates. Z. Geomorphol. 39Ž. 1 , 1±28. Chichester, 460 pp. Schluter,È T., 1994. The Precambrian of East Africa. 177 pp. Thomas, M.F., Thorpe, M., 1985. Environmental change and Scotese, C.R., 1991. Jurassic and Cretaceous plate tectonic recon- episodic etchplanation in the humid tropics of Sierra Leone: structions. Palaeogeogr. Palaeoclimatol. Palaeoecol. 87, 493± the Koidu etchplain. In: Douglas, I., Spencer T.Ž. Eds. , Envi- 501. ronmental Change and Tropical Geomorphology, Chap. 12. Shackleton, N.J., 1987. Oxygen isotopes, ice volume and sea George Allen, London, pp. 239±267. level. Q. Sci. Rev. 6, 183±190. van Damme, D., Pickford, M., 1995. The late Cenozoic ampullari- Spooner, C.M., Hepworth, J.V., Fairbairn, H.W., 1970. Whole- daeŽ. mollusca, gastropoda of the Albertine Rift Valley rock Rb±Sr investigation of some east African granulites. Ž.Uganda±Zaire . Hydrobiologia 316, 1±32. Geol. Mag. 107, 511±521. van Stratten, H.P., 1977. Morphotectonic investigations along the Taylor, D.M., 1990. Late Quaternary pollen records from two Western Rift in the Masindi area, Bunyoro District, Uganda. Ugandan mires: evidence for environmental change in the Geol. Rund. 66, 217±228. Rukiga Highlands of southwest Uganda. Palaeogeogr. Palaeo- Veevers, J.J., 1977. Rifted arch basins and post-breakup rim climatol. Palaeoecol. 80, 283±300. basins on passive continental margins. Tectonophysics 41, Temple, P., 1970. The lakes of Uganda. In: Ominde, S.H.Ž. Ed. , T1±T5. Studies in East African Geography and Development. Heine- Wopfner, H., Kreuser, T., 1986. Evidence for Late Palaeozoic mann, London, pp. 86±98. glaciation in southern Tanzania. Palaeogeogr. Palaeoclimatol. Thomas, M.F., 1965. An approach to some problems of landform Palaeoecol. 56, 259±275.