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The sedimentary basins of Tanzania - reviewed
Journal of Africaa Earth Sciences, Vol. 13, No. 3/4, pp. 291-297, 1991.
0899-5362/91 $3.00 + 0.(30 © 1991 Pergamon Press pl
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The sedimentary basins of Tanzania - reviewed E. I. ~[BEDE
University of Dar-es-Salaam, Box 35052, DSM, Tanzania (Accepted for publication 15th February, 1990) AbsU'aet- The sedimentarybasins of Tanzania have been classifiedinto four morphotectonicgroups: the coastal basin, the Karoo rift basins, basins found within the present East African rift valley and the cratonic sag basins. Except for the cratonic sag basins, each of these basin group has been affectedby rifting at one time or another. The geology of each basin is discussed, structural evolution is evaluated and the prospectivity is thence looked into. Coal is exploitedat Songwe-Kiwiracoalfieldand is foundin potentiallyeconomicquantities in other Karoo basins. Prospecting for hydrocarbonresourceshas been going on since the 50s. Gas has been discoveredin Songosongoand Mnazi bay fields, uneconomicalquantities ofoil have also been reported in Songosongo.Being basicallyrift basins which have reached different stages of development, source rocks normally associated with Initial-rifting, synrifting as well as post-riftingprocessesare probablywelldeveloped.Resevoirrocks, traps and caprocks are normally not rare in such tectonicenvironments. Thermal gradients associatedwith the rifting stageare normaly high to effect maturation of source rocks even at low sedimentary thicknesses. Studies done so far are still inconclusive, because while testing has mainly been focused on structural traps stratigraphic traps seems to be more promising.
INTRODUCTION
A s e d i m e n t a r y b a s i n is defined a s a n e l e m e n t of t h e E a r t h ' s s u r f a c e w h e r e s e d i m e n t s have acc u m u l a t e d for a significant time (Stonley, 1981). The scale of t h e b a s i n varies greatly from o c e a n b a s i n s to small p o n d s , b u t for a long time e a r t h s c i e n t i s t s h a v e b e e n restricting t h e m s e l v e s to r e a s o n a b l y b i g s e d i m e n t filled d e p r e s s i o n s ( I 0 0 k m long a n d above) with evidence of deep s u b s i d e n c e a n d significant d e f o r m a t i o n (Helwig, 1985). The d e v e l o p m e n t of t h e plate tectonic t h e o r y h a s offered a n e w w a y of explaining t h e evolution of s e d i m e n t a r y b a s i n s (Dewey a n d Bird, 1970; Dewey, 1972 a n d McKenzie, 1972). The t h e o r y is also t h o u g h t to b e suggestive of guidelines for oil a n d g a s exploration (Thompson, 1967). Putting a b a s i n in its p r o p e r c l a s s m a y prove to be a worthwhile exercise b e c a u s e it h e l p s in the q u i c k a n a l y s i s of t h e t e c t o n i c h i s t o r y of t h e b a s i n especially if one tries to c o m p a r e it with o t h e r similar b a s i n s in t h e world w h i c h have b e e n t h o r o u g l y studied. As Klemme (1980} suggests, w h e n l i n k e d to t h e v a r i a b i l i t y of p e t r o l e u m characteristics, b a s i n classification m a y a p p r a i s e the p e t r o l e u m potential of n e w frontier b a s i n s or f u r t h e r d e v e l o p m e n t in old b a s i n s . This p a p e r a t t e m p t s to s y n t h e s i z e t h e geod y n a m i c m o d e l of T a n z a n i a n b a s i n s a n d their g e n e s i s in relation to plate tectonic setting. It also tries to relate their s t r u c t u r a l d e v e l o p m e n t to prospectlvity a n d c o m p a r e t h e m to similar b a s i n s
in t h e world. The w o r k is a n o u t c o m e of T a n z a n i a n b a s i n a n a l y s i s carried o u t in 1986 b y NORAD a n d TPDC to which the a u t h o r participated a s a m e m b e r of staff from t h e University of D a r - e s - S a l a a m . Only b a s i n s with significant s e d i m e n t a r y t h i c k n e s s are considered. Figure 1 shows the sedimentary b a s i n s of Tanzania. The b a s i n s have tectonicaly b e e n classified into f o u r g r o u p s . E a c h g r o u p h a s b e e n defined o n t h e b a s i s of form, s e d i m e n t fill, tectonic setting a n d the interelated tectonic events a s p r o p o s e d b y Helwig (1985). The b a s i n s include t h e c o a s t a l b a s i n c o n s i d e r e d a s a passive continental margin; b a s i n s g r o u p e d together a s Karoo rlft b a s i n s c o n s i d e r e d a s a b o r t e d rlft basins; b a s i n s f o u n d within t h e p r e s e n t E a s t African rlft valley, a n d the b a s i n s refered to a s cratonic sag b a s i n s w h i c h are b a s i n s f o u n d within the stable a r e a s w h i c h s h o w n o evidence ofrift relate tectonic activities. Figure 3 is t h e p r o p o s e d classification of Tanzanian basins. Apart from r o c k s of Karoo age no r o c k s of Paleozoic age have b e e n r e c o g n i s e d so far in T a n z a n i a , b u t r o c k s of t h e B u k o b a n s y s t e m (ca 9 5 0 Ma), w h i c h crop o u t extensively in the vicinity of t h e Late Tertiary rlft valleys a s well as cratonic s a g b a s i n s might b e of interest for p e t r o l e u m exploration in this country. F o r e m o s t , B u k o b a n r o c k s are p r e d o m i n a n t l y clastic a n d a t t a i n c o n s i d e r a b l e t h i c k n e s s e s . Secondly, t h e stratigraphic boundary between Karoo and B u k o b a n r o c k s h a s no w h e r e b e e n observed, p r o b a b l y r o c k s refered to t h e two g r o u p s are of t h e
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s a m e age in s o m e p a r t s of Tanzania. This m a y prove to be a factor w h e n evaluating t h e reservoir as well a s t h e s o u r c e potentials of the basins. Karoo s e d i m e n t a t i o n is r e c o r d e d to have b e g u n during Late Carboniferous followed by a c o n t i n e n t a l rifting p h a s e d u r i n g Permian. The rifting p h a s e c o n t i n u e d progressively into Middle J u r a s s i c , b e c o m i n g less i n t e n s e d u r i n g Late
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The sedimentary basins of Tanzania - reviewed
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during Cretaceous a n d Tertiary. They represent Middle J u r a s s i c s e d i m e n t s are overlain by Upper part of intracratonlc Karoo troughs, (aborted rifts) J u r a s s i c m a r l s m a r k i n g a trangressive p h a s e d i s c u s s e d by Kreuser a n d Semkiwa (1987) a n d which c o n t i n u e d into Lower Cretaceous. This Kreuser et al. (1988) or t h e intraplate rlft b a s i n s of t r a n s g r e s s i o n w a s f o l l o w e d b y a s e r i e s of Helwig (1985). The b a s i n s are filled with u p to t r a n g r e s s i v e a n d r e g r e s s i v e p h a s e s w h i c h 8 k m of c o n t i n e n t a l s e d i m e n t s of fluviatile, characterized t h e m i d d l e p a r t of Cretaceous, lacustrine, deltaic a n d s w a m p y origin. Distinct before it w a s followed by a n o t h e r t r a n g r e s s i o n zones of economical coal s e a m s occur in some towards the e n d of Cretaceous c o n t i n u i n g into basins. Karoo s e d i m e n t s lie u n c o n f o r m a b l y on top Lower Tertiary. Mid Tertiary regression, m a r k e d by of the b a s e m e n t of m e t a m o r p h i c rocks with local the absence of Oligocene in some" .wells (Kent, a n d m i n o r faulting. The b a s i n s are comparable to 1974), was followed by Late Ollgocene/Miocene Karoo b a s i n of S o u t h Africa d e s c r i b e d by trangression said to have b e e n acco/npanied with Woodward (1966) which contain the major coal intensive tectonic activities related to the rifting r e s o u r c e s of t h e African continent. In the S o u t h activities inland at this time (Schlich.'et al., 1974). African Karoo basins, h y d r o c a r b o n occurrences This w a s followed by a regressive p h a s e which lead are k n o w n in the form of parafflnic diatremes, to t h e e s t a b l i s h m e n t of the p r e s e n t coastlines. Being a passive continental margin, s u b s i d e n c e m i n o r oil s e e p s a n d n o n c o m m e r c i a l n a t u r a l gas deposits, e n c o u n t e r e d in some exploration in this b a s i n m u s t have b e e n controlled by two activities, first t h e isostatic s u b s i d e n c e d u r i n g t h e wells. The NE t r e n d i n g R u h u h u basin, east of lake rifting stage, secondly, by the t h e r m a l s u b s i d e n c e Nyasa, is one of the b a s i n s falling u n d e r this group. as a result of cooling of the newly g e n e r a t e d oceanic Here, t h e presence o f t h e oldest Karoo b e d s in crust. Traps will m a i n l y be structural, related to Tanzania is d e m o n s t r a t e d by the existence of the rifting stage, while stratigraphic t r a p s will glacial tfllites dated as Late Carboniferous to mainly be related to t h e passive cooling s u b s i d e n c e Early P e r m i a n (Wopfner a n d Kreuser, 1986). The stage e.g. t h e Mid-Cretaceous transgressive a n d stratigraphy of this b a s i n h a s b e e n reviewed by regressive s e q u e n c e s which form reservoir in t h e Kreuser a n d Semkiwa (1987) a n d C a s s h y a p et al. Songosongo gas field. These will include shelf (1987). A broad belt of Karoo rocks, extending sediments, deltas, s u b m a r i n e fans a n d turbidites. n o r t h e a s t w a r d s from t h e Mozambique b o r d e r Comparison of this basin to t h e coastal basin of a r o u n d S o n g e a to t h e coastal basin, (Selous Kenya (Mbede, 1987) a n d continental shelfs of basin, Fig. I) also falls u n d e r this group. The west m a r g i n of Africa is feasible. thick s e d i m e n t a r y c o n t e n t of this b a s i n is well d e m o n s t r a t e d in t h e aeromagnetic m a p (Batterham B a s i n s f o u n d w t t h i n t h e E. A. rift v a l l e y etal., 1983, fig. 9). In this m a p the basin b r a n c h e s These include b a s i n s which form part of the into two s u b - b a s i n s refered by Spence (1957) as tectonically active East African rlft system. Hydroeastern a n d w e s t e r n basins. Kflosa b a s i n (Fig. 1) or c a r b o n discoveries reported in other rift b a s i n s Mikumi a n d Nyakatltu basin of Kreuser (1984) is s u c h as t h o s e of S h e v r o n in t h e rift b a s i n s fault b o u n d e d on b o t h sides against the b a s e m e n t of s o u t h e r n S u d a n (Schull, 1984) a n d t h o s e forming high g r o u n d s a n d Karoo s e d i m e n t s are described in the small rift b a s i n s of China by covered by n o n e m a r i n e Neogene in some parts. C h a o y u a n i (1985) lead to extensive s t u d i e s to be The stratigraphy of Selous basin is reviewed by focused on t h e East African rift b a s i n s (Rosendahl Hankel (1987) w h o refers to it as Luwegu basin. a n d Livingstone, 1983 a n d Gentz, 1985). In Tanzania, these b a s i n s include t h e R u k w a a n d The C o a s t a l b a s i n R u a h a b a s i n containing u p to 7 a n d 2 k m of T h i s i n c l u d e s t h e w h o l e T a n z a n i a c o a s t a l s e d i m e n t a r y fill respectively, (Pierce a n d Lipkov, belt f r o m t h e K e n y a n b o r d e r (north) to t h e 1987); Lake T a n g a n y i k a a n d Nyasa in which more Mozambiquan b o r d e r (south). It includes a series t h a n 3 a n d 2 k m have b e e n reported respectively of s u b - b a s i n s e.g. Ruvu, Mandawa, R u v u m a (Rosendahl et al., 1986; Ebinger et al., 1987). b a s i n s a n d t h e Islands offshore. The oldest outR e c e n t s t u d i e s , (Crossley a n d Crow, 1980; croping rocks here are of Karoo age including Rosendahl a n d Livingstone, 1983; Ebinger et oiL, t h e T a n g a outcrop, o u t c r o p west of R u v u in 1984; Rosendahl etal., 1986; Ebinger, 1989) have Ngerengere area a n d t h o s e m a p p e d f u r t h e r S o u t h revealed t h a t b a s i n s within t h e East African rift a r o u n d M a n d a w a area. In thIs basin, Karoo rocks valleys are s e g m e n t e d into a series of alternating are fault b o u n d e d to the west against the base- assymetrical, extensional s p o o n - s h a p e d b a s i n s or m e n t , while to t h e east they are overlain by Middle s u b - b a s i n s linked to one a n o t h e r by a ductile J u r a s s i c m a r i n e rocks which overstep the base- deformation within t h e a c c o m m o d a t i o n zones. m e n t in some p a r t s giving first record of t h e These b a s i n s are b o r d e r e d by well developed development of full m a r i n e conditions in the basin. border faults, approximately 100 k m long with a
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throw of 1 - 6 kin. Each b a s i n is suggested to be a u t o n o m o u s with a distinct depocenter, s e d i m e n t thickness, h e a t flow a n d s t r u c t u r a l style (Ebinger, 1989). Stratigraphicaly a n d sedlmentologicaly the basins reveal depositional components associated with continental m a r g i n s including shelf s e d i m e n t s , wedges, deltas, s u b m a r i n e fans, turbidites and slumps (Rosendahl and Livingstone, 1983; Ebinger et al., 1987) giving provision for development of good stratigraphic traps. Both B u k o b a n a n d Karoo rocks crop o u t within a n d in t h e vicinity of t h e p r e s e n t rift valleys. This indicates t h a t Bukoban, as well as Karoo basins, extended far b e y o n d t h e p r e s e n t rifts a n d possibly t h e y u n d e r l y the p r e s e n t lake beds. Hence prospects in these b a s i n s will include Bukoban, Karoo a n d Tertiary. High geothermal gradients normally associated with rifting could have been responsible for t h e p r e m a t u r e m a t u r a t i o n of source rocks even at low s e d i m e n t thicknesses. Two wells drilled by Amoco Production C o m p a n y in R u k w a b a s i n in 1987, p e n e t r a t e d a t h i c k sequence of red b e d s overlying Karoo rocks a n d underlying the lake b e d s of Mio/Pliocene to Recent age which cover t h e rift lakes. Red b e d s in Rukwa area crop out as small outcrops in several p a r t s of the b a s i n a n d have for long b e e n correlated to one a n o t h e r basically on lithological grounds. Early investigators assigned t h e s e beds to the u p p e r section of Karoo (Scholz, 1914; Stockley, 1938, McConnel, 1947 a n d 1950). Later it w a s t h o u g h t that they were m u c h y o u n g e r t h a n Karoo (e.g. Spurr, 1951). Spence (1954) while investigating the Galula coal field correlated the red beds of R u k w a T r o u g h to t h e r e d s a n d s t o n e s (the D i n o s a u r beds) of Malawi r e g a r d e d by Dixey (1928) as Cretaceous. R u s s i a n geologists reported moluscs and reptilian fossils including C e r a t o m y a W i m m i s e n s s e s N u c l a sp. a n d H o m o y a I n o r n a t a Sowerbry etc in t h e red beds o u t c r o p at U s e v i a n o r t h w e s t of L. R u k w a . Matrenitzky et al. (1972) s u g g e s t e d J u r a s s i c (Kallovian) age for these beds. The red beds found by A m m o c o in t h e two wells have b e e n dated as Late Miocene on palynological g r o u n d s {Morley et a/., 1989). Preliminary results of s a m p l e s collected in J u n e 1989 indicate t h a t the Songwe valley outcrop is also of Miocene age. It r e m a i n s a question of f u r t h e r palynological s t u d i e s in Rukwa basin, w h e t h e r t h e red b e d s from different outcrops are of s a m e age or if they represent different depositional events. Cratonic sag basins These b a s i n s differ from other b a s i n s d i s c u s s e d above by the fact t h a t they lie totally on the n o n e t h i n n e d c o n t i n e n t a l c r u s t a n d are not related to any r l l i n g activity (Fig. 3d). They represent intra-
plate passive b a s i n s of Helwig (1985). Lake Victoria a n d Malagaras b a s i n s are here considered to fall u n d e r this group. T h e y are filled by Tertiary s e d i m e n t s w h i c h lie o n t o p of Mid Tertiary erosional surfaces of Miocene to Pliocene age (Quenel et al., 1959). Lake Victoria which is only 10 000 years c o n t a i n s only u p to I 0 0 m thick of Q u a t e n a r y sediments. Tertiary s e d i m e n t s in these b a s i n s are u n d e r l a i n in s o m e p a r t s by rocks B u k o b a n System. B u k o b a n rocks are c o m p o s e d of slightly m e t a m o r p h o s e d clastic s e d i m e n t s containing interbeds of s a n d s t o n e s , siltstones a n d shales interlayered with b a s a l t s a n d gabbros in some p a r t s (Quennel, 1956). A significant section (about 230 m) of gray to black s h a l e s rich in m a c e r a t e d organic debries of c a r b o n a c e o u s n a t u r e is described by Haligan {1961) a r o u n d Uvinza area. B u k o b a n rocks are t h o u g h t to be older t h a n 1010 Ma a n d y o u n g e r t h a n 815 Ma (Cahen et al., 1984). The dating w a s m a i n l y b a s e d on correlation to similar a n d c o n t i n u o u s b e d s in B u r u n d i where K-Ar dating w a s carried o u t in t h e gabbro a n d basalt layers found interlayered with t h e sedim e n t s . The exact age of B u k o b a n s e d i m e n t s is not yet clear b u t t h e middle sectionof BuanJi series is about 650 Ma (Maloney a n d Downie, 1972), so t h e u p p e r section is obviously m u c h y o u n g e r t h a n that. STRUCTURAL EVOLUTION
It h a s for long b e e n d e m o n s t r a t e d t h a t the p r e s e n t s h a p e of Africa w a s largely established during Late P r e c a m b r i a n / E a r l y Paleozoic by the Pan-African orogeny, (Kennedy, 1964 a n d C a h e n et al., 1984). This event (Pan-African) divided the continental l a n d m a s s into stable cratonic nuclei b o u n d e d by zones of relative s t r u c t u r a l weakness. B u k o b a n rocks of Late P r e c a m b r i a n to probably Early Paleozoic age are said to have b e e n laid down in shallow b a s i n s r e s u l t i n g from w a r p i n g of Pre-Bukoban Surfaces, (Quennel, 1956). These surfaces are regarded as first manifestation of "oasinal swell" type of s t r u c t u r e s , said to have b e e n features of African t e c t o n i c s since Late Precambrian (Quennei et al., 1959). The whole of Early to Mid-Paleozoic w a s a p e n e p l a n a t i o n era characterized by regional uplift, extensive erosion a n d deposition over eroded b a s e m e n t in the local downwarps. Except for the cratonic sag basins, s e d i m e n t a r y b a s i n s of Tanzania have b e e n affected by rifting activity at least once, d u r i n g Karoo a n d / o r Late Tertiary rifting. Karoo s e d i m e n t a t i o n w a s initiated in the Late C a r b o n i f e r o u s w h e n glacial tillites were deposited. The rifting p h a s e is t h o u g h t to have started by the e n d of Carboniferous. This resulted f r o m r e g i o n a l d o m a l uplift d u e to t h e r m a l
The sedimentary basins of Tanzania - reviewed
295
e x p a n s i o n a n d c o n v e c t i v e u p w e l l i n g w h i c h within East African ri~ valley, t h e rifting acUvity r e s u l t e d into t h i n n i n g of c o n t i n e n t a l c r u s t . was reactivated d u r i n g Mid-Terttary w h e n faulting Evidence of uplifting h a s n o t yet b e e n established, took place. The rke(ed t r o u g h s were f u r t h e r downit is only infered from t h e theory of rifling as faulted a n d occupied by lakes in s o m e parts. Midexamplifled in m o d e r n rifts a n d t h e existence of Tertiary surfaces of Tanzania were also locally uplifted margins. The domal uplift resulted in w a r p e d into g e n t l e b a s i n s a n d swells. T h i s extension a n d formaUon of g r a b e n s a n d half m e c h a n i s m , similar to w h a t h a p p e n e d d u r i n g g r a b e n s d u r i n g Permian. These were probably B u k o b a n time defined locations for cratonic sag similar to t h o s e described by Rosendahl et oL basins. The sagging a n d s u b s i d e n c e m a y have (1986) a n d Ebinger (1989) in t h e w e s t e r n rift resulted from t h e t h e r m a l m e t a m o r p h i c c h a n g e s s y s t e m . 1000 to 3 0 0 0 m e t e r s of c o n t i n e n t a l in the lower lithosphere w i t h o u t a n y a p p a r e n t s e d i m e n t s were deposited in lacustrine, fluvial. extension or c o m p r e s s i o n a m e c h a n i s m similar to deltaic to s w a m p y environment. The rifting p h a s e that d i s c u s s e d by Helwig (I 985). It m i g h t as well be c o n t i n u e d progressively into Lower J u r a s s i c . related to the extensive forces on t h e two a r m s c a u s i n g periodic m o v e m e n t along b o u n d i n g faults. of the East African rift system. Figure 2 is the This g e n e r a t e d fluvtatfle s y s t e m s which deposited structural m a p of Tanzania, a n d t h e s t r u c t u r a l u p w a r d fining megacycles, consiting of coarse evolution of T a n z a n i a n b a s i n s is s u m m a r i z e d in basal b e d s p a s s i n g u p w a r d s into high energy Fig. 4. braided s t r e a m deposits, s u c c e e d e d by low energy ECONOMIC CONSIDERATIONS m e a n d e r s t r e a m deposits, flood plain a n d s w a m p y environment. Llthofactes variations within Karoo The structural evolution of T a n z a n i a n b a s i n s are t h o u g h t to be d u e to differences in the rate of s u b s i d e n c e a n d s e d i m e n t a t i o n within g r a b e n s a n d h a s p r o d u c e d a wide variety of lithostratigraphic half grabens. Periodic m a r i n e invasion from t h e u n i t s w h i c h c a n provide siginfficant h y d r o North '~vhere o p e n m a r i n e conditions already carbon reservoir u n d e r favourable conditions of existed" lead to restricted m a r i n e b a s i n s or gulfs in generation a n d migration. Both s t r u c t u r a l a n d fast s u b s i d i n g b a s i n s in which evaporites a n d stratigraphic t r a p s s h o u l d be well developed. Structural t r a p s will be m a i n l y related to the extenblack s h a l e s were deposited. At the e n d of Karoo times, m o v e m e n t along sive rifting p h a s e s while stratigraphic t r a p s will be b o u n d i n g f a u l t s r e s u l t e d into f u r t h e r d o w n related to depositional p h a s e s as d i s c u s s e d in the f a u l t i n g a n d p r e s e r v a t i o n of Karoo r o c k s . text. An u n d e r g r o u n d coal m i n e h a s b e e n operating T e c t o n t s m b e c a m e less i n t e n s e d u r i n g Upper Jurassic. The first p h a s e of opening u p of Indian for years at Ilirna (north-west of lake Nyasa). Ocean is believed to have t a k e n place in Late Recently a Chinees t e a m o p e n e d a n o t h e r m i n e J u r a s s i c / L o w e r Cretaceous. w h e n Madagascar adjacent to it. Coal deposits also exists in other started to move s o u t h w a r d s relative to Africa Karoo b a s i n s s u c h as K e t e w a k a - M c h u c h u m a , (Rabinowtz et aL, 1983). Oceanic c r u s t created by Ngaka, NJuga etc (Mackinlay, 1965). Besides this m o v e m e n t formed the floor of the proto Indian researches done in 1980's (Wopfner a n d Kreuser, Ocean. S u b s i d e n c e a c c o m p a n i e d by e a s t w a r d regional tilting c o n t i n u e d as a result of t h e r m a l cooling of t h e newly formed oceanic crust. From t h e n o n w a r d s the coastal basin developed as a passive continetal margin, being covered by a series of transgressive a n d regressive s e q u e n c e s prograding e a s t w a r d s towards the open m a r i n e b a s i n s of t h e developing I n d i a n Ocean. The later part of Paleogene is m a r k e d by significant tectonic reactivation which c o n t i n u e d into Neogene. This w a s r e s p o n s i b l e for t h e m a s s i v e s t r u c t u r a l inversion which left m o s t of the s t r u c t u r a l highs in their p r e s e n t positions (Mbede, /n review). Late P a l e o g e n e t e c t o n i c activities were also a c c o m p a n i e d with intensified tilting of the p r e s e n t o n s h o r e areas as well as rapid s u b s i d e n c e a n d deposition in t h e developing offshore s t r u c t u r a l features, s u c h as Zanzibar a n d Pemba channels. The Karoo rift b a s i n s s h o w little or no evidence of Legend No or few deposits a n y f u r t h e r m o v e m e n t after Late Jurassic. Basins Fig. 4. Structural evolution of Tanzanian sedimentary basins,
2%
E. I. MBEDE
1986; Kreuser and Semkiwa, 1987; Casshyap et a/., 1987; Kreuser etal., 1988), little h a s been done regarding the economic development of these deposits. This is partialy b e c a u s e of the remoteness of the area in which t h e y are located. Recent studies (Kreuser et aL, 1988), indicate t h a t Karoo coal beds in R u h u h u basin could have provided source rocks for economic quantities of gas. This indicates t h a t Karoo rocks m a y prove to be potential source beds in other basins as well. Bukoban sandstones could have provided reservoirs for Karoo source b e d s where t h e y underly them. Oil a n d gas exploration h a s for long been focused on the coastal basin. Gas has been discovered at S o n g o s o n g o a n d Mnazi b a y (KaJato, 1982). Development plan for the Songosongo gas field is u n d e r w a y by Kilwa-Masoko Ammonia Company (KILAMCO). KILAMCO is expecting to utilise the gas either locally for power generation and fertilisee industries or for export (Mwalyego and Moshi, 1987). Though no commercial oil deposit h a s been reported, quite good source rocks are believed to be probably well developed at least locally in the coastal basin (Nzori, pers. comm., 1989). The coastal b a s i n c a n still be c o n s i d e r e d as a n exploration target besides recent relinquishment of all c o n c e s s i o n s (Hartman, 1987). Detailed considerations of depositional cycles in relation to generation, migration a n d a c c u m m u l a t i o n of hydrocarbons at each particular stage of basin development will give a good picture of traps and reservoirs. Stratigraphic traps m a y prove to be more prospective t h a n s t r u c t u r a l traps which have so far been given priority. This is because recent tectonics in this basin m a y have dispersed most of the early formed reserves through migration along fault lines. Isotopic comparison of light hydrocarbons from off shows and the gas in Songosongo have revealed t h a t the two have been in contact at least once. Tracing t e m p e r a t u r e and pressure curves on s t r u c t u r e s east a n d n o r t h - e a s t of Songoson~o gas field s u c h as Nyuni, has shown t h a t f o r m a t i o n s acting as reservoir a n d the predicted source in Songosongo, within these s t r u c t u r e s fall within the oil window depth (Nzori, pers. comr~, 1989). Hence there is a possibility of finding liquid hydrocarbons east of the present gas fields. To u n d e r s t a n d the prospectivity of the basins found within the East African rift valley detailed s t u d y of each basin or sub-basin is required. Since each basin is a u t o n o m o u s with a different s e d i m e n t o l o g i c a l a n d t h e r m a l h i s t o r y , it is evident t h a t their prospectivity also varies. The fact t h a t t h e b a s i n s are a s s y m e t r i c a l a n d alternate, indicates that their areas of m a x i m u m generation and a c c u m u l a t i o n also alternate in
similar manner. Huc (1987, fig. 11) demonstrates t h e c o n c e n t r a t i o n of o r g a n i c m a t t e r at t h e depocenter of Mrembe basin in Lake Tanganyika, this shows that concentration of organic m a t t e r in each basins will vary according to the shlll of the depocenters. Presence or absence of hydrocarbons in one basin does not garantee their existence or none existence in the other. Some basins might be highly prospective while others axe not, some rich in off, some in gas some in both (Zori, pers. comrm, 1989). T e r t i a r y p r o s p e c t s in c r a t o n i c sag b a s i n s , developed u n d e r low geothermal gradient and with low Tertiary s e d i m e n t a r y t h i c k n e s s e s should be considered low. But the thick B u k o b a n section croping out on the sides of these basins is of interest. Bukoban rocks include coarse s a n d s t o n e formations eg. the Uruwira s a n d s t o n e s which c a n provide potential reservoirs if t h e y underly Karoo a n d Tertiary source beds where available. The argellaceous formations s u c h as Nyanza Shales can provide potential source bed if available at depth and are rich in organic matter. Acknow/edgements - Dr. P. W y c i s k of SFB 69, Berlin and Dr. S. Kapillma of the University of Dar-esS a l a a m reviewed the manuscript and helped to improve it. Dr. P. Legge helped in the preparation of the manuscript. The useful discussion with Mr. Nzori is appreciated. REFERENOES
Batterham, P. M., Bullock, S. d. and Hopgood, D. N. 1983. Tanzania: Intergrated Interpretation of Aerom a g n e t i c and Radlometric Maps for Mineral Exploration. Appl~d Earth ScL, Trans. Sect. B, 92, 83-92, London. Cahen, L., Shelling, N.J., Delhal, J. and Vail, J. R. 1984. The Geochronology and evolution of Africa, Oxford, 512 p. Casshyap, S. M., Kreuser, T. and Wopfner, H. 1987. Analysis of cyclical sedimentation in the Lower Permian Mchuchuma Coalfield (Tanzania), Geol. Rundshau, 76(3), 869-883. Chaoyuan, H. 1985, Geological characteristics and oil exploration of small depresions in the eastern China. Geo/ogy, 13, 303-306. Crossley, R. and Crow, M. J. 1980. The Malawi Rffl. In: Geodynamic Evolution of the Afro-Arablan rift system, Rome Accademia Nationale Lincei, 77-87. Dewey, J. F. 1972. Plate tectonics. ScL America, may, 56-68. Dewey, J. F. and Bird, J. M. 1970. Mountain belts and the new global tectonics. J. Geophys. Res., 62, 2625-2647. Dixey, F. 1928. The Dinosaur Beds oflake Nyasa, Trans. Roy. Soc. S. Africa, XVI, 55 p. Ebinger, C. J. 1989. Tectonic Development of The Western Branch of East African rl/1 system. Geol. Soc. Am. Bu//., 101, 885-903.
The sedimentary basins of Tanzania - reviewed Ebinger, C. J., Crow, M. J., R o s e n d a h l , B. R., Livlngstone, D. L. and LeFournier, J. 1984. Structural evolution oflake Malawi, Afi-lca. No/ure, 308, 627- 629. Ebinger, C. J., Rosendahl, B. ]~ and Reynolds, D. J. 1987. Tectonic model of the Malaw/ Rift, Africa. Tectonophyslcs, 141, 215-235. Gentz, J. 1985. Tressure hunt in Africa. ~ ~ , 10, 1-5. Haligan, R. 1961. The Geology of part of Mpanda and Klgoma Districts, Rocks. Geol. Surv. Tanganyika, 9. Hankel, O. 1987. Llthostratigraphlc subdivision of the Karoo rocks of the Luwegu Basin (Tanzania) and their biostratigraphic classification based on m i c r o f l o r a s , m a c r o f l o r a s , fossil woods a n d vertebrates. Ge~./hau/sch~_~ Band 76(2|, 539-565. Hartman, J. B. 1987. Oil and Gas Development in Central and Southern Africa in 1986. AA/~, 71(IOB), 190-225. Helwlg, J. A. 1985. Or/gin and classification of sedimentary basins, Proceedings of the 17th Annual offshore Technology Conference, 1, 21-32. Huc, A. Y. 1987. Aspect of deposition processes of organic matter in sedimentary basins, Org. Geochem., 16(1-3), 824-836. KaJato, H. K. 1982. Gas strike spu~. Search for oil in Tanzania. O1/and Gas J., March 15th, 123-130. Kennedy, W. Q. 1984. The structural differentiation of Africa (500 my) tectonic episode. Annm Rep. Res. Inst. African Geol., Unhr. Leeds, $, 48-49. Kent, P. E. 1974. Leg 25 Results in relation to East Coastal stratigraphy, In: Initial reports of Deep Sea Dr////ng Project, x ' r v , 679-654. Klemme, H. D. 1980. Petroliferous b a s i n s and characteristics. J. Petrol. G e ~ , 3(2), 187-207. Kreuser, T. 1984. Karoo basins In Tanzania In: (Klerx J. K. and J. Michot Eds), African Geology Mus. R. Afr. Cent. Tervuren, 231-246. Kreuser, T. and Semk/wa, P. M. 1987. Geometry and depositlonal history of Karoo (Permian) Coal basin (Muchuchuma Ketewaka) In: S W Tanzania, N. J. Pa/eont. M/~, 2, 69-98, Stuttgart. Kreuser, T., Schramedei, R. and Rullk6tter, J. 1988. Gas-prone source rocks from Kartm basins in Tanzania. J. Pert. Geol., 11(2), 169-184. Maloney, R. and Downle, C. 1972. Palynomorphs from the BuanJl Series Bukoban System and theft stratigmphlc implication. ~ y of Africa and the ~ ~ j islands, Cape Town, 7, 18 p. Matrenlzky, T., Smlrnov, V., Pentelcov, V., Tolochkov, V., Trlfan, M., and Zhukov, S. 1973. Geology and minerals of central part of the Western Rift, Unpubl. reports. Madini, Dodoma. McConnel, R. B. 1947. The geology of NamweleMkomolo coalfield, Uflpa district, Dept. of lands and mines. Ge~. Divlskm TanganySca, 27, 58 p. McConnel, R. B. 1950. Investigations of Uflpa coalfields. Geol. Sur~. Dept., Ann. Rep., Tanganyika, 10 p. McKenzie, D. P. 1972. Plate tectonics, In: (Robertson E. C. Ed.}, The nature of the solki earth, Ge~hy. J. Roy. Astron. Soc., 18, 1-32. Mckinlay, A. C. M. 1965. Coalfields and Coal Resources of Tanzania. Ge~. Surv. Tanganylka Bull., 38, 82 p.,
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Dodoma. Mbede, E. I. 1987. A review of the hydrocarbon potential of Kenya. J. Afr/. Earth Sc/., 6(3), 313-322. Morley, C. F~, Wescott, W. A., Cunningham, S. M. and Haper, R. M. 1989. Recent Exploration in Lake Rukwa, East African Rift, 28th International Geological Congress. Abstract, 2-462 to 2-463. Mwalyego, Y. S. a n d Moshi, W. A. E. K. 1987. Songosongo Gas. Apraisal, Drilling and plans for use. SADC Energy, 13, 12-13. Peirce, J. W. a n d Lipkov, L. 1987. S t r u c t u r a l interpretation of the Rukwa Rift, Tanzania. Geophysics, 53(6), 824-836. Quennel, A. M. 1956. The Bukoban System of East Africa, Proceedings of the 20th International Geological Congress, 20th session, Part 1,281-307. Quennel, A. M., Mckinlay, A. C. M., and Altken, W. G. 1959. Summary of the Geology of Tanganyika, Part I, Introduction and stratigraphy. Mere. Geol. Surv. Tanganyika, 64 p. Rabinowttz, P. D., Coffin, M. F. and Falvey, D. 1983. The separation of Madagascar and Africa. Sc/ence, 2 2 2 , 67-69. Rosendahl, B. R. and Livingstone, D.A. 1983. Rift lakes of East Africa: New seislmlc data and lmplicaUon of future research. Eptsode, 83, 14-19. Rosendahl, B. R., Reynolds, D. J., Lober, P. M., Bugress, C. F., McGlll, J., Scott, D., Lamblase, J. J. and Derksen, S. J. 1986. Structural expression of rlRing: lesson from lake Tanganylka, In: (Frostick et al. Eds), Sedimentation in African Riffs, Geol. Soc. special publ., 25, 29-34. Schlich, R., Simpson, E. S. W. and Vallier, T. L 1974. Regional aspects of Deep Sea Drilling in Western Indian Ocean, In: Int. Rep. of DSDP, XXV, 743-759. Scholz, E. 1914. Beitrag zu Geologie der Sudwestlichen Graben-Gebiet Deutsch Ostafrica. Tanganylka. Annot~ 2, 10p. Schull, T. J. 1984. Off exploration in the non-marine rift basins of interior Sudan: Presented to Can. Petr. Geol. in Calgary. Spence, J. 1954. The geology of the Galula coalfield. Geo/. Surv. Dept. BULL,25, 34 p. Spence, J. 1957. The Geology of part of the Eastern Province of Tanganylka. Bull. Geol. Stwv. Tanganyika, 62, 28 p. Spun', A. M. 1951. New occurences of coal in the Rukwa Rift Valley. Geol. Surv., Tanganyika. Min. Res. pamphlet, 57, 5p. Stocldey, G. M. 1938, The geology of parts of the Tabora Kigoma and Uflpa District NW L. Rukwa. Ge~. Divs., Dept. of lands and mines, Tanganylka, 20, 35 p. Stonley, R. ]981. Petroleum, the sedimentary basin. Economic Geology and Ge~tectonics, 51-71. Thompson, T. L. 1967. Plate tectonics in Oil and Gas exploration of Continental Margins. AA/~, 60(3), 1463-1501. Woodward, J. E. 1966. Stratigraphy and oil occurences In the Karoo basin, South Africa, Proc. Petrol. 8th Commonwealth Min. Met. Congr., 1965, 5, 45-56. Wopfner, H. and Kreuser, T. 1986. Evidence for late Paleozoic glaciation in southern Tanzania. Pa/eogeoyraphy, Paleocllmatok~gy, Palaeoecology, 56, 259-275, Amsterdam.
0899-5362/91 $3.00 + 0.(30 © 1991 Pergamon Press pl
Printed/n Thailand
The sedimentary basins of Tanzania - reviewed E. I. ~[BEDE
University of Dar-es-Salaam, Box 35052, DSM, Tanzania (Accepted for publication 15th February, 1990) AbsU'aet- The sedimentarybasins of Tanzania have been classifiedinto four morphotectonicgroups: the coastal basin, the Karoo rift basins, basins found within the present East African rift valley and the cratonic sag basins. Except for the cratonic sag basins, each of these basin group has been affectedby rifting at one time or another. The geology of each basin is discussed, structural evolution is evaluated and the prospectivity is thence looked into. Coal is exploitedat Songwe-Kiwiracoalfieldand is foundin potentiallyeconomicquantities in other Karoo basins. Prospecting for hydrocarbonresourceshas been going on since the 50s. Gas has been discoveredin Songosongoand Mnazi bay fields, uneconomicalquantities ofoil have also been reported in Songosongo.Being basicallyrift basins which have reached different stages of development, source rocks normally associated with Initial-rifting, synrifting as well as post-riftingprocessesare probablywelldeveloped.Resevoirrocks, traps and caprocks are normally not rare in such tectonicenvironments. Thermal gradients associatedwith the rifting stageare normaly high to effect maturation of source rocks even at low sedimentary thicknesses. Studies done so far are still inconclusive, because while testing has mainly been focused on structural traps stratigraphic traps seems to be more promising.
INTRODUCTION
A s e d i m e n t a r y b a s i n is defined a s a n e l e m e n t of t h e E a r t h ' s s u r f a c e w h e r e s e d i m e n t s have acc u m u l a t e d for a significant time (Stonley, 1981). The scale of t h e b a s i n varies greatly from o c e a n b a s i n s to small p o n d s , b u t for a long time e a r t h s c i e n t i s t s h a v e b e e n restricting t h e m s e l v e s to r e a s o n a b l y b i g s e d i m e n t filled d e p r e s s i o n s ( I 0 0 k m long a n d above) with evidence of deep s u b s i d e n c e a n d significant d e f o r m a t i o n (Helwig, 1985). The d e v e l o p m e n t of t h e plate tectonic t h e o r y h a s offered a n e w w a y of explaining t h e evolution of s e d i m e n t a r y b a s i n s (Dewey a n d Bird, 1970; Dewey, 1972 a n d McKenzie, 1972). The t h e o r y is also t h o u g h t to b e suggestive of guidelines for oil a n d g a s exploration (Thompson, 1967). Putting a b a s i n in its p r o p e r c l a s s m a y prove to be a worthwhile exercise b e c a u s e it h e l p s in the q u i c k a n a l y s i s of t h e t e c t o n i c h i s t o r y of t h e b a s i n especially if one tries to c o m p a r e it with o t h e r similar b a s i n s in t h e world w h i c h have b e e n t h o r o u g l y studied. As Klemme (1980} suggests, w h e n l i n k e d to t h e v a r i a b i l i t y of p e t r o l e u m characteristics, b a s i n classification m a y a p p r a i s e the p e t r o l e u m potential of n e w frontier b a s i n s or f u r t h e r d e v e l o p m e n t in old b a s i n s . This p a p e r a t t e m p t s to s y n t h e s i z e t h e geod y n a m i c m o d e l of T a n z a n i a n b a s i n s a n d their g e n e s i s in relation to plate tectonic setting. It also tries to relate their s t r u c t u r a l d e v e l o p m e n t to prospectlvity a n d c o m p a r e t h e m to similar b a s i n s
in t h e world. The w o r k is a n o u t c o m e of T a n z a n i a n b a s i n a n a l y s i s carried o u t in 1986 b y NORAD a n d TPDC to which the a u t h o r participated a s a m e m b e r of staff from t h e University of D a r - e s - S a l a a m . Only b a s i n s with significant s e d i m e n t a r y t h i c k n e s s are considered. Figure 1 shows the sedimentary b a s i n s of Tanzania. The b a s i n s have tectonicaly b e e n classified into f o u r g r o u p s . E a c h g r o u p h a s b e e n defined o n t h e b a s i s of form, s e d i m e n t fill, tectonic setting a n d the interelated tectonic events a s p r o p o s e d b y Helwig (1985). The b a s i n s include t h e c o a s t a l b a s i n c o n s i d e r e d a s a passive continental margin; b a s i n s g r o u p e d together a s Karoo rlft b a s i n s c o n s i d e r e d a s a b o r t e d rlft basins; b a s i n s f o u n d within t h e p r e s e n t E a s t African rlft valley, a n d the b a s i n s refered to a s cratonic sag b a s i n s w h i c h are b a s i n s f o u n d within the stable a r e a s w h i c h s h o w n o evidence ofrift relate tectonic activities. Figure 3 is t h e p r o p o s e d classification of Tanzanian basins. Apart from r o c k s of Karoo age no r o c k s of Paleozoic age have b e e n r e c o g n i s e d so far in T a n z a n i a , b u t r o c k s of t h e B u k o b a n s y s t e m (ca 9 5 0 Ma), w h i c h crop o u t extensively in the vicinity of t h e Late Tertiary rlft valleys a s well as cratonic s a g b a s i n s might b e of interest for p e t r o l e u m exploration in this country. F o r e m o s t , B u k o b a n r o c k s are p r e d o m i n a n t l y clastic a n d a t t a i n c o n s i d e r a b l e t h i c k n e s s e s . Secondly, t h e stratigraphic boundary between Karoo and B u k o b a n r o c k s h a s no w h e r e b e e n observed, p r o b a b l y r o c k s refered to t h e two g r o u p s are of t h e
291
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J u r a s s i c . Along t h e c o a s t full m a r i n e conditions e s t a b l i s h e d d u r i n g Middle J u r a s s i c c o n t i n u e d into Tertiary. B a s i n s g r o u p e d h e r e a s Karoo rift b a s i n s s h o w little or n o evidence of activity after t h e e n d of Karoo m o v e m e n t s . T h e rifting p h a s e a c c o m p a n i e d with i g n e o u s activities w a s active again in t h e p r e s e n t rift valleys d u r i n g late Mid Tertiary. Cratonic sag b a s i n s h a v e a s e p a r a t e history altogether, being controlled by warping t h r o u g h o u t t h e i r history.
KENYA
GEOTECTONIC TYPE OF TANZANIAN ]8~SINS Wig. s) Karoo rift b a s i n s T h e s e include Karoo b a s i n s in w h i c h faults are said to h a v e c a u s e d d i s p l a c e m e n t of i k m or m o r e d u r i n g P e r m i a n to Early J u r a s s i c b u t practically s h o w little or no evidence of s u b s e q u e n t activity
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s a m e age in s o m e p a r t s of Tanzania. This m a y prove to be a factor w h e n evaluating t h e reservoir as well a s t h e s o u r c e potentials of the basins. Karoo s e d i m e n t a t i o n is r e c o r d e d to have b e g u n during Late Carboniferous followed by a c o n t i n e n t a l rifting p h a s e d u r i n g Permian. The rifting p h a s e c o n t i n u e d progressively into Middle J u r a s s i c , b e c o m i n g less i n t e n s e d u r i n g Late
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The sedimentary basins of Tanzania - reviewed
293
during Cretaceous a n d Tertiary. They represent Middle J u r a s s i c s e d i m e n t s are overlain by Upper part of intracratonlc Karoo troughs, (aborted rifts) J u r a s s i c m a r l s m a r k i n g a trangressive p h a s e d i s c u s s e d by Kreuser a n d Semkiwa (1987) a n d which c o n t i n u e d into Lower Cretaceous. This Kreuser et al. (1988) or t h e intraplate rlft b a s i n s of t r a n s g r e s s i o n w a s f o l l o w e d b y a s e r i e s of Helwig (1985). The b a s i n s are filled with u p to t r a n g r e s s i v e a n d r e g r e s s i v e p h a s e s w h i c h 8 k m of c o n t i n e n t a l s e d i m e n t s of fluviatile, characterized t h e m i d d l e p a r t of Cretaceous, lacustrine, deltaic a n d s w a m p y origin. Distinct before it w a s followed by a n o t h e r t r a n g r e s s i o n zones of economical coal s e a m s occur in some towards the e n d of Cretaceous c o n t i n u i n g into basins. Karoo s e d i m e n t s lie u n c o n f o r m a b l y on top Lower Tertiary. Mid Tertiary regression, m a r k e d by of the b a s e m e n t of m e t a m o r p h i c rocks with local the absence of Oligocene in some" .wells (Kent, a n d m i n o r faulting. The b a s i n s are comparable to 1974), was followed by Late Ollgocene/Miocene Karoo b a s i n of S o u t h Africa d e s c r i b e d by trangression said to have b e e n acco/npanied with Woodward (1966) which contain the major coal intensive tectonic activities related to the rifting r e s o u r c e s of t h e African continent. In the S o u t h activities inland at this time (Schlich.'et al., 1974). African Karoo basins, h y d r o c a r b o n occurrences This w a s followed by a regressive p h a s e which lead are k n o w n in the form of parafflnic diatremes, to t h e e s t a b l i s h m e n t of the p r e s e n t coastlines. Being a passive continental margin, s u b s i d e n c e m i n o r oil s e e p s a n d n o n c o m m e r c i a l n a t u r a l gas deposits, e n c o u n t e r e d in some exploration in this b a s i n m u s t have b e e n controlled by two activities, first t h e isostatic s u b s i d e n c e d u r i n g t h e wells. The NE t r e n d i n g R u h u h u basin, east of lake rifting stage, secondly, by the t h e r m a l s u b s i d e n c e Nyasa, is one of the b a s i n s falling u n d e r this group. as a result of cooling of the newly g e n e r a t e d oceanic Here, t h e presence o f t h e oldest Karoo b e d s in crust. Traps will m a i n l y be structural, related to Tanzania is d e m o n s t r a t e d by the existence of the rifting stage, while stratigraphic t r a p s will glacial tfllites dated as Late Carboniferous to mainly be related to t h e passive cooling s u b s i d e n c e Early P e r m i a n (Wopfner a n d Kreuser, 1986). The stage e.g. t h e Mid-Cretaceous transgressive a n d stratigraphy of this b a s i n h a s b e e n reviewed by regressive s e q u e n c e s which form reservoir in t h e Kreuser a n d Semkiwa (1987) a n d C a s s h y a p et al. Songosongo gas field. These will include shelf (1987). A broad belt of Karoo rocks, extending sediments, deltas, s u b m a r i n e fans a n d turbidites. n o r t h e a s t w a r d s from t h e Mozambique b o r d e r Comparison of this basin to t h e coastal basin of a r o u n d S o n g e a to t h e coastal basin, (Selous Kenya (Mbede, 1987) a n d continental shelfs of basin, Fig. I) also falls u n d e r this group. The west m a r g i n of Africa is feasible. thick s e d i m e n t a r y c o n t e n t of this b a s i n is well d e m o n s t r a t e d in t h e aeromagnetic m a p (Batterham B a s i n s f o u n d w t t h i n t h e E. A. rift v a l l e y etal., 1983, fig. 9). In this m a p the basin b r a n c h e s These include b a s i n s which form part of the into two s u b - b a s i n s refered by Spence (1957) as tectonically active East African rlft system. Hydroeastern a n d w e s t e r n basins. Kflosa b a s i n (Fig. 1) or c a r b o n discoveries reported in other rift b a s i n s Mikumi a n d Nyakatltu basin of Kreuser (1984) is s u c h as t h o s e of S h e v r o n in t h e rift b a s i n s fault b o u n d e d on b o t h sides against the b a s e m e n t of s o u t h e r n S u d a n (Schull, 1984) a n d t h o s e forming high g r o u n d s a n d Karoo s e d i m e n t s are described in the small rift b a s i n s of China by covered by n o n e m a r i n e Neogene in some parts. C h a o y u a n i (1985) lead to extensive s t u d i e s to be The stratigraphy of Selous basin is reviewed by focused on t h e East African rift b a s i n s (Rosendahl Hankel (1987) w h o refers to it as Luwegu basin. a n d Livingstone, 1983 a n d Gentz, 1985). In Tanzania, these b a s i n s include t h e R u k w a a n d The C o a s t a l b a s i n R u a h a b a s i n containing u p to 7 a n d 2 k m of T h i s i n c l u d e s t h e w h o l e T a n z a n i a c o a s t a l s e d i m e n t a r y fill respectively, (Pierce a n d Lipkov, belt f r o m t h e K e n y a n b o r d e r (north) to t h e 1987); Lake T a n g a n y i k a a n d Nyasa in which more Mozambiquan b o r d e r (south). It includes a series t h a n 3 a n d 2 k m have b e e n reported respectively of s u b - b a s i n s e.g. Ruvu, Mandawa, R u v u m a (Rosendahl et al., 1986; Ebinger et al., 1987). b a s i n s a n d t h e Islands offshore. The oldest outR e c e n t s t u d i e s , (Crossley a n d Crow, 1980; croping rocks here are of Karoo age including Rosendahl a n d Livingstone, 1983; Ebinger et oiL, t h e T a n g a outcrop, o u t c r o p west of R u v u in 1984; Rosendahl etal., 1986; Ebinger, 1989) have Ngerengere area a n d t h o s e m a p p e d f u r t h e r S o u t h revealed t h a t b a s i n s within t h e East African rift a r o u n d M a n d a w a area. In thIs basin, Karoo rocks valleys are s e g m e n t e d into a series of alternating are fault b o u n d e d to the west against the base- assymetrical, extensional s p o o n - s h a p e d b a s i n s or m e n t , while to t h e east they are overlain by Middle s u b - b a s i n s linked to one a n o t h e r by a ductile J u r a s s i c m a r i n e rocks which overstep the base- deformation within t h e a c c o m m o d a t i o n zones. m e n t in some p a r t s giving first record of t h e These b a s i n s are b o r d e r e d by well developed development of full m a r i n e conditions in the basin. border faults, approximately 100 k m long with a
294
E. I. MBEDE
throw of 1 - 6 kin. Each b a s i n is suggested to be a u t o n o m o u s with a distinct depocenter, s e d i m e n t thickness, h e a t flow a n d s t r u c t u r a l style (Ebinger, 1989). Stratigraphicaly a n d sedlmentologicaly the basins reveal depositional components associated with continental m a r g i n s including shelf s e d i m e n t s , wedges, deltas, s u b m a r i n e fans, turbidites and slumps (Rosendahl and Livingstone, 1983; Ebinger et al., 1987) giving provision for development of good stratigraphic traps. Both B u k o b a n a n d Karoo rocks crop o u t within a n d in t h e vicinity of t h e p r e s e n t rift valleys. This indicates t h a t Bukoban, as well as Karoo basins, extended far b e y o n d t h e p r e s e n t rifts a n d possibly t h e y u n d e r l y the p r e s e n t lake beds. Hence prospects in these b a s i n s will include Bukoban, Karoo a n d Tertiary. High geothermal gradients normally associated with rifting could have been responsible for t h e p r e m a t u r e m a t u r a t i o n of source rocks even at low s e d i m e n t thicknesses. Two wells drilled by Amoco Production C o m p a n y in R u k w a b a s i n in 1987, p e n e t r a t e d a t h i c k sequence of red b e d s overlying Karoo rocks a n d underlying the lake b e d s of Mio/Pliocene to Recent age which cover t h e rift lakes. Red b e d s in Rukwa area crop out as small outcrops in several p a r t s of the b a s i n a n d have for long b e e n correlated to one a n o t h e r basically on lithological grounds. Early investigators assigned t h e s e beds to the u p p e r section of Karoo (Scholz, 1914; Stockley, 1938, McConnel, 1947 a n d 1950). Later it w a s t h o u g h t that they were m u c h y o u n g e r t h a n Karoo (e.g. Spurr, 1951). Spence (1954) while investigating the Galula coal field correlated the red beds of R u k w a T r o u g h to t h e r e d s a n d s t o n e s (the D i n o s a u r beds) of Malawi r e g a r d e d by Dixey (1928) as Cretaceous. R u s s i a n geologists reported moluscs and reptilian fossils including C e r a t o m y a W i m m i s e n s s e s N u c l a sp. a n d H o m o y a I n o r n a t a Sowerbry etc in t h e red beds o u t c r o p at U s e v i a n o r t h w e s t of L. R u k w a . Matrenitzky et al. (1972) s u g g e s t e d J u r a s s i c (Kallovian) age for these beds. The red beds found by A m m o c o in t h e two wells have b e e n dated as Late Miocene on palynological g r o u n d s {Morley et a/., 1989). Preliminary results of s a m p l e s collected in J u n e 1989 indicate t h a t the Songwe valley outcrop is also of Miocene age. It r e m a i n s a question of f u r t h e r palynological s t u d i e s in Rukwa basin, w h e t h e r t h e red b e d s from different outcrops are of s a m e age or if they represent different depositional events. Cratonic sag basins These b a s i n s differ from other b a s i n s d i s c u s s e d above by the fact t h a t they lie totally on the n o n e t h i n n e d c o n t i n e n t a l c r u s t a n d are not related to any r l l i n g activity (Fig. 3d). They represent intra-
plate passive b a s i n s of Helwig (1985). Lake Victoria a n d Malagaras b a s i n s are here considered to fall u n d e r this group. T h e y are filled by Tertiary s e d i m e n t s w h i c h lie o n t o p of Mid Tertiary erosional surfaces of Miocene to Pliocene age (Quenel et al., 1959). Lake Victoria which is only 10 000 years c o n t a i n s only u p to I 0 0 m thick of Q u a t e n a r y sediments. Tertiary s e d i m e n t s in these b a s i n s are u n d e r l a i n in s o m e p a r t s by rocks B u k o b a n System. B u k o b a n rocks are c o m p o s e d of slightly m e t a m o r p h o s e d clastic s e d i m e n t s containing interbeds of s a n d s t o n e s , siltstones a n d shales interlayered with b a s a l t s a n d gabbros in some p a r t s (Quennel, 1956). A significant section (about 230 m) of gray to black s h a l e s rich in m a c e r a t e d organic debries of c a r b o n a c e o u s n a t u r e is described by Haligan {1961) a r o u n d Uvinza area. B u k o b a n rocks are t h o u g h t to be older t h a n 1010 Ma a n d y o u n g e r t h a n 815 Ma (Cahen et al., 1984). The dating w a s m a i n l y b a s e d on correlation to similar a n d c o n t i n u o u s b e d s in B u r u n d i where K-Ar dating w a s carried o u t in t h e gabbro a n d basalt layers found interlayered with t h e sedim e n t s . The exact age of B u k o b a n s e d i m e n t s is not yet clear b u t t h e middle sectionof BuanJi series is about 650 Ma (Maloney a n d Downie, 1972), so t h e u p p e r section is obviously m u c h y o u n g e r t h a n that. STRUCTURAL EVOLUTION
It h a s for long b e e n d e m o n s t r a t e d t h a t the p r e s e n t s h a p e of Africa w a s largely established during Late P r e c a m b r i a n / E a r l y Paleozoic by the Pan-African orogeny, (Kennedy, 1964 a n d C a h e n et al., 1984). This event (Pan-African) divided the continental l a n d m a s s into stable cratonic nuclei b o u n d e d by zones of relative s t r u c t u r a l weakness. B u k o b a n rocks of Late P r e c a m b r i a n to probably Early Paleozoic age are said to have b e e n laid down in shallow b a s i n s r e s u l t i n g from w a r p i n g of Pre-Bukoban Surfaces, (Quennel, 1956). These surfaces are regarded as first manifestation of "oasinal swell" type of s t r u c t u r e s , said to have b e e n features of African t e c t o n i c s since Late Precambrian (Quennei et al., 1959). The whole of Early to Mid-Paleozoic w a s a p e n e p l a n a t i o n era characterized by regional uplift, extensive erosion a n d deposition over eroded b a s e m e n t in the local downwarps. Except for the cratonic sag basins, s e d i m e n t a r y b a s i n s of Tanzania have b e e n affected by rifting activity at least once, d u r i n g Karoo a n d / o r Late Tertiary rifting. Karoo s e d i m e n t a t i o n w a s initiated in the Late C a r b o n i f e r o u s w h e n glacial tillites were deposited. The rifting p h a s e is t h o u g h t to have started by the e n d of Carboniferous. This resulted f r o m r e g i o n a l d o m a l uplift d u e to t h e r m a l
The sedimentary basins of Tanzania - reviewed
295
e x p a n s i o n a n d c o n v e c t i v e u p w e l l i n g w h i c h within East African ri~ valley, t h e rifting acUvity r e s u l t e d into t h i n n i n g of c o n t i n e n t a l c r u s t . was reactivated d u r i n g Mid-Terttary w h e n faulting Evidence of uplifting h a s n o t yet b e e n established, took place. The rke(ed t r o u g h s were f u r t h e r downit is only infered from t h e theory of rifling as faulted a n d occupied by lakes in s o m e parts. Midexamplifled in m o d e r n rifts a n d t h e existence of Tertiary surfaces of Tanzania were also locally uplifted margins. The domal uplift resulted in w a r p e d into g e n t l e b a s i n s a n d swells. T h i s extension a n d formaUon of g r a b e n s a n d half m e c h a n i s m , similar to w h a t h a p p e n e d d u r i n g g r a b e n s d u r i n g Permian. These were probably B u k o b a n time defined locations for cratonic sag similar to t h o s e described by Rosendahl et oL basins. The sagging a n d s u b s i d e n c e m a y have (1986) a n d Ebinger (1989) in t h e w e s t e r n rift resulted from t h e t h e r m a l m e t a m o r p h i c c h a n g e s s y s t e m . 1000 to 3 0 0 0 m e t e r s of c o n t i n e n t a l in the lower lithosphere w i t h o u t a n y a p p a r e n t s e d i m e n t s were deposited in lacustrine, fluvial. extension or c o m p r e s s i o n a m e c h a n i s m similar to deltaic to s w a m p y environment. The rifting p h a s e that d i s c u s s e d by Helwig (I 985). It m i g h t as well be c o n t i n u e d progressively into Lower J u r a s s i c . related to the extensive forces on t h e two a r m s c a u s i n g periodic m o v e m e n t along b o u n d i n g faults. of the East African rift system. Figure 2 is the This g e n e r a t e d fluvtatfle s y s t e m s which deposited structural m a p of Tanzania, a n d t h e s t r u c t u r a l u p w a r d fining megacycles, consiting of coarse evolution of T a n z a n i a n b a s i n s is s u m m a r i z e d in basal b e d s p a s s i n g u p w a r d s into high energy Fig. 4. braided s t r e a m deposits, s u c c e e d e d by low energy ECONOMIC CONSIDERATIONS m e a n d e r s t r e a m deposits, flood plain a n d s w a m p y environment. Llthofactes variations within Karoo The structural evolution of T a n z a n i a n b a s i n s are t h o u g h t to be d u e to differences in the rate of s u b s i d e n c e a n d s e d i m e n t a t i o n within g r a b e n s a n d h a s p r o d u c e d a wide variety of lithostratigraphic half grabens. Periodic m a r i n e invasion from t h e u n i t s w h i c h c a n provide siginfficant h y d r o North '~vhere o p e n m a r i n e conditions already carbon reservoir u n d e r favourable conditions of existed" lead to restricted m a r i n e b a s i n s or gulfs in generation a n d migration. Both s t r u c t u r a l a n d fast s u b s i d i n g b a s i n s in which evaporites a n d stratigraphic t r a p s s h o u l d be well developed. Structural t r a p s will be m a i n l y related to the extenblack s h a l e s were deposited. At the e n d of Karoo times, m o v e m e n t along sive rifting p h a s e s while stratigraphic t r a p s will be b o u n d i n g f a u l t s r e s u l t e d into f u r t h e r d o w n related to depositional p h a s e s as d i s c u s s e d in the f a u l t i n g a n d p r e s e r v a t i o n of Karoo r o c k s . text. An u n d e r g r o u n d coal m i n e h a s b e e n operating T e c t o n t s m b e c a m e less i n t e n s e d u r i n g Upper Jurassic. The first p h a s e of opening u p of Indian for years at Ilirna (north-west of lake Nyasa). Ocean is believed to have t a k e n place in Late Recently a Chinees t e a m o p e n e d a n o t h e r m i n e J u r a s s i c / L o w e r Cretaceous. w h e n Madagascar adjacent to it. Coal deposits also exists in other started to move s o u t h w a r d s relative to Africa Karoo b a s i n s s u c h as K e t e w a k a - M c h u c h u m a , (Rabinowtz et aL, 1983). Oceanic c r u s t created by Ngaka, NJuga etc (Mackinlay, 1965). Besides this m o v e m e n t formed the floor of the proto Indian researches done in 1980's (Wopfner a n d Kreuser, Ocean. S u b s i d e n c e a c c o m p a n i e d by e a s t w a r d regional tilting c o n t i n u e d as a result of t h e r m a l cooling of t h e newly formed oceanic crust. From t h e n o n w a r d s the coastal basin developed as a passive continetal margin, being covered by a series of transgressive a n d regressive s e q u e n c e s prograding e a s t w a r d s towards the open m a r i n e b a s i n s of t h e developing I n d i a n Ocean. The later part of Paleogene is m a r k e d by significant tectonic reactivation which c o n t i n u e d into Neogene. This w a s r e s p o n s i b l e for t h e m a s s i v e s t r u c t u r a l inversion which left m o s t of the s t r u c t u r a l highs in their p r e s e n t positions (Mbede, /n review). Late P a l e o g e n e t e c t o n i c activities were also a c c o m p a n i e d with intensified tilting of the p r e s e n t o n s h o r e areas as well as rapid s u b s i d e n c e a n d deposition in t h e developing offshore s t r u c t u r a l features, s u c h as Zanzibar a n d Pemba channels. The Karoo rift b a s i n s s h o w little or no evidence of Legend No or few deposits a n y f u r t h e r m o v e m e n t after Late Jurassic. Basins Fig. 4. Structural evolution of Tanzanian sedimentary basins,
2%
E. I. MBEDE
1986; Kreuser and Semkiwa, 1987; Casshyap et a/., 1987; Kreuser etal., 1988), little h a s been done regarding the economic development of these deposits. This is partialy b e c a u s e of the remoteness of the area in which t h e y are located. Recent studies (Kreuser et aL, 1988), indicate t h a t Karoo coal beds in R u h u h u basin could have provided source rocks for economic quantities of gas. This indicates t h a t Karoo rocks m a y prove to be potential source beds in other basins as well. Bukoban sandstones could have provided reservoirs for Karoo source b e d s where t h e y underly them. Oil a n d gas exploration h a s for long been focused on the coastal basin. Gas has been discovered at S o n g o s o n g o a n d Mnazi b a y (KaJato, 1982). Development plan for the Songosongo gas field is u n d e r w a y by Kilwa-Masoko Ammonia Company (KILAMCO). KILAMCO is expecting to utilise the gas either locally for power generation and fertilisee industries or for export (Mwalyego and Moshi, 1987). Though no commercial oil deposit h a s been reported, quite good source rocks are believed to be probably well developed at least locally in the coastal basin (Nzori, pers. comm., 1989). The coastal b a s i n c a n still be c o n s i d e r e d as a n exploration target besides recent relinquishment of all c o n c e s s i o n s (Hartman, 1987). Detailed considerations of depositional cycles in relation to generation, migration a n d a c c u m m u l a t i o n of hydrocarbons at each particular stage of basin development will give a good picture of traps and reservoirs. Stratigraphic traps m a y prove to be more prospective t h a n s t r u c t u r a l traps which have so far been given priority. This is because recent tectonics in this basin m a y have dispersed most of the early formed reserves through migration along fault lines. Isotopic comparison of light hydrocarbons from off shows and the gas in Songosongo have revealed t h a t the two have been in contact at least once. Tracing t e m p e r a t u r e and pressure curves on s t r u c t u r e s east a n d n o r t h - e a s t of Songoson~o gas field s u c h as Nyuni, has shown t h a t f o r m a t i o n s acting as reservoir a n d the predicted source in Songosongo, within these s t r u c t u r e s fall within the oil window depth (Nzori, pers. comr~, 1989). Hence there is a possibility of finding liquid hydrocarbons east of the present gas fields. To u n d e r s t a n d the prospectivity of the basins found within the East African rift valley detailed s t u d y of each basin or sub-basin is required. Since each basin is a u t o n o m o u s with a different s e d i m e n t o l o g i c a l a n d t h e r m a l h i s t o r y , it is evident t h a t their prospectivity also varies. The fact t h a t t h e b a s i n s are a s s y m e t r i c a l a n d alternate, indicates that their areas of m a x i m u m generation and a c c u m u l a t i o n also alternate in
similar manner. Huc (1987, fig. 11) demonstrates t h e c o n c e n t r a t i o n of o r g a n i c m a t t e r at t h e depocenter of Mrembe basin in Lake Tanganyika, this shows that concentration of organic m a t t e r in each basins will vary according to the shlll of the depocenters. Presence or absence of hydrocarbons in one basin does not garantee their existence or none existence in the other. Some basins might be highly prospective while others axe not, some rich in off, some in gas some in both (Zori, pers. comrm, 1989). T e r t i a r y p r o s p e c t s in c r a t o n i c sag b a s i n s , developed u n d e r low geothermal gradient and with low Tertiary s e d i m e n t a r y t h i c k n e s s e s should be considered low. But the thick B u k o b a n section croping out on the sides of these basins is of interest. Bukoban rocks include coarse s a n d s t o n e formations eg. the Uruwira s a n d s t o n e s which c a n provide potential reservoirs if t h e y underly Karoo a n d Tertiary source beds where available. The argellaceous formations s u c h as Nyanza Shales can provide potential source bed if available at depth and are rich in organic matter. Acknow/edgements - Dr. P. W y c i s k of SFB 69, Berlin and Dr. S. Kapillma of the University of Dar-esS a l a a m reviewed the manuscript and helped to improve it. Dr. P. Legge helped in the preparation of the manuscript. The useful discussion with Mr. Nzori is appreciated. REFERENOES
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