Chapter 10 The coastal basins of somalia, kenya and tanzania

Chapter 10

The Coastal Basins of Somalia, Kenya and Tanzania

E.I. MBEDE and A. DUALEH

INTRODUCTION

The western Indian Ocean seaboard is an Atlantic type of continental margin, hence the sedimentary basins involved are typical pull-apart basins of Klemme (1980). The margin has, however been subjected to transform movements through Late Jurassic to Late Cretaceous times when Madagascar was moving southwards relative to Africa. It is thus referred to as a transform continental margin by, among others, Bosellin (1986) and Mascle et al. (1987). The margin seems to have been an emergent and stable block during the whole of Palaeozoic time. Sedimentation starts on top of a peneplaned Precambrian basement surface made up of highly metamorphosed rocks. These crop out to the west of coastal sediments in what is called the Mozambiquan belt in Tanzania and Kenya (Figs. 6 and 9). Further north, in Somalia, basement rocks crop out as oval shaped areas in southern Somalia and along the northern Somali main escarpment (Fig. 2). Here they contain low grade (Inda series) and medium to high grade (Old Formation) metamorphic rocks related to the Pan-African tectonothermal event that terminated with the intrusion of granites, granodiorites, synites and abundant dikes (D'Amico et al., 1982). Basement exposures along the northern Somali coast, and along the escarpment are attributed to the separation of Arabia from Africa during the Miocene. Whereas exposures in the Bur-Acaba and Nogal areas are considered to be reactivated Mesozoic structures, similar to other structural uplifts recorded further south. At least four sedimentary cycles, each of which was probably associated with a major tectonic event, have been recorded on the east African margin during Phanerozoic time. First is the precursory tectonism of the opening of Indian Ocean and the fragmentation of East Gondwanaland (Late Carboniferous to Early Permian). This lead to the formation of continental rifts which subsequently hosted continental deposits. Marine connections during this time appear

at different ages in Tanzania, Kenya and Somalia. These reflect physical limitations to marine incursion in the isolated fault separated restricted troughs of the proto "Malgash Gulf". Salt diapirs in Kenya and Somalia, and the Mandawa salt basin of Tanzania, are considered to be of this cycle. The second phase started with the deposition of basal arenaceous sandstone unit overlain by marine beds. This phase is related to the major faulting phase during the Early Jurassic, which indicates reactivation of basement before the deposition of the arkosic Ngerengere Beds in Tanzania and their equivalents, the Manzeras Sandstones in Kenya, and the Adigrat Formation in Somalia and Ethiopia. A strong marine transgression, which probably culminated during the Middle Jurassic (Bathonian), affected the whole of East Africa. Wholly marine conditions were already established in N.E. Kenya and Somalia by the Early Jurassic, whereas further south, in SE Kenya and Tanzania, this did not take place until the Middle Jurassic. This indicates that the sea was slowly encroaching the margin from the north and east, where open marine conditions already existed (Fig. 13). The southward drift of Madagascar relative to Africa occurred along a short spreading centre and along the Davie fracture zone, a transform fault believed to have been active from Late Jurassic (156 m.y.) to Early Cretaceous (130 m.y.) time. A Late Jurassic transgression was associated with tectonism related to this event. Another major transgression flooded most of East Africa in Aptian and Late Palaeocene to Early Eocene times. These two event could be related to the development of the Owen fracture zone and to the widening of the Indian Ocean at the close of Early Cretaceous and Early Tertiary to Late Eocene times. The fourth cycle includes tectonism related to the drift of southern Arabia away from Africa (northern Somalia) and the development of the East African rift system during Oligocene and Miocene times. This resulted in the total withdrawal of the sea

African Basins. Sedimentary Basins of the World, 3 edited by R.C. Selley (Series Editor: K.J. Hsti), pp. 211-233. 9 1997 Elsevier Science B.V., Amsterdam. All rights reserved.

212

E.I. MBEDE and A. DUALEH

from the present areas of northern Somalia (pre-drift doming), and in the reactivation of sedimentation in the areas bordering the Indian Ocean coastal belt. It also caused the formation of the present Gulf of Aden and the main physiographic features of northern Somalia. A Late Eocene to Oligocene regression occurs along the east African margin. This paper attempts to synthesize the geology and stratigraphic evolution of the coastal basins of the east Africa margin (Fig. 1). The principal data used include well logs, geophysical, geochemical data and literature available. The geology of each country is discussed separately so as to give emphasis to the local stratigraphic variations within the region, even though the margin evolved as one unit. The structural evolution is then looked at as one unit, and finally there is a section on economic considerations. Besides gas discovered in Tanzania, no commercial petroleum reserves have been reported so far in the region. Oil and gas shows have been reported everywhere, and the basin is considered to be a low potential gas prone province (Chatellier and Slevin, 1988), but we think that the conclusion is too premature 0~

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with the present intensity of data available. Other geological resources, including gypsum, common salt, kaolinite, limestones and other building materials, are exploited at the moment, while heavy mineral beach sands, as well as palaeo-placers are reported to be abundant. The basins discussed in this paper include only those bordering the present Indian Ocean. They include the Somali Embayment, the Somali Coastal Basin, the Luug-Mandera Basin, the Kenya Coastal Basin, the Selous-Ruvu-Tanga Basin and Lindi Rift Basin to the south. These basins are separated by basement highs, either cropping out, or concealed beneath a thin sedimentary cover.

REVIEW OF THE GEOLOGY OF THE SOMALI COASTAL BASIN Introduction

The first epeirogenic movement to affect the region formed a series of intersecting basins separated by structural highs. The latter include the 40 ~

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THE COASTAL BASIN OF SOMALIA, KENYA AND TANZANIA

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Bur-Acaba uplift of southern Somalia, and the Nogal and Hargesya-Ergavo uplifts of northern Somalia (Fig. 2). These highs formed the boundaries of the

present Somali sub-basins, and controlled the pattern of sedimentation throughout. Two main sedimentary basins, separated by a basement ridge (the Bur-

214

E.I. MBEDE and A. DUALEH

Acaba uplift), occur in southern Somalia. These are the Mesozoic Luug Mandera Basin, which trends N N E - S S W and the Somali Coastal Basin trending parallel to the Indian Ocean coast. This contains both Mesozoic and Tertiary sediments. The northern part of the Somali Coastal Basin is overprinted by the E - W elongated Somali Embayment, which also contains both Mesozoic and Tertiary sediments. A tentative correlative chart of these three basins is shown in Fig. 5. Pre-Jurassic rock do not crop out in southern Somalia and none of the wells drilled in it reaches the basement. The older rock penetrated are of Triassic to Early Jurassic age, and were recorded in Brava-1 well drilled in the coastal basin (Fig. 2). The Hol-1 well, drilled at the axial part of Luug-Mandera Basin, bottomed in a fluvio-deltaic sequence of Hettangian-Toarcian age. Marine sedimentary rocks predating the first wide spread regional transgression were recorded in the Obbia-1 well of the Somali Embayment. Lower Jurassic neritic carbonates, the Didimut Beds, were recorded in the western margin of the Luug-Mandera Basin (Beltrand and Pyre, 1973). This depositional event is regarded to be linked to a phase of continental rifting. The first widespread transgression covered the horn during Early to Middle Jurassic time. Below is the description of stratigraphy and sedimentology of the Somali basins bordering the present Indian Ocean.

containing Middle Jurassic to Early Cretaceous ammonites were penetrated in the Brava-1 well (Fig. 2). This sequence, now called the Brava Formation, is correlatable with the Upper Jurassic shales that crop out in coastal Kenya (Beltrand and Pyre, 1973). The Brava Formation is unconformably overlain by the Gumburo Group (Upper Cretaceous). It contains over 1000 m of light gray shales, sandstones and siltstones, with rare limestone interbeds deposited in an inner middle neritic environment with strong deltaic influence. The Barren Beds, of Late Palaeocene to Middle Eocene age, include a maximum thickness of about 3000 m of predominantly shallow marine sandstones partially intercalated with siltstones and shales. Lignitic shales occur in upper part of this unit indicating inner neritic near shore depositional environment with deltaic influence. The Somali Merca Formation (Miocene to Pliocene)consists of mainly medium to fine grained sandstones and white to cream microcrystalline limestones, with thin shale and anhydritic interbeds of littoral to shallow marine environment. These older formations do not crop out because they are covered by Recent alluvial and eolian sediments along the coastal belt. The description of the formations is therefore based on subsurface data.

The Luug-Mandera Basin The existence of thick Pre-Jurassic sediments in the axial part of Luug-Mandera Basin is indicated by the discrepancy between section inferred from geophysical data and those measured in penetrated sections (Beltrand and Pyre, 1973). Little is known

Somali Coastal Basin (Fig. 3) Over 2000 m of predominantly greenish shales, occasionally interbedded with limestone bands, and

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THE COASTAL BASIN OF SOMALIA, KENYA AND TANZANIA from this section because of a lack of direct complete evidence, however, it is believed that it starts with continental conglomerates and sandstones comparable with the Karroo Series of northeastern Kenya, followed by an evaporitic sequence. Surface sections cropping out along the southwestern flank of the Luug Mandera Basin were studied thoroughly by, among others, Beltrand and Pyre (1972), and was later elaborated by Buscaglione and Fazzuoli (1987). On the basis of these works, the Middle Jurassic to Early Cretaceous section of this basin has been divided into four formations. The Baidabo Formation (Pleinsbachian to Bathonian) consists of over 800 m of predominantly thick bedded oolitic and algal limestones, interbedded with varicolored shales, resting unconformably on the Bur-Acaba crystalline basement. The basal 20 m of this formation is a coarse grained quartzitic sandstone (the Deleb Member) that passes laterally and vertically into varicolored shales (the Uanai Member). This is followed by detrital, generally oolitic, limestone with shale interbeds and abundant shell fragments (the Baidabo and Goloda members). These were deposited in continental to shallow shelf environments. The Baidabo Formation is unconformably overlain by the Anole Formation (Callovian to Oxfordian), consisting of gray calcareous shales, marly limestones and fossiliferous calcilutites, with abundant ammonites and belemnites. The Anole Formation corresponds to the maximum extension of the sea in this basin and its depositional environment ranges from shallow to deep shelf environments. The Uegit Formation (Kimmeridgian to Portlandian) consists of cyclically repeated oolitic, oncolitic, and bioclastic limestones, marls and ferruginous sandstones. It is thought to have been deposited on a restricted platform. It unconformably overlies the Anole Formation, and has a thickness of about 350 m. The base of the unit is marked by cross-bedded sandstones, indicative of the beginning of a regressive phase. Towards the end of the Jurassic the LuugMandera Basin became isolated from the Somali Coastal Basin. The Bur-Acaba region became totally emergent and acted as a physical barrier between the two main basins (Beltrand and Pyre, 1973). By Late Jurassic to Early Cretaceous time an evaporitic basin was established in the low areas of the basin, resulting to the deposition of the lagoonal Garbaharre Formation (Portlandian to Lower Cretaceous), while continental sandstones were deposited on the flanks. The Garbaharre Formation includes two members. The lower Busul Member (tentatively referred to the Late Portlandian) consists of about 300 m of yellowish bioclastic packstones and grainstones, gray bioclastic wackestones, quartzose sandstones and yellowish dolomites. The upper Mao Member consists of alternating gypsum and anhy-

215

drite levels with thin beds of shales, calcarenites, dolomites and cross-bedded sandstone, and is about 310 m thick. In the southwestern part of the basin predominantly reddish coarse to medium grained quartzose sandstones (the Amber Beds) crop out. These are about 180 m thick, consisting of nearshore to continental deposits. No index fossil had been found in it, and it has tentatively been referred to as Early Cretaceous. From then onwards the basin ceased to subside.

The Somali Embayment (Fig. 4) None of the wells drilled in the coastal part of the basin reached the basement, however, the Garade-More-1 well drilled on the northern flank of the basin reached the basement. The Adigrat Formation is the basal formation of Somalia. Its age is controversial because it is contains only poor or non diagnostic fossils. In the Garade-More-1 well the basal 80 m of the Adigrat Formation consists of fine to very fine grained carbonate-cemented sandstone interbedded with continental shales, overlain by 40 m of interbedded dark marl and very fine-grained argillaceous tidal flat dolomites. The Adigrat Formation is regionally diachronous, and its upper limit lies within Lower to Middle Jurassic (Dualeh, 1986). The Hamenlei Formation of Bathonian to Oxfordian age, described at the type section in the eastern Ogaden basin, is a white well bedded, mainly oolitic, fossiliferous limestones having thickness of about 210 m (Barnes, 1976). In the Garade-More-I well it is about 1093 m thick, consisting of mainly, oolitic and pseudo-oolitic packstones and grainstones interbedded with silty mudstones and dolomites. In the Obbia- 1 well 2175 m of dark gray shales and gray argillaceous limestones were penetrated without reaching the base. It is difficult at the moment to define the lower limit of the unit because it is transgressive. The depositional environment is back reef shoal in the Ogaden, tidal flat in the GaradeMore-1 area and basinal in the Obbia-I well, where the lowermost part penetrated is believed to belong to the Pleinsbachian-Toarcian stages. The Urandab Formation (Kimmeridgian) unconformably overlies the Hamenlei Formation. It crops out also in the eastern Ogaden where it consists of 55 m of gray to green gypsum bearing shales with intercalations of argillaceous limestones (Barnes, 1976). In the Garade-More-1 well the Urandab Formation consists of 95 m of gray to green slightly laminated glauconitic and pyritic marls with the lower 18 m consisting of oolitic interclasts. The maximum thicknesses recorded for this formation is in the subsurface of the eastern part of the Somali Embayment, where it is made up of 500-700 m of basinal dark gray shales and gray, marly limestone beds.

216

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THE COASTAL BASIN OF SOMALIA, KENYA AND TANZANIA The formation was deposited in a tidal flat complex in the Garade-More-1 well, is basinal in the mid Somali Embayment and of shallow marine origin in the eastern Ogaden. The Gabredarre Formation (Tithonian) was deposited in the Somali Embayment as a basinal sequence. In the Obbia-1 well it consists of 343 m of dark gray and dark brown shale with some gray fine crystalline limestones. This contrasts with the 410 m in the type section in east Ogaden, where it consists of partially oolitic yellowish, fine crystalline limestones with interbedded marly gypsum and limestones (Barnes, 1976). A regression began throughout the region during the Tithonian. In the south and central Somalia and southern Ogaden an evaporitic formation (Main Gypsum Formation) is generally regarded as Tithonian to Albian in age on the basis of the stratigraphic position. Also the Gabredarre Formation is totally missing from the Garade-More-1 well on the northern flank of the Somali Embayment, this was probably due to epeirogenic movements. The type locality of the Main Gypsum Formation is in the southern Ogaden. It consists of around 200 m of gypsum with marls and intercalation of calcareous lagoonal shales. Eastwards it grades laterally into the Cotton Formation (Lower Cretaceous). The type section is in the Cotton-1 well where it consists of fore-reef limestones and middle neritic shales. In the Somali Embayment, the Cotton Formation is a deep marine limestone. The Cotton Formation is overlain by the Gumbro Group (Cenomanian to Maastrichtian). In the Obbia well of the Somali Embayment this consists of 640 m of the light coloured, fossiliferous and porous limestones, with a few beds of dark gray shales. In the eastern Ogaden this unit consists of fossiliferous, lignitic and pyritic dark gray shales deposited in deep quiet waters, with restricted circulation (Clift, 1956). In the southern Ogaden and southern central Somalia, the Gumbro Series has been divided into four formations, each with a distinct characteristic lithology. The Mustahil Formation in the Ogaden is about 200 m thick, consisting of alternating white to yellow lenticular marly limestones and marls with gypsum at the top. The Ferfer Formation type section is also in the Ogaden, where it consists of about 200 m of gypsum with calcareous marly and shaly intercalations. The Beletutuen Formation type section is at Beletutuen in south central Somalia where it is about 415 m thick, mainly limestone bearing gypsum with shale and sandstone beds. The type section of the Jesomma Formation is in south-central Somalia and consists of 350-400 m of fine to coarse grained continental sandstones with local gypsum beds at the base. This formation marks the end of the Cretaceous cycle in Somalia. Tertiary sedimentation is believed to have started with the deposition of the Anrado Forma-

217

tion during the Early Eocene. At the type section in Nogal (northern Somalia), the Anrado Formation consists of 550 m of fine crystalline compact, light brown limestones with local thin gray, shaly beds of shallow marine environment. The Taleh Formation (Middle Eocene) was deposited in an evaporitic basin in Nogal, central Somalia and northwestern Somalia. At its type section in Nogal, the unit is 450 m thick and consists mainly of gypsum, anhydrite and shales with intercalations of limestone and cherty, marly beds. Its facies changes from being evaporitic in Nogal eastwards to deep marine fine clastics offshore. The Late Eocene section is called the Karkar Formation. In the Nogal area this is about 400 m thick, consisting of chalky limestone with intercalations of paper shales and gypsum (near shore facies with a hypersaline lagoonal episode). Eastwards, it changes to deep water facies, where in the Obbia-1 well 360 m of interbedded gray micaceous shales of Late Eocene age occur. The Karkar Formation marks the last major transgression in the region. Subsequently marine sedimentation was restricted to the present coast and offshore areas. About 731 m of coarse grained friable sandstones and greenish silty shales with chalky porous limestones have been penetrated in the Obbia-1 well, these are referred to as undifferentiated Miocene deposits.

GEOLOGICAL REVIEW OF THE KENYA COASTAL BASIN Introduction

The Kenyan Coastal Basin is dissected by a number of reactivated Mesozoic structures (Fig. 6). The basin is considered by Reeves et al., (1987) as the southern arm of end Jurassic/Early Cretaceous rifting which lead to the opening up of Indian Ocean. The Somali Coastal Basin is the northern arm, and the Anza Graben to the northwest failed to open. This is when Madagascar is considered to have migrated southwards. The stratigraphy of the basin has recently been reviewed by among others Cannon et al., (1981) and Rais-Assa (1986, 1988). Figure 8 compares the stratigraphic terminology for the basin according to different authors. The oldest and most extensive outcropping sedimentary rocks locally called the Duruma Series or Duruma Sandstones are of Karroo age, faulted to the west against the basement, while to the east they either disappear under the cover of post-Karroo rocks, or are faulted against them. Salt diapirs detected by geophysical data offshore (Figs. 6 and 7) are thought to be of this age and equivalent to the Mandawa salt basin of southern Tanzania. To the north, Karroo rocks thin beneath the cover of non-marine Neogene, un-

218 derlying the Cretaceous and Tertiary sediments in the southern part of Anza Graben. They reappear again north of Wajir in the Luug-Mandera Basin discussed later. Karroo rocks are overlain by limestones of Middle Jurassic age (the Kambe Formation) which marks the beginning of the post-Karroo marine phase. Karroo

The basal part of Duruma Series, referred to as the Taru Grits (Caswell, 1953, 1956) or the Taru Formation (Cannon, 1981) is made up of coarse grained, poorly sorted, feldspathic, fluvial sandstones. The formation reaches a maximum thickness of 2700 m, and has been dated as Upper Carboniferous to Permian on the basis of the fresh water bivalve Paleonodante fischeri known only from beds of Upper Carboniferous to Late Permian age (Walter and Linton, 1973). The Taru Formation is overlain by the Maji ya Chumvi Formation, consisting of fine grained laminated gray, silty shales and flaggy sandstones with ripple marking, cross-bedding and frequent sun cracking and rain pitting, often intercalated with carbonaceous beds. The Maji ya Chumvi Formation is considered by Walter and Linton (1973) to represent a swampy to lagoonal depositional environment between Late Permian and Early Triassic age (Rais-Assa, 1988). The overlying Mariakani Formation, though much coarser than the former, is also made up of fine grained to medium grained flaggy sandstones, micaceous siltstones and silty shales arranged in upward-coarsening deltaic sequences. Together with the overlying highly micaceous, black shale with coaly beds intercalations, the Mariakani Formation is dated as Middle to Late Triassic. The Manzeras Sandstone Formation of the uppermost Duruma Series is composed of coarse grained and cross-bedded sandstones. The lower part shows an upward-coarsening sequence of probably deltaic origin while the upper part is eolian. The contact between the Manzeras Sandstone and the underlying Matolani Formation is quite distinct to the south. Here it is described as unconformable with evidence of active faulting and fracturing at the top of the Manzeras Formation (Rais-Assa 1988). The Upper Manzeras shows progressive discordance with many synsedimentary faults and synsedimentary submeridinal synclinal structures. These movements, also recorded at the end of Karroo deposition of the Ngerengere Beds, are said to have originated from epeirogenic uplift of the margin related to the initial stage of continental rifting which was followed by an erosive phase. This is probably the End Jurassic to Early Cretaceous rifting described by Reeves et al. (1987) before Madagascar started drift-

E.I. MBEDE and A. DUALEH ing southwards. The Manzeras Sandstones, which mark the end of deposition in the Karroo Basin in Kenya have not yet been dated, but Caswell (1953) attributes them to the Upper Triassic, while Cannon et al. (1981) dates them as Lower Jurassic. Post Karroo

Apart from the Manzeras sandstones, no Lower Jurassic rocks have been described from coastal Kenya. Their absence has been interpreted as either due to continued deposition of the Manzeras, or a break in sedimentation (Rais-Assa, 1988). The Kambe Formation (Middle Jurassic) unconformably overlies the Manzeras Formation. It is faulted in some parts to the west against the underlying Karroo rocks. Lithologically it consists of dark gray reefal to lagoonal, pisolitic and oolitic limestones interbedded with the calcareous Posidonia shales of Westermann (1975), reaching up to 65 m. They are interpreted as deposited well below the photic zone in quiet poorly aerated waters. The limestones are characteristic of shallow water environment. The Kibiongoni Beds are grouped together with the Kambe Limestone as the Kambe Formation by recent workers (Rais-Assa, 1988). They lie unconformably on top of the Kambe Limestone. They contain boulders of both the Manzeras sandstones and the Kambe limestones, so they are clearly younger. They are also lithologically distinct from the Kambe limestones. They start with a basal conglomerate followed by unfossiliferous sandstone bearing ripple marks, rain pits and, on top, are the silty shales of restricted, presumably estuarine origin (Westermann, 1975). The Kambe Limestones have been dated as Bajocian/Bathonian, but the age is still questionable. The Kibiongoni beds are believed to be of Bathonian/Callovian age. The Kambe Formation grades upwards into Upper Jurassic shales including the Miritin, Rabai and Changamwe shales of Caswell (1953). These shales have also been referred to by Cannon et al. (1981) and Rais-Assa (1988) as the Mto Mkuu Formation. The basal part of the Mto Mkuu Formation described by Walter and Linton (1973) contains pebbles of both the Manzeras and Kambe Formations. It is cross-bedded. The formation is thought to be deltaic in origin. Apart from a small patch of Freretown limestone north of Mombasa no Cretaceous outcrop has been reported along coastal Kenya (Haughton, 1963). More than 4000 m of Cretaceous sediments are reported at the depocentre of the Lamu Embayment (Walter and Linton, 1972) and a thick sequence of Cretaceous sediments was encountered at Deep Sea Drilling Project (DSDP) site 241 east of the Lamu depocentre. The Lower Cretaceous clastics of coastal Kenya are made up of series of quartzites of Neo-

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THE COASTAL BASIN OF SOMALIA, KENYA AND TANZANIA containing planktonic foraminifera, deposited in a deep quiet water environment in the depocentre, while along the margin limestones containing a very sparse benthonic fauna are reported. Tertiary sequences are well developed in the Lamu Embayment being quite thick at the depocentre. Leg 25 of DSDP found an almost complete Tertiary sequence, with only part of the Oligocene and Upper Eocene section missing. Palaeocene sediments do not crop out in coastal Kenya, but are present at depth, being quite thick in the central part of the basin wedging out northwards and westwards, to where Middle Eocene sediments lie directly on top of Cretaceous beds. Lithologically the Middle Eocene sediments consist of dense micritic limestones interbedded with dark gray to brown shales deposited in a shallow marine environment. The Middle Eocene to Oligocene sediments crop out along coastal Kenya as a belt of low lying hills running parallel to the present coast. South of the Lamu Embayment they are argillaceous and feldspathic sandstones, poorly sorted and friable, with variable development of limestones, deposited in fluviatile, littoral and deltaic environments. Further north evidence of marine environment disappears and a sequence of variegated unfossiliferous red green mudstones and poorly sorted sandstones appears. The Upper Eocene rocks reported in the Anza and Bahati wells (Fig. 6) are barren continental beds, similar to those described in the Somalia Coastal Basin described earlier. Miocene beds are well represented in wells drilled in the Lamu Embayment. At the depocentre they are mainly limestones, dolomitic in some parts, with veins of anhydrites. Traced northwards and westwards they become sandy grading into variegated mudstones. They represent a transgressive phase, which began at the end of Oligocene. Pliocene to Quaternary deposits are mainly unfossiliferous laid down in fluvial and eolian environment. Along the coast the succession is of reefal limestone of Palaeogene to Neogene age, while offshore marine deposition has been taking place. Figure 7 is the section across the Kenyan coast.

THE GEOLOGICAL REVIEW OF TANZANIACOASTAL BASIN Introduction The main sub-basins distinguished in coastal Tanzania include the Selous-Ruvu-Tanga rift basin, along which a NNW-SSW fault trend is predominant, and the Lindi Rift Basin, to the south of which the Mandawa salt basin forms part, containing a NNW-SSE Karroo fault trend (Fig. 13). Both basins are Mesozoic to Tertiary in age and are crossed

221

by a number of north-south structural highs, while E - W is another remarkable fault trend in the basins. The major structural features are shown in Fig. 9. The rifting process, which began during early stages of the eastern Gondwanaland break up, continued progressively into the Early Jurassic. Periodic movements along bounding faults lead to the cyclic deposition of continental, fluviatile and lacustrine sequences, with periodic marine incursions in areas of high subsidence rates. These resulted in the development of restricted marine basins or gulfs where black shales and evaporites were deposited. At the close of the Early Jurassic tectonism along bounding faults became less intense, and a shallow marine transgression followed during the Middle Jurassic as a result of continued subsidence and tilting. An overall regression, associated with a number of tectonically influenced sea level fluctuations is remarkable from the early part of Cretaceous. A major regional unconformity is recognized at the base of the Aptian and Lower Albian. Activities at this time may be related to the continuous wrench faulting which lead to the southward movement of Madagascar. The Late Cretaceous and Early Tertiary was a period of tectonic stability, while significant tectonic reactivation began in the Late Palaeogene and continued into the Neogene. These movements were related to the development of the modern East Africa Rift system, which resulted in massive structural inversions and intensified eastward tilting of the present onshore areas, where they were accompanied by rapid subsidence and deposition. The development of offshore basins such as the Mafia and Zanzibar channel is believed to have taken place at this time. Selous-Ruvu-Tanga Basin This basin lies within the NNE-SSW trending rift (Tanga fault trend), the area described includes the area from 11~ S northwards to the Kenyan border together with the offshore areas (Fig. 9). The Karroo sediments in this basin consist of a complex series of continental fluviatile systems developed along the downthrown flanks of normal faults of Late Carboniferous to Early Mid Jurassic age. Movements along the faults were periodic and initiated erosive phases which generated fluvial systems which deposited upward-fining megacycles with basal conglomerates. These pass upwards into high energy braided stream deposits, and are succeeded by low energy meandering stream, flood plain and deltaic to swampy deposits. At least two Karroo megacycles are recognized within this basin. The older Karroo sediments include the Lower Permian Hatambulo Formation, described by Hankel (1987) around Stigler's Gorge. These are composed of tightly cemented feldspathic sandstones of deltaic and la-

222

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with red interbeds and gray silty claystones. Sediments in this area are considered to be lacustrine to braided stream deposits, while evidence of occasional marine incursions, coaly beds and deltaic influence exists. Geophysical data reveal thicknesses of above 3000 m (Kent et al., 1971 and Kent and Pyre, 1973), while recent seismic interpretations indicate that significant amounts of Triassic sediments must have been removed by Jurassic erosion. Seismic interpretation in this area also revealed complex sequences within the Ngerengere Beds, separated by disconformities, indicating different cycles of deposition in response to tectonic control by the bounding faults. In the central part of the basin at Ngerengere, the Ngerengere Beds consist of arkosic sandstones, with occasional limestones, and shales reflecting a high energy environment, becoming much quieter with time. There is evidence for a marine incursion

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224 before the full marine environment was established during Bajocian/Bathonian time. The intermittent faulting that influenced the cyclic deposition of the Ngerengere beds almost ceased during the Bajocian. Continued subsidence lead to the development of a shallow epicontinental sea by the end of the Bajocian and left the Karroo grabens along coastal Tanzania flooded with marine waters overstepping the basement. On the crests of basement ridges reefs developed in Bathonian and Callovian times. Around Tanga, the Ngerengere Beds are overlain by 340 m of compact well bedded oolitic to pisolitic limestones (the Amboni Limestone), of Bajocian/Bathonian age. Around Msata, Lugoba and Msolwa reefal limestone, or reworked rubbly limestone, directly overly basement, while at Kidugalo and Kidunda calcareous sandstones and sandy oolitic limestone of Bajocian to Callovian age are recorded. Between Msata and Msolwa, however, dark gray Posidonia shales, considered to be the lateral equivalent of Middle Jurassic limestones, directly overly the basement. Posidonia shales have also been recorded at depth in the wells drilled in the north of this area. The oolitic and argillaceous reefal limestones mentioned above indicate a near shore environment with a limited clastic supply during the Middle Jurassic. Deep drilling indicates that the limestone is replaced basinwards by deep marine shales. The dark gray Posidonia shales described above were deposited in a restricted back reef environment. The Middle Jurassic transgression continued into the Late Jurassic, but this was a rather quiet phase, and deposition was mainly marine with predominantly clastic sediments. These show an upward decrease in grain size from sand to clay throughout the Kimmeridgian along coastal Tanzania (Fig. 11). This low energy environment prevailed in most parts of coastal Tanzania for the whole of the Late Jurassic, while fluviatile deposition was taking place in some parts of the basin. Around Tanga, 700 m of Oxfordian to Kimmeridgian interbedded shales, sands and marls are observed, while over 1000 m of sandstones, limestones and mudstones are exposed along the Wami River to the south. The PuguMusanga high is believed to have been emergent at this time, and marginal Upper Jurassic coastal facies have been recorded in the Kisangire-1 and Kisarawe-1 wells drilled on top of this high (Figs. 9 and 10). To the west restricted water circulation lead to the deposition of neritic to bathyal deposits around Wangiyongo (Kajato, 1982). Further south, in the Selous Rift, a new fluvial system developed and deltaic deposition prevailed on the Callovian shelf. This delta continued to prograde northwards during Late Jurassic into Neocomian time. In general there is an overall shallowing of the sea from the

E.I. MBEDE and A. DUALEH central part of the basin northwards and southwards, while to the east deep marine deposition continued. The Late Jurassic transgression was followed by a period of regression starting in the Neocomian. This regression resulted in the deposition of fluviatile sandstones that form a major reservoir along coastal Tanzania. In the Kisarawe-1 well 500 m of Neocomian sandstones were encountered, and Wangiyongo borehole found more than 700 m of deltaic bituminous sandstones interbedded with clay that graded downward into Upper Jurassic marine shales. Along the Bagamoyo road septarian limestones and conglomeratic sandstones crop out, indicating some tectonic activity at this time. The Neocomian regression was followed by a period of fluctuating water level, open marine conditions prevailed in response to a gradual subsidence and rising of the sea level. This Middle Cretaceous deposition developed into a full marine transgression during the Late Cretaceous. Tectonic activity at this time may have resulted from the influence of both continued wrench faulting, as Madagascar continued to move southwards, and the effects of the break-up of West Gondwanland (the separation of South America from Africa). In this basin the AIbian to Cenomanian sequence overlaps Neocomian sediments, are themselves transgressed by Senonian strata, indicating locally complex disconformities. A distinct regional unconformity related to the worldwide to Austrian unconformity can be observed at the base of the Aptian and Lower Albian section. In the Kisarawe-1 well, Aptian siltstones and interbedded shales, indicate deposition in an open marine to outer shelf environment. To the north the Aptian sediments consist of upward-fining sequences of gray calcareous claystones, with minor siltstones and limestones, deposited in a marine mid-shelf environment. Aptian sediments are overlain by Albian limestones, marls and mudstones. These extend from Kisarawe to Musanga in the south, and to Wami in the north, where 900 m of marine clays, sandstones and limestones of Aptian to Turonian age crop out along the river. Eastwards, in the Pemba-5 well more than 800 m of Upper Senonian silty mudstones and rare limestones were deposited in an outer shelf environment. Upper Cretaceous outer shelf sediments are also recorded in the Zanzibar-1 well, Rasimachuis-1 (Campanian-Maastrichtian) and Ruaruke- 1 (Turonian-Campanian) and also exposed east of Msata. 900 m of Albian sediments were recorded in the Chalinze borehole. In the Maneromango area to the south Neocomian sandstones are overlain by a complex series of lithofacies deposited in inner shelf environment. The basal conglomerate is Aptian in Agwe. To the east, more than 1000 m of Albian to Maastrichtian sediments unconformably overlie the Neocomian strata. They are coarse grained, indicat-

THE COASTAL BASIN OF SOMALIA, KENYA AND TANZANIA

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ing a shallowing of the sea in this area towards the end of the Cretaceous. In general the Mid to Upper Cretaceous sediments in this basin indicate deep marine conditions (Kent, 1971). It is likely that the Upper Cretaceous sea transgressed westwards beyond the existing rift boundaries. To the south, a sequence of gently dipping sandstone crops out extensively across the Selous rift from the Rufiji River southwards to the Mozambiquan boarder. It oversteps Karroo sediments to directly overlie basement and is overlain by Miocene sediments. It has been correlated the Aptian Makonde Beds on lithological similarities (Spence, 1957). Other workers place these rocks at the top of the Karroo Series, while others refer to them as Middle to Upper Jurassic in age. The Upper Cretaceous transgression continued into the Early Tertiary with a minor regional hiatus at the Palaeocene-Late Cretaceous boundary. Palaeocene sediments are mainly deep marine dark gray to green clays with occasional sandy and silty intercalations. Gas shows were reported in Palaeocene beds of the Zanzibar-l, Pemba-5 and Mafia-1 wells. These sediments indicate shallowing of the sea. An ensuing regressive phase started during the Mid Eocene and continued into the Oligocene, whose strata are largely absent along the Tanzanian coast. This regression may have resulted from isostatic adjustment due to active sea floor spreading related to development of the Owen fracture and widening of the Indian Ocean. It reflects building up of broad continental shelf and slope at a faster rate than the subsidence which was probably continuous from the Late Cretaceous. Though there are

abundant instances of lateral facies variation over a short distance, clastic deposition was the most dominant mode of deposition. Miocene sediments overstep older beds, indicating renewed transgression and tectonic activity contemporaneous with the development of the modem East African rift system. Uplift and basinward tilting of hinterlands, together with intensive erosion and rapid basinal subsidence, resulted in the deposition of large volumes of sediments and in the development of a large prograding delta across the Dar-es-Salaam Embayment and Zanzibar channel. This is accompanied by synsedimentary faults similar to those observed on a seismic section across the Zanzibar channel (Fig. 10). In the Pemba-5 well to the north, there is evidence of deltaic deposition in Oligocene to Lower Miocene sediments. The Middle to Upper Miocene sediments consist of marine limestones, silty mudstones and sands indicate distance from the delta. The E - W trending Rufiji depression is believed to have formed at this time. The final regression resuited in a widespread unconformity over Miocene deposits. Marine Pliocene deposition was limited to the present day offshore areas. Onshore Neocomian to Miocene beds are overlain by regressive estuarine fluviatile Plio-Pleistocene sands in existing depression such as the Ruvu valley and the Rufiji depression. The Lindi Rift Basin The Lindi rift is a huge basin located in southeast Tanzania, of which the Mandawa salt basin forms

226

E.I. MBEDE and A. DUALEH

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part (Fig. 9). A dominant N-S structural trend can clearly be seen. In the Mandawa Basin the Pande and Kizimbani highs and the Mandawa anticline are examples. The prevalent Karroo fault direction here is N N W - S S E (Lindi fault trend) while NNE-SSW and E - W trends are also observed. The Karroo litho-

facies in this basin are more distinct and more varied due to differences in the subsidence rates within half grabens. On the eastern flank of the Matumbi hills crop out 3000 m of highly faulted Karroo proximal fanglomerates, massive red to brown sandstones, siltstones and mudstones attributable to braided and meandering stream deposits. Nodular limestones and sandstones indicate occasional marine incursions. To the south at Mandawa evidence of marine incursion is more obvious because early Karroo clastics are followed by transitional continental to marine beds then lagoonal shales and evaporitic sequence of Triassic age. Thicknesses and the distribution of the Ngerengere Beds in this basin are complicated by extensive faulting. Around the Matumbi hills, Middle Jurassic oolitic and reefal limestones were deposited on top of the basement, indicating shallow near shore marine environments, similar to those described earlier from the Ruvu Selous Basin to the north. Southwards, in Mandawa, there are Middle Jurassic marine marls with subordinate oolitic limestones. A local intra Mid-Jurassic unconformity occurs between Bajocian and Bathonian sediments indicating early salt movement in the basin. The salt pillow left by these movement controlled Late Jurassic and Cretaceous deposition in southern Tanzania. A change to littoral and eventually neritic environment can be observed. In Mandawa shales intercalated with evaporites were first deposited, grading into marls, sandstones and then into continental sandstones. Lagoonal shales were still deposited in a restricted marine basin west of the anticline. Northwards, around Matumbi, marine and deltaic buff sandstones and clays were deposited during the Upper Jurassic. Basinwards, deposition of deep marine clays took place. Around Songosongo Upper Jurassic marine clays are reported to have probably produced gas from Cretaceous reservoirs (Kajato, 1982). The End Jurassic to Early Cretaceous left lateral movement that split the Kizimbani and Pande highs are thought to have also caused salt flow which resulted in the formation of the N-S trending Mandawa anticline. These movements are thought to have been related to the southward drift of Madagascar. They also resulted in a local transgression to the west during the Early Cretaceous when Neocornian sandstones, siltstones and occasionally limestones, were deposited. The Neocomian transgression became more widespread during the Lower Aptian with initial deposition of conglomeratic sandstones and siltstones southwards and westwards of the Mandawa anticline. These are the Makonde beds. Reefal Orbitolina limestones were deposited east and west of the Kiturika hinge line east of Mandawa, these limestones extend southwards to Lindi, and pass laterally into Upper Aptian Kihuluhulu and Lower Albian Kigongo marls (Fig.

THE COASTAL BASIN OF SOMALIA, KENYA AND TANZANIA

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11). South of Mandawa there are Upper Aptian and Albian clays with septarian nodules. The widespread Mid-Cretaceous transgression continued into the Late Cretaceous, facilitated by the eastward tilting and subsidence of the basin. Deep marine environments were established just east of the Mandawa anticline where argillaceous facies intercalated with bands of arranaceous beds were deposited. Palaeocene sediments in this basin indicate the last phase of the Cretaceous transgression, with a noticeable unconformity at the Late Cretaceous/ Palaeocene boundary. Sediments deposited during Palaeocene are mainly clays, marls, siltstones, sands, reefal and algal limestones changing laterally into marls and clays basinward. The beginning of the Eocene Period is associated with tectonic activities and marks the onset of a regression that continued into the Oligocene. Mid and Upper Eocene sediments reflect continental slope build up probably caused by basement faulting. By the Late Eocene a continental shelf was established where platform carbonates were deposited. Gas shows are reported from Eocene limestone reservoir in Songosongo. An angular unconformity occurs at the Middle and Upper Eocene boundary at Mandawa. Oligocene sediments are limited in their distribution and were probably eroded prior to Miocene faulting related to the development of the modem East African rift system. Between Lindi and Kilwa Miocene sediments are well developed and are strongly transgressive, extending to the N.E. of Mandawa. The Pliocene was another phase of local transgression, which deposited the continental Mikindani Beds. These are mainly sandstones, gravels and clays laterally merging with marine reefal limestones and sandy clays.

STRUCTURAL EVOLUTION

The structural development of the eastern margin of Africa is related to the fragmentation of eastern

227

Gondwanaland and the birth of Indian Ocean. This is demonstrated by the correlation of sedimentary units from deep sea drilling well locations offshore and those cropping out onshore (Wolfgang et al., 1974). Faulting controlled subsidence and sedimentation, while flexure dominated the region through out the time. The Indian Ocean, however, is one of the least understood seaways in the world. The western Indian Ocean basin, the so called Somali Basin, has recently been intensively studied by among others Beltrand and Pyre, (1973), Schlich et al., (1974), Kent, (1972), Mascle et al., (1987), Rabinowtz et al., (1982, 1983), Bosellin, (1986) and Dualeh and Naim, (in press). The story of its development can be summarized as follows. The first stage of Indian Ocean development is believed to have taken place from the Late Jurassic to Mid Cretaceous when Madagascar is believed to have migrated southwards (Rabinowtz et al., 1983). Oceanic crust created by this movement must have formed the floor of the proto-Indian ocean at that time. The ocean that existed in the region during Early to Middle Jurassic time, the "Neotethys sea", is considered to have been an epicontinental sea (Cannon et al, 1981). The second stage took place in two phase, the first during the Late Cretaceous, when a rift separated Mascarene from Madagascar. During the second phase in the Palaeocene a new branch of the southern rift separated the Indian subcontinent from the Seychelles microcontinent along the Carlsberg Ridge, leading to the closure of the "Neotethys sea". It is the first stage of Indian Ocean development that affected most of the area discussed in this paper, however, the impact of earlier and later events can be seen in sedimentary units. The development of the eastern margin of Africa began with the formation of an intracontinental rift during the early stages of the eastem Gondwanaland fragmentation. Initial faulting is believed to have taken place during the Late Carboniferous if not Early Permian. Rocks of this age have been recorded in Kenya and Tanzania, while in Somalia Karroo rocks are questionably dated as Triassic. Early tectonism resulted into the formation of NNE-SSW, NNW-SSE and in some places E - W trending faultbounded basins and sub-basins. Continental clastic sediments were deposited in these constantly subsiding grabens and half-grabens while continued subsidence lead to partial marine incursion in the fast subsiding sub-basins. Marine influence increased throughout the Jurassic leading to the establishment of full marine conditions in the region during the Middle Jurassic. Magnetic anomalies in the Gulf of Somalia (Rabinowtz et al., 1983) show that Madagascar started to move southwards from East Africa 156 m.y. ago (M 25). Figure 13 is a palaeogeographic recon-

228

E.I. MBEDE and A. DUALEH

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struction of Madagascar during Late Jurassic-Early Cretaceous times. This phase was associated with Upper Jurassic to Early Cretaceous marine shales recorded almost everywhere in the region. Marine deposition terminated in the failed rift basins such as the Luug-Mandera Basin. N-S trending gravity highs in Tanzania, as well as in Kenya, are thought to indicate strong crustal changes caused by early transform movement (Rabinowtz, 1983). Madagascar is believed to have attained its final position 130 m.y. ago (M 9). Figure 14 shows the position of Madagascar relative to other major physiographic features in the Cretaceous Period. This when the continental margin was fully formed, and the first expression of the Owen fracture zone ap-

peared. Progressive eastwards shift of the depocentre is observed from the Cretaceous into the Tertiary as a result of a combination of continued regional sag associated with sea floor spreading of Indian ocean. By the Palaeocene to Early Eocene India is believed to have started moving northwards. This marks the last widespread marine episode. The Mid Eocene to Oligocene is a regressive period in the region and this is the time when the "Neotethys Sea" is believed to have been subducted. This regional regression has been related to the pre-rift doming of the Afro-Arabian shield. The last phase of tectonic activity in the region began in the Miocene and continues to the present day. This phase is related to the opening up of the Gulf of Aden and the Red Sea, and the

THE COASTAL BASIN OF SOMALIA, KENYA AND TANZANIA

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establishment of the East African rift system to the west. Major structural patterns related to this tectonic history are shown in Figs. 6, 9 and 12. No compressive events have been recorded in the region so far. Some anticlines, like Mandawa, have been related to salt movement, while other structural features, like the NNE-SSW trending Sengt anticline, and Garbaharry anticline of the Luug-Mandera Basin,

were formerly related to either salt movements, or to movement of Upper Jurassic shales (Pyre, 1972). According to Carmingnani et al. (1983), however, the anticlines are thought to be caused by wrenching associated with a transpressional component of fault movement. This is probably the case for other anticlines observed elsewhere in the region.

230 ECONOMIC GEOLOGY

Hydrocarbon prospectivity No systematic source rock studies have been done so far in the region, however, seeps are known in some parts of the basin (Tabajar, Kenya and Wangiyongo, Tanzanzia). A gas discovery, together with oil shows, has been reported from Songosongo and Mnazi Bay, Tanzania, and gas shows have been recorded in Tertiary reservoirs in most of the wells drilled in the region. These all indicate the existence of mature source rocks in the region. Exploration done so far has been inexhaustive. The few wells that have been drilled mainly tested structural traps that can be considered risky in this region due to updip migration during fault reactivation. Very little testing of stratigraphic traps has been done if any. In the early 1980s Robertson Research Group collected samples from deep wells in the region for geochemical studies, their results have not been published. The mainly lacustrine, fluvial, deltaic to swampy deposits of the early Karroo sequence can be expected to have locally developed source beds rich in either type I or type III kerogen. Whereas the late Karroo sequence contains restricted marine deposits in some parts can be expected to have well developed rich source beds. The source character of Karroo beds in the region is very little known, because the formation has not been reached in the deep wells drilled so far. Permian samples studied by BEICIP (1984) in Kenyan Karroos gave TOC values of up to 0.9% while Kamen-Kaye and Barnes (1979), described shales of this age in Somalia and N.E. Kenya Karroo sections, and suggested that they may have produced the gas found in the neighbouring Ethiopia. Outcrop samples described by Kreuser, (1984) from Tanga, Rufiji, Mikumi and Nyakatitu sections gave TOC values between 0.3-2.4% of structureless vitrinitic material, while east of Rufiji at depth TOC values of above 2% are recorded in Upper Triassic shales. In Mandawa marine shales of Karroo age contain sapropelic oil prone kerogen with a fair amount of extractable oil and TOC values of above 1%. The thick post-Karroo sequence is said to have developed in total marine environment accompanied with a series of transgressions and regressions which could have been favorable for good marine source bed development, with either paralic type II kerogen or type III kerogen where deltaic conditions existed. The Middle Jurassic Posidonia marine shales and equivalent interfingering shallow water limestones, have good amorphic algal material with sapropelic oil prone kerogen and good TOC values. Upper Jurassic shales from wells drilled in central part of Tanzania give average TOC values of 0.5%, with

E.I. MBEDE and A. DUALEH humic gas prone kerogen in some parts. They are believed to have generated the gas in the Songosongo gas field (Kajato, 1982). In Kenya Upper Jurassic shales have been penetrated in only one well (Garissa-1) where they contain type IV kerogen. Surface samples studied by different oil companies between 1974-1983 reveal that the shales of this age have very low TOC values, containing immature amorphic and herbaceous materials with type II oil prone kerogen and abundant coaly type III kerogen. The thick marine fossiliferous calcareous shales of the Brava Formation (Jurassic to Cretaceous) in Somalia exhibit favourable source character. They have organic contents of between 0.26-0.72% in the Jurassic section, and 1.2% in the Cretaceous section. This is in the form of oil prone kerogen with an average vitrinite reflectivity of 0.66Ro. Albian to Palaeocene clays and shales overlain by a thick Neogene sequence are well developed in Zanzibar and Pemba, and to some extent in the Mafia channels (Fig. 10). Their source character is not reported, but gas shows have been recorded from Tertiary reservoirs in some wells. The carbonaceous shales and lignites described in the thick pile of deltaic Miocene sediments of the Zanzibar channel could provide another source for this gas. The Lower Cretaceous section is not a source rock along coastal Kenya, though it might be basinward where the Upper Cretaceous sediments have source characteristics similar to Tertiary sections, with mainly detrital Type IV organic matter associated with Type III humic Kerogen. The latter limits their potential for liquid hydrocarbon generation (Mbede, 1987). In Somalia Cretaceous and Tertiary shales offer favourable source beds in the vicinity of potential reservoirs. The petrophysical character of sedimentary formations in the region is not yet well established. The stratigraphic evolution of the basins and sub-basins, however, suggest that several lithostratigraphic units could be significant hydrocarbon reservoirs under favorable conditions of generation and migration. At outcrop Karroo sandstones show very poor reservoir character, being mainly feldspathic and poorly sorted. Great burial depths before Tertiary inversions might have contributed to the destruction of porosity and permeability within Karroo sediments. The Upper Karroo section, however, is expected to be of good reservoir potential. The Adigrat Formation provides a gas reservoir in Ethiopia, while the Manzeras sandstones of Kenya are expected to have good reservoir character, being partially eolian. The Ngerengere Beds of Tanzania around Tanga give good neutron porosities in some wells, while in Mandawa they are described by Kajato (1982) to be of good reservoir potential. Middle Jurassic depositional environments are suggestive of good potential reservoir development

THE COASTAL BASIN OF SOMALIA, KENYA AND TANZANIA in most parts of East Africa, being mainly reefal, oolitic and fossiliferous limestones. The Lower Cretaceous sediments are known to be regressive and, in Tanzania, Neocomian fluviatile and deltaic sandstones are potential reservoirs. In the Songosongo field gas is found in a deltaic Albian sand which unconformably overlies extensive Neocomian continental sands (Kajato, 1982). In the northern part of Kenya Lower Cretaceous quartzitic sandstones are expected to have good reservoir character (Mbede, 1987), whereas in Somalia the Cretaceous section is mainly argillaceous, though locally some potential reservoir beds can be observed within the Gumburo Group of the Somali Embayment. Mid-Cretaceous lithofacies were deposited under fluctuating sea level prior to the regional Late Cretaceous transgression. They are expected to have stratigraphic traps within the Albian-Aptian section. Though the Upper Cretaceous is known to be transgressive almost everywhere in the region, significant Cenomanian and Turonian regressions deposited potential reservoirs. Kajato (1982) suggests that Upper Cretaceous sands are potential reservoir within the Mafia channel structure. There are several potential Tertiary reservoirs in the region, mainly along and seaward of the present coastline. Continental barren beds of northeastern Kenya and Somalia can prove to be of good reservoir character, while a gas blow-out in 1981 was reported from Eocene limestones in the Songosongo gas field, Tanzania. Palaeogene reservoirs have been reported in Zanzibar 1, Pemba-5 and Mafia-1 wells. The Mnazi Bay gas discovery is in Miocene sands, while gas shows have been recorded in Miocene sands of the Rasimachuis-1, Zanzibar-1, Pemba-5, Mafia- 1 and Kisarawe- 1 wells. Tertiary prospects are also widely reported in Somalia and Kenya, but their development is limited compared to that in Tanzania (Figs. 3, 4, 7 and 10). The tectonic development of the basins in this region leaves no doubt that both structural and stratigraphic traps must be well developed. Structural traps will mainly be related to the tensional forces related to early rifting processes. The later opening up of the Indian Ocean generated structures related to compression associated with wrench movements. Stratigraphic traps will be mainly related to the post-Karroo transgressive and regressive sequences. Middle Jurassic evaporites and shales can provide sealing within restricted Karroo basins, while Upper Jurassic shales can provide sealing for Lower and Middle Jurassic reservoirs. Upper Cretaceous shales can provide sealing for Lower and Middle Cretaceous reservoirs as exemplified in the Songosongo gas field. Widespread deep water deposits exist within the Tertiary sequence, including Miocene shales capping the Lower Miocene gas reservoir in Mnazi Bay.

231

Tensional forces have also occurred throughout the history of the region. Although partly good source rocks are at least developed locally in the region and reservoir potential are recognized, no commercial oil has been reported so far, despite drilling most major structures. The high geothermal gradients normally associated with rifting are expected to have speeded up maturation of potential source beds within Karroo formations. Reservoir potential is poor in these rocks, but traps are abundant, though sealing is a problem. The thick post-rift sequence on top of the Karroo sediments might have contributed to the overmaturity of Karroo source beds and to the destruction of porosity and permeability. Petroleum may have migrated into early traps, however, and then have remigrated into younger traps during later movements. Maturation within the post-Karroo sequence is believed to have been slow, because of a low geothermal gradient. This idea is supported by the immature Upper Jurassic outcrop sample collected in Kenya. Maturity may be expected to increase basinward. Jurassic source beds have not been reached in most wells drilled. In Kenya Lower Cretaceous source rocks have been found to be overmature, while Mid Cretaceous source rocks are just at the oil window (Mbede, 1987). Tertiary shales are immature in most wells except in Tanzania, where Tertiary development is extraordinary (Fig. 10). Here a burial curve reconstruction has proved that the maturity of source beds within the Pugu-Musanga structural axis is higher than one would expect for normal subsidence to their present depths. This is suggestive of Mid Miocene inversion which is believed to have resulted in the removal of several thousands of metres of section. Tertiary beds are suspected to have generated oil which migrated into traps within the vicinity, but Miocene inversion might have caused significant redistribution and eventual loss of hydrocarbons by erosion or leakage. Hence structural traps in this region are considered to be more risky than stratigraphic traps which need detailed geophysical mapping and are expensive to explore (Nagati, 1996).

Other mineral deposits Exploitation of other mineral resources is active around the region. Early Jurassic to Recent limestones are used by cement factories. Each country has at least one cement factory. Limestones, together with shales, are also used locally for the production of facing stones, aggregates and other building materials. High grade gypsum, interbedded with Triassic to Jurassic shales occur in the Mandawa Basin in south Tanzania. No exploitation of this deposit has ever taken place, but near Malindi (Kenya) gypsum is exploited on a small scale for the cement in-

232

dustry. The cement industry in Tanzania is supplied with gypsum from Recent lake bed deposits found together with clastic and limy materials. Kaolinite suitable for paper and rubber manufacture is produced from Late Miocene kaolinitic sandstones in the Pugu hills (Tanzania). Clean sands obtained as by-product of this kaolin industry is suitable for use as glass sands and is expected to be used by the Mbagala glass industry in Dar-es-Salaam. Beach sands are considered to be not so good for glass industry due to less of the 30-70 mesh, and a high proportion of impurities such as carbonate grains and refractory minerals (zircon and kyanite). A number of heavy mineral deposits exist but they have received little attention by prospectors so far. Workable amounts of ilmenite, rutile, zircon and leucoxene are reported. No exploitation of heavy mineral is going on at the moment, though reconnaissance exploration is known to have been carried out. More than 72 million tons of proved reserves were estimated (Halse, 1980) in Kenya and a mining license has been awarded. In Tanzania a pilot plant for heavy mineral concentration was established by STAMICO (the State Mining Corporation) in the 1970's around Dar-es-Salaam, but was later abandoned. Production of salt from sea water by solar evaporation is the major source of common salt within the countries, while the widespread sedimentary basins also provide aquifers for groundwater resources especially in the arid areas of northeastern Kenya and Somalia.

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