Geology and geophysics of the pdf

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, basi

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

Cretaceous-Paleogene 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

(Chapter 8), variations in boundary fault angle (Chapter 7),the influence of preexisting fabrics on rift structure (Chap-ter 9), and structural controls on sedimentation (Chapter13) This chapter provides the background information tothe specific aspects of rifting listed above, by presentingkey geological results derived from hydrocarbon exploration

in the area The reasons for the lack of exploration successare not discussed in detail in this section, but are incorpo-rated into Chapter 14

Abstract

The Turkana area comprises a string of half graben basins, most of which have been imaged by

seis-mic reflection data (a total of 2,267 km of 60-fold vibroseis data) and two tested by hydrocarbon

explo-ration 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 7 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 1 km near the boundary fault Overlying the Lokone Shale are

fluvio-deltaic sandstones that grade upwards from arkosic sandstones to sandstones having a significant

vol-caniclastic 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

deposit-ed 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

consider-able 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.

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 2

Geology and Geophysics of the

Western Turkana Basins, Kenya

C.K Morley W.A Wescott

Department of Petroleum Geoscience D.M Stone

University of Brunei Darussalam R.M Harper

Negara Brunei Darussalam S.T Wigger

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

northwest-southeast 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 Sudaneserifts through the western Turkana area was suspected Theavailable gravity data (e.g., Swain and Khan 1978) on thewestern side was relatively sparse and did not show thepresence of well developed sedimentary basins From geo-logical maps the terrain appears to have little hydrocarbonpotential since it is dominated by igneous rocks and Pre-

20 Morley et al.

Figure 1 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.

cambrian basement, with only small patches of coarse

clas-tic rocks cropping out (mostly termed Turkana Grits)

Con-fronted 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

region-al seismic reflection lines reveregion-aled basins up to 7 km thick

(including lines TVK-2 and 3, Figures A4 and A6)

Conse-quently 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

Fig-ures 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

asso-ciated 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

gen-erally 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

been studied in great detail (e.g., Patterson et al 1970,Brown and Feibel 1986, 1988, Boschetto et al 1992) Sev-eral field parties were organized by Amoco to field checkgeological maps made from aerial photographs (Figure 3),measure stratigraphic and sedimentary sections, and definestratigraphic ages using radiometric dating for igneous rocksand micropaleontology, palynology, or vertebrate fossils forsedimentary rocks The surface geology along the TVK seis-mic lines was mapped and used in the seismic interpreta-tion Water well drilling rigs were also used to determine thestratigraphy in some critical but poorly exposed areas In

1993, two wildcat wells were drilled by Shell Kenya PC whichtested a deeper part of the stratigraphy in two basins

of the Kenya Rift Investigations into the deep crustal andmantle structure of the rift by the KRISP group using seis-mic 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 LakeBogoria (Henry et al 1990) and thins passing along the riftaxis 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 theKenya Rift

The regional cross section on Figure 1 shows the mainfeatures of the rift based on outcrop, gravity, seismic reflec-tion and refraction data There are a series of dominantlyeast-dipping half graben boundary faults These faults arethought to pass at depth into a zone of ductile shearing(pure shear) in the lower crust The thin crust and impor-tant volcanic activity suggests heat flow is high in theTurkana area This is supported by the geothermal gradient

of 4.2°C/100 m in the Loperot-1 well Consequently, thebrittle-ductile transition zone is likely to be relatively shal-low—probably between about 10–12 km in depth This esti-mate was derived from better constrained data from thecentral Kenya Rift where the continental crust is thicker(about 35 km, Henry et al 1990), heat flow is high (50–100mWm2, Wheildon et al 1994) and the brittle-ductile transi-tion 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 orb) but does not fit well with models that offset the surfacerift from the upwards deflection of the Moho due to a low-angled detachment fault that penetrates the entire crust(e.g., Bosworth 1987; Figure 5c)

The cross section in Figure 1 provides a deceptively ple picture of the rift because, in detail, the structuralgeometries and basin evolution are complex It does, how-ever show the main features of the rift, which are domi-nantly a series of west-thickening half graben basins bound-

sim-Geology and Geophysics of the Western Turkana Basins, Kenya 21

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

geo-logical 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

check-ing 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

sedimen-tology 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.

TABLE 1 History of major exploration surveys undertaken 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

associ-ated 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

south-ern-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 inthree main areas:

1 On the western side of the map it forms the footwallblock to the Lokichar Fault The eastern edge of the base-ment (where the town of Lokichar is situated) approxi-mately marks the trace of the Lokichar Fault

2 The Lokone Horst area–this area is called a horst becausethe basement is surrounded by much younger lavas andsedimentary rocks The term horst is not, however, cor-rect The western margin of this basement outlier is actu-ally onlapped by coarse arkosic sandstones and small hol-lows in the basement surface are filled by thinlimestones The eastern margin is defined by a large fault(Lokone Fault) which is one of the boundary faults to theKerio Basin The north northeast-south southwest strike

of the boundary fault, follows the strike of foliationswithin the basement

3 The Lariu Range on the shores of Lake Turkana—in a verysimilar structural setting to the Lokone Horst—are base-ment outcrops bounded by faults on the eastern margin,and overlain by coarse arkosic sandstones or lavas on thewestern side The region represents the flexural margin tothe Kerio Basin

The sedimentary rocks that overlie Precambrian basementare best exposed west of the Lokone Horst (see followingsection) They represent the hinged margin deposits of theLokichar Basin Sandstones are coarse, commonly conglom-eratic, and channelized, with erosive bases (Wescott et al.1993) Individual beds range up to several meters in thick-ness The sandstones commonly display cross and troughcross bedding and water escape features They are predom-inantly fluvial, probably braided stream, deposits Thingreen 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 poorlyknown In places, vertebrate and microfossils indicate theexposed sequence ranges from late Oligocene to middleMiocene in age

Overlying the sandstones are lava flows In the AuwerwerHills the flows are of middle Miocene age and form regular-

ly layered outcrops that dip a few degrees towards the west.Passing northwards the igneous rocks change character con-siderably 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 aswell as lava flows These too are of middle Miocene age Onthe western margin of the Napedet Hills are clastic sedi-mentary rocks with a considerable volcaniclastic componentthat overlie the lavas Their age is poorly known but is prob-ably late Miocene-Pliocene Along trend with the NapedetHills are the Lothidok Hills They, however, comprise anolder volcanic and volcaniclastic sequence of lateOligocene-Miocene age The eastern margin of the LothidokHills is a boundary fault margin to the Turkana Basin Igneous rocks at the southern margin of the map com-prise extensive Pliocene lavas that have flowed over thesouthern portions of the older basins described above.The major outcrops described above do provide cluesabout the distribution of the basin geometries, but it is dif-

24 Morley et al.

Figure 4 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.

ficult to make a convincing picture of the basins from

sur-face data alone The multiple types of geological and

geo-physical 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

sed-iments flank the Kerio River These sedsed-iments—silts,

mud-stones, 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 7 km throw and 10

km extension), and to the east by a flexural margin

con-sisting 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

sand-stones overlying crystalline basement, followed by

fluvio-deltaic sandstones containing less feldspar and more

vol-canic-derived material (Figures 8 and 9) (Joubert 1966) At

the top of the sequence are lavas with interbedded

sedi-ments 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-ure 2), but the paleocurrent directions and absence of canic clasts indicate the northern area was not a source forthe (basement-derived) arkosic sandstones that form much

vol-of the early basin fill

Outcrops are good, but patchy, in the Lokichar Basin.Typically sandstones crop out, but the faster weatheringshales are rare Some small shale outcrops were found, par-ticularly along seismic line TVK-12, where igneous intru-sions within the shale were largely responsible for exposingand 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 totalorganic carbon tends to be relatively high in the bakedzones (samples yielded about 2.5% TOC), while away fromthe baked zones TOC content is less than 1% This can beattributed to weathering breaking down the unbaked shalesfaster than the baked ones In order to obtain unweatheredsamples and stratigraphic information on the shales at lowcost, a shothole rig (a Mayhew 2000) from the seismicacquisition crew was used to drill 14 shallow wells to depths

of 90 m

Six of these wells were drilled along seismic line TVK-12and six more were located on line TVK-110, 5 km to thesouth (Figure 8) For a flexural margin location the shalesare remarkably thick, correlative and continuous alongstrike (Figure 11) The unweathered shales yielded well pre-

Geology and Geophysics of the Western Turkana Basins, Kenya 25

Figure 5 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 = horizon- tal scale

26 Morley et al.

Figure 6 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).

served microfossils that provided important age

informa-tion Palynomorphs of Margocloporites verwijhei,

Echitri-porites Kwaense, Racemonocolpites hians, Praedapollis

pro-trudentiporatus 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

(Fig-ure 8) The correlation is based on lithology, stratigraphic

position, common palynomorphs, organic content, and

seis-mic character Using patchy outcrops and the shallow well

data, the shale section can be mapped along strike for at

least 5 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 sibility that the shales extended further eastwards into theKerio Basin

pos-The shallow well data provided an late Oligocene-earlyMiocene age for the shales—significantly older than datesfor other half grabens in the Kenya Rift Yet earlier initia-tion of rifting seems possible because strata below thosecorrelated to the wells show apparent onlap onto theassumed top Precambrian basement reflection However,given the very rapid depositional rates in rifts (perhaps 2–3mm/yr), the age difference between the dated and undatedsections could be very small

The Loperot-1 exploration well confirmed that theOligocene-Miocene shales had expanded in thickness fromthe outcrops towards the Lokichar Fault In the well theywere about 340 m thick (Figure 13), and seismic data indi-cates they may be as thick as 1,350 m approaching the

Geology and Geophysics of the Western Turkana Basins, Kenya 27

Figure 7 Regional structure map of the Turkana basins from seismic reflection data The structure is dominated by half grabens

boundary fault (Figure 14) Hence, the shales were

deposit-ed during a period of markdeposit-ed basin subsidence, which was

probably controlled by boundary fault activity

The Loperot-1 well also encountered a second

fluvio-lacustrine sequence of early Oligocene to possible Eocene

age, not seen at the surface (Figure 13) This mixed

sand-stone-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):

1 The oldest basin fill comprises Paleogene-lower Miocene

sedimentary rocks At the surface they are

predominant-ly fluvio-deltaic, arkosic, pebbpredominant-ly sandstones and fringe

the Lokone Horst (Figure 8) A well exposed section

through the outcrops in given in Figure 9 Hence, the

name Lokone Formation is proposed Two important shale

units have been identified within this formation Theolder shale, known only from the Loperot-1 well has beennamed the Loperot Shale Member, while the youngershale, identified in shallow wells, outcrops (Figures 8 and

9 section 5), and the Loperot-1 well is named the LokoneShale Member The Lokone Formation is likely to be dom-inated by sandstones on the flexural margin and by lacus-trine shales towards the boundary fault margin

2 The sandstone dominated sequence overlying the LokoneFormation is characterized by a considerable volcaniclas-tic component, including tuff or reworked tuffaceousunits (see Figure 9, section 1 for a typical stratigraphicsection) It is referred to as the Auwerwer Sandstone For-mation

3 Basalt flows directly overlie the Auwerwer SandstoneFormation (Figure 9, section 1) and are well exposed inthe Auwerwer Hills, hence they are called the AuwerwerBasalts Other formations or members may exist in thesubsurface, but their presence has not yet been con-firmed In particular, the presence of coarse conglomer-atic fans fringing the hanging wall of the Lokicharboundary fault is suspected

28 Morley et al.

Figure 8 Location map for

Lokichar Basin.

30 Morley et al.

Figure 10 Geological cross sections through the Lokichar Basin based on seismic reflection lines (Figure 10a this page, 10b facing page)

Geology and Geophysics of the Western Turkana Basins, Kenya 31

Models for rift stratigraphy developed for the Reconcavo

Basin, Brazil, (Ghignone and Andrade 1970), the Triassic rift

basins of the eastern U.S.A (Manspeizer 1988), and the

Ridge Basin, California (Crowell 1974) seem applicable to

the Lokichar Basin In general, the models show lacustrine

shales expanding in thickness towards the deep, basinal

areas These shales are capped above, below, and laterally

by coarse-grained sediments (Figure 14—detailed

discus-sion of the interpretation of Figure 14 is given in Chapter

13) The half graben shape and great thickness of section

(up to 7 km) indicate in like manner to the other models

that a boundary fault controlled basin thickness and

paleo-water depths (Morley 1989)

Source Rock Potential

The majority of the maturation data from the shallow

wells demonstrate that the Lokone Shale Member was never

buried to oil generating depths (Figure 11) The thermal

alteration index (TAI), in samples, ranges from 3–5

Howev-er, the higher values were probably caused by proximity to

adjacent intrusions and are not representative of the

over-all maturity of the section Vitrinite data from the

Loperot-1 well yielded Ro values from 0.32–0.37%, showing the

shales to be thermally immature Evidence for immaturity is

further supported by alkane gas chromatography which

showed an abundance of polycyclic alkanes and the

odd/even predominance of normal alkanes in the range

n-C24 to n-C32

Organic carbon within the section averaged from0.05–8.6%—selected pieces of shale were as rich as 11%.Values in the shales were consistently greater than 1% TOC.Amorphous Type 1 kerogen, indicated by the aromatic ratioand a high ratio of the areas of the methylene and methylgroups, was present in the source-quality samples (Figure11) In the Loperot-1 shallow well (line TVK-12, VP 480),organic material between 70–73 m showed bright yellowfluorescence under ultraviolet light, indicating hydrogen-rich, oil-prone composition Rock-Eval pyrolysis revealedhigh hydrogen indices from 788–1,020 mg HC/g TOC,demonstrating a high convertibility to hydrocarbons and thepresence of oil-prone kerogen

In the Loperot-1 exploration well the Lokone ShaleMember also contained marginal to excellent source rocks(1–17% TOC, average 4.5%) present over the interval1,050–1,390 m (Figure 15) The Loperot Shale Memberbetween 2,410–2,600 m (Eocene?-Oligocene) had TOC val-ues between 0.2–3.3%, and was highly mature to post-mature (Ro= 1.1%)

Seismic Correlations

The highest mapped horizon, V, is the base of the erwer Basalts, and the P horizon is close to the top of theLokone Formation The top of Precambrian basement can bemapped with a fair degree of confidence Strike-lines, inparticular line TVK-100, show that the timing of activityalong the Lokichar Fault changes passing northwards (Fig-

Auw-32 Morley et al.

Figure 11 Stratigraphy of shallow wells drilled along lines TVK-12 and TVK-110, see Figure 8 for location of wells Source rock mation of lacustrine shale samples taken from the wells and outcrops are illustrated as are FTIR spectra of weathered and unweath- ered samples, and elemental analysis as noted

infor-ures 16, A14, and A15) The strike line shows a basin in its

southern end, an antiformal central area, and deepening

towards another basin to the north In the southern area,

the section below the V horizon (middle Miocene to

Paleo-gene) expands into the basin This section becomes

consid-erably 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 byevidence from dip lines The lines crossing the NorthLokichar Basin (lines between TVK-3 and TVK-102W, FiguresA3–A5, A11, and A12) display a lower reflector package,below the V horizon that thickens to the east (Figure 10b).Overlying the V horizon the reflectors expand westwardsinto the Lokichar Fault In the North Lokichar Basin thelower sequence (probably early Miocene-Paleogene) appears

to have been associated with a west-dipping boundary

Geology and Geophysics of the Western Turkana Basins, Kenya 33

Figure 12 a(upper) Correlation of shallow well data to seismic line TVK-12 b(lower) Correlation of shallow well data to seismic line TVK-110 See Figure 8 for location of lines and Figure 11 for the stratigraphy of the wells.

fault This fault is not well imaged but must lie under the

Napedet Hills volcanic province Then, in the late Miocene,

the basin changed “polarity” and the later section

expand-ed into an east-dipping fault Changing basin geometries

with time can be seen in the changing thickening patterns

of isopach maps for the Paleogene-middle Miocene and late

Miocene-Pliocene intervals (Figure17a, b)

Structural Geometry

Extension in the Lokichar Basin is concentrated along

the boundary fault There is very little extension along

minor faults In places, particularly the southern part of the

basin, virtually 100% of the seismically visible extension

occurs along the boundary fault There is no good reason to

suppose that faults beyond seismic resolution in such

cir-cumstances contribute a significant amount to crustal

extension (e.g., Marrett and Allmendinger 1991)

Many of the minor faults seen on the seismic data do notappear to offset the top of basement reflection (Figure 18).The most important fault set is dominantly antithetic to theboundary fault The faults are listric in cross section andform one or more detachments within the lacustrine shales.They are not associated with gravity structures such as shalediapirs or folds and thrusts Instead, they probably form adetachment that terminates at the Lokichar Fault ( Figure10) To the north a steeper antithetic fault is developedthat does penetrate basement, however, its location is sim-ilar to listric faults to the south Minor faults die out with-

in the rift section, commonly just above the top of theLoperot Shale Member The two antithetic fault sets formtrends oblique to the boundary fault Overlapping fault setsgive an overall curved shape to the minor fault map pattern(Figure 7) The development of detached fault systems is acommon characteristic of some other rift segments in East

Geology and Geophysics of the Western Turkana Basins, Kenya 35

Figure 14 a) Correlation of Loperot-1 well to lines TVK-12 and TVK-13 (merged lines, see Figure 7 for location) b) stratigraphic model for the Lokichar Basin based on well, outcrop data, and the seismic character of lines TVK-12 and TVK-13.

Sedimentary-a.

b.

Africa (e.g., Rukwa Rift [Chapter 5], southeastern Anza

Graben [Chapter 4]), and has been described from the Sudan

rifts by Mann (1989)

One of the striking features of the Lokichar Fault is that

it changes dip and timing of activity considerably passing

along strike (see Chapter 8) The North Lokichar Basin is

characterized by a steeply dipping fault segment (Figures

A3, A4, and A5) that was active during the late

Miocene-Pliocene, whereas the Lokichar Basin has a low-angled fault

segment active during the Paleogene-middle Miocene

(Fig-ures A1 and A6) Between the two areas is a transition zone

(saddle) where the boundary fault becomes very low angled

and there are numerous basement-involved minor faults

that produce a number of tilted fault blocks (Figures A8, A9,

of about 65 km The saddle between the two basins extendsalong strike for about 25–30 km, and seismic coverage ofthe North Lokichar Basin extends along strike about 40 km(Figures 6, 7, and 16) There are no wells in the basin whichmakes outcrops in the Napedet and Lothidok Hills veryimportant correlation points (Figures 2 and 3) Seismic datashows that the North Lokichar Basin is characterized by adeeper reflection package which thickens to the east, and

an overlying reflection package which thickens westwards,into the northerly continuation of the Lokichar Fault

of an east-dipping late Miocene-Pliocene fault, whose ity caused the footwall to be uplifted and eroded

activ-Boschetto et al (1992) divided the stratigraphy of theLothidok Hills into three main units: the Kalakol Basalts,the Lothidok Formation, and the Turkwel Beds The TurkwelBeds are of late Miocene-Pliocene age and represent the

36 Morley et al.

Figure 15 Distribution

of TOC in the Loperot

Shale, Loperot-1 well,

based on sidewall core

samples (see Figure 13

for sample location

within well).

Figure 16 Geological cross section based on line TVK-100 This strike line illustrates how the

Lokichar fault controls two basinal areas, which are separated by a central high or saddle.

The southern basin (Lokichar Basin) is of Paleogene-middle Miocene age, while the northern

basin (North Lokichar Basin) is of late Miocene to Pliocene age.