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Late Cainozoic drainage evolution in the Zambezi basin: Geomorphological evidence from the Kalahari rim
Journal of African Earth Sciences, Vol. 7, No. 4, pp. 611~518, 1988
0731-7247/88 $3.00+0.00 Pergamon Press plc
Printed in Great Britain
L A T E C A I N O Z O I C D R A I N A G E E V O L U T I O N IN THE Z A M B E Z I BASIN : G E O M O R P H O L O G I C A L E V I D E N C E F R O M THE K A L A H A R I RIM
David S.G. Thomas* and Paul A. Shaw *Department of Geography, University of Sheffield, Sheffield 510 2TN, U.K. Department of Environmental Science, University of Botswana, Gaborone, Botswana
(Received f o r publication 28 February 1987) Abstract--Tke development of the Zambezi drainage system is discussed within the framework of the postGondwana tectonic evolution of southern Africa. An internal drainage system, including the proto-Upper Zambezi, has been progressively captured during the late Cainozoic by a more agressive coastwise system. Supporting geomorphic evidence is presented from the eastern Kalahari rim. Drainage alignments and gradients, and terrace sequences are discussed. Lacustrine features found on the present watershed between the Middle Zambezi and internal systems are described and explalned as remnants of the former proto-Dpper Zambezi tributary system. Their interaction with linear dune a c t i v i t y is also examined. Despite the problems of dating the drainage changes described, i t is concluded that their elucidation is important in understanding sedimentation and landform development in the eastern part of the Kalahari basin.
INTRODU CTION
a c c o r d with the model of p o s t - G o n d w a n a l a n d t e c t o n i c e vol ut i on of the s u b - c o n t i n e n t , proposed by, a m o n g s t others, De Swardt and Bennet (1974). This was f i r s t eluded to by Krenkel (1928), and r e c o g n i s e s t h e s i g n i f i c a n c e of re gi ona l v a r i a t i o n s in the r a t e s and n a t u r e of t e c t o n i c m o v e m e n t s in s out he rn and c e n t r a l Africa. The final division of Gondwanaland in t he m i d - J u r a s s i c l e f t southern A f r i c a with an u p l i f t e d m a r g i n a l hingeline, o f t e n e q u a t e d with the G r e a t E s c a r p m e n t (De Swardt and Bennet 1974), and a g e n t l y downwarped i n t e r i o r basin, which l a t e r provided t he s e t t i n g for t he deposition of t he K a l a h a r i s y s t e m (Fig. 1), including t he e x t e n s i v e K a l a h a r i sand (Thomas 1984). From this t e c t o n i c f r a m e w o r k , a d u a l i t y of d r a i n a g e evolved; r e l a t i v e l y short s t r e a m s , with s t e e p g r a d i e n t s , e x t e n d i n g from the hi nge l i ne to t he c oa s t, and an e n d o r e i c s y s t e m flowing in the opposite d i r e c t i o n from t he hi nge l i ne to t he i nt e ri or. The O ka va ngo is t h e only major river which r e m a i n s from this s y s t e m today. Ignoring t he d e t a i l s imposed by c l i m a t i c c ha nges and f u r t h e r t e c t o n i c a c t i v i t y , d r a i n a g e e v o l u t i o n since t he l a t e J u r a s s i c has t h e r e f o r e e s s e n t i a l l y been a proc e s s of p r o g r e s s i v e c a p t u r e of the i n t e r n a l s y s t e m by t h e more a g r e s s i v e e x o r e i c s y s t e m (e.g. De H e i nz e l i n 1963). Evi de nc e for this c o m e s from t he Cunene in Angola (B e e t z 1933), Fish R i ve r in N a m i b i a (Wellington 1955), O ra nge - Vaal s y s t e m (Schmitz 1968, Smit 1977) as well as from the Zambezi. The c o n c e p t of an e x t e n s i v e e a r l y pos t -G ondw a na e ndore i c d r a i n a g e s y s t e m b e c o m e s more s i g n i f i c a n t when it is re c ogni s e d t h a t such a s y s t e m is one of the few l e a s a b l e m e c h a n i s m s t h a t can a c c o u n t for the i ni t i a l deposition of t he K a l a ha r i System s e d i m e n t s . It has long been re c ogni s e d t h a t the Upper Z a m b e z i , above t he Victoria Falls, and t he Middle Z a m b e z i mus t have e vol ve d as s e p a r a t e s y s t e m s only to be linked r e l a t i v e l y r e c e n t l y . Du Toit (1927, 1933), Wellington (1955), Bond (1963) and Grove (1969) a l l proposed t h a t the Upper Z a m b e z i f o r m e r l y c ont i nue d its s o u t h e r l y course, crossing w ha t is now B ot s w a na to join the Orange s y s t e m (Lister 1979) or Limpopo (Fig. lb; Wellington 1955, Bond 1963. Bond (1963) c i t e s t h e s m a l l c a t c h m e n t , s m a l l c ha nne l and l a rge deep va l l ey of the p r e s e n t Limpopo as e v i d e n c e of it once being p a r t of a l a r g e r d r a i n a g e s y s t e m . The Middle Z a m b e z i was a s e p a r a t e river, being p a r t of t he Shire s y s t e m (Bond 1963). The s out hw a rd course of the prot o-U ppe r Z a m b e z i was di s rupt e d by g e n t l e e a r t h m o v e m e n t s in t he middle
I n t e r e s t has been growing over the past two d e c a d e s in e n v i r o n m e n t a l and c l i m a t i c c h a n g e s in c e n t r a l s o u t h e r n Africa, with much of the r e c e n t r e s e a r c h in th e region d i r e c t e d t o w a r d s e v e n t s in the l a t e Q u a t e r n a r y (e.g. Cooke 1980, Cooke and Verst a ppe n 1984), and p a l a e o h y d r o l o g i c a l d e v e l o p m e n t s in p a r t i c u l a r . There is, also, however, a l e g a c y of r e s e a r c h which has a t t e m p t e d to place c e n t r a l A f r i c a n d r a i n a g e e v o l u t i o n and e n v i r o n m e n t a l c h a n g e s within a longer t i m e p e r s p e c t i v e , p a r t l y in r e c o g n i t i o n of the i m p o r t a n c e of major t e c t o n i c e v e n t s for the shaping of s u b - c o n t i n e n t as a whole. In c e n t r a l Africa, e a r t h s c i e n t i s t s have been f a s c i n a t e d by t he e v o l u t i o n of the Z a m b e z i d r a i n a g e s y s t e m . The c o n t r a s t s b e t w e e n the upper and middle r e a c h e s of its course (e.g. Du Toit 1933) and t he d e v e l o p m e n t of the V i c t o r i a Falls e.g. (Dixey 1945) have c o n t r i b u t e d to this i n t e r e s t . In its upper reaches, for a l m o s t a third of its t o t a l length, t h e Z a m b e z i flows a c r o s s the n o r t h e a s t e r n e x t r e m i t y of the K a l a h a r i basin. This basin is occupied by K a l a h a r i Sy stem s e d i m e n t s , d o m i n a t e d at the s u r f a c e by the K a l a h a r i sand which e x t e n d s over 2.5 mil km 2 of c e n t r a l and southern Africa. The K a l a h a r i has been described as a vast basin of i n t e r n a l d r a i n a g e (Cooke 1980), which has received s e d i m e n t s since a t l e a s t the e a r l y T e r t i a r y (Dingle et al 1983). Y e t today one of t h e most n o t a b l e a s p e c t s of the s o u t h e r n half of this basin ( l a t i t u d e 18°8 to 27°S) is the v i r t u a l a b s e n c e of s u r f a c e drain age. T h e r e f o r e the e v o l u t i o n of d r a i n a g e in this region is an i m p o r t a n t f a c t o r when t h e depo s i t i on of t h e s e C a i n o z o i c s e d i m e n t s is considered. In this paper, t h e r e f o r e , we f i r s t l y e x a m i n e t he d e v e l o p m e n t of ideas, which have o f t e n been s p e c u l a t i v e , about the e v o l u t i o n of the Z a m b e z i drainage. Secondly, we p r e s e n t g e o m o r p h o l o g i c a l e v i d e n c e for l a t e C a i n o z o i c d e v e l o p m e n t s in the p a l a e o d r a i n a g e of the e a s t e r n K a l a h a r i rim, and e x a m i n e t h e i r s i g n i f i c a n c e . THE E V O L U T I O N OF THE Z A M B E Z I SYSTEM
The p r e s e n t course of the Z a m b e z i is r e l a t i v e l y young, d a t i n g from the P l i o c e n e (e.g. L i s t e r 1979) or even th e m i d - P l e i s t o c e n e (Bond 1975), and has evol ve d through the e f f e c t s of d r a i n a g e c a p t u r e and t e c t o n i c a c t i v i t y . Early t h e o r i e s of the e v o l u t i o n of the Z a m b e z i s y s t e m , put f o r w a r d by Du Toit, Dixey and Wellington, have c o n t r i b u t e d s i g n i f i c a n t l y to l a t e r views on t he su bject. The p r i n c i p l e s of t h e i r ideas a r e r e a d i l y in
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DAVID S. G. THOMAS and PAUL A. SHAW The hi s t ory of t he Middle Z a m b e z i is e qua lly i m p o r t a n t to the o v e r a l l e vol ut i on of the d r a i n a g e in this region. R e ne w e d downwarping along t he a n c i e n t G w e m b e and C h i c o a troughs c a us e d t he d e v e l o p m e n t of the proto-Middle Z a m b e z i (Bond 1963, Williams 1975, L i s t e r 1979). This river cut w e s t w a r d s along the trough line, through basal K a l a ha ri and Karoo beds (Bond 1963), with l oc a l base l e ve l s being e s t a b l i s h e d when r e s i s t a n t bands of p r e - C a m b r i a n g n e i s s e s w er e e n c o u n t e r e d below t he karoo s e d i m e n t s (Lister 1979). Wellington (1955, p. 471) proposed t h a t the Luangwa R i ve r was a t r i b u t a r y of the prot o-U ppe r Za mbe zi, flowing s o u t h - w e s t w a r d s through t he G w e mbe trough before being c a p t u r e d as p a r t of the e vol ut i on of the Middle Z a m b e z i (Fig. 1). If so, this and the e x t e n s i o n of the Middle Z a m b e z i above the C hi c oa Gorge, must h a v e post d a t e d t he C r e t a c e o u s , for a t t h a t t i m e pa r t of t he d r a i n a g e in the a r e a c e n t r e d upon a shallow
Kalah ari, probably a c o m b i n a t i o n of u p l i f t in the v i c i n i t y of the M a k g a d i k g a d i basin and downwarping in the O k a v a n g o region (Du Toit 1954, Bond 1963). This t h e o r y is given support by undisputed e v i d e n c e of t e c t o n i s m in t h e a r e a today (e.g. R e e v e s 1972, Hutchins e t al 1976, Cooke 1980). The s e v e r a n c e of the Upper Z a m b e z i course ma y have caused endorism in the s y s t e m (e.g. Du Toit 1926), prior to the link with the Middle Z a m b e z i being e s t a b l i s h e d through the Livingstone syncline. This c r e a t e s the i n t e r e s t i n g possibility of a source of w a t e r for an a n c e s t r a l , p r e - P l e i s t o c e n e , Lake P a l e a o M a k g a d i k g a d i (Fig. lc). Evidence for such an a n c i e n t lake can be seen on Landsat i m a g e r y of n o r t h e r n Botswana (Mallick e t al 1981). The linking of the two a n c e s t r a l c o m p o n e n t s of the Z a m b e z i o c c u r r e d be fore the l a t e P l e i s t o c e n e , but not e a r l i e r than the P l i o c e n e (see Dixey 1950, Bond 1975, L i s t e r 1979).
(hinge line)
b.
C.
/
Fig. 1. Present day drainage in southern Africa. Earlier patterns are represented in b) and c): b) post-dates the division of Gondwanaland; c) existed prior to the union of the proto-Middle and proto-Upper Zambezi rivers, during the Pliocene early Pleistocene.
Late Cainozoic drainage evolution in the Zambesi basin d e p r e s s i o n in c e n t a l n o r t h e r n Z i m b a b w e , in w h i c h t h e c a l c a r e o u s G o k w e Beds w e r e d e p o s i t e d ( L i s t e r 1979, S u t t o n 1979). F o l l o w i n g t h e e s t a b l i s h m e n t of t h e p r e s e n t Z a m b e z i course, further downwarping along the Gwembe trough led t o r e n e w e d i n c i s i o n ( D i x e y 1945). This w a s p a r t of a m o r e g e n e r a l r e j u v e n a t i o n of c o a s t w a r d f l o w i n g r i v e r s , c a u s e d by r e n e w e d u p l i f t a l o n g t h e c o a s t a l h i n g e l i n e s (De S w a r d t and B e n n e t 1974). C o n s e q u e n t l y , t h e P l e i s t o c e n e e r o s i o n a c h i e v e d by t h e M i d d l e Z a m b e z i has b e e n d e s c r i b e d as ' i m m e n s e ' ( D i x e y 1950, p. 11). This w a s r e s p o n s i b l e for t h e r a p i d e v o l u t i o n of t h e V i c t o r i a F a l l s and, in t h e s p a c e of a b o u t a q u a r t e r to h a l f m i l l i o n y e a r s (Bond 1975), t h e f o r m a t i o n of t h e i n c i s e d Middle Z a m b e z i G o r g e . The K a f u e R i v e r , in Z a m b i a , has a l s o e v o l v e d an unusual course, taking an abrupt eastward turn at H e z h i t e z h i . This c a n a l s o be e x p l a i n e d by c a p t u r e , r e l a t e d to t h e e v o l u t i o n of t h e Z a m b e z i ( W i l l i a m s 1975). Bond (1963) n o t e d how t h e e a r l y K a f u e f l o w e d s o u t h - w e s t w a r d s to join t h e U p p e r Z a m b e z i . The c o u r s e w a s a l o n g a gap, 2 km w i d e and a t a n a l t i t u d e of a b o u t 1070 m.a.s.1, w h i c h now c r o s s e s t h e p r e s e n t K a f u e - Z a m b e z i w a t e r s h e d , b e t w e e n H e z h i t e z h i and M a m b o v a . This is now a n a l l u v i a l f l a t , p o o r l y d r a i n e d by t h e N a n z h i l a and S i m a t e n g a s t r e a m s , h u t t h e a l l u v i u m is u n d e r l a i n by c o u r s e c u r r e n t - b e d d e d f l u v i a l g r a v e l s ( C l a r k 1950). The c a p t u r e of t h i s r i v e r c a n n o t s i m p l y be e x p l a i n e d in t h e s a m e m a n n e r as t h a t of t h e U p p e r Z a m b e z i . The p r e s e n t e a s t e r l y f l o w i n g s e c t i o n of t h e K a f u e , b e l o w H e z h i t e z h i , has u n d e r g o n e d r a i n a g e r e v e r s a l ( D i x e y 1955), o n c e b e i n g p a r t of t h e s y s t e m j o i n i n g t h e U p p e r Z a m b e z i . In t h i s r e s p e c t t h e p r o c e s s m u s t h a v e b e e n s i m i l a r to, but p o s t - d a t i n g , t h e c a p t u r e of t h e L u a n g w a . W i l l i a m s (1975) n o t e s how, on t h e d i v i d e b e t w e e n t h e Middle Z a m b e z i and t h e K a f u e , t h e s o u t h f l o w i n g t r i b u t a r i e s of t h e f o r m e r a r e m o r e a g g r e s s i v e t h a n t h o s e f l o w i n g n o r t h to t h e l a t t e r . I t therefore seems likely that the Kafue system was c a p t u r e d by a M i d d l e Z a m b e z i t r i b u t a r y . This v i e w is c o m p l e t e l y in k e e p i n g w i t h t h e r a p i d P l e i s t o c e n e d e v e l o p m e n t of t h e Middle Z a m b e z i e n v i s a g e d by D i x e y (1950). A l t h o u g h s p e c u l a t i o n h a s p l a y e d a m a j o r role in t h e d e v e l o p m e n t of t h e i d e a s d e s c r i b e d a b o v e , c e n t r a l
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A f r i c a n i c h t h y o l o g i c a l s t u d i e s s u p p o r t t h e t h e o r y of s e p a r a t e e v o l u t i o n of t h e p r o t o U p p e r and M i d d l e Z a m b e z i s y s t e m s . Fish s p e c i e s in t h e U p p e r Z a m b e z i and K a f u e f o r m p a r t of an ' O k a v a n g i a n ' f a u n a , d i s t i n c t f r o m t h a t of t h e Middle Z a m b e z i (e.g. J a c k s o n 1961). Bond (1963) a n d Jubb (1964) s u g g e s t e d t h a t t h e V i c t o r i a F a l l s w e r e an o b s t a c l e t o i n t e g r a t i o n of t h e t w o fish p o p u l a t i o n s , but i t has s i n c e b e e n d e m o n s t a t e d t h a t t h e b a r r i e r t o i n t e g r a t i o n was e c o l o g i c a l , n o t p h y s i c a l . F o l l o w i n g t h e c o n s t r u c t i o n of t h e K a r i b a D a m , s l o w water habitats, prevoiusly limited to the Upper Zambezi, w e r e a v a i l a b l e in t h e m i d d l e r e a c h e s of t h e r i v e r . This f a c i l i t a t e d a s u c c e s s f u l i n v a s i o n b e l o w t h e V i c t o r i a F a l l s by A l e s t e s l a t e r a l l y , p r e v i o u s l y c o n f i n e d t o t h e U p p e r Z a m b e z i and t h e K a f u e (Balon 1971, 1974). DRAINAGE DEVELOPMENT EASTERN KALAHARI
ON THE RIM
I m p o r t a n t e v i d e n c e of d r a i n a g e d e v e l o p m e n t in t h e U p p e r and Middle Z a m b e z i c a t c h m e n t s occurs in n o r t h w e s t e r n Z i m b a b w e and n o r t h e a s t e r n B o t s w a n a . D r a i n a g e in t h i s a r e a t o d a y is d i v i d e d b e t w e e n t w o v e r y d i f f e r e n t s y s t e m s : a low d e n s i t y s y s t e m of emphemeral streams draining west towards the middle K a l a h a r i r e g i o n , and m o r e n u m e r o u s and a g r e s s i v e t r i b u t a r i e s of t h e Middle Z a m b e z i (Fig. 2). The w a t e r s h e d between these systems, coinciding approximately with t h e b o r d e r b e t w e e n B o t s w a n a and Z i m b a b w e , r e f l e c t s t h e e x t e n t of h e a d w a r d e r o s i o n by r i g h t - b a n k t r i b u t a r i e s of t h e Middle Z a m b e z i i n t o t h e e a s t e r n f l a n k of t h e K a l a h a r i basin. This is r e l a t e d t o t h e r e n e w e d i n c i s i o n of t h e Middle Z a m b e z i a l o n g t h e G w e m b e T r o u g h (e.g. D i x e y 1945, 1950) in t h e U p p e r P l e i s t o c e n e , and h a s had t h e e f f e c t of c a u s i n g t h e b a s i n r i m t o h a v e t h e a p p e a r a n c e of a p l a t e a u e d g e , a t 1050 - 1100 m.a.s.1. D r a i n a g e p a t t e r n s of t h e Middle Z a m b e z i tributary system E a s t of K a l a h a r i Middle Z a m b e z i w a t e r s h e d , in n o r t h w e s t e r n Z i m b a b w e , d i s p a r i t i e s b e t w e e n d r a i n a g e p a t t e r n s t o t h e e a s t and t o t h e w e s t of t h e G w a y i r i v e r h a v e b e e n d i s c u s s e d by T h o m a s (1983/4). The Matetsi and Deka systems have incised westwards i n t o t h e r i m of t h e K a l a h a r i basin, e x p o s i n g u n d e r l y i n g
~au eposJts -ession a Flats
Fig. 2. Drainage in northeast Botswana and northwest Zimbabwe.
614
DAVID S. G. THOMAS and PAUL A. SHAW Evi de nc e to support this t h e o r y c o m e s from t e r r a c e s e q u e n c e s in Sikumi vlei and on t he N g w e z u m b a River. Sikumi vlei is a short, 15 km long w e s t bank t r i b u t a r y of t he Gwayi, its floor 40m below t he K a l a h a r i p l a t e a u and s i t u a t e d just to t he e a s t of t he boundary of Hwange N a t i o n a l P a rk (Fig. 3). It is t y p i c a l of many Z i m b a b w e a n dambos (Whitlow 1984), having a r e l a t i v e l y broad, low g r a d i e n t floor, c o n t a i n i n g r e l a t i v e l y t h i c k p e a t de pos i t s and poor channel development with l i t t l e l a t e r a l flow. Three k i l o m e t r e s from t he c o n f l u e n c e with the Gwayi, a t e r r a c e sequence is exposed on th e north side of t he valley, which a t this point is cu t into t he Karoo s a nds t one t h a t unde rl i e s t he K a l a h a r i sand. The s e q u e n c e is shown in Fig. 3 and de s c ri be d in t a b l e 1. It is unknown w h e t h e r t he basal K a l a h a r i beds, r e p r e s e n t e d by c o n g l o m e r a t e s and s i l i c e ous sandstone, were de pos i t e d in a va l l e y with the s a m e a l i g n m e n t as t he one p r e s e n t today, or w h e t h e r t he y a r e more e x t e n s i v e , with the p r e s e n t v a l l e y c ut down into them. As Karoo s a nds t one is exposed higher up t he v a l l e y side, above t he out c rop of basal K a l a h a r i Beds, it is s u g g e s t e d t h e i r depostion was within a p r e - e x i s t i n g valley. This is a na l oge ous with t he s i t u a t i o n in the Middle Z a m b e z i va l l e y de s c ri be d by Dixey (1950). The s u r f a c e of much of the t e r r a c e s e q u e n c e is m a n t l e d by a poorly s ort e d colluvium. This is ma i nl y composed of a n g u l a r s i l c r e t e pi e c e s in a m a t r i x of re w ork ed K a l a h a r i sand, de ri ve d from the p l a t e a u s urfa c e . The most s i g n i f i c a n t units of t he s e que nc e , however, a re t he upper and lower g r a v e l s and t he w h i t e and pink alluvia, which a re all fl uvi a l deposits. It is beyond the bounds of reason to a c c o u n t for t h e s e de posits in t e r m s of fl uvi a l a c t i v i t y r e l a t e d to the p r e s e n t vlei. F u r t h e r m o r e , t h e r e is not a s u i t a b l e source for t h e g r a v e l s within its K a l a h a r i p l a t e a u c a t c h m e n t . The second s e c t i o n of fluvial g r a v e l s c o m e s from t he N g w e z u m b a in north e a s t e r n Botswana. This e p h e m e r a l sand ri ve r drains w e s t w a r d s from one of the a l l u v i a l a r e a s on t he Zi mba bw e - Botswana to t he Mababe depression (Shaw 1985). Although t he l ack of d e t a i l e d he i ght d a t a re c l ude s g r e a t e r a c c u r a c y , this s t r e a m falls a p p r o x i m a t e l y 100m over t he 70 km from s ourc e to t he Mababe depression, giving a mean g r a d i e n t of 1.43m km - I . At N g w e z u m b a dam in Chobe N a t i o n a l Park, basal K a l a ha ri beds a re exposed on the southern side of the p r e s e n t channel. On the north side, a s e c t i o n displays 3.5m of loosely c e m e n t e d bedded g r a v e l s and alluvium
Karoo s e d i m e n t s and b a s a l t s with the local base l e ve l c o n t r o l l e d by the height of the Middle Z a m b e z i . These riv ers have very s t e e p g r a d i e n t s , the M a t e t s i having a mean g r a d i e n t of 4.34m km -1 over the 122 km from the w a t e r s h e d to the Z a m b e z i , and the Deka a g r a d i e n t of 4. 69m km -1. Incision below the K a l a h a r i p l a t e a u is up to a m a x i m u m depth of 500m, and has c r e a t e d a s e r i e s of K a l a h a r i o u t l i e r s to the north and e a s t of th e p a l t e a u rim. The Gwayi, which joins the Z a m b e z i a t Devil's Gorge has, on the other hand, cut s o u t h - e a s t w a r d s into t h e p l a t e a u . This river is n o t a b l e for th e l a c k of s i g n i f i c a n t l e f t bank t r i b u t a r i e s , w h e r e a s it is joined from t h e e a s t by r e m a r k a b l e s t r a i g h t s u b - p a r a l l e l s t r e a m s . The l a r g e s t , the Shangani, has incised 130m through the K a l a h a r i s e d i m e n t s near its c o n f l u e n c e with t h e Gwayi. The others, however, display only l i m i t e d d o w n c u t t i n g , with the Tshongokwe, Kana and Mazola having r e l a t i v e l y low g r a d i e n t s (1.94m km1 for the Kana) and courses mainly flowing on K a l a h a r i sand. F u r t h e r north, the Sengwa, Lutope and Mbumbusi, which are not t r i b u t a r y to the Gwayi, also display s i g n i f i c a n t s t r a i g h t w e s t w a r d c o m p o n e n t s in t h e i r courses. Thomas (1983/4) proposed t h a t the s t r a i g h t , subp a r a l l e l drainage to the e a s t of the Gwayi had evol ve d upon a l i n e a r dune l a n d s c a p e on the K a l a h a r i sand. We can now add t h a t t h e s e rivers were f o r m e r l y pa rt of the t r i b u t a r y s y s t e m of the p r o t o - U p p e r Zambe z i , which is in a c c o r d with the h i s t o r y of regional d r a i n a g e e v o l u t i o n put forward by L i s t e r (1979). Headward erosion by the Gwayi, in c o n j u n c t i o n with the evoluti on of the Middle Zambezi drainage, has contributed s i g n i g i c a n t l y to the p r o g r e s s i v e c a p t u r e of the e a s t west s y s t e m , f o r m e r l y t r i b u t a r y to the proto -U ppe r Zamb ezL This also a c c o u n t s for t h e lack of s i g n i f i c a n t Gwayi w e s t bank t r i b u t a r i e s , for south of the D e ka s y s t e m , the w a t e r s h e d of the Middle Z a m b e z i n e t w o r k is p a r a l l e l , and a few k i l o m e t r e s w e s t of the G w a yi valley.
Terrace sequences.
It may t h e r e f o r e be h y p o t h e s i s e d t h a t the s t r e a m s to the w e s t of the w a t e r s h e d , for e x a m p l e t he N g w e z u m b a , draining into the Mabahe depression, and th e Nata, which e n t e r s Sue Pan, a r e r e m n a n t s of the l e f t bank t r i b u t a r i e s of the proto-Upper Zambe z i .
i
SIKUMI ~ t
~
? Wm 29
Kalahar,
Karoo Sandslone Kb
Vie deposts ]
Fig. 3. The terrace sequence at Sikumi Vlei. Inset: location of Sikumi Vlei.
Late Cainozoic drainage evolution in the Zambesi basin
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MAX, T H I C K N E S S OF UNIT
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Kb
B a s a l K a l a h a r l bed~. S i l i c e o u s s a n d s t o n e a n d ferrtI~inous c e m e n t e d conglomerate
im
Ks
Kalahari
--
G 1
Upper gravel terrace. Unconsolidated, p r i m a r i l y q u a r t z p e b b l e s , u p to 5 c m in d i a m e t e r
im
G 2
Lower gravel terrace. Largely unconsolidated, but some cementing at base. M i x e d l l t h o l o g i e s inc. q u a r t z , j a s p e r , agate, b a s a l t , chert. R o u n d e d , max. d i a m e t e r 6cm, av. 2cm.
l. S m
A 1
Pale alluvium,
im
A 2
Pink alluvium. 70% sand
C
Colluvium. Reworked Kalaharl sand and angular silcrete fragments. Inc. 8 r a v e l s d o w n s l o p e f r o m G i.
sand.
615
On plateau
4% clay,
surface
2 6 % silt,
am
0.4m
Table 1. Description of Sikumi Vlei terrace sediments (see figure 3).
(Fig. 4) which a r e c l e a r l y d i s t i n c t from t h e K a l a h a r i c o n g l o m e r a t e . The g r a v e l s a r e l a r g e l y composed of well rounded g r e e n and w h i t e s i l c r e t e s , jasper, c h e r t and a g a t e , and more a n g u l a r pebbles of K a l a h a r i c o n g l o m e r a t e . Some of the c l a s t s in the g r a v e l s e c t i o n a r e up to 0.3m in d i a m e t e r . The s i l c r e t e and c o n g l o m e r a t e pebbles were probably d e r i v e d from sources within the K a l a h a r i p l a t e a u , and both are exposed in s i t u in s e c t i o n s in t h e p r e s e n t N g w e z u m b a c h a n n e l A g a t e is found in a s s o c i a t i o n with Karoo basalt, of which t h e r e are s m a l l e x p o s u r e s
Pedogenic calcrele, max 1 5 m ihick
-O-'-a....%'1 o o % o %o ,~0
l ol
1- t 5 m
ooooo oq9o |/ co o 0
0
°° * 0
0
Gravels, poorly sorted, in fine m a t r i x
0.25m
Alluv,um
03Ore
Mixed gravels in fine matr,x
0 70 m
Mainly fine alluvium, some c~asts
O.60m
Graded gravels, fineing upwards Matrix deficmnt, some coarse sand
in t he K a z u m a depression. The d e g r e e of rounding of t he g r a v e l s s u g g e s t s c o n s i d e r a b l e fl uvi a l t r a n s p o r t . Although t he N g w e z u m b a was an a c t i v e r i v e r during l a t e Q u a t e r n a r y high l a k e s t a g e s in t he Mababe depression (Shaw 1985), only fine d e l t a i c s e d i m e n t s w e re deposited. We t h e r e f o r e s u g g e s t t h a t ~he g r a v e l s a r e bedload de pos i t s a s s o c i a t e d with high flow a t a t i m e when t he N g w e z u m b a had a more e x t e n s i v e catchment. O t he r g r a v e l e xpos ure s oc c ur as thin s u r f a c e s pre a d s f u r t h e r down t h e course of t he N g w e z u m b a , and on t he K a l a h a r i p l a t e a u edge ne a r P a n d a m a t e n g a . It is not possible, however, to distinguish t h e s e from in s i t u w e a t h e r e d basal K a l a h a r i c o n g l o m e r a t e . We c ons i de r though t h a t t h e Sikumi vlei t e r r a c e s and N g w e z u m b a dam g r a v e l s a re e v i d e n c e for t he e x i s t e n c e of more e x t e n s i v e d r a i n a g e into north B ot s w a na from an e a s t e r l y d i r e c t i o n , p r e - d a t i n g incision of, and c a p t u r e by, the p r e s e n t Middle Z a m b e z i t r i b u t a r i e s . Our hypot he s i s is supported by the a s y m m e t r y of t he Gwayi t r i b u t a r y ne t w ork, and the s t r o n g e a s t - w e s t d i r e c t i o n a l t r e n d of the ri ve rs to t he e a s t of the Gwayi (Thomas 1983/4), and is in a g r e e m e n t with t he published t h e o r i s i n g c o n c e r n i n g t he e v o l u t i o n of the Z a m b e z i d r a i n a g e s y s t e m in the l a t e C a i noz oi c . S U R F A C E SEDIMENTS A N D D R A I N A G E C H A N G E
0
O15m Fine ravels Little matr=w UNCONFORMll~ Karoo basalt
Fig. 4. Gravel sequence at Ngwezumba dam.
Mallick e t al (1981) i d e n t i f i e d and mapped from La nds a t i m a g e r y 'possible l a c u s t r i n e d e v e l o p m e n t s ' (p. 28) in e a s t e r n Botswana. These a re s i t u a t e d on t he w a t e r s h e d b e t w e e n t he i n t e r n a l and Middle Z a m b e z i d r a i n a g e s y s t e m s , and a re h i t h e r t o u n i n v e s t i g a t e d . They a re s i g n i f i c a n t in t he c o n t e x t of this study b e c a u s e t h e i r d e v e l o p m e n t in t h a t l o c a t i o n can not be e x p l a i n e d in t e r m s of the p r e s e n t d r a i n a g e of the region. The l a r g e s t of these, known as t he K a z u m a Depression w he re it e x t e n d s into Zimbabwe, c o v e r s a bout 1000 km (Mallick e t al 1981), w hi l s t 50 km sout17 a second, near P a n d a m e t e n g a , e x t e n d s over about
616
DAVID S. G. THOMAS and PAUL A. SHAW
800 km 2 (Fig. 2). O t h e r s m a l l e r , l a c u s t r i n e d e v e l o p m e n t s o ccur on t h e n o r t h e r n edge of the sand p l a t e a u in Hwange N a t i o n a l Park, Zimbabwe. In all c a s e s t he m a r g i n s of t h e depressions a r e being eroded, as pa rt of the r e t r e a t of the K a l a h a r i rim e f f e c t e d by the Middle Z a m b e z i s y s t e m . In places, Karoo b a s a l t s and basal K a l a h a r i Beds a r e exposed on the depression floors. It is not surprising, t h e r e f o r e , t h a t soils a r e in part composed of w e a t h e r e d basalt, and t h e y have been d e s c r i b e d as such in a r e c e n t FAO r e p o r t (FAO 1986), and an e a r l i e r one produced by th e Rhodesian Soil Survey (Sweet 1970). However, over l a r g e a r e a s t h e y also c o n t a i n a s i g n i f i c a n t a l l u v i a l c o m p o n e n t , d e r i v e d a t l e a s t in part from r e w o r k e d K a l a h a r i sand (Thomas 1987). The floors of t he depressions a r e up to 20m below the h e i g h t of t he surrounding sand p l a t e a u . Because of t h e i r w a t e r s h e d position, th e origin of t h e s e l a c u s t r i n e f e a t u r e s can only be e x p l a i n e d as part of the f o r m e r w e s t w a r d draining p r o t o - U p p e r Z a m b e z i t r i b u t a r y s y s t e m . Although t h e y have not been i n v e s t i g a t e d in t h e field, s i m i l a r l a c u s t r i n e d e v e l o p m e n t s can be i d e n t i f i e d from L an dsat i m a g e r y of Western Zambia. These f e a t u r e s occur within the K a l a h a r i sand too, and a r e found in a s s o c i a t i o n with e a s t bank t r i b u t a r i e s of t he Z a m b e z i . For e x a m p l e , the Bovu River, which e n t e r s the Z a m b e z i from due north about 10 km e a s t of
Kazangula, c ros s e s a f l a t a l l u v i a l - f l o o r e d depression of some 100 km -2, before a b r u p t l y d e v i a t i n g from its s o u t h w e s t w a r d o r i e n t a t i o n to decend t he flank of downwarped Z a m b e z i trough. The e v o l u t i o n of many f e a t u r e s of t he h y d r o l o g i c a l s y s t e m of t he Middle K a l a h a r i has been c o n t r o l l e d by f a u l t i n g and g e n t l e t e c t o n i c a c t i v i t y . The 1978 P h o t o g e o l o g i c map of Botswana i n d i c a t e s t h a t p a r t s of t he s out he rn m a r g i n s of t he K a z uma and P a n d a m e t e n g a l a ke f l a t s a r e f a u l t c o n t r o l l e d . However, t he 1984 Botswana g e o l o g i c a l map shows t h a t in t h e v i c i n i t y of t he f l a t s t h e r e a r e r e l a t i v e l y few f a u l t s in the underlying ba s a l t , and t h e r e f o r e t e c t o n i s i s m alone can not a c c o u n t for t h e i r l o c a t i o n . From La nds at i m a g e r y , it can be seen t h a t t h e i r ma rgi ns a re p a r t i a l l y defined by l i n e a r dunes, p a r t of t he e x t e n s i v e fossilised e rg which c o n t i n u e s e a s t w a r d s on t he sand p l a t e a u into Zi mba bw e (Thomas 1984). The r e l a t i o n s h i p b e t w e e n dunes and l a c u s t r i n e f e a t u r e s is in f a c t complex. The e dge s of t he bounding dunes have been t r i m m e d by w a t e r a c t i o n , and process is probably c o n t i n u i n g to a l i m i t e d e x t e n t today, for during ye a rs of good local rains the depressions may be t e m p o r a r i l y inundated by shallow w a t e r bodies. However, s t a b i l i s e d dunes also e x t e n d down onto t h e l a ke fl a t s , and on the s out he rn m a r g i n of the P a n d a m e t e n g a depression, a w a t e r - t r i m m e d dune s t u m p
Fig. 5. Aerial photograph showing r e l i c t linear dune stump overlying a former channel of the Nungu River, eastern Botswana.
Late Cainozoic drainage evolution in the Zambesi basin be seen o v e r l y i n g a n old abandoned chann e l of the Nungu R i v e r (Fig. 5). There have c l e a r l y been a l t e r n a t i o n s b e t w e e n periods of a e o l i a n a c t i v i t y and periods of p e r m a n e n t or s e m i - p e r m a n e n t w a t e r in the depressio ns since the P l i o c e n e , but we a r e c u r r e n t l y unable to shed any light on the d e t a i l s of chronologies. can
DISCUSSION
D r a i n a g e evolution in c e n t r a l A f r i c a has e voke d i n t e r e s t since the t i m e of L i v i n g s t o n e (1858). During the ensu£ng period t h e r e has been c o n s i d e r a b l e s p e c u l a t i o n , but a g e n e r a l concensus of opinion, t h a t the p r e s e n t Z a m b e z i course was e s t a b l i s h e d no e a r l i e r t h a t the Pliocene, and perhaps as l a t e as the mi ddl e P l i e s t o c e n e . The p r e s e n t s y s t e m of t r i b u t a r i e s has been e v o l v i n g since then and r e p r e s e n t s a p roc e s s of s u p e r i m p o s i t i o n on and c a p t u r e of the p r e - Z a m b e z i d r a i n a g e t h a t had evolved since the J u r a s s i c . This p ro cess has been a f f e c t e d and in p a r t d e t e r m i n e d by t e c t o n i c m o v e m e n t s which have o p e r a t e d in favour of e n h a n c i n g the power of t h e Middle Z a m b e z i and its t r i b u t a r i e s , r e s u l t i n g in the c a p t u r e of the protoUpper Zamb ezi, and its t r i b u t a r y s y s t e m . Thus Wiliams (1975) d e s c r i b e s the c a p t u r e of the Kafue by a l e f t bank Middle Z a m b e z i t r i b u t a r y , and we assign t he c a p t u r e of the w e s t w a r d flowing rivers in n o r t h e r n Z i m b a b w e to s t e e p g r a d i e n t right bank t r i b u t a r i e s of the Middle Zambezi, including the G w a y i , D e k a and M a t e t s i . The e f f e c t has been to d e p r i v e the few w e s t w a r d flowing s t r e a m s in n o r t h e r n B o t s w a n a of t h e i r f o r m e r h e a d w a t e r s , which are now e a s t - b a n k t r i b u t a r i e s of the Gwayi. Both t h e s e s y s t e m s r e t a i n the r e l a t i v e l y low g r a d i e n t s i n h e r i t e d from t h e i r a n c e s t r a l protoUpper Z a m b e z i t r i b u t a r i e s . Their c a p t u r e has also had the e f f e c t of i s o l a t i n g the l a c u s t r i n e d e v e l o p m e n t s south of th e Z ambezi, on the Botswana - Zimbabwe border, from the s t r e a m s which previously drained t o w a r d s them. As well as d e t e r m i n i n g d r a i n a g e p a t t e r n s , this pro cess is c u r r e n t l y causing the w e s t w a r d r e t r e a t of th e e a s t e r n rim of the K a l a h a r i p l a t e a u , exh umi nu underly ing Karoo b a s a l t s previously r e v e a l e d in t he Hwange a r e a . In broader t e r m s , the d r a i n a g e c h a n g e s are p a r t of the r e p l a c e m e n t of the c e n t r a l s o u t h e r n A f r i c a n e n d o r e i c s y s t e m by a more e x t e n s i v e e x o r e i c one (c.f. De Swardt and Bennet 1974), r e f l e c t e d in the g r a d u a l w e s t w a r d m o v e m e n t of the w a t e r s h e d dividing the right bank t r i b u t a r i e s of the middle Z a m b e z i and to day's i n t e r n a l d r a i n a g e . D a t i n g the major c h a n g e s d e s c r i b e d above, and t h e r e f o r e the Sikumi t e r r a c e s and the N g w e z u m b a grav els, in a n y t h i n g o t h e r than g e n e r a l t e r m s is d i f f i c u l t . From the s t u d i e s of the t e c t o n i c e v o l u t i o n of t he s u b c o n t i n e n t , t h e y obviously o c c u r r e d no e a r l i e r t ha n t h e l a t e Pliocene, and before the l a t e Q u a t e r n a r y e n v i r o n m e n t a l changes i d e n t i f i e d from p a l a e o h y d r o l o g i c a l s t u d i e s in the Middle K a l a h a r i (Shaw and Cooke 1986). During the period in which the e a s t - west d r a i n a g e flowed into n o r t h e r n Botswana and the p r o t o - U p p e r Z a m b e z i , it is probable t h a t major c h a n g e s in flow r e g i m e s o ccurred. This is i n d i c a t e d by t h e v a r i a t i o n s in the size of m a t e r i a l f o r m i n g the d i f f e r e n t t e r r a c e s within th e Sikumi sequence, and the f a c t t h a t periods of incision o c c u r r e d to c r e a t e the t e r r a c e s . P r e s u m a b l y , t e c t o n i c m o v e m e n t s c a u s i n g c h a n g e s in base l e v e l and c l i m a t i c c h a n g e s were responsible. In the a b s e n c e of f u r t h e r c o r r o b o r a t i n g e v i d e n c e , however, it is c u r r e n t l y not possible to m a k e f u r t h e r d e d u c t i o n s about r i v e r r e g i m e s . R e l a t i o n s h i p s b e t w e e n d r a i n a g e d e v e l o p m e n t and the K a l a h a r i beds are p o t e n t i a l l y of i n t e r e s t , but the age of t h e s e s e d i m e n t s l a r g e l y r e m a i n s a m a t t e r of s p e c u l a t i o n (e.g. Dingle e t al 1983). That rive rs to the e a s t of the Gwayi were d u n e - a l i g n e d , and l a k e depressions now on the K a l a h a r i rim were a t l e a s t p a r t i a l l y d e l i m i t e d by the s a m e c o n t r o l may be more s i g n i f i c a n t . The p r o b l e m s of d i r e c t l y d a t i n g the dune s y s t e m s (e.g. Thomas 1984, Shaw and Cooke 1986) and the o c c u r r e n c e of m u l t i p l e periods of a r i d i t y during
617
the P l e i s t o c e n e in t he K a l a h a r i basin do not, however, a s s i s t in o b j e c t i v e a s s e s s m e n t s of t he a ge of t h e s e f e a t u r e s or of t he d r a i n a g e changes. N e v e r t h e l e s s , in s pi t e of t h e s e problems, the i m p o r t a n c e of t he d r a i n a g e c ha nge s e v i d e n c e d by the l a c u s t r i n e d e v e l o p m e n t s and the t e r r a c e s e q u e n c e s in n o r t h w e s t e r n B ot s w a na and n o r t h e a s t e r n Zi mba bw e can not be over looked. Not only do t he y r e f l e c t major a s p e c t s of the C a i n o z o i c t e c t o n i c and hydrologic e v o l u t i o n of t he s u b - c o n t i n e n t , but it is only through t he i r r e c o g n i t i o n and i n t e r p r e t a t i o n t h a t a b e t t e r u n d e r s t a n d i n g of c e r t a i n a s p e c t s of re gi ona l l a n d f o r m s and s e d i m e n t s can begin.
Acknowledgeg~nts-DSGT would like to thank The Royal Society, London, for financial assistance which enabled fieldwork contributing to this research to be carried out. PAS would similarly like to thank the Research and Publications Committee, University of Botswana.
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De Heinzelin, J. 1963. A tentative Paleogeographic map of Neogene Africa. In: Howell, F.C. and Bouliere, F. (eds): African ecology and human evolution Viking Fund Publications in Anthropology. Aldine, Chicago. 648-654. De Swardt, A.M.J. and Bennet, G. 1974. Structural and physiographic evolution of Natal since the Late Jurassic. Trans. Geol. Soc. S. Air. 77: 309-322. Dingle, R.V., Siesser, W.G. and Newton, A.R. 1983. Nesozoic and T e r t i a r y geology of southern Africa. Bal kema, Rotterdam. Dixey, F. 1945. The geomorphology of Northern Rhodesia. Trans, Geol. Soc. 5. Af. 48: 9-45. Dixey, F. 1950. Part 1. Geology. In: Clark, J.D. ~ stone age cuIturns of Northern Rhodesia. S. Af. Archeol. Soc. 929. Du Toit, A.L. 1926. The Kalahari and some of i t s S. Af. ,]. Sci. 24: ~ - I 0 1 . Du Toit,
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evolution of South Africa. S. Af. Geogrl. J. 16: 3-20.
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Du Toit, A.L. 1954. The geology of South Africa 3rd. edn. Oliver and Boyd, Edinburgh. 463p. FAO 1986. Report on the reconnaissance soil survey: Kasane sheet. FAO / UR devei, project / Govt. Botswana BOT/80/O03. Grove, A.T. 1969. Landforms and climatic change in the Kalahari and Ngamiland. Geog1. J. 135: 191-212. Hutchins, D.G., Hutton, S.M. and Jones, C.R. 1976. The geology of the Okavango Delta. ProcL~=dings of the s~posium on the Okavan9o Delta and i t s future use. Botswana Society, Gaborone. 15-19. Jackson, P.B.N. 1961. ~ fishes of Northern RIKxJesia. A check l i s t of indigenous species. Government Printer, Lusaka. 140p. Jubb, R.A. 1964. S~e fishes of the Victoria Falls region. In: Fagan, B.N. (nd) A Hand~k to the Victoria Falls, the Hatoka G~)rge and part of the Upper Zamhezi River. Comm. Pres. Nat. Hist. Mon. Northern Rhodesia. 129-140. Krenkel, E. 1928. Geologie Afrikas Vo]. 2. Leipzig, Germany. 677p. Lister, L . A . 1979. The geomorphic evolution Rhodesia. Trans. Geol. SOc. S. Af. 82: 363-3/0.
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Livingstone, D. 1858. Hissionary travels and researches in South Africa. New York. Mallick, D.I.J., Habgood, F. and Skinner, A.C. 1981. A geological interpretation of Landsat imagery and air photography of Botswana. Overseas ~ o I . #iner. Res. 56: 36p. Reeves, C.V. 1972. Rifting in the Kalahari? Nature 237: 9596. Williams,
G.J. 1975. Geomorphology of the Southern Province
Editors Note: Due to the changeover to new format, it has proved necessary to proceed to publication without receiving the authors' final corrections. The Editor and publishers have made every effort to ensure that the text is accurately reproduced here.
Schmitz, G. 1968. The plateau area of South Africa. Sc~itz, South Africa. ]5p. Shaw, P.A. 1985. Late Quaternary landforms and environmental change in northwest Botswana: the evidence of Lake Ngami and the Mababe Depression. Trams Inst. Br. Geogr. N.S. 10: 333-346. Shaw, P.A. and Cooke, H.J. 1986. Geomorphic evidence for the late Quaternary palaeoclimates of the middle Kalahari of northern Botswana. Cat~a 13: 349-359. Smit, P.J. 1977. Die geohidrnlogie in die ~ranggebied yam die Nelopo r i v i e r in die noordelike KalaHari. Unpubl. Ph, D. Thesis, University of the Orange Free State, South Africa. Sutton, E.R. 1979. The geology of the Mafungabusi area. Bull. Rhed. Geol. Surv. 81: 318p. Sweet, C.P. 1970. Report on the soils of the N.W. section of Wankie National Park. Dept. Res. Special Services, Branch of Chem. and Soil Sci. Report CS/3/2/32. Salisbury, Rhodesia. 16p. Thomas, D.S.G. 1983/4. Geomorphic evolution and river channel orientation in north west Zimbabwe. Prec. ~ | . Ass. Ziab. 15: 12-22. Thomas, D.S.G. 1984. Ancient ergs of the former arid zones of Zimbabwe, Zambia and Angola. Trans. Inst. mr. ~ r . N.S. g: 15-88. Thomas, D.S.G. 1987. Discrimination of depositional environments, using sedimentary characteristics, in the Rega KaIahari, central southern Africa. In: Frostick, L.E. and Reid, I. (eds): Desert sedilents, ancient and lodere geol. Soc. Lond. Spec. Pub1. Wellington, J.H. 1955. Southern Africa, a geoxjraphicaI study. VoI~e 1 Physical Geography. Cambridge. 528p.
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L A T E C A I N O Z O I C D R A I N A G E E V O L U T I O N IN THE Z A M B E Z I BASIN : G E O M O R P H O L O G I C A L E V I D E N C E F R O M THE K A L A H A R I RIM
David S.G. Thomas* and Paul A. Shaw *Department of Geography, University of Sheffield, Sheffield 510 2TN, U.K. Department of Environmental Science, University of Botswana, Gaborone, Botswana
(Received f o r publication 28 February 1987) Abstract--Tke development of the Zambezi drainage system is discussed within the framework of the postGondwana tectonic evolution of southern Africa. An internal drainage system, including the proto-Upper Zambezi, has been progressively captured during the late Cainozoic by a more agressive coastwise system. Supporting geomorphic evidence is presented from the eastern Kalahari rim. Drainage alignments and gradients, and terrace sequences are discussed. Lacustrine features found on the present watershed between the Middle Zambezi and internal systems are described and explalned as remnants of the former proto-Dpper Zambezi tributary system. Their interaction with linear dune a c t i v i t y is also examined. Despite the problems of dating the drainage changes described, i t is concluded that their elucidation is important in understanding sedimentation and landform development in the eastern part of the Kalahari basin.
INTRODU CTION
a c c o r d with the model of p o s t - G o n d w a n a l a n d t e c t o n i c e vol ut i on of the s u b - c o n t i n e n t , proposed by, a m o n g s t others, De Swardt and Bennet (1974). This was f i r s t eluded to by Krenkel (1928), and r e c o g n i s e s t h e s i g n i f i c a n c e of re gi ona l v a r i a t i o n s in the r a t e s and n a t u r e of t e c t o n i c m o v e m e n t s in s out he rn and c e n t r a l Africa. The final division of Gondwanaland in t he m i d - J u r a s s i c l e f t southern A f r i c a with an u p l i f t e d m a r g i n a l hingeline, o f t e n e q u a t e d with the G r e a t E s c a r p m e n t (De Swardt and Bennet 1974), and a g e n t l y downwarped i n t e r i o r basin, which l a t e r provided t he s e t t i n g for t he deposition of t he K a l a h a r i s y s t e m (Fig. 1), including t he e x t e n s i v e K a l a h a r i sand (Thomas 1984). From this t e c t o n i c f r a m e w o r k , a d u a l i t y of d r a i n a g e evolved; r e l a t i v e l y short s t r e a m s , with s t e e p g r a d i e n t s , e x t e n d i n g from the hi nge l i ne to t he c oa s t, and an e n d o r e i c s y s t e m flowing in the opposite d i r e c t i o n from t he hi nge l i ne to t he i nt e ri or. The O ka va ngo is t h e only major river which r e m a i n s from this s y s t e m today. Ignoring t he d e t a i l s imposed by c l i m a t i c c ha nges and f u r t h e r t e c t o n i c a c t i v i t y , d r a i n a g e e v o l u t i o n since t he l a t e J u r a s s i c has t h e r e f o r e e s s e n t i a l l y been a proc e s s of p r o g r e s s i v e c a p t u r e of the i n t e r n a l s y s t e m by t h e more a g r e s s i v e e x o r e i c s y s t e m (e.g. De H e i nz e l i n 1963). Evi de nc e for this c o m e s from t he Cunene in Angola (B e e t z 1933), Fish R i ve r in N a m i b i a (Wellington 1955), O ra nge - Vaal s y s t e m (Schmitz 1968, Smit 1977) as well as from the Zambezi. The c o n c e p t of an e x t e n s i v e e a r l y pos t -G ondw a na e ndore i c d r a i n a g e s y s t e m b e c o m e s more s i g n i f i c a n t when it is re c ogni s e d t h a t such a s y s t e m is one of the few l e a s a b l e m e c h a n i s m s t h a t can a c c o u n t for the i ni t i a l deposition of t he K a l a ha r i System s e d i m e n t s . It has long been re c ogni s e d t h a t the Upper Z a m b e z i , above t he Victoria Falls, and t he Middle Z a m b e z i mus t have e vol ve d as s e p a r a t e s y s t e m s only to be linked r e l a t i v e l y r e c e n t l y . Du Toit (1927, 1933), Wellington (1955), Bond (1963) and Grove (1969) a l l proposed t h a t the Upper Z a m b e z i f o r m e r l y c ont i nue d its s o u t h e r l y course, crossing w ha t is now B ot s w a na to join the Orange s y s t e m (Lister 1979) or Limpopo (Fig. lb; Wellington 1955, Bond 1963. Bond (1963) c i t e s t h e s m a l l c a t c h m e n t , s m a l l c ha nne l and l a rge deep va l l ey of the p r e s e n t Limpopo as e v i d e n c e of it once being p a r t of a l a r g e r d r a i n a g e s y s t e m . The Middle Z a m b e z i was a s e p a r a t e river, being p a r t of t he Shire s y s t e m (Bond 1963). The s out hw a rd course of the prot o-U ppe r Z a m b e z i was di s rupt e d by g e n t l e e a r t h m o v e m e n t s in t he middle
I n t e r e s t has been growing over the past two d e c a d e s in e n v i r o n m e n t a l and c l i m a t i c c h a n g e s in c e n t r a l s o u t h e r n Africa, with much of the r e c e n t r e s e a r c h in th e region d i r e c t e d t o w a r d s e v e n t s in the l a t e Q u a t e r n a r y (e.g. Cooke 1980, Cooke and Verst a ppe n 1984), and p a l a e o h y d r o l o g i c a l d e v e l o p m e n t s in p a r t i c u l a r . There is, also, however, a l e g a c y of r e s e a r c h which has a t t e m p t e d to place c e n t r a l A f r i c a n d r a i n a g e e v o l u t i o n and e n v i r o n m e n t a l c h a n g e s within a longer t i m e p e r s p e c t i v e , p a r t l y in r e c o g n i t i o n of the i m p o r t a n c e of major t e c t o n i c e v e n t s for the shaping of s u b - c o n t i n e n t as a whole. In c e n t r a l Africa, e a r t h s c i e n t i s t s have been f a s c i n a t e d by t he e v o l u t i o n of the Z a m b e z i d r a i n a g e s y s t e m . The c o n t r a s t s b e t w e e n the upper and middle r e a c h e s of its course (e.g. Du Toit 1933) and t he d e v e l o p m e n t of the V i c t o r i a Falls e.g. (Dixey 1945) have c o n t r i b u t e d to this i n t e r e s t . In its upper reaches, for a l m o s t a third of its t o t a l length, t h e Z a m b e z i flows a c r o s s the n o r t h e a s t e r n e x t r e m i t y of the K a l a h a r i basin. This basin is occupied by K a l a h a r i Sy stem s e d i m e n t s , d o m i n a t e d at the s u r f a c e by the K a l a h a r i sand which e x t e n d s over 2.5 mil km 2 of c e n t r a l and southern Africa. The K a l a h a r i has been described as a vast basin of i n t e r n a l d r a i n a g e (Cooke 1980), which has received s e d i m e n t s since a t l e a s t the e a r l y T e r t i a r y (Dingle et al 1983). Y e t today one of t h e most n o t a b l e a s p e c t s of the s o u t h e r n half of this basin ( l a t i t u d e 18°8 to 27°S) is the v i r t u a l a b s e n c e of s u r f a c e drain age. T h e r e f o r e the e v o l u t i o n of d r a i n a g e in this region is an i m p o r t a n t f a c t o r when t h e depo s i t i on of t h e s e C a i n o z o i c s e d i m e n t s is considered. In this paper, t h e r e f o r e , we f i r s t l y e x a m i n e t he d e v e l o p m e n t of ideas, which have o f t e n been s p e c u l a t i v e , about the e v o l u t i o n of the Z a m b e z i drainage. Secondly, we p r e s e n t g e o m o r p h o l o g i c a l e v i d e n c e for l a t e C a i n o z o i c d e v e l o p m e n t s in the p a l a e o d r a i n a g e of the e a s t e r n K a l a h a r i rim, and e x a m i n e t h e i r s i g n i f i c a n c e . THE E V O L U T I O N OF THE Z A M B E Z I SYSTEM
The p r e s e n t course of the Z a m b e z i is r e l a t i v e l y young, d a t i n g from the P l i o c e n e (e.g. L i s t e r 1979) or even th e m i d - P l e i s t o c e n e (Bond 1975), and has evol ve d through the e f f e c t s of d r a i n a g e c a p t u r e and t e c t o n i c a c t i v i t y . Early t h e o r i e s of the e v o l u t i o n of the Z a m b e z i s y s t e m , put f o r w a r d by Du Toit, Dixey and Wellington, have c o n t r i b u t e d s i g n i f i c a n t l y to l a t e r views on t he su bject. The p r i n c i p l e s of t h e i r ideas a r e r e a d i l y in
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612
DAVID S. G. THOMAS and PAUL A. SHAW The hi s t ory of t he Middle Z a m b e z i is e qua lly i m p o r t a n t to the o v e r a l l e vol ut i on of the d r a i n a g e in this region. R e ne w e d downwarping along t he a n c i e n t G w e m b e and C h i c o a troughs c a us e d t he d e v e l o p m e n t of the proto-Middle Z a m b e z i (Bond 1963, Williams 1975, L i s t e r 1979). This river cut w e s t w a r d s along the trough line, through basal K a l a ha ri and Karoo beds (Bond 1963), with l oc a l base l e ve l s being e s t a b l i s h e d when r e s i s t a n t bands of p r e - C a m b r i a n g n e i s s e s w er e e n c o u n t e r e d below t he karoo s e d i m e n t s (Lister 1979). Wellington (1955, p. 471) proposed t h a t the Luangwa R i ve r was a t r i b u t a r y of the prot o-U ppe r Za mbe zi, flowing s o u t h - w e s t w a r d s through t he G w e mbe trough before being c a p t u r e d as p a r t of the e vol ut i on of the Middle Z a m b e z i (Fig. 1). If so, this and the e x t e n s i o n of the Middle Z a m b e z i above the C hi c oa Gorge, must h a v e post d a t e d t he C r e t a c e o u s , for a t t h a t t i m e pa r t of t he d r a i n a g e in the a r e a c e n t r e d upon a shallow
Kalah ari, probably a c o m b i n a t i o n of u p l i f t in the v i c i n i t y of the M a k g a d i k g a d i basin and downwarping in the O k a v a n g o region (Du Toit 1954, Bond 1963). This t h e o r y is given support by undisputed e v i d e n c e of t e c t o n i s m in t h e a r e a today (e.g. R e e v e s 1972, Hutchins e t al 1976, Cooke 1980). The s e v e r a n c e of the Upper Z a m b e z i course ma y have caused endorism in the s y s t e m (e.g. Du Toit 1926), prior to the link with the Middle Z a m b e z i being e s t a b l i s h e d through the Livingstone syncline. This c r e a t e s the i n t e r e s t i n g possibility of a source of w a t e r for an a n c e s t r a l , p r e - P l e i s t o c e n e , Lake P a l e a o M a k g a d i k g a d i (Fig. lc). Evidence for such an a n c i e n t lake can be seen on Landsat i m a g e r y of n o r t h e r n Botswana (Mallick e t al 1981). The linking of the two a n c e s t r a l c o m p o n e n t s of the Z a m b e z i o c c u r r e d be fore the l a t e P l e i s t o c e n e , but not e a r l i e r than the P l i o c e n e (see Dixey 1950, Bond 1975, L i s t e r 1979).
(hinge line)
b.
C.
/
Fig. 1. Present day drainage in southern Africa. Earlier patterns are represented in b) and c): b) post-dates the division of Gondwanaland; c) existed prior to the union of the proto-Middle and proto-Upper Zambezi rivers, during the Pliocene early Pleistocene.
Late Cainozoic drainage evolution in the Zambesi basin d e p r e s s i o n in c e n t a l n o r t h e r n Z i m b a b w e , in w h i c h t h e c a l c a r e o u s G o k w e Beds w e r e d e p o s i t e d ( L i s t e r 1979, S u t t o n 1979). F o l l o w i n g t h e e s t a b l i s h m e n t of t h e p r e s e n t Z a m b e z i course, further downwarping along the Gwembe trough led t o r e n e w e d i n c i s i o n ( D i x e y 1945). This w a s p a r t of a m o r e g e n e r a l r e j u v e n a t i o n of c o a s t w a r d f l o w i n g r i v e r s , c a u s e d by r e n e w e d u p l i f t a l o n g t h e c o a s t a l h i n g e l i n e s (De S w a r d t and B e n n e t 1974). C o n s e q u e n t l y , t h e P l e i s t o c e n e e r o s i o n a c h i e v e d by t h e M i d d l e Z a m b e z i has b e e n d e s c r i b e d as ' i m m e n s e ' ( D i x e y 1950, p. 11). This w a s r e s p o n s i b l e for t h e r a p i d e v o l u t i o n of t h e V i c t o r i a F a l l s and, in t h e s p a c e of a b o u t a q u a r t e r to h a l f m i l l i o n y e a r s (Bond 1975), t h e f o r m a t i o n of t h e i n c i s e d Middle Z a m b e z i G o r g e . The K a f u e R i v e r , in Z a m b i a , has a l s o e v o l v e d an unusual course, taking an abrupt eastward turn at H e z h i t e z h i . This c a n a l s o be e x p l a i n e d by c a p t u r e , r e l a t e d to t h e e v o l u t i o n of t h e Z a m b e z i ( W i l l i a m s 1975). Bond (1963) n o t e d how t h e e a r l y K a f u e f l o w e d s o u t h - w e s t w a r d s to join t h e U p p e r Z a m b e z i . The c o u r s e w a s a l o n g a gap, 2 km w i d e and a t a n a l t i t u d e of a b o u t 1070 m.a.s.1, w h i c h now c r o s s e s t h e p r e s e n t K a f u e - Z a m b e z i w a t e r s h e d , b e t w e e n H e z h i t e z h i and M a m b o v a . This is now a n a l l u v i a l f l a t , p o o r l y d r a i n e d by t h e N a n z h i l a and S i m a t e n g a s t r e a m s , h u t t h e a l l u v i u m is u n d e r l a i n by c o u r s e c u r r e n t - b e d d e d f l u v i a l g r a v e l s ( C l a r k 1950). The c a p t u r e of t h i s r i v e r c a n n o t s i m p l y be e x p l a i n e d in t h e s a m e m a n n e r as t h a t of t h e U p p e r Z a m b e z i . The p r e s e n t e a s t e r l y f l o w i n g s e c t i o n of t h e K a f u e , b e l o w H e z h i t e z h i , has u n d e r g o n e d r a i n a g e r e v e r s a l ( D i x e y 1955), o n c e b e i n g p a r t of t h e s y s t e m j o i n i n g t h e U p p e r Z a m b e z i . In t h i s r e s p e c t t h e p r o c e s s m u s t h a v e b e e n s i m i l a r to, but p o s t - d a t i n g , t h e c a p t u r e of t h e L u a n g w a . W i l l i a m s (1975) n o t e s how, on t h e d i v i d e b e t w e e n t h e Middle Z a m b e z i and t h e K a f u e , t h e s o u t h f l o w i n g t r i b u t a r i e s of t h e f o r m e r a r e m o r e a g g r e s s i v e t h a n t h o s e f l o w i n g n o r t h to t h e l a t t e r . I t therefore seems likely that the Kafue system was c a p t u r e d by a M i d d l e Z a m b e z i t r i b u t a r y . This v i e w is c o m p l e t e l y in k e e p i n g w i t h t h e r a p i d P l e i s t o c e n e d e v e l o p m e n t of t h e Middle Z a m b e z i e n v i s a g e d by D i x e y (1950). A l t h o u g h s p e c u l a t i o n h a s p l a y e d a m a j o r role in t h e d e v e l o p m e n t of t h e i d e a s d e s c r i b e d a b o v e , c e n t r a l
613
A f r i c a n i c h t h y o l o g i c a l s t u d i e s s u p p o r t t h e t h e o r y of s e p a r a t e e v o l u t i o n of t h e p r o t o U p p e r and M i d d l e Z a m b e z i s y s t e m s . Fish s p e c i e s in t h e U p p e r Z a m b e z i and K a f u e f o r m p a r t of an ' O k a v a n g i a n ' f a u n a , d i s t i n c t f r o m t h a t of t h e Middle Z a m b e z i (e.g. J a c k s o n 1961). Bond (1963) a n d Jubb (1964) s u g g e s t e d t h a t t h e V i c t o r i a F a l l s w e r e an o b s t a c l e t o i n t e g r a t i o n of t h e t w o fish p o p u l a t i o n s , but i t has s i n c e b e e n d e m o n s t a t e d t h a t t h e b a r r i e r t o i n t e g r a t i o n was e c o l o g i c a l , n o t p h y s i c a l . F o l l o w i n g t h e c o n s t r u c t i o n of t h e K a r i b a D a m , s l o w water habitats, prevoiusly limited to the Upper Zambezi, w e r e a v a i l a b l e in t h e m i d d l e r e a c h e s of t h e r i v e r . This f a c i l i t a t e d a s u c c e s s f u l i n v a s i o n b e l o w t h e V i c t o r i a F a l l s by A l e s t e s l a t e r a l l y , p r e v i o u s l y c o n f i n e d t o t h e U p p e r Z a m b e z i and t h e K a f u e (Balon 1971, 1974). DRAINAGE DEVELOPMENT EASTERN KALAHARI
ON THE RIM
I m p o r t a n t e v i d e n c e of d r a i n a g e d e v e l o p m e n t in t h e U p p e r and Middle Z a m b e z i c a t c h m e n t s occurs in n o r t h w e s t e r n Z i m b a b w e and n o r t h e a s t e r n B o t s w a n a . D r a i n a g e in t h i s a r e a t o d a y is d i v i d e d b e t w e e n t w o v e r y d i f f e r e n t s y s t e m s : a low d e n s i t y s y s t e m of emphemeral streams draining west towards the middle K a l a h a r i r e g i o n , and m o r e n u m e r o u s and a g r e s s i v e t r i b u t a r i e s of t h e Middle Z a m b e z i (Fig. 2). The w a t e r s h e d between these systems, coinciding approximately with t h e b o r d e r b e t w e e n B o t s w a n a and Z i m b a b w e , r e f l e c t s t h e e x t e n t of h e a d w a r d e r o s i o n by r i g h t - b a n k t r i b u t a r i e s of t h e Middle Z a m b e z i i n t o t h e e a s t e r n f l a n k of t h e K a l a h a r i basin. This is r e l a t e d t o t h e r e n e w e d i n c i s i o n of t h e Middle Z a m b e z i a l o n g t h e G w e m b e T r o u g h (e.g. D i x e y 1945, 1950) in t h e U p p e r P l e i s t o c e n e , and h a s had t h e e f f e c t of c a u s i n g t h e b a s i n r i m t o h a v e t h e a p p e a r a n c e of a p l a t e a u e d g e , a t 1050 - 1100 m.a.s.1. D r a i n a g e p a t t e r n s of t h e Middle Z a m b e z i tributary system E a s t of K a l a h a r i Middle Z a m b e z i w a t e r s h e d , in n o r t h w e s t e r n Z i m b a b w e , d i s p a r i t i e s b e t w e e n d r a i n a g e p a t t e r n s t o t h e e a s t and t o t h e w e s t of t h e G w a y i r i v e r h a v e b e e n d i s c u s s e d by T h o m a s (1983/4). The Matetsi and Deka systems have incised westwards i n t o t h e r i m of t h e K a l a h a r i basin, e x p o s i n g u n d e r l y i n g
~au eposJts -ession a Flats
Fig. 2. Drainage in northeast Botswana and northwest Zimbabwe.
614
DAVID S. G. THOMAS and PAUL A. SHAW Evi de nc e to support this t h e o r y c o m e s from t e r r a c e s e q u e n c e s in Sikumi vlei and on t he N g w e z u m b a River. Sikumi vlei is a short, 15 km long w e s t bank t r i b u t a r y of t he Gwayi, its floor 40m below t he K a l a h a r i p l a t e a u and s i t u a t e d just to t he e a s t of t he boundary of Hwange N a t i o n a l P a rk (Fig. 3). It is t y p i c a l of many Z i m b a b w e a n dambos (Whitlow 1984), having a r e l a t i v e l y broad, low g r a d i e n t floor, c o n t a i n i n g r e l a t i v e l y t h i c k p e a t de pos i t s and poor channel development with l i t t l e l a t e r a l flow. Three k i l o m e t r e s from t he c o n f l u e n c e with the Gwayi, a t e r r a c e sequence is exposed on th e north side of t he valley, which a t this point is cu t into t he Karoo s a nds t one t h a t unde rl i e s t he K a l a h a r i sand. The s e q u e n c e is shown in Fig. 3 and de s c ri be d in t a b l e 1. It is unknown w h e t h e r t he basal K a l a h a r i beds, r e p r e s e n t e d by c o n g l o m e r a t e s and s i l i c e ous sandstone, were de pos i t e d in a va l l e y with the s a m e a l i g n m e n t as t he one p r e s e n t today, or w h e t h e r t he y a r e more e x t e n s i v e , with the p r e s e n t v a l l e y c ut down into them. As Karoo s a nds t one is exposed higher up t he v a l l e y side, above t he out c rop of basal K a l a h a r i Beds, it is s u g g e s t e d t h e i r depostion was within a p r e - e x i s t i n g valley. This is a na l oge ous with t he s i t u a t i o n in the Middle Z a m b e z i va l l e y de s c ri be d by Dixey (1950). The s u r f a c e of much of the t e r r a c e s e q u e n c e is m a n t l e d by a poorly s ort e d colluvium. This is ma i nl y composed of a n g u l a r s i l c r e t e pi e c e s in a m a t r i x of re w ork ed K a l a h a r i sand, de ri ve d from the p l a t e a u s urfa c e . The most s i g n i f i c a n t units of t he s e que nc e , however, a re t he upper and lower g r a v e l s and t he w h i t e and pink alluvia, which a re all fl uvi a l deposits. It is beyond the bounds of reason to a c c o u n t for t h e s e de posits in t e r m s of fl uvi a l a c t i v i t y r e l a t e d to the p r e s e n t vlei. F u r t h e r m o r e , t h e r e is not a s u i t a b l e source for t h e g r a v e l s within its K a l a h a r i p l a t e a u c a t c h m e n t . The second s e c t i o n of fluvial g r a v e l s c o m e s from t he N g w e z u m b a in north e a s t e r n Botswana. This e p h e m e r a l sand ri ve r drains w e s t w a r d s from one of the a l l u v i a l a r e a s on t he Zi mba bw e - Botswana to t he Mababe depression (Shaw 1985). Although t he l ack of d e t a i l e d he i ght d a t a re c l ude s g r e a t e r a c c u r a c y , this s t r e a m falls a p p r o x i m a t e l y 100m over t he 70 km from s ourc e to t he Mababe depression, giving a mean g r a d i e n t of 1.43m km - I . At N g w e z u m b a dam in Chobe N a t i o n a l Park, basal K a l a ha ri beds a re exposed on the southern side of the p r e s e n t channel. On the north side, a s e c t i o n displays 3.5m of loosely c e m e n t e d bedded g r a v e l s and alluvium
Karoo s e d i m e n t s and b a s a l t s with the local base l e ve l c o n t r o l l e d by the height of the Middle Z a m b e z i . These riv ers have very s t e e p g r a d i e n t s , the M a t e t s i having a mean g r a d i e n t of 4.34m km -1 over the 122 km from the w a t e r s h e d to the Z a m b e z i , and the Deka a g r a d i e n t of 4. 69m km -1. Incision below the K a l a h a r i p l a t e a u is up to a m a x i m u m depth of 500m, and has c r e a t e d a s e r i e s of K a l a h a r i o u t l i e r s to the north and e a s t of th e p a l t e a u rim. The Gwayi, which joins the Z a m b e z i a t Devil's Gorge has, on the other hand, cut s o u t h - e a s t w a r d s into t h e p l a t e a u . This river is n o t a b l e for th e l a c k of s i g n i f i c a n t l e f t bank t r i b u t a r i e s , w h e r e a s it is joined from t h e e a s t by r e m a r k a b l e s t r a i g h t s u b - p a r a l l e l s t r e a m s . The l a r g e s t , the Shangani, has incised 130m through the K a l a h a r i s e d i m e n t s near its c o n f l u e n c e with t h e Gwayi. The others, however, display only l i m i t e d d o w n c u t t i n g , with the Tshongokwe, Kana and Mazola having r e l a t i v e l y low g r a d i e n t s (1.94m km1 for the Kana) and courses mainly flowing on K a l a h a r i sand. F u r t h e r north, the Sengwa, Lutope and Mbumbusi, which are not t r i b u t a r y to the Gwayi, also display s i g n i f i c a n t s t r a i g h t w e s t w a r d c o m p o n e n t s in t h e i r courses. Thomas (1983/4) proposed t h a t the s t r a i g h t , subp a r a l l e l drainage to the e a s t of the Gwayi had evol ve d upon a l i n e a r dune l a n d s c a p e on the K a l a h a r i sand. We can now add t h a t t h e s e rivers were f o r m e r l y pa rt of the t r i b u t a r y s y s t e m of the p r o t o - U p p e r Zambe z i , which is in a c c o r d with the h i s t o r y of regional d r a i n a g e e v o l u t i o n put forward by L i s t e r (1979). Headward erosion by the Gwayi, in c o n j u n c t i o n with the evoluti on of the Middle Zambezi drainage, has contributed s i g n i g i c a n t l y to the p r o g r e s s i v e c a p t u r e of the e a s t west s y s t e m , f o r m e r l y t r i b u t a r y to the proto -U ppe r Zamb ezL This also a c c o u n t s for t h e lack of s i g n i f i c a n t Gwayi w e s t bank t r i b u t a r i e s , for south of the D e ka s y s t e m , the w a t e r s h e d of the Middle Z a m b e z i n e t w o r k is p a r a l l e l , and a few k i l o m e t r e s w e s t of the G w a yi valley.
Terrace sequences.
It may t h e r e f o r e be h y p o t h e s i s e d t h a t the s t r e a m s to the w e s t of the w a t e r s h e d , for e x a m p l e t he N g w e z u m b a , draining into the Mabahe depression, and th e Nata, which e n t e r s Sue Pan, a r e r e m n a n t s of the l e f t bank t r i b u t a r i e s of the proto-Upper Zambe z i .
i
SIKUMI ~ t
~
? Wm 29
Kalahar,
Karoo Sandslone Kb
Vie deposts ]
Fig. 3. The terrace sequence at Sikumi Vlei. Inset: location of Sikumi Vlei.
Late Cainozoic drainage evolution in the Zambesi basin
UNIT .
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MAX, T H I C K N E S S OF UNIT
DESCRIPTION .
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Kb
B a s a l K a l a h a r l bed~. S i l i c e o u s s a n d s t o n e a n d ferrtI~inous c e m e n t e d conglomerate
im
Ks
Kalahari
--
G 1
Upper gravel terrace. Unconsolidated, p r i m a r i l y q u a r t z p e b b l e s , u p to 5 c m in d i a m e t e r
im
G 2
Lower gravel terrace. Largely unconsolidated, but some cementing at base. M i x e d l l t h o l o g i e s inc. q u a r t z , j a s p e r , agate, b a s a l t , chert. R o u n d e d , max. d i a m e t e r 6cm, av. 2cm.
l. S m
A 1
Pale alluvium,
im
A 2
Pink alluvium. 70% sand
C
Colluvium. Reworked Kalaharl sand and angular silcrete fragments. Inc. 8 r a v e l s d o w n s l o p e f r o m G i.
sand.
615
On plateau
4% clay,
surface
2 6 % silt,
am
0.4m
Table 1. Description of Sikumi Vlei terrace sediments (see figure 3).
(Fig. 4) which a r e c l e a r l y d i s t i n c t from t h e K a l a h a r i c o n g l o m e r a t e . The g r a v e l s a r e l a r g e l y composed of well rounded g r e e n and w h i t e s i l c r e t e s , jasper, c h e r t and a g a t e , and more a n g u l a r pebbles of K a l a h a r i c o n g l o m e r a t e . Some of the c l a s t s in the g r a v e l s e c t i o n a r e up to 0.3m in d i a m e t e r . The s i l c r e t e and c o n g l o m e r a t e pebbles were probably d e r i v e d from sources within the K a l a h a r i p l a t e a u , and both are exposed in s i t u in s e c t i o n s in t h e p r e s e n t N g w e z u m b a c h a n n e l A g a t e is found in a s s o c i a t i o n with Karoo basalt, of which t h e r e are s m a l l e x p o s u r e s
Pedogenic calcrele, max 1 5 m ihick
-O-'-a....%'1 o o % o %o ,~0
l ol
1- t 5 m
ooooo oq9o |/ co o 0
0
°° * 0
0
Gravels, poorly sorted, in fine m a t r i x
0.25m
Alluv,um
03Ore
Mixed gravels in fine matr,x
0 70 m
Mainly fine alluvium, some c~asts
O.60m
Graded gravels, fineing upwards Matrix deficmnt, some coarse sand
in t he K a z u m a depression. The d e g r e e of rounding of t he g r a v e l s s u g g e s t s c o n s i d e r a b l e fl uvi a l t r a n s p o r t . Although t he N g w e z u m b a was an a c t i v e r i v e r during l a t e Q u a t e r n a r y high l a k e s t a g e s in t he Mababe depression (Shaw 1985), only fine d e l t a i c s e d i m e n t s w e re deposited. We t h e r e f o r e s u g g e s t t h a t ~he g r a v e l s a r e bedload de pos i t s a s s o c i a t e d with high flow a t a t i m e when t he N g w e z u m b a had a more e x t e n s i v e catchment. O t he r g r a v e l e xpos ure s oc c ur as thin s u r f a c e s pre a d s f u r t h e r down t h e course of t he N g w e z u m b a , and on t he K a l a h a r i p l a t e a u edge ne a r P a n d a m a t e n g a . It is not possible, however, to distinguish t h e s e from in s i t u w e a t h e r e d basal K a l a h a r i c o n g l o m e r a t e . We c ons i de r though t h a t t h e Sikumi vlei t e r r a c e s and N g w e z u m b a dam g r a v e l s a re e v i d e n c e for t he e x i s t e n c e of more e x t e n s i v e d r a i n a g e into north B ot s w a na from an e a s t e r l y d i r e c t i o n , p r e - d a t i n g incision of, and c a p t u r e by, the p r e s e n t Middle Z a m b e z i t r i b u t a r i e s . Our hypot he s i s is supported by the a s y m m e t r y of t he Gwayi t r i b u t a r y ne t w ork, and the s t r o n g e a s t - w e s t d i r e c t i o n a l t r e n d of the ri ve rs to t he e a s t of the Gwayi (Thomas 1983/4), and is in a g r e e m e n t with t he published t h e o r i s i n g c o n c e r n i n g t he e v o l u t i o n of the Z a m b e z i d r a i n a g e s y s t e m in the l a t e C a i noz oi c . S U R F A C E SEDIMENTS A N D D R A I N A G E C H A N G E
0
O15m Fine ravels Little matr=w UNCONFORMll~ Karoo basalt
Fig. 4. Gravel sequence at Ngwezumba dam.
Mallick e t al (1981) i d e n t i f i e d and mapped from La nds a t i m a g e r y 'possible l a c u s t r i n e d e v e l o p m e n t s ' (p. 28) in e a s t e r n Botswana. These a re s i t u a t e d on t he w a t e r s h e d b e t w e e n t he i n t e r n a l and Middle Z a m b e z i d r a i n a g e s y s t e m s , and a re h i t h e r t o u n i n v e s t i g a t e d . They a re s i g n i f i c a n t in t he c o n t e x t of this study b e c a u s e t h e i r d e v e l o p m e n t in t h a t l o c a t i o n can not be e x p l a i n e d in t e r m s of the p r e s e n t d r a i n a g e of the region. The l a r g e s t of these, known as t he K a z u m a Depression w he re it e x t e n d s into Zimbabwe, c o v e r s a bout 1000 km (Mallick e t al 1981), w hi l s t 50 km sout17 a second, near P a n d a m e t e n g a , e x t e n d s over about
616
DAVID S. G. THOMAS and PAUL A. SHAW
800 km 2 (Fig. 2). O t h e r s m a l l e r , l a c u s t r i n e d e v e l o p m e n t s o ccur on t h e n o r t h e r n edge of the sand p l a t e a u in Hwange N a t i o n a l Park, Zimbabwe. In all c a s e s t he m a r g i n s of t h e depressions a r e being eroded, as pa rt of the r e t r e a t of the K a l a h a r i rim e f f e c t e d by the Middle Z a m b e z i s y s t e m . In places, Karoo b a s a l t s and basal K a l a h a r i Beds a r e exposed on the depression floors. It is not surprising, t h e r e f o r e , t h a t soils a r e in part composed of w e a t h e r e d basalt, and t h e y have been d e s c r i b e d as such in a r e c e n t FAO r e p o r t (FAO 1986), and an e a r l i e r one produced by th e Rhodesian Soil Survey (Sweet 1970). However, over l a r g e a r e a s t h e y also c o n t a i n a s i g n i f i c a n t a l l u v i a l c o m p o n e n t , d e r i v e d a t l e a s t in part from r e w o r k e d K a l a h a r i sand (Thomas 1987). The floors of t he depressions a r e up to 20m below the h e i g h t of t he surrounding sand p l a t e a u . Because of t h e i r w a t e r s h e d position, th e origin of t h e s e l a c u s t r i n e f e a t u r e s can only be e x p l a i n e d as part of the f o r m e r w e s t w a r d draining p r o t o - U p p e r Z a m b e z i t r i b u t a r y s y s t e m . Although t h e y have not been i n v e s t i g a t e d in t h e field, s i m i l a r l a c u s t r i n e d e v e l o p m e n t s can be i d e n t i f i e d from L an dsat i m a g e r y of Western Zambia. These f e a t u r e s occur within the K a l a h a r i sand too, and a r e found in a s s o c i a t i o n with e a s t bank t r i b u t a r i e s of t he Z a m b e z i . For e x a m p l e , the Bovu River, which e n t e r s the Z a m b e z i from due north about 10 km e a s t of
Kazangula, c ros s e s a f l a t a l l u v i a l - f l o o r e d depression of some 100 km -2, before a b r u p t l y d e v i a t i n g from its s o u t h w e s t w a r d o r i e n t a t i o n to decend t he flank of downwarped Z a m b e z i trough. The e v o l u t i o n of many f e a t u r e s of t he h y d r o l o g i c a l s y s t e m of t he Middle K a l a h a r i has been c o n t r o l l e d by f a u l t i n g and g e n t l e t e c t o n i c a c t i v i t y . The 1978 P h o t o g e o l o g i c map of Botswana i n d i c a t e s t h a t p a r t s of t he s out he rn m a r g i n s of t he K a z uma and P a n d a m e t e n g a l a ke f l a t s a r e f a u l t c o n t r o l l e d . However, t he 1984 Botswana g e o l o g i c a l map shows t h a t in t h e v i c i n i t y of t he f l a t s t h e r e a r e r e l a t i v e l y few f a u l t s in the underlying ba s a l t , and t h e r e f o r e t e c t o n i s i s m alone can not a c c o u n t for t h e i r l o c a t i o n . From La nds at i m a g e r y , it can be seen t h a t t h e i r ma rgi ns a re p a r t i a l l y defined by l i n e a r dunes, p a r t of t he e x t e n s i v e fossilised e rg which c o n t i n u e s e a s t w a r d s on t he sand p l a t e a u into Zi mba bw e (Thomas 1984). The r e l a t i o n s h i p b e t w e e n dunes and l a c u s t r i n e f e a t u r e s is in f a c t complex. The e dge s of t he bounding dunes have been t r i m m e d by w a t e r a c t i o n , and process is probably c o n t i n u i n g to a l i m i t e d e x t e n t today, for during ye a rs of good local rains the depressions may be t e m p o r a r i l y inundated by shallow w a t e r bodies. However, s t a b i l i s e d dunes also e x t e n d down onto t h e l a ke fl a t s , and on the s out he rn m a r g i n of the P a n d a m e t e n g a depression, a w a t e r - t r i m m e d dune s t u m p
Fig. 5. Aerial photograph showing r e l i c t linear dune stump overlying a former channel of the Nungu River, eastern Botswana.
Late Cainozoic drainage evolution in the Zambesi basin be seen o v e r l y i n g a n old abandoned chann e l of the Nungu R i v e r (Fig. 5). There have c l e a r l y been a l t e r n a t i o n s b e t w e e n periods of a e o l i a n a c t i v i t y and periods of p e r m a n e n t or s e m i - p e r m a n e n t w a t e r in the depressio ns since the P l i o c e n e , but we a r e c u r r e n t l y unable to shed any light on the d e t a i l s of chronologies. can
DISCUSSION
D r a i n a g e evolution in c e n t r a l A f r i c a has e voke d i n t e r e s t since the t i m e of L i v i n g s t o n e (1858). During the ensu£ng period t h e r e has been c o n s i d e r a b l e s p e c u l a t i o n , but a g e n e r a l concensus of opinion, t h a t the p r e s e n t Z a m b e z i course was e s t a b l i s h e d no e a r l i e r t h a t the Pliocene, and perhaps as l a t e as the mi ddl e P l i e s t o c e n e . The p r e s e n t s y s t e m of t r i b u t a r i e s has been e v o l v i n g since then and r e p r e s e n t s a p roc e s s of s u p e r i m p o s i t i o n on and c a p t u r e of the p r e - Z a m b e z i d r a i n a g e t h a t had evolved since the J u r a s s i c . This p ro cess has been a f f e c t e d and in p a r t d e t e r m i n e d by t e c t o n i c m o v e m e n t s which have o p e r a t e d in favour of e n h a n c i n g the power of t h e Middle Z a m b e z i and its t r i b u t a r i e s , r e s u l t i n g in the c a p t u r e of the protoUpper Zamb ezi, and its t r i b u t a r y s y s t e m . Thus Wiliams (1975) d e s c r i b e s the c a p t u r e of the Kafue by a l e f t bank Middle Z a m b e z i t r i b u t a r y , and we assign t he c a p t u r e of the w e s t w a r d flowing rivers in n o r t h e r n Z i m b a b w e to s t e e p g r a d i e n t right bank t r i b u t a r i e s of the Middle Zambezi, including the G w a y i , D e k a and M a t e t s i . The e f f e c t has been to d e p r i v e the few w e s t w a r d flowing s t r e a m s in n o r t h e r n B o t s w a n a of t h e i r f o r m e r h e a d w a t e r s , which are now e a s t - b a n k t r i b u t a r i e s of the Gwayi. Both t h e s e s y s t e m s r e t a i n the r e l a t i v e l y low g r a d i e n t s i n h e r i t e d from t h e i r a n c e s t r a l protoUpper Z a m b e z i t r i b u t a r i e s . Their c a p t u r e has also had the e f f e c t of i s o l a t i n g the l a c u s t r i n e d e v e l o p m e n t s south of th e Z ambezi, on the Botswana - Zimbabwe border, from the s t r e a m s which previously drained t o w a r d s them. As well as d e t e r m i n i n g d r a i n a g e p a t t e r n s , this pro cess is c u r r e n t l y causing the w e s t w a r d r e t r e a t of th e e a s t e r n rim of the K a l a h a r i p l a t e a u , exh umi nu underly ing Karoo b a s a l t s previously r e v e a l e d in t he Hwange a r e a . In broader t e r m s , the d r a i n a g e c h a n g e s are p a r t of the r e p l a c e m e n t of the c e n t r a l s o u t h e r n A f r i c a n e n d o r e i c s y s t e m by a more e x t e n s i v e e x o r e i c one (c.f. De Swardt and Bennet 1974), r e f l e c t e d in the g r a d u a l w e s t w a r d m o v e m e n t of the w a t e r s h e d dividing the right bank t r i b u t a r i e s of the middle Z a m b e z i and to day's i n t e r n a l d r a i n a g e . D a t i n g the major c h a n g e s d e s c r i b e d above, and t h e r e f o r e the Sikumi t e r r a c e s and the N g w e z u m b a grav els, in a n y t h i n g o t h e r than g e n e r a l t e r m s is d i f f i c u l t . From the s t u d i e s of the t e c t o n i c e v o l u t i o n of t he s u b c o n t i n e n t , t h e y obviously o c c u r r e d no e a r l i e r t ha n t h e l a t e Pliocene, and before the l a t e Q u a t e r n a r y e n v i r o n m e n t a l changes i d e n t i f i e d from p a l a e o h y d r o l o g i c a l s t u d i e s in the Middle K a l a h a r i (Shaw and Cooke 1986). During the period in which the e a s t - west d r a i n a g e flowed into n o r t h e r n Botswana and the p r o t o - U p p e r Z a m b e z i , it is probable t h a t major c h a n g e s in flow r e g i m e s o ccurred. This is i n d i c a t e d by t h e v a r i a t i o n s in the size of m a t e r i a l f o r m i n g the d i f f e r e n t t e r r a c e s within th e Sikumi sequence, and the f a c t t h a t periods of incision o c c u r r e d to c r e a t e the t e r r a c e s . P r e s u m a b l y , t e c t o n i c m o v e m e n t s c a u s i n g c h a n g e s in base l e v e l and c l i m a t i c c h a n g e s were responsible. In the a b s e n c e of f u r t h e r c o r r o b o r a t i n g e v i d e n c e , however, it is c u r r e n t l y not possible to m a k e f u r t h e r d e d u c t i o n s about r i v e r r e g i m e s . R e l a t i o n s h i p s b e t w e e n d r a i n a g e d e v e l o p m e n t and the K a l a h a r i beds are p o t e n t i a l l y of i n t e r e s t , but the age of t h e s e s e d i m e n t s l a r g e l y r e m a i n s a m a t t e r of s p e c u l a t i o n (e.g. Dingle e t al 1983). That rive rs to the e a s t of the Gwayi were d u n e - a l i g n e d , and l a k e depressions now on the K a l a h a r i rim were a t l e a s t p a r t i a l l y d e l i m i t e d by the s a m e c o n t r o l may be more s i g n i f i c a n t . The p r o b l e m s of d i r e c t l y d a t i n g the dune s y s t e m s (e.g. Thomas 1984, Shaw and Cooke 1986) and the o c c u r r e n c e of m u l t i p l e periods of a r i d i t y during
617
the P l e i s t o c e n e in t he K a l a h a r i basin do not, however, a s s i s t in o b j e c t i v e a s s e s s m e n t s of t he a ge of t h e s e f e a t u r e s or of t he d r a i n a g e changes. N e v e r t h e l e s s , in s pi t e of t h e s e problems, the i m p o r t a n c e of t he d r a i n a g e c ha nge s e v i d e n c e d by the l a c u s t r i n e d e v e l o p m e n t s and the t e r r a c e s e q u e n c e s in n o r t h w e s t e r n B ot s w a na and n o r t h e a s t e r n Zi mba bw e can not be over looked. Not only do t he y r e f l e c t major a s p e c t s of the C a i n o z o i c t e c t o n i c and hydrologic e v o l u t i o n of t he s u b - c o n t i n e n t , but it is only through t he i r r e c o g n i t i o n and i n t e r p r e t a t i o n t h a t a b e t t e r u n d e r s t a n d i n g of c e r t a i n a s p e c t s of re gi ona l l a n d f o r m s and s e d i m e n t s can begin.
Acknowledgeg~nts-DSGT would like to thank The Royal Society, London, for financial assistance which enabled fieldwork contributing to this research to be carried out. PAS would similarly like to thank the Research and Publications Committee, University of Botswana.
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