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

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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-plana- tion 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
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