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Morley, C.K., W.A Wescott, D.M Stone, R.M Harper, S.T Wigger, R.A Day, and F.M Karanja, 1999, Geology and Geophysics of the Western Turkana Basins, Kenya, in C.K Morley ed., Geoscience of Rift Systems—Evolution of East Africa: AAPG Studies in Geology No 44, p 19–54 Chapter Geology and Geophysics of the Western Turkana Basins, Kenya C.K Morley Department of Petroleum Geoscience University of Brunei Darussalam Negara Brunei Darussalam W.A Wescott D.M Stone R.M Harper S.T Wigger R.A Day Amoco Production Company Houston, Texas, U.S.A F.M Karanja Shell Exploration and Production Kenya Nairobi, Kenya Abstract The Turkana area comprises a string of half graben basins, most of which have been imaged by seismic reflection data (a total of 2,267 km of 60-fold vibroseis data) and two tested by hydrocarbon exploration wells The Lokichar Basin is the oldest known basin (late Paleogene-middle Miocene) and has a simple half graben geometry Basin fill is up to km thick and is comprised of Paleogene-middle Miocene fluvio-deltaic sands punctuated by episodic thick lacustrine shale and sandstone deposition Two of these thick lacustrine shales were identified in the Loperot-1 well In particular, the upper lacustrine sequence (Oligocene-early Miocene), called the Lokone Shale Member, is well developed The shales are rich in TOC (4.5% average) and thicken to km near the boundary fault Overlying the Lokone Shale are fluviodeltaic sandstones that grade upwards from arkosic sandstones to sandstones having a significant volcaniclastic component of middle Miocene age The uppermost basin fill is middle Miocene age lava flows Northward the lower, arkosic sequence passes into a volcanic-dominated succession of similar age (late Oligocene-early Miocene) and is exposed in the Lothidok Hills The volcanics seem to have been deposited in an east-thickening half graben (Lothidok Basin) At the end of the middle Miocene, both the Lokichar and Lothidok Basins were abandoned and a new string of half graben basins formed (North Lokichar Basin, Turkana Basin and the Kerio Basin) The North Lokichar Basin is a west-thickening basin superimposed on the Lothidok Basin, and the Kerio Basin is a west-thickening series of half graben basins that probably had a middle Miocene-Paleogene history of extension as well Sediments with a considerable volcaniclastic component and lava flows fill these basins No deep water lacustrine conditions are known Extension continued into the Pliocene, particularly in the Turkana Basin and the northern part of the Kerio Basin Finally, the old half grabens were abandoned and a belt of minor fault swarms and volcanic centers was established east of Lake Turkana and in the southern part the lake Pliocene, and possibly Holocene, tectonic inversion affected the Turkana and Kerio Basins (Chapter 8), variations in boundary fault angle (Chapter 7), the influence of preexisting fabrics on rift structure (Chapter 9), and structural controls on sedimentation (Chapter 13) This chapter provides the background information to the specific aspects of rifting listed above, by presenting key geological results derived from hydrocarbon exploration in the area The reasons for the lack of exploration success are not discussed in detail in this section, but are incorporated into Chapter 14 INTRODUCTION The Turkana basins are the most important area in the Kenya Rift for studying a long-lived portion of the rift system and how it interacted with the older CretaceousPaleogene Anza Graben (see Chapter 4) Surface, seismic reflection, and well data acquired over the region make this area of the rift highly significant in terms of understanding basin inversion in the region (Chapter 12), basin evolution 19 20 Morley et al Figure Topographic and gravity maps of the central and northern Kenya Rift (gravity map after Morley et al 1992) Regional cross section through the southern Turkana area The rationale for exploring for hydrocarbons in northern Kenya was the search for Cretaceous basins similar to the producing basins in Sudan (e.g., Schull 1988) Prior to the acquisition of seismic reflection data, the large northwestsoutheast trending negative gravity anomaly of the Anza Graben (Figure 1) on the eastern side of Lake Turkana gave early encouragement that a significant sedimentary basin was present The extent of related basins on the western side of the lake was unknown when exploration began, although some linkage of the Anza Graben to the Sudanese rifts through the western Turkana area was suspected The available gravity data (e.g., Swain and Khan 1978) on the western side was relatively sparse and did not show the presence of well developed sedimentary basins From geological maps the terrain appears to have little hydrocarbon potential since it is dominated by igneous rocks and Pre- Geology and Geophysics of the Western Turkana Basins, Kenya cambrian basement, with only small patches of coarse clastic rocks cropping out (mostly termed Turkana Grits) Confronted by the possibility of deep basins existing, but with all the basic play ingredients (source, seal, reservoir, trap, timing of hydrocarbon migration) unknown, Amoco embarked on an extensive multi-disciplinary program to try to answer the many exploration questions posed by the area (Table 1) Amoco gathered additional gravity data on the western side of Lake Turkana, which enabled a cluster of potential basins to be identified between Lodwar and the southern end of Lake Turkana (Figure 2) An initial grid of ten regional seismic reflection lines revealed basins up to km thick (including lines TVK-2 and 3, Figures A4 and A6) Consequently a more extensive seismic grid was acquired (Table 1) Depending upon terrain, the lines are spaced between 3km and 10 km Apart from four lines across the Lotikipi Plain and two along the western coast of Lake Turkana, most of the TVK lines (TVK-1 to 13 and TVK-100 to 143, see Figures A1–A26 for examples) cover the western Turkana basins In total there are 2,267 km of 60-fold vibroseis data The data quality ranges from good to poor, the latter associated with shallow basement blocks or surface volcanics Gravity and magnetic data were gathered along all seismic lines Examples of the seismic data are shown in Appendix A as Figures A1-A26 More recent seismic data, infilling parts of the TVK grid, has been acquired by both Amoco and Shell but is not part of the data presented herein (Table 1) The region is very large, has poor accessibility, and generally the surface geology had only been described on a reconnaissance basis The “local” exceptions are Miocene to Holocene vertebrate fossil localities, some of which have TABLE History of major exploration surveys undertaken in the Turkana area 1984: Project PROBE seismic reflection survey of Lake Turkana Approximately 1,400 km of multichannel data, 10 km line spacing 1985–1986 (5/85–2/86): Reconnaissance seismic program by Amoco west of Lake Turkana and Lotikipi Plains, accompanied by gravity and magnetic surveys, and geological mapping and sampling along the seismic lines Aerial photo interpretation of the Lotikipi Plains area 1987–1988: Detailed seismic program in west Turkana basins A total of 1,407 km of 60-fold vibroseis data was acquired by Amoco Gravity and magnetic surveys, and geological mapping and sampling conducted along the seismic lines Aerial photo interpretation and field checking produced geological map of western Turkana (Figure 3) Shallow wells drilled on potential source rock interval, Lokichar Basin Igneous rock samples collected for K/Ar whole rock age dating Surface stratigraphy and sedimentology of the basins examined 1990: After Shell farmed into the area, they acquired 500 km of vibroseis data over four prospective areas KRISP seismic refraction survey of the Kenya Rift, including the Turkana area 1991: Shell acquires an additional 581 km of vibroseis data in two areas 1992: Shell drilled the Loperot-1 and Eliye Springs-1 wells 21 been studied in great detail (e.g., Patterson et al 1970, Brown and Feibel 1986, 1988, Boschetto et al 1992) Several field parties were organized by Amoco to field check geological maps made from aerial photographs (Figure 3), measure stratigraphic and sedimentary sections, and define stratigraphic ages using radiometric dating for igneous rocks and micropaleontology, palynology, or vertebrate fossils for sedimentary rocks The surface geology along the TVK seismic lines was mapped and used in the seismic interpretation Water well drilling rigs were also used to determine the stratigraphy in some critical but poorly exposed areas In 1993, two wildcat wells were drilled by Shell Kenya PC which tested a deeper part of the stratigraphy in two basins REGIONAL STRUCTURE The Turkana area is the widest know part of the East African Rift System, with the exception of the Afar Triangle At its widest the rift consists of 3–4 major half grabens spanning a distance of about 150 km (Figure 1) The Turkana area is topographically low, relative to other parts of the Kenya Rift Investigations into the deep crustal and mantle structure of the rift by the KRISP group using seismic refraction data indicates that the continental crust may be thinned from about 40 km on the rift flanks (Maguire et al 1994, Braile et al 1994), to about 35 km beneath Lake Bogoria (Henry et al 1990) and thins passing along the rift axis towards the north The crust may be as thin as 18–20 km under the western Turkana area (Figure 4, e.g., Mechie et al 1994), making it the area of thinnest crust in the Kenya Rift The regional cross section on Figure shows the main features of the rift based on outcrop, gravity, seismic reflection and refraction data There are a series of dominantly east-dipping half graben boundary faults These faults are thought to pass at depth into a zone of ductile shearing (pure shear) in the lower crust The thin crust and important volcanic activity suggests heat flow is high in the Turkana area This is supported by the geothermal gradient of 4.2°C/100 m in the Loperot-1 well Consequently, the brittle-ductile transition zone is likely to be relatively shallow—probably between about 10–12 km in depth This estimate was derived from better constrained data from the central Kenya Rift where the continental crust is thicker (about 35 km, Henry et al 1990), heat flow is high (50–100 mWm2, Wheildon et al 1994) and the brittle-ductile transition is thought to lie between 12–16 km in depth (Tongue et al 1994) The zone of lower crustal stretching, defined by the KRISP refraction survey, lies below the surface rift This is consistent with the McKenzie rift model (Figure 5a or b) but does not fit well with models that offset the surface rift from the upwards deflection of the Moho due to a lowangled detachment fault that penetrates the entire crust (e.g., Bosworth 1987; Figure 5c) The cross section in Figure provides a deceptively simple picture of the rift because, in detail, the structural geometries and basin evolution are complex It does, however show the main features of the rift, which are dominantly a series of west-thickening half graben basins bound- Figure Regional geological map of northern Kenya compiled from Landsat, aerial photo, and field data Volcanic age dates were compiled from the following: Amoco field work, locations 23–43 and 56–67, Morley et al (1992); 1–18 from Zanettin et al (1983); 19–22 from Boschetto (1988, 1992); 44–49 from Brotzu et al (1984), 50–55 from McDougall and Watkins (1988); 68–70 from Patterson et al (1970); and 71–73 from Baker et al (1971) 22 Morley et al Figure Geological map over the southwestern Turkana basins, made from aerial photo interpretation and field mapping 24 Morley et al Figure Regional map of the Kenya Rift with superimposed depth to Moho contours (derived from refraction data by KRISP 1991) The crust thins in a northerly direction, with the greatest thinning occurring in the Turkana area ed on their eastern sides by major boundary faults The western-most fault trend is the Elgayo-Turkwel Fault There is no steep gradient in the Bouguer gravity anomaly associated with this trend in the Turkana area, hence, it is assumed that basement is shallow To the east lies the Lokichar Fault zone, which bounds two large negative anomalies (Lokichar and North Lokichar Basins) separated by a gravity high (Figure 1) Further east lie the Kerio and Turkana Basins, both of which extend northwards under Lake Turkana, where seismic data acquired by Project PROBE revealed deep rift basins (Figure 6, Dunkleman et al 1989) On the eastern side of Lake Turkana and crossing the southern-most part of the lake is a belt of closely spaced minor faults (Kino Sogo Fault Belt) cutting Pliocene lava flows (Figure 2) They represent the youngest structural zone within the Kenya Rift GEOLOGICAL MAP OF THE WESTERN TURKANA BASINS The map of the surface geology of the western Turkana basins, Figure 3, was made from aerial photo interpretation and fieldwork The main mapped rock units are crystalline Precambrian basement, Oligocene-Pliocene igneous rocks, arkosic “grits” (Turkana Grits), and sandstones with a large volcaniclastic component Precambrian basement appears in three main areas: On the western side of the map it forms the footwall block to the Lokichar Fault The eastern edge of the basement (where the town of Lokichar is situated) approximately marks the trace of the Lokichar Fault The Lokone Horst area–this area is called a horst because the basement is surrounded by much younger lavas and sedimentary rocks The term horst is not, however, correct The western margin of this basement outlier is actually onlapped by coarse arkosic sandstones and small hollows in the basement surface are filled by thin limestones The eastern margin is defined by a large fault (Lokone Fault) which is one of the boundary faults to the Kerio Basin The north northeast-south southwest strike of the boundary fault, follows the strike of foliations within the basement The Lariu Range on the shores of Lake Turkana—in a very similar structural setting to the Lokone Horst—are basement outcrops bounded by faults on the eastern margin, and overlain by coarse arkosic sandstones or lavas on the western side The region represents the flexural margin to the Kerio Basin The sedimentary rocks that overlie Precambrian basement are best exposed west of the Lokone Horst (see following section) They represent the hinged margin deposits of the Lokichar Basin Sandstones are coarse, commonly conglomeratic, and channelized, with erosive bases (Wescott et al 1993) Individual beds range up to several meters in thickness The sandstones commonly display cross and trough cross bedding and water escape features They are predominantly fluvial, probably braided stream, deposits Thin green and red silts and mudstones form interbeds Recovery of microfossils from these oxidized sediments is very poor, consequently the ages of many outcrops are very poorly known In places, vertebrate and microfossils indicate the exposed sequence ranges from late Oligocene to middle Miocene in age Overlying the sandstones are lava flows In the Auwerwer Hills the flows are of middle Miocene age and form regularly layered outcrops that dip a few degrees towards the west Passing northwards the igneous rocks change character considerably In the Napedet and Kamutile Hills (Figure 3) there is great variety to the igneous rocks, there are dike swarms, vent complexes, considerable quantities of pyroclastics as well as lava flows These too are of middle Miocene age On the western margin of the Napedet Hills are clastic sedimentary rocks with a considerable volcaniclastic component that overlie the lavas Their age is poorly known but is probably late Miocene-Pliocene Along trend with the Napedet Hills are the Lothidok Hills They, however, comprise an older volcanic and volcaniclastic sequence of late Oligocene-Miocene age The eastern margin of the Lothidok Hills is a boundary fault margin to the Turkana Basin Igneous rocks at the southern margin of the map comprise extensive Pliocene lavas that have flowed over the southern portions of the older basins described above The major outcrops described above provide clues about the distribution of the basin geometries, but it is dif- Geology and Geophysics of the Western Turkana Basins, Kenya 25 ficult to make a convincing picture of the basins from surface data alone The multiple types of geological and geophysical data required for an improved understanding of the basins are discussed in the following sections In general, the igneous and sedimentary rock outcrops represent the flexural or hinge margin of one basin, and the footwall uplift block to the boundary fault of an adjacent basin The main basinal areas are covered by Recent deposits An extensive area of young (Holocene ?) lacustrine sediments flank the Kerio River These sediments—silts, mudstones, and limestones (including stromatolites and oyster beds)—indicate that Lake Turkana once covered a much larger area The Lokichar, North Lokichar, Kerio, and Turkana Basins are covered by seismic reflection data and are examined in detail in the following sections A quick reference to the regional subsurface geology, Figure 6, is provided which approximately illustrates basin geometries from seismic data and also shows how offshore Lake Turkana seismic data correlates with the onshore data LOKICHAR BASIN Introduction The Lokichar Basin is approximately 30 km wide and 60 km long, and currently has the best quality subsurface data of any basin in the East African Rift System It demonstrates many of the classic characteristics of half graben basins It is a relatively simple half graben bounded to the west by the Lokichar boundary fault (with up to km throw and 10 km extension), and to the east by a flexural margin consisting of Precambrian basement outcrops of the Lokone Horst (Figures 3, 6–10, A1, A2, and A6–A10) The flexural margin is the footwall of the Lokone boundary fault, which lies on the eastern margin of the Lokone Horst Movement on this fault caused the flexural margin to be isostatically uplifted and eroded, resulting in the exposure of extensive sedimentary outcrops on the western flanks of the Lokone basement Horst (Figure 8) Basin Stratigraphy The basin’s surface outcrops are only representative of the flexural margin sediments This is typical for most rift exposures—deep basin sediments tend not to be exposed Outcrop stratigraphy consists of arkosic fluvio-deltaic sandstones overlying crystalline basement, followed by fluviodeltaic sandstones containing less feldspar and more volcanic-derived material (Figures and 9) (Joubert 1966) At the top of the sequence are lavas with interbedded sediments of middle Miocene age The exposed sequence lacks evidence for extensive, thick shales which could act as seals or source rocks Paleocurrent directions from cross bedding in the fluvial grits indicate a source area to the south to southeast Since the main boundary fault trends north-south, and lies to the west, these data suggest dominantly axial drainage systems Volcanic activity was intense in the area north of the Lokichar Basin during the late Oligocene-early Miocene (Fig- Figure Models for crustal extension: a pure shear model (McKenzie 1978), b simple shear-pure shear model, and c simple shear model (Wernicke 1985) Vertical scale = horizontal scale ure 2), but the paleocurrent directions and absence of volcanic clasts indicate the northern area was not a source for the (basement-derived) arkosic sandstones that form much of the early basin fill Outcrops are good, but patchy, in the Lokichar Basin Typically sandstones crop out, but the faster weathering shales are rare Some small shale outcrops were found, particularly along seismic line TVK-12, where igneous intrusions within the shale were largely responsible for exposing and preserving the shales at the surface These shales are be referred to as the Lokone Shales The shales were baked by the intrusions, but not significantly altered The total organic carbon tends to be relatively high in the baked zones (samples yielded about 2.5% TOC), while away from the baked zones TOC content is less than 1% This can be attributed to weathering breaking down the unbaked shales faster than the baked ones In order to obtain unweathered samples and stratigraphic information on the shales at low cost, a shothole rig (a Mayhew 2000) from the seismic acquisition crew was used to drill 14 shallow wells to depths of 90 m Six of these wells were drilled along seismic line TVK-12 and six more were located on line TVK-110, km to the south (Figure 8) For a flexural margin location the shales are remarkably thick, correlative and continuous along strike (Figure 11) The unweathered shales yielded well pre- 26 Morley et al Figure Regional map of the Turkana region with cross sections based on seismic data illustrating the basic subsurface basin geometries of the region The offshore Lake Turkana seismic is that of Project PROBE (e.g., Dunkleman et al 1989) Geology and Geophysics of the Western Turkana Basins, Kenya 27 Figure Regional structure map of the Turkana basins from seismic reflection data The structure is dominated by half grabens served microfossils that provided important age information Palynomorphs of Margocloporites verwijhei, Echitriporites Kwaense, Racemonocolpites hians, Praedapollis protrudentiporatus and Retibrevitricoporites protrudens suggest a late Oligocene to early Miocene age The presence of abundant freshwater algae (Pediastrum and Botryococcus) point to a non-marine, lacustrine environment Good stratigraphic correlation can be made from the rocks imaged on line TVK-12 to those on line TVK-110 (Figure 8) The correlation is based on lithology, stratigraphic position, common palynomorphs, organic content, and seismic character Using patchy outcrops and the shallow well data, the shale section can be mapped along strike for at least km Exposures of shales were not observed north of line TVK-12 Black to dark gray shales were observed as far south as Namadang (Figure 8) Despite the location of the Lokone Shales close to the flexural margin of the basin (Fig- ure 12), they are at least 100 m thick, suggesting the possibility that the shales extended further eastwards into the Kerio Basin The shallow well data provided an late Oligocene-early Miocene age for the shales—significantly older than dates for other half grabens in the Kenya Rift Yet earlier initiation of rifting seems possible because strata below those correlated to the wells show apparent onlap onto the assumed top Precambrian basement reflection However, given the very rapid depositional rates in rifts (perhaps 2–3 mm/yr), the age difference between the dated and undated sections could be very small The Loperot-1 exploration well confirmed that the Oligocene-Miocene shales had expanded in thickness from the outcrops towards the Lokichar Fault In the well they were about 340 m thick (Figure 13), and seismic data indicates they may be as thick as 1,350 m approaching the 28 Morley et al Figure Location map for Lokichar Basin boundary fault (Figure 14) Hence, the shales were deposited during a period of marked basin subsidence, which was probably controlled by boundary fault activity The Loperot-1 well also encountered a second fluviolacustrine sequence of early Oligocene to possible Eocene age, not seen at the surface (Figure 13) This mixed sandstone-claystone interval extends from 1,900–2,757 m, and between 2,410 and 2,600 m has source rock potential It is this interval which displays onlap onto the top Precambrian basement reflection Previously all sedimentary rocks stratigraphically below the lava flows were called Turkana Grits (e.g., Joubert 1966, Williamson and Savage 1986) With the new subsurface data it is possible to subdivide and date the stratigraphy more accurately The following stratigraphic terms are proposed (Figure 8): The oldest basin fill comprises Paleogene-lower Miocene sedimentary rocks At the surface they are predominantly fluvio-deltaic, arkosic, pebbly sandstones and fringe the Lokone Horst (Figure 8) A well exposed section through the outcrops in given in Figure Hence, the name Lokone Formation is proposed Two important shale units have been identified within this formation The older shale, known only from the Loperot-1 well has been named the Loperot Shale Member, while the younger shale, identified in shallow wells, outcrops (Figures and section 5), and the Loperot-1 well is named the Lokone Shale Member The Lokone Formation is likely to be dominated by sandstones on the flexural margin and by lacustrine shales towards the boundary fault margin The sandstone dominated sequence overlying the Lokone Formation is characterized by a considerable volcaniclastic component, including tuff or reworked tuffaceous units (see Figure 9, section for a typical stratigraphic section) It is referred to as the Auwerwer Sandstone Formation Basalt flows directly overlie the Auwerwer Sandstone Formation (Figure 9, section 1) and are well exposed in the Auwerwer Hills, hence they are called the Auwerwer Basalts Other formations or members may exist in the subsurface, but their presence has not yet been confirmed In particular, the presence of coarse conglomeratic fans fringing the hanging wall of the Lokichar boundary fault is suspected Figure 20 Correlations of basin stratigraphy in western Turkana (after Morley et al 1992) In general there is a trend for the Paleogene-middle Miocene section to be more volcanic-prone passing northwards, and to contain more basement derived sedimentary rocks to the south East-west variability is poorly constrained A more widespread volcaniclastic component to the sedimentary rocks occurs after the middle Miocene The schematic cross section illustrates the along-strike transition from predominantly basement-derived sedimentary rocks in the Lokichar Basin to a volcanic dominated basin fill in the Lothidok Basin, separated by the saddle area (Maguire et al 1985, an unpublished Amoco in-house report) 40 Morley et al Geology and Geophysics of the Western Turkana Basins, Kenya 41 Figure 21 Location map for the Kerio Basin 1,917 m yielded a date of 5.1± 0.2 Ma If it is a sill, then the surrounding rocks must be at least as old as this date The Miocene-Pliocene boundary probably lies in the vicinity of 1,900–2,000 m The thickness of the section testifies to the important late rift activity in the area The well provides a good correlation point for the offshore Lake Turkana seismic data acquired by Project PROBE, and indicates most of the section imaged offshore is probably latest MiocenePliocene (Dunkleman et al 1989) The sequence in the well is dominantly fluvio-lacustrine sands and shales, and is similar to surface outcrops which lie to the north There are no indications of a well developed lacustrine sequence that would support the existence of an extensive, long-lived proto-lake Turkana during the Pliocene Two points of correlation within the southern Kerio Basin come from the shallow wells Lothagam-1 and Kerio-1 (Figures 27 and 28) Four cutting samples from the Lothagam1 well were analyzed for age-dates and paleoenvironments Samples between 48–54 m and 81–87.5 m are probably of late Miocene-Pliocene age Lacustrine conditions are indicated by the presence of freshwater algae Pediastrum and Botryococcus Four samples from the Kerio-1 well were also examined but were essentially barren or contained recent specimens (contamination) The lacustrine sections from both wells were completely different from the Lokone shallow wells in their poor organic content and absence of source rock potential In the southern Kerio Basin, middle to early Miocene volcanics form the eastern flexural margin in the Lariu Range The presence of arkosic grits below the volcanics in the Lariu Range (Wescott et al 1993, Figure 21) suggests that stratigraphy similar to the Lokichar Basin might be present in the southern Kerio Basin (Figure 20) The arkosic grits may expand considerably into major faults However, poor penetration due to volcanic layers may preclude imaging the lower section on the seismic data Seismic Correlation It is not possible to correlate seismic horizons from the Lokichar Basin across to the Kerio and Turkana Basins because of the (basement) structural high running from the Lokone Horst to the Napedet Hills that separates the Lokichar Basin from the other basins (Figure 7) This high is bounded on the eastern side by a series of east-dipping boundary faults (Figure 29) The Kerio Basin is not deeply eroded except in the Lariu Range area where the tie between seismic data and outcrop data is poor (Figures 26 and A2) One key correlation is the Orange, or O, horizon which forms a bright reflection on seismic data (Figures A16–A21) In the southern Turkana Basin the O horizon was penetrated in the Eliye Springs-1 well, later yielding a radiometric age of 5.1 Ma from samples of an igneous intrusion or lava flow close to the location of the reflection In the southern portion of the Kerio Basin (line TVK-12, Figure A2) the O horizon correlates to a lava flow dated at 4.3 ±1.9 Ma (Morley et al 1992) While these volcanic horizons probably represent separate bodies, the O horizon does appear to lie close to the Miocene-Pliocene boundary It is possible to distinguish faults that were dominantly active before and after O horizon deposition (Figures 30, A1–A5, A16–A25) The southern part of the Kerio Basin was primarily active before the Pliocene (Figures 17, A1, A2, A16, and A17) while the northern region and Turkana Basin were dominantly active during the Pliocene to Holocene 42 Morley et al Figure 22 Section of seismic line TVK-102E illustrating the correlation to the surface outcrops of Lothagam Hill (see Figures and 21 for location and Figure A22 for the full line) The absence of deep well and outcrop data on the deeper section makes understanding the pre-O horizon history in the Turkana and Kerio Basins difficult It is apparent that significantly faulted, rotated, and eroded basins exist below the O horizon (Figures 31, 32, A27, and A28) Further, the age of the older section is unsure It might represent a Paleogene-middle Miocene or late Miocene phase of deformation, or both For seismic lines near Lothagam Hill it is possible to tie the outcrop data into the subsurface information On seismic line TVK-102E (Figure 23), an irregular reflection lies Figure 23 Section of seismic line TVK-102E illustrating the section that probably represents middle Miocene volcanics overlying early Miocene volcanics and sediments (see Figure A22 for the full line) about 200–300 ms below the O horizon and separates a poorly reflective zone below, from a highly reflective zone above This reflection probably represents the top of middle Miocene volcanics observed near the base of the Lothagam Hill outcrops (Patterson et al 1970) The poor reflectivity below the O horizon is likely to be due to a thick section of mixed volcanic flows and volcaniclastic rocks similar to those described in outcrop from the Lothidok Hills by Boschetto et al (1992) Hence, in the Northern Kerio Basin it appears that the late Miocene section is thin (200–400 m) and undergoes little expansion It is the Pliocene and Figure 24 Well logs and stratigraphic column for the Eliye Springs-1 well (see Figure for well location) Geology and Geophysics of the Western Turkana Basins, Kenya 43 44 Morley et al Figure 25 Tie between Eliye Springs-1 well and line TVK-141 (also see Figure for location) middle Miocene-Paleogene sections that undergo the greatest thickness changes (Figure 17) In the Southern Kerio Basin the seismic data shows that between the middle Miocene volcanics and the O horizon there is a considerable expansion of section, indicating late Miocene age fault activity Two isopach maps for the preand post-O horizon basin fills (Figure 17) show that there is a north-south transition in the Kerio Basin between dominantly Miocene-Pliocene expansion of section in the south and dominantly Pliocene expansion of section in the north Significant rotation of the older section can be observed (Figures A1, A17, A27, A28, and Figure 31) The strike line TVK-11 (Figures A27 and A28) shows the division between the pre-and post-O horizon tectonic particularly well, with many minor faults terminating near the O horizon and not passing into the younger section In a segment of TVK-110 (Figure 32) and on TVK-132 (Figure A17) three major tectonic events can be observed The first is the most highly rotated reflection package It displays only moderate expansion into extensional faults The second package is more strongly wedge shaped and indicative of a time of important extension and rotation On both seismic lines the rotational event is terminated at the Orange horizon, marking a change in structural style near the Miocene-Pliocene boundary On line TVK-110 (Figure 32) the O horizon is an unconformity that seals older faults On line TVK-132 (Figure A17) extension continued after the Orange horizon, and the overlying section expands into the faults It is not possible to tie the first two packages directly to outcrops—so their age is unknown—they could both be of Miocene age Alternatively, the deeper package could be of Paleogene age East of these lines, in the Lariu Range, arkosic grits are present beneath middle-early Miocene volcanics (Wescott et al 1993) They are undated, but it is very possible that they correspond with the early MioceneOligocene section observed in the Lokichar Basin, and form the lower tectonic unit seen on lines TVK-110 and TVK-132 (Figure 26) A number of seismic lines in the Kerio and Turkana Basins show structural styles, including folds, that are incompatible with simple extensional tectonics In particular, the northern Kerio Basin is strongly affected (Figures A18–A21, A27, and A28) These non-extensional structures are interpreted to be associated with basin inversion that occurred during the Pliocene, near or at the end of rifting (Figure 31) Chapter 13 examines the inversion structures in detail DISCUSSION Traditionally the period of tectonic activity associated with the Kenya Rift has been Neogene-Recent (e.g., Baker and Wohlenberg 1971), with older volcanism (30 Ma.) recognized in the Turkana area (Baker 1986) Volcanism generally appears to have preceded extension and evidence for Figure 26 Regional geological cross sections based on seismic lines The O horizon lies approximately at the base of the Pliocene In general, the timing of deformation is younger to the west This is particularly well seen in the section for lines TVK-13 and TVK-12, where the Lokichar Basin is of Paleogene-middle Miocene age, the Kerio Basin is a mixture of Paleogene-Miocene and Pliocene age, and in southern Lake Turkana, the section is dominantly Pliocene and younger Eastward decreasing age of deformation is less apparent further north (TVK-102, E and W) where all the basins may contain older Paleogene-Miocene section as well as Pliocene and younger section Considerable structural reorganization occurred between the middle and late Miocene and the Miocene and Pliocene Geology and Geophysics of the Western Turkana Basins, Kenya 45 46 Morley et al Figure 27 Well log for shallow well Lothagam-1 (see Figure 21 for location) The section of line TVK-11 illustrates the location of the Lothagam-1 well on a relatively high, eroded portion of the line (see Figure 21 for location) Figure 28 Section of seismic line TVK-119 illustrating the location of the shallow well Kerio-1 on an eroded footwall uplift (see Figure 21 for location) Geology and Geophysics of the Western Turkana Basins, Kenya 47 40 Ma and an important early phase of extension and sea floor spreading began about 30 Ma (Girdler 1991) One characteristic of the Sudanese-Anza Rift System is the very minor amount of associated igneous activity, yet in the Lotikipi Plain and northern Turkana area very extensive Paleogene volcanics are present (Chapter 4, Figure 34) This suggests an East African Rift influence on the Paleogene history Hence, the Turkana region is not only a region where two separate rift systems cross, it is also in the transition zone between an older, more developed part of the East African Rift System, the Ethiopian Rift, and the younger central and southern Kenya Rift (Hendrie et al 1994, Ebinger and Ibrahim 1994) Looking in detail at the Lokichar Basin it is not possible to find an obvious tectonic boundary at the base of the early Miocene and state this is the beginning of the Kenya Rift episode The clearest switch in half graben geometry in the North Lokichar Basin occurs not at the base of the Miocene, but in the middle Miocene, where the polarity changes Hence, no clear separation of Paleogene and Neogene rifting events is possible—there is no widespread end of the Paleogene rifting unconformity A table of tectonic events in different parts of northern Kenya is presented in Figure 35 and illustrates the progressive changes and overlapping timing of tectonic events in different basins The most well defined unconformity occurs between the Muranachok Grits (age unknown) and the overlying Oligocene-Miocene volcanics In the Lokichar Basin, the presence to two separate lacustrine source rocks (Oligocene-Miocene and possibly Eocene) suggest two tectonic pulses, since major episodes of deep-lacustrine sedimentation are commonly controlled by boundary fault activity (Morley 1989, Lambiase and Bosworth 1995) These observations indicate that if the rifting events associated with the Sudanese-Anza Rift System and the East African Rift System can be separated, the break lies within the Oligocene (probably around 32–36 Ma.) DISCUSSION OF EXPLORATION RESULTS Figure 29 Map of the main boundary fault planes in western Turkana, mapped from seismic data The map illustrates the dominance of east-dipping boundary faults which control west-thickening half grabens In a few areas polarity reversals are present deformation prior to the middle Miocene within the central and southern Kenya Rift is poor to nonexistent (Barker 1986) The Anza Graben developed from the Cretaceous to the Paleogene (e.g., Bosworth 1992, Bosworth and Morley 1994) Hence in the “cross-over” area between the two rifts (i.e., the Turkana region) the simplest sequence of events is to relate the Paleogene deformation to the Anza-Sudanese Rift System (Figure 33) and the Neogene-Recent deformation to the Kenya Rift system This, however, is probably an over simplification, because the Ethiopian rift to the north also contains sedimentary and igneous rocks of Paleogene age that are related to the development of the East African Rift (Ebinger et al 1993) Extension around the Gulf of Aden-Red Sea-Ethiopian Rift triple junction began around The Turkana region illustrates a number of important points concerning exploration in rifts Firstly, is the extreme stratigraphic variability between individual half grabens This aspect is particularly well demonstrated in the region because the variability involves volcanic as well as sedimentary rocks Hence, every half graben needs to be treated separately, either because of lateral changes in sediment or volcanic source areas (compare, for example, the timeequivalent Lokichar and Lothidok Basins), or because of lateral changes in the timing of extension (e.g., Lokichar and North Lokichar Basins) This lateral variability is important for understanding source rock distribution The only basin known to have a well established lacustrine source rock is the Lokichar Basin In a relatively small area (perhaps 100 km2) the thick (average 500 m), high TOC shale is capable of generating in the region of 10–20 billion barrels (1.6–3.2 × 109 m3) of oil Such potential is a considerable incentive for exploration The problem of the regional distribution of source rocks in 48 Morley et al Figure 30 Cross sections through the Kerio and Turkana Basins based on seismic lines The sections illustrate that passing northwards the older Miocene-Paleogene section and deformation become less important and the younger Pliocene section (post-O horizon) becomes more important Structural inversion of the basins also becomes more significant northwards The passage from the Kerio Basin to the Turkwel Basin is marked by a change in the polarity of the Pliocene and younger basin fill (Figure 30a this page, 30b facing page) Geology and Geophysics of the Western Turkana Basins, Kenya 49 50 Morley et al Figure 31 Portion of TVK134 illustrating major tectonic unconformities (uc) and events The O reflection lies approximately at a time of change in the basin (base Pliocene, top Miocene) where minor faults were abandoned and the overlying section only expanded into the boundary fault (East Kerio River Fault) Inversion on the East Kerio River Fault occurred during the Pliocene Figure 32 (above) Detail of TVK-110 illustrating evidence for different tectonic phases Interval is probably Pliocene in age, and illustrates that in the southern part of the Kerio Basin the Pliocene is largely post-extension and onlaps the older rift topography Within the Miocene-Paleogene section two phases of rotation (intervals and 2) can be detected The actual timing of these phases is unknown, they may correspond with the EoceneOligocene and Oligocene-middle Miocene events in the Lokichar Basin Alternatively, they may correspond to Oligocene-middle Miocene and middle Miocene-late Miocene events Figure 33 (right) Map of central, northern, and eastern Kenya illustrating the main areas with evidence for Paleogene rifting Figure 34 Tectonic evolution of the northern and central Kenya Rift from the Paleogene to the present day There is an overall eastward migration of volcanic and structural activity with time However, in detail the structural evolution of the basins is more complex and the pattern less apparent Geology and Geophysics of the Western Turkana Basins, Kenya 51 52 Morley et al Figure 35 Timing chart of tectonic and basin fill events in the Turkana region, based on data discussed in both Chapters and the area has yet to be solved It does demonstrate, however, the remarkable potential productivity of these lacustrine source rocks The main exploration challenge in the Lokichar Basin was finding a large enough trap The half graben structural style of the Lokichar Basin is not suited to generating tilted fault block traps—the classic exploration targets in rifts Finding large structural traps was a significant exploration problem and it was necessary to search for stratigraphic components to increase trap size beyond the structural closure In other parts of the Turkana area more tilted fault block traps were present, however, inversion structures were also commonly present Since they are late features there is a question concerning the possible timing of these structures, with respect to migration, even if source rocks are present The other question concerning trap viability was whether the section had thick enough shales, and in the correct position to produce lateral seals across tilted fault blocks at potential reservoir horizons (see Chapter 14) Finally, the quality of the potential reservoir rock is variable Sandstones which contain igneous and pyroclastic material are prone to undergoing diagenetic porosity and permeability reduction by generation of clay minerals at shallow depths of burial Consequently, basement-derived sandstones were thought to be the primary potential reservoirs, although they contained feldspars which could also undergo alteration with burial The basement-derived sandstones tend to lie relatively deep in the section and may pass laterally into the well developed lacustrine shale sequences Hence a possible juxtaposition of source, reservoir, and seal is somewhat favorable (see Chapter 14 for a more detailed discussion) Exploration to date has discovered favorable and unfavorable aspects of the petroleum potential of the region The lateral variability in stratigraphy, timing of structural and volcanic events, extensional fault geometry, and tectonic inversion clearly indicate that different areas of the rift will have considerable variation in potential plays and Geology and Geophysics of the Western Turkana Basins, Kenya liabilities However, there is still not enough detailed subsurface information to be confident that the region has been fully evaluated SUMMARY There are five main areas where the contact between crystalline Precambrian basement and sedimentary rocks can be observed: The Lapurr Range, Muranachok, Mount Porr, Lariu Range, and the Lokone Horst (Figure 2) In all cases the sediments are dominantly arkosic grits (fluvial) that are notoriously difficult to age date (e.g., Williamson and Savage 1986, Boschetto et al 1992) However, dinosaur bones from the Lapurr Range (Arambourg and Wolff 1969) indicate a Cretaceous age for the most northerly grits Palynology studies yielded an Oligocene-early Miocene age for the most southerly grit exposures west and south of the Lokone Horst The other three areas remain undated The arkosic nature of the grits indicates they were derived from basement rocks They invariably underlie the earliest volcanic flows in the area The presence of Paleogene rocks in the Loperot-1 well and Paleogene activity in the southeastern Anza Graben suggests rifting during the Paleogene in Northern Kenya was important and formed a string of basins as illustrated in Figures 33 and 34 These basins include some of the regions of poorly dated arkosic grit basins and the basins underlying the Lotikipi Plain (Chapter 3) The volcanic flows which overlie the arkosic grits vary considerably in age (Figures 34 and 35) As a generalization, the oldest flows occur in the northern part of the Turkana area and become younger in a southerly direction The oldest flows in the northern Turkana area are Oligocene age (Zanettin et al 1983) and range from about 33–36 Ma to 14–13 Ma Over the same time period in the Lokichar Basin (to the south), arkosic grits and lacustrine shales were being deposited in a half graben basin (Figures 20 and 33) During the early Miocene the arkosic grits were replaced by fluvio-lacustrine sediments composed of both basement and volcanic-derived material, and only in the middle Miocene (approximately 15–10 Ma) were lava flows present Consequently, the onset of volcanic activity becomes considerably younger to the south and east The Lothidok Hills (Boschetto et al 1992) display the most southerly record of early Miocene volcanic activity South of the Lothidok Hills (Kerio and North Lokichar Basins), following the lava flows of the middle Miocene, clastic material having a considerable volcanic-derived component was deposited in a half graben setting The two key exposures for these (late Miocene-Pliocene) sediments are the western side of the Napedet Hills and Lothagam Hill Both sections are dominated by fluvio-deltaic sandstones, with some lacustrine shale intervals Some relatively minor volcanic activity also occurred during this time Then from about 3–0.5 Ma., extensive volcanic flows occurred at the southern end of the Lokichar and Kerio Basins, the southern part of Lake Turkana, and the entire area east of Lake Turkana (Figures and 34) This marked an important shift in volcanic activity away from the Turkana basins and into the Anza Graben area 53 At least two phases of inversion can be identified on seismic reflection data One of more restricted extent occurs within the Pliocene, the other occurred very late (probably within 0.4 Ma.) after deposition of the basin fill observed on the onshore seismic data (see Chapter 12) REFERENCES CITED Arambourg, C., and R.G Wolff, 1969, Nouvelles donnees paleontologique sur l’ages des “gres du Lubur” (Turkana Grits) a l’ouest du Lac Rudolphe Comptes Rendus Societe Geologique de France, v 6, p 190–202 Baker, B.H., 1986, Tectonics and volcanism of the southern Kenya Rift valley and its influence on rift sedimentation, in L.E Frostick, R.W Renaut, I Reid, and J.-J Tiercelin, eds., Sedimentation in the African rifts, Geological Society of London Special Publication, v 25, p 45–57 Baker, B.H., L.A.J Williams, and F.J Fitch, 1971, Sequence and geochronology of the Kenya Rift volcanics: Tectonophysics, v 11, p 191–215 Baker, B.H., and J Wohlenberg, 1971, Structure and evolution of the Kenya Rift valley: Nature, v 229, p 538–542 Boschetto, H.B., 1988, Geology of the Lothidok Range, northern Kenya, Unpublished M.Sc Thesis, University of Utah, 202 pp Boschetto, H.B., F.H Brown, and I McDougall, 1992, Stratigraphy of the Lothidok Range, northern Kenya, and K/Ar ages of its Miocene Primates: Journal of Human Evolution, v 22, p 47–71 Bosworth, W., 1987, Off-axis volcanism in the Gregory Rift, East Africa: implications for models of continental rifting: Geology, v 15, p 397–400 Bosworth, W., 1992, Mesozoic and early Tertiary rift tectonics in East Africa: Tectonophysics, v 209, p 115–137 Bosworth, W., and C.K Morley, 1994, Structural and stratigraphic evolution of the Anza Rift, Kenya: Tectonophysics, v 236, p 93–115 Braile, L.W., B Wang, C.R Daudt, G.R Keller, and J.P Patel, 1994, Modeling the 2-D seismic velocity structure across the Kenya Rift: Tectonophysics, v 236, p 251–269 Brotzu, P., L Morbidelli, M Nicoletti, E.M Piccirillo, and G Traversa, 1984, Miocene to Quaternary volcanism in Eastern Kenya: sequence and chronology: Tectonophysics, v 101, p 75–86 Brown, F.H., and C.S Feibel, 1986, Revision of the lithostratigraphic nomenclature in the Koobi Fora region, Kenya: Journal of the Geological Society of London, v 143, p 297–210 Brown, F.H., and C.S Feibel, 1988, “Robust” hominids and Plio-Pleistocene paleogeography of the Turkana Basin, Kenya and Ethiopia, in F.E Grine (ed.), Evolutionary history of the “Robust” Australopithecines, Aldine de Gruyter: New York, p 325–341 Crowell, J.C., 1974, Origin of Late Cenozoic basins in Southern California, in W.R Dickinson ed., Tectonics and sedimentation, SEPM Special Publication 22: Tulsa, p 190–204 Dunkleman, T.J., B.R Rosendahl, and J.A Karson, 1989, Structure and stratigraphy of the Turkana Rift from seis- 54 Morley et al mic reflection data: Journal of African Earth Sciences, 8, p 489–510 Ebinger, C.J., T Yemane, G Woldegabriel, J.L Aronson, and R.C Walter, 1993, Late Eocene-Recent volcanism and faulting in the southern main Ethiopian rift: Journal of the Geological Society of London, v 150, p 99–108 Ebinger, C.J., and A Ibrahim, 1994, Multiple episodes of rifting in Central and East Africa: A re-evaluation of gravity data: Geologische Rundschau, v 83, p 689–702 Ghignone, J.I., and G Andrade, 1970, General geology and major oil fields of Reconcavo Basin, Brazil, in M.E Halbouty, ed., Geology of giant petroleum fields, AAPG Memoir 14, p 337–358 Girdler, R W., 1991, The Afro-Arabian rift system-an overview Tectonophysics, 197, 139-153 Hendrie, D.B., N.J Kusznir, C.K Morley, and C.J Ebinger, 1994, Cenozoic extension in northern Kenya: a quantitative model of rift basin development in the Turkana area: Tectonophysics, v 236, p 409–438 Henry, W.J., J Mechie, P.K.H Maguire, M.A Kahn, C Prodehl, G.R Keller, and J Patel, 1990, A seismic investigation of the Kenya Rift valley: Geophysical Journal International, v 43, p 687–692 Joubert, P., 1966, Geology of the Loperot area: Report of the Geological Survey of Kenya, v 74 KRISP Working Group, 1991, The Kenya Rift: pure shear extension above a mantle plume: Nature, v 345, p 223–227 Lambiase, J.J., and W Bosworth, 1995, Structural controls on sedimentation in continental rifts, in J.J Lambiase, ed., Hydrocarbon habitat in rift basins, Geological Society Special Publication 80, p 117–144 Maguire, P.K.H., C.J Swain, R Masotti, and M.A Kahn, 1994, A crustal and uppermost mantle cross sectional model of the Kenya Rift derived from seismic and gravity data: Tectonophysics, v 236, p 217–249 Mann, C.D., 1989, Thick-skin and thin-skin detachment faults in continental Sudanese rift basins: Journal of African Earth Sciences, v 8, p 307–322 Manspeizer, W., 1988, Triassic-Jurassic rifting and opening of the Atlantic: An overview, in W Manspeizer, ed., Triassic-Jurassic rifting, developments in geotectonics 22, Elsevier: Amsterdam, p.41–79 Marrett, R., and R.W Allmendinger, 1991, Estimates of strain due to brittle faulting: sampling of fault populations: Journal of Structural Geology, v 13, p 735–738 McDougall, I., and R.T Watkins, 1988, Potassium-argon ages of volcanic rocks from northeast of Lake Turkana, northern Kenya: Geological Magazine, v 125, p 15–23 McKenzie, D., 1978, Some remarks on the development of sedimentary basins: Earth and Planetary Science Letters, v 40, p 25–32 Mechie, J., G.R Keller, C Prodehl, S Gaciri, L.W Braile, W.D Mooney, D Gajewski, and K.-J Sandmeier, 1994, Crustal structure beneath the Kenya Rift from axial profile data: Tectonophysics, v 236, p 179–200 Morley, C.K., 1989, Extension, detachments, and sedimentation in continental rifts (with particular reference to East Africa): Tectonics, v 8, p 1175–1192 Morley, C.K., R.A Nelson, T.L Patton, and S.G Munn, 1990, Transfer zones in the East African rift system and their relevance to hydrocarbon exploration in rifts: AAPG Bulletin, v 74, p 1234–1253 Morley, C.K., W.A Wescott, D.M Stone, R.M Harper, S.T Wigger, and F.M Karanja, 1992, Tectonic evolution of the northern Kenya Rift: Journal of the Geological Society of London, v 149, p 333–348 Patterson, B., A.K Behrensmeyer, and W.D Sill, 1970, Geology and fauna of a new Pliocene locality in north-western Kenya: Nature, v 226, p 918–921 Schull, T.J., 1988, Oil exploration in the non-marine basins of Interior Sudan: AAPG Bulletin, v 72, p 1128–1142 Swain, C., and M.A Khan, 1978, Gravity measurements in Kenya: Geophysical Journal of the Royal Astronomical Society, 53, 427–429 Tongue, J., P Maguire, and P Burton, 1994, An earthquake study in the Lake Baringo basin of the central Kenya Rift: Tectonophysics, v 236, p 151–164 Wernicke, B., 1985, Uniform sense simple shear of the continental lithosphere: Canadian Journal of Earth Sciences, v 22, p 108–125 Wescott, W.A., C.K Morley, and F.M Karanja, 1993, Geology of the “Turkana Grits” in the Lariu Range and Mt Porr areas, southern Lake Turkana, northwestern Kenya: Journal of African Earth Science, v 16, p 425–435 Wheildon, J., P Morgan, K.H Williamson, T.R Evans, and C.A Swanberg, 1994, Heat flow in the Kenya Rift zone: Tectonophysics, v 236, p 131–149 Williamson, P.G., and R.J.G Savage, 1986, Early rift sedimentation in the Turkana Basin, north Kenya, in L.E Frostick, R.W Renaut, I Reid, and J.-J Tiercelin, Sedimentation in the African rifts, Geological Society of London Special Publication, 25, p 267–284 Zanettin, B., E Justin Visentin, G Bellieni, E.M Piccirillo, and F Rita, 1983, The volcanics of northern Turkana Basin (Kenya): Age, succession and structural evolution: Elf-Aquitaine, Bulletin de Centres Recherches Exploration-Production, v 7, p 249–255 [...]... Paleogene) expands into the basin This section becomes considerably thinner in the northern part of the line compared with the southern part The section overlying the V horizon (late Miocene to Pliocene) in the southern area is very thin and thickens markedly towards the northern part of the line Observations made from the strike lines are backed up by evidence from dip lines The lines crossing the North... The highest mapped horizon, V, is the base of the Auwerwer Basalts, and the P horizon is close to the top of the Lokone Formation The top of Precambrian basement can be mapped with a fair degree of confidence Strike-lines, in particular line TVK-100, show that the timing of activity along the Lokichar Fault changes passing northwards (Fig- Geology and Geophysics of the Western Turkana Basins, Kenya... volcanic rocks at the base (Patterson et al 1970, Figure 20) Most of the sequence is fluvio-deltaic with some ephemeral lake deposits The sandstones are dominantly composed of quartz, however, they differ from the arkosic sands by their red, purple, and orange colored cements, and the presence of tuffaceous horizons which point to the contribution of volcanic sources Near the base of the Pliocene a thick... age, and illustrates that in the southern part of the Kerio Basin the Pliocene is largely post-extension and onlaps the older rift topography Within the Miocene-Paleogene section two phases of rotation (intervals 1 and 2) can be detected The actual timing of these phases is unknown, they may correspond with the EoceneOligocene and Oligocene-middle Miocene events in the Lokichar Basin Alternatively, they... (Arambourg and Wolff 1969) indicate a Cretaceous age for the most northerly grits Palynology studies yielded an Oligocene-early Miocene age for the most southerly grit exposures west and south of the Lokone Horst The other three areas remain undated The arkosic nature of the grits indicates they were derived from basement rocks They invariably underlie the earliest volcanic flows in the area The presence of. .. also in the transition zone between an older, more developed part of the East African Rift System, the Ethiopian Rift, and the younger central and southern Kenya Rift (Hendrie et al 1994, Ebinger and Ibrahim 1994) Looking in detail at the Lokichar Basin it is not possible to find an obvious tectonic boundary at the base of the early Miocene and state this is the beginning of the Kenya Rift episode The. .. Lothagam Hill (see Figures 7 and 21 for location and Figure A22 for the full line) The absence of deep well and outcrop data on the deeper section makes understanding the pre-O horizon history in the Turkana and Kerio Basins difficult It is apparent that significantly faulted, rotated, and eroded basins exist below the O horizon (Figures 31, 32, A27, and A28) Further, the age of the older section is unsure... evolution of the basins is more complex and the pattern less apparent Geology and Geophysics of the Western Turkana Basins, Kenya 51 52 Morley et al Figure 35 Timing chart of tectonic and basin fill events in the Turkana region, based on data discussed in both Chapters 2 and 3 the area has yet to be solved It does demonstrate, however, the remarkable potential productivity of these lacustrine source rocks The. .. lines The O horizon lies approximately at the base of the Pliocene In general, the timing of deformation is younger to the west This is particularly well seen in the section for lines TVK-13 and TVK-12, where the Lokichar Basin is of Paleogene-middle Miocene age, the Kerio Basin is a mixture of Paleogene-Miocene and Pliocene age, and in southern Lake Turkana, the section is dominantly Pliocene and younger... Paleogene rocks in the Loperot-1 well and Paleogene activity in the southeastern Anza Graben suggests rifting during the Paleogene in Northern Kenya was important and formed a string of basins as illustrated in Figures 33 and 34 These basins include some of the regions of poorly dated arkosic grit basins and the basins underlying the Lotikipi Plain (Chapter 3) The volcanic flows which overlie the arkosic