The Stratigraphy and Structure of the Kamasia Range, Kenya Rift Valley G. R. Chapman, S. J. Lippard & J. E. Martyn

Home , Elgeyo escarpment, Kabarnet, Rift valley

.ll geol. Soc. Lond. Vol. 135, 1978, pp. 265-281, 9 figs. Printed in Northern Ireland

The stratigraphy and structure of the Kamasia Range, Kenya Rift Valley

G. R. Chapman, S. J. Lippard & J. E. Martyn

SUMMARY: The Kamasia Range (or Tugen Hills) is an uplifted, faulted and arched block lying between the Elgeyo Escarpment, which forms the western boundary of the Kenya Rift, and the axial Baringo-Suguta trough between 0°15'N and l°30'N in the northern part of the rift. Metamorphic basement rocks are exposed in the central part of the Kamasia area where they are overlain by 3000 m of Miocene lavas and sediments ranging in age from 16 Ma to 7 Ma. This is the thickest sequence of rocks of this age exposed in the Kenya Rift. Downwarping occurred from the beginning of this period, but the first major fault movements occurred at about 7 Ma (late Miocene) forming the Elgeyo and Kamasia fault-scarps. Later volcanicity was mainly confined to the area east of the Kamasia, although extensive flood trachyte lavas covered practically the whole area east of the Elgeyo at one time. A second period of major fault movement occurred in the late Pliocene-Pleistocene (2-0.5 Ma) uplifting the Kamasia Range, forming the western dip-slope and downfaulting the axial graben of the rift to the ea,:;t. The overall structure of the Kamasia is a broad arch cut by large faults, with maximum displacement of about 4000 m, just to the west of its axis, but in detail the structures are complex. The numerous faults are normal dip-slip type and show an en echelon and obliquely intersecting pattern with dominant directions 000-040 ° and 330-340 ° . Folding on axes perpendicular to the fault planes produced half-domes and basins indicating a secondary compressive stress directed along the rift. It is proposed that the majority of the faults developed as vertical fractures in horizontal rock sequences and were then rotated by continued extension and up-arching to produce complex tilted fault-block systems both synthetic and antithetic to the main rift structure.

The northern part of the Kenya Rift extends from the The Kenya Rift is associated throughout its length Equator to about 2°N in a generally N-S to NNE- with large volumes of Cainozoic alkaline volcanics and SSW direction (Fig. 1). In the southern half of this sediments (King 1970, Williams 1969, 1970, Baker et area the rift is about 50-60 km wide and forms a fairly al. 1972). The oldest volcanics within the rift occur in well-defined depression bounded by marginal fault- northern Kenya and have been dated at about 20- scarps. The largest of these is the Elgeyo Escarpment, 25 Ma (Baker et al. 1971). The Kamasia area exposes a with an average height of about 1500 m, which forms thick sequence of Miocene volcanics and sediments the western boundary between 0°15'N and 1°25'N. dating back to about 16 Ma (King & Chapman 1972). The Kamasia Range (or Tugen Hills) is a north-south These crop out extensively along the crest of the range range of hills some 30 km wide and 75 km long within (Fig. 2). Part of the sequence can be correlated with the rift. The main backbone of the Kamasia lies about that of the Elgeyo Escarpment; the greater part of 25-30 km east of the Elgeyo Escarpment. At its high- both sequences consists of plateau phonolite flood est point, in the Saimo area, it reaches a comparable lavas. These lavas, which on the rift margins have a altitude to the rift shoulder (about 2500 m). Between relatively limited age-range (10-14 Ma, Baker et al. the Elgeyo and Kamasia escarpments is a wide depres- 1971, Lippard 1973), are not only about three times sion occupied by the valley of the upper Kerio River thicker in the Kamasia area but also the eruptions and the western slopes of the Kamasia. For most of spanned a longer period (16-7Ma). After these the length of the range the western dip-slope rises phonolite eruptions an important period of rift faulting fairly uniformly from the valley floor to the crest of occurred in the Kamasia-Elgeyo area. This formed the the range with an average slope of 6-7 ° . The eastern Kamasia Range which has persisted as a positive to- side, by contrast, is steep and rugged, consisting of a pographic feature ever since. A second major fault series of fault-scarps 1000-1500 m high. The broken movement episode at about 2-0.5 Ma reactivated the eastern foothills pass eastward into the axial Baringo main Kamasia faults and produced the present-day trough of the rift. The eastern flank of the rift between elevation of the range. Volcanism and tectonism post- 0 ° and I°N is marked by several en echelon stepped dating these movements have been confined to a nar- fault-scarps, none of which is more than about 500 m row area around Lake Baringo, which is part of the high. The morphology of the rift is thus markedly axial Quaternary trough of the rift (Baker 1965, Baker asymmetric in this part. et al. i972).

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J. Thompson (1884) crossed the Kamasia and late Pliocene volcanics and sediments were tilted and Elgeyo in 1883 and reported, mistakenly, that the faulted during the second main rift-faulting episode Kamasia was composed of crystalline metamorphic which produced the final uplift of the range. The rocks. Gregory (1921) visited the range in his earlier post-Kabarnet trachyte movement on the main East African expedition (1892-3) and showed an es- Kamasia (Saimo) Fault is at least 2000 m. Undisturbed sentially correct interpretation of the structure and later Pleistocene to Recent sediments and lava~ lie succession. Bailey Willis (1936) later described the horizontally across a planar erosion surface on older Baringo-Elgeyo area in general but failed to recognise tilted formations in the eastern foothills, but farther the importance of the Kamasia dip-slope and the east are faulted around Lake Baringo. Elgeyo Fault. Shackleton (1951) described the sediments along the Elgeyo Escarpment and showed a cross-section across the area confirming Gregory's ear- Basement gneisses and schists lier observations. The present work was begun in 1965 as part of a The metamorphic basement rocks outcrop along the larger project to investigate the geology of the north- Elgeyo Escarpment, in the Kito Pass area in the north ern part of the Kenya Rift under the direction of and in a small area at the foot of the Saimo Escarp- Professor B. C. King. The present area comprises the ment in the central part of the Kamasia (Fig. 2). They whole of Quarter Degree Sheet 34 NE (a geological consist of para- and orthogneisses, amphibolites, map of this area, compiled by the authors, was pub- schists and local marbles and quartzites belonging to lished by the Geological Survey of Kenya in 1974) and the Mozambique 'System'. Granite pegmatites are most of Sheet 34 SE. The latter was mapped by the common. They have undergone polyphase deforma- Geological Survey of Kenya (Walsh 1969), but was tion but generally have a predominant steep foliation completely remapped and revised by one of the au- trending between 320 ° and 010 ° . Isotopic determina- thors (SJL). tions on similar rocks in southern Kenya give ages of 650-450 Ma (Cahen & Shelling 1966).

Stratigraphy Cainozoic stratigraphy ot the Elgeyo The rift in the Kamasia-Elgeyo area was initiated as a Escarpment downwarped trough in the middle Miocene (about 16 Ma) in an area of considerable pre-existing relief, part of which can be reconstructed from a study of the Kimwarer Formation (Kimwarer beds of basement surface along the Elgeyo Escarpment. Infil- Walsh 1969) ling of the trough usually commenced with sediments Thesecomprise up to 60 m of sediments lying on an but was rapidly followed by outpourings of basanite eroded surface of the basement at the southern end of and phonolite lavas, more than 1200 m thick in the the Elgeyo Escarpment. They are divided into a lower Kamasia area. At the southern end of the Elgeyo member of conglomerates and-sands and an upper Escarpment similar rocks of comparable age are about containing basaltic tufts. 800 m thick. These lavas were followed by large volumes of plateau phonolite lavas which infilled the Elgeyo Formation (Elgeyo basalts of Walsh trough and flooded on to the Uasin Gishu Plateau during the period 14.5-12 Ma. Later phonolite flows, 1969) erupted during the period 11-7 Ma and interbedded These are massive lavas and agglomerates with a with thick sedimentary formations, were confined to maximum thickness of 580 m found only locally at the the rift as a result of further downwarping, and possi- southern end of the Elgeyo. The main rock types are bly movement along the Elgeyo Fault. A major period basanites, tephrites, limburgites and analcitites. of faulting occurred at the end of phonolite eruptions Isotopic ages of the Elgeyo Formation (15.1 and (7 Ma). This involved about 2000 m of vertical move- 15.6 Ma (Baker et al. 1971) confirm a general correla- ment on the main Kamasia fault and formed the tion with the 'early' volcanics in the Kamasia area. 'proto-Kamasia' range. After considerable erosion, but no great time interval, there followed eruptions of Chof Phonolite Formation local basalts and widespread 'flood' trachytes (Kabar- net trachyte). At the southern end of the Kamasia a This sequence of thin phonolite lavas has a max- large central trachyte volcano (Kapkut) was built up at imum thickness of 339 m and lies conformably on the this time (7-6.5 Ma). Further fault movements were Elgeyo Formation. The lavas are petrographically and followed by the eruption of a thick, widespread basalt geochemically distinct from the overlying Uasin Gishu formation (Kaparaina basalt) now exposed in the east- phonolites (Lippard 1973). An isotopic age of 15.0 Ma ern foothills of the Kamasia. These lavas and overlying has been obtained (Chapman & Brook, in press).

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FZG. 3. Correlation diagram of Miocene (pre-7 Ma) sequences in the Kamasia, N Elgeyo and S Elgeyo areas. Thickness and time scales are approximate. Dots---sediments; cross-hatchingmbasic and intermediate lavas; diagonal lines--phonolite lavas; vertical lines--distinctive phonolite flow correlated from the Kamasia to the Elgeyo area. Tambach Formation ( Tambach beds or Plateau to the west. It comprises seven flows with a sediments o[ earlier authors) maximum aggregate thickness of 480 m and ranging in These sediments were described by Murray-Hughes age from 14.5 Ma to 12 Ma (Lippard 1973). The low- (1933) and Shackleton (1951). They are exposed for est flow is a Kenya-type phonolite (Prior 1903, Lip- 30 km along the Elgeyo Escarpment north of 0°30'N. pard 1973) that is directly correlated with the basal The maximum thickness is 380 m but there are rapid member of the Tiim Phonolite Formation of the variations in lithology and thickness related to the Kamasia area (Fig. 3). irregularity of the underlying basement surface. The lower part consists largely of conglomerates and sands Cainozoic stratigraphy of the derived chiefly from the basement but with some Kamasia Range pebbles of phonolite and phonolitic nephelinite in the south. The direction of sediment transport was mainly Kamego Formation from the west. The upper part consists of laminated shales up to 180 m thick with fish remains (Tilapia sp). This unit of quartzites and siltstones rests on an A thin phonolite flow, similar to the Chof phonolite irregular surface of basement rocks in the Saimo area. type, occurs near the base of the sequence. In addi- It is overlain unconformably by sediments belonging tion, there are beds of trachytic/phonolitic tufts and a to the Sidekh Phonolite Formation. The maximum number of small intrusions of ankaramite and basa- thickness of these beds is 80 m. Pollen collected from nite. them is of Tertiary aspect (C. Downie pets. com.), but their exact age is uncertain. Uasin Gishu Phonolite Formation ( Uasin Gishu phonolites of Shackleton 1951 ) Sidekh Phonolite Formation This widespread sequence of 'flood' phonolite lavas These rocks crop out extensively along the Saimo crops out along the Elgeyo Escarpment for 75 km and Escarpment and form the whole of the Sidekh Range covers an area of 6500 sq km on the Uasin Gishu to the north. In the northerh area they consist of ten

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to twelve phonolite flows with two sedimentary units shales with well-preserved fish (Tilapia sp) form the totalling 1200 m. In the Saimo section they are 700 m lower part of the ~uccession. thick and consist of five to six flows and some 80 m of interbedded sediments (Fig. 3). All the lavas are Ewalel Phonolite Formation plateau-type phonolites (Lippard 1973). The basal sediments are arkosic grits and breccias resting on the Phonolite lavas and tufts, varying from 180 to 600 m Kamego Formation or the basement gneiss. Higher in thickness overlie the Tiim phonolites and Ngorora sedimentary units are mainly fine-grained tufts, shales beds in the Kamasia area. North of 0°30'N the forma- and mudstones. The Sigatgat Complex (McClenaghan tion consists of one to three lava flows but southwards 1971) occurs just to the north of the area and consists the proportion of pyroclastics increases until tufts of a thick pile of phonolitic tufts and agglomerates cut make up more than 70 per cent, many of these are by phonolite dykes and plugs and is the probable ignimbrites. Isotopic age determinations (Chapman & source of the Sidekh phonolites which thicken in that Brook, in press) and stratigraphic considerations, show direction. Isotopic age determination~ of 16.0 Ma from that these phonolites are younger than the bulk of the one of the lower flows, and 14.4 Ma from the upper Miocene 'plateau' phonolites in Kenya and range in part (Chapman & Brook, in press) indicate a general age down to about 7 Ma. correlation with the Elgeyo, Chof Phonolite and Tam- bach formations (Fig. 3). Eron Basalt Formation These basalt lavas, locally 150m thick, crop out Noroyan/ Saimo Formation widely in the southern part of the Kamasia area below Two horizons of basic to intermediate lavas overlie the Kabarnet and Kapkut trachytes and unconforma- the Sidekh phonolites. They overlap only over a dis- bly overlie the Ewalel phonolites. tance of 2 km along the Saimo Escarpment where they Kabarnet Trachyte Formation (Kabarnet are separated by a single flow of phonolite. The lower trachytes.of Walsh 1969) (Noroyan) member consists of up to six flows of analcite hawaiite and analcite mugearite totalling 105 m. These trachyte flood lavas cover a large area of the The Saimo member comprises numerous thin flows of Kamasia dip slope, for the most part resting uncon- basanite and tephrite, with a maximum thickness of formably on various units of the older phonolite se- 240m. quence, and also as inliers to the east of the main range. The number of flows in vertical sequence varies from one to six, the greatest thickness in a single Tiim Phonolite Formation section being 355 m. The original extent of the lavas These phonolite lavas are at least 1050 m thick in cannot have been less than 1800 sq km and individual the central Kamasia area and thin northwards to flows can be traced over several hundred sq km. Thin 300 m at I°N. In the type area the sequence consists of interbeds of sediments, including the fossiliferous eleven to fourteen flows of plateau-type phonolite, Mpesida beds (Bishop et al. 1971) occur, but are except for a single flow of Kenya-type phonolite at the restricted to the eastern inliers. Several consistent age base (Fig. 3). K/Ar age determinations (Chapman & determinations at close to 7 Ma have been obtained Brook, in press) show the general equivalence of the from the Kabarnet trachytes (Baker et al. 1971, Chap- Tiim phonolites to the Uasin Gishu phonolites. At the man & Brook, in press). northern end of the Saimo Range a 255 m thick intercalation of fossiliferous tuffaceous sediments, the Kapkut Trachyte Volcano Muruyur beds (Bishop et al. 1971), occurs within the Tiim phonolites. The highland area at the southern end of the Kamasia area, which connects to the southern end of the Elgeyo Escarpment, is formed by the eroded Ngorora Formation remnant of a central volcano built largely of trachyte This is an extensive sequence of sediments, up to lavas (Kapkut trachytes). These have been dated 360m thick, the palaeontology, stratigraphy and isotopically at about 7 Ma (Chapman & Brook, in sedimentology of which has been described by Bishop press). & Chapman (1970) and Bishop & Pickford (1975). The formation everywhere shows a correlation be- Kaparaina Basalt Formation tween thickness and tectonic position (Fig. 7). The thickest section occurs on the downthrown side of the This is a widespread and thick (up to 600 m) se- Kito Pass Fault, while thin, condensed sequences quence of predominantly alkaline olivine basalt lavas. occur on tectonic 'highs' along the crest of the They are mainly confined to the eastern foothills of Kamasia Range. Further west laminated tuffaceous the Kamasia. In the extreme northeast of the area the

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Lukeino member, consisting of fossiliferous sands, zontal and are affected only by very young grid silts, diatomites and tufts, forms the base of the forma- faulting. tion and rests on weathered Kabarnet trachyte. The Kaparaina lava sequence includes some flows of hawaiite, mugearite and trachyte. Age determinations Kapthurin Formation (Kapthurin beds of McCall et on the iavas have been summarised by Chapman & al. 1967) Brook (in press) and range from 5.4 Ma to 3.9 Ma. In These sediments consist mainly of red earths, silts the central part of the Kaparaina Range there is a and gravels which rest on an erosion surface cut across swarm of NNE-trending dykes and abundant coarse the tilted Chemeron and older formations. They have agglomerates indicating a local centre of eruptions. a maximum thickness of about 200 m. Interfluve out- liers link the main outcrop near Lake Baringo to local Kaperyon Formation piedmont fan deposits at the foot of the Saimo Escarp- ment. T l~ey are the result of rapid erosion of the This consists of bedded pumice tufts and diatomites. Saimo Scarp, rejuvenated during early Pleistocene An increase in abundance and size of lapilli and blocks faulting, and were deposited in braided rivers. Two towards the northwest indicates derivation from the beds of pumice tuff occur in the sequence with the Ribkwo Volcano (McClenaghan 1971, Webb & Baringo trachyte resting partly on the lower one. The Weaver 1975). The overlying Ribon Trachyte is an Kapthurin beds have been described by Martyn & eroded remnant of one of the later flank eruptions of Tobias (1967), Leakey (1969), Bishop et al. (1971) the volcano. This flow has yielded isotopic ages of and Tallon (1976). Palaeontological evidence indicates 4.9 Ma and 3.7 Ma (Chapman & Brook, in press). The that most of the sequence is younger than upper younger age is more consistent with the local stratig- Middle Pleistocene. raphy. Kerio Valley beds Chemeron Formation (Chemeron beds of McCall et al. 196 7) These sediments infill the floor of the Kerio Valley to an unknown depth. They are fluvial gravels and silts These sediments crop out to the west of Lake similar in appearance to the Kapthurin beds but no Baringo and were first described by Gregory (1921) tufts or fossil remains have been found. They grade who postulated that they were deposited in a former laterally into piedmont fan deposits at the foot of the "Lake Kamasia'. After further work by Solomon & Elgeyo Escarpment and are largely derived from its Leakey (1931) and Fuchs (1950), the division into erosion in Pleistocene to Recent times. Chemeron beds and Kapthurin beds, for the younger unconformably overlying sediments, was introduced by McCall et al. (1967). Cainozoic structures The Chemeron beds have two main outcrop areas east and west of the Kaparaina Range. They rest The main trend of the faults in the Elgeyo-Kamasia generally conformably on the Kaparaina basalts, but area is 000-040 ° (Fig. 4), this direction dominating the there is evidence for an erosional break at the base. entire rift from 0°30'N to Lake Turkana (Fig. 1). The maximum thickness is 195 m. From facies and South of 0°30'N the direction is 340 °, this being the thickness evidence it seems probable that the two northern angle of the well-known 'dog-leg' in the outcrop areas represent separate depositional basins. central part of the Kenya Rift (King 1970, Baker et al. The Chemeron beds contain a rich vertebrate fauna 1972). This abrupt change in direction is seen along (Martyn & Tobias 1967, Bishop et al. 1971). the Elgeyo Escarpment. In the Kamasia a number of faults show northwest trends, but the general trend of Pleistocene lavas the range is almost due north. This is the result of the dominating influence of the major faults. The eastern boundary of the Kamasia area is A close correspondence of basement trends, mainly marked by the western margin of the Quaternary lavas a steep gneissic foliation, with the trend of the Elgeyo and sediments in the rift centre. In the Baringo area Fault is seen along most of the length of the escarp- these were formerly broadly grouped as the 'Lake ment. Similarly the Kito Pass Fault (Fig. 2) is approxi- Hannington-Dispei phonolites' (McCall et al. 1967), mately parallel to the basement structure. The Saimo but many lava types were erupted during this period. Fault is however slightly oblique to the local trend of The Chemakilani lavas (2.0Ma) and the Ndau the basement. mugearite (1.5 Ma) pre-date the lower Pleistocene Sinuosity is a feature of many faults, as noted by faulting, while the Loyamarok trachyphonolite Robson (1971) in the Gulf of Suez Rift. An extreme (0.5 Ma) and the Baringo trachyte (0.25 Ma) overstep example is provided by the faults in the Sumet area across on to the Kaparaina basalts, are practically hori- which curve through more than 100 ° of arc.

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N 0 50 100 150 km L ' [ JL [ 1 .... 1 i l, ' I , , , t~l

ELGEYO KAMASIA KAMASIA FAULT FAULTS FAULTS (S. of LAT. 0045 N ) ( N. of LAT. 0045 N )

FIG. 4. Rose diagrams of fault directions (heights of columns proportional to fault lengths irrespective of throw).

Faults with large displacements (i.e. more than Description of the structures by area about 500 m) in the Kamasia area are generally about 2.5 km apart across strike and tend to be arranged en Uasin Gishu Plateau echelon (Fig. 5). Where seen the fault planes com- monly show a perpendicular relationship to the dip of The plateau area, west of the Elgeyo Escarpment, is the strata, suggesting that block tilting has occurred unaffected by Cainozoic faulting. It shows a gentle tilt (see below). The majority of the faults downthrow to of about 1° away from the rift. Since the uppermost the east, towards the centre of the rift, but there are phonolite flow covering the plateau is tilted, the some that downthrow in the opposite direction. These plateau surface is a tectonic slope formed in post- antithetic faults generally occur on the eastern limbs of Miocene time. Immediately west of the Elgeyo the structural arches. basement surface beneath the lavas is highly irregular Although,the rift structures appear to be dominated and sharply downwarped towards the rift. These ir- by rrorpaal faults, particularly when viewed in east- regularities were largely infilled by the Miocene sedi- west cross sections (Fig. 5), deformation of the ments and lavas so that by the time of eruption of the Cainozoic rocks by folding is of considerable impor- upper Uasin Gishu phonolites (14-12 Ma) these lavas tance in the Kamasia area. were able to flow without interruption westwards The entire Kamasia structure may be described as across on to the plateau area. an arch dislocated by large normal faults located just to the west of its axis. The apparent result has been to The Elgeyo Fault elevate the west limb to form the present main part of the range and to depress the crest and eastern limb Gregory (1921) showed that the Elgeyo Escarpment which now form the eastern foothills. Both to the formed the line of a major fault and he suggested that to north and south of the Kamasia proper the faulted the north of the Equator the western boundary of the arch structure is replaced by an asymmetric anticline rift bifurcated with the 'Plateau of Kamasia' standing with a steep, antithetically faulted eastern limb. between the two branches. Shackleton (1951) showed Smaller scale folds with axes approximately perpen- the fault in cross section and postulated a curved dicular to the faults are common in the Kamasia area fault-plane to account for the apparent rotation of the and are described as fault-arches and fault-basins. Kamasia block. By extrapolation from the known Examples from the eastern foothills are described thickness of Miocene and Pliocene rocks in the below. In addition, sharp flexures may be caused by Kamasia, the throw on the fault must be at least 'drag' usually on the downthrow side of the fault and 3000 m at 0°30'N. This decreases rapidly south of on axes parallel to the fault plane. These often occur 0°20'N, where the southern end of the fault is marked where there are sediments on one, or both, sides of by splay faults. There is no fault at the southern end of the fault. the Kerio Valley, the structure being replaced by a

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monocline. Northwards the sub-volcanic surface on number of faults of smaller throw separated by a series the downthrow side rises from a maximum depression of narrow parallel horsts and graben along the crest of in the Tambach area until basement rocks are exposed the range. In about 10 km the structure changes from on both sides of the fault plane at about l°30'N, some a single large fault scarp to a complex system of tilted 30 km north of the map area. The throw appears to blocks and antithetic and synthetic faults. Further decrease northwards but this cannot be proved owing south around Tenges (Fig. 6.) the structure reverts to to the absence of Cainozoic rocks. The inclination of en echelon westward-tilted blocks and synthetic faults. the fault plane is conjectural as it is nowhere exposed. These are relatively small structures compared to those further north, and Pliocene lavas crop out right across this southern part of the range. The Kamasia dip-slope and the Kerio Valley

This is the structurally depressed area lying between The eastern foothills the Elgeyo and Kamasia faults. The western side of the Kamasia Range is for the most part a gently The main structural feature of the low hills to the inclined dip-slope of the Kabarnet trachytes. West of east of the main range of the Kamasia is the the Saimo area the dip-slope is convex, increasing in Kaparaina Arch. This is an 8-10 km wide anticline dip towards the Kerio, but elsewhere it is concave, trending NNE which can be traced for about 55 km particularly in the north where the dip of the trachytes between 0°30'N and I°N. In the Kaparaina Range is locally towards the east near the valley floor. The itself (Fig. 2) the structure is relatively little faulted but eastern margin of the dip-slope is for the most part the is locally intruded along its axis by the Kaparaina dyke main Kamasia faults which means that the movements swarm. North of this the axis is continued by a linear during the main faulting episodes can be simply con- hors~-graben complex characterised by folding on axes sidered as a rotation of the Kamasia block between normal to the faulting. The uplifted parts of these the Elgeyo and Kamasia faults. However, in the Saimo structures, described as 'fault-arches and domes', are area the post-Kabarnet trachyte rotation of the dip- expressed as inliers of Kabarnet trachyte (Fig. 8), slope was about a hinge-line running parallel to, but forming whale-backed ridges. The most spectacular of about 10 km west of, the Saimo Fault. On the Saimo" these is the Kokwomur mass which is a tilted horst Range horizontal Kabarnet trachytes bank up against terminated at its southern end by an E-W fault, and at an eroded relief produced after the pre-7 Ma faulting its northern end by a sharp downward flexure. Further episode. south, river incision of the Yatya half-dome structure has caused the exposure of Ewalel phonolites beneath the trachytes (Fig. 2). South of 0°30'N the western The main Kamasia Range limb of the Kaparaina Arch is completely suppressed The Kito Pass Fault (Fig. 5) is the most northerly of and all the pre-Pleistocene formations dip away from the main Kamasia faults which occur en echelon along the Kamasia main range towards the east. the length of the range. It has a throw of more than 2000 m and brings up an extensive area of basement rocks on the upthrow side. It dies out southwards in Chronology of the faulting the Sumet area where it is replaced to the east en echelon by the Saimo Fault. The northern part of this Miocene structure is marked by a complex of strongly curved faults in the Sumet area (Fig. 5). Further south it is a There is little evidence for pre-volcanic or early to single fracture with a. displacement of about 4000 m middle. Miocene faulting in the Kamasia area, the only and dominates the central part of the Kamasia Range. possible example being a buried basement scarp paral- A detailed study of this structure shows (Fig. 6): lel to the Kito Pass Fault. Middle Miocene downwarp- (a) The fault movement has involved both up-arching ing in the vicinity of the Elgeyo Escarpment is shown on the upthrown block and downwarping on the by the thinning and overlap of the lower volcanic units downthrow side. to the west away from the rift. The character, distribu- (b) The base of the Tiim phonolites (about 14 Ma) is tion and thickness of the Tambach sediments (c. 16- depressed to at least 1000 m below sea-level on 14 Ma) are not compatible with the existence of a fault the downthrow side. This horizon is at least 600 m scarp. In addition the later Uasin Gishu phonolite above the base of the Cainozoic sequence. flows (14-12Ma) passed uninterruptedly across the (c) Almost all the up-arching took place in the line of the present Elgeyo Fault. In the Kamasia area Miocene (pre-7 Ma). deposition of the Ngorora beds (10-12Ma) was (d) Control on sedimentation was exercised by the affected by contemporaneous fault movements fault from at least 14 Ma to the present day (Fig. amounting to about 300 m (Fig. 7); however, there are 7). no major unconformities within the Miocene (16- Further south the Saimo structure breaks up into a 7 Ma) sequence in the central Kamasia area.

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~ ~_ KITO PASS FAULT

_...-. ~ _7_-~- .___~. ~-. . ~SAIMO FAULTA

N t

- -FAULt

~f iI

Vertical scale - horizontal E-W scale lOJ~ $ / L ,

FIG. 5. Structural block diagram of the Kamasia main range structure (The datum plane is the base of the Tiim phonolites).

The first major faulting episode occurred at about Pliocene - Pleistocene 7 Ma and caused the marked unconformity in the Kamasia area between the Kabarnet trachytes and A local unconformity beneath the Kaparaina basalts older formations (Fig. 9). It can probably be correlated (5.5-4.5 Ma) in the southern part of the area is attri- over a wide area of the Kenya Rift and corresponds to buted to faulting, but more important movements the movements described as 'late Miocene' or 'early followed these eruptions. These formed the Kaparaina Pliocene' (Kent 1944, Baker 1965, Baker et al. 1972). Arch and the basins in which the Chemeron (4-2 Ma) On the Saimo Fault the movement was about 2000 m and Kaperyon beds were deposited. (Fig. 6), more than half the total displacement. The present relief of the Kamasia is largely the

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SAIMO _ T IIM SUMET -

, I I

:r/'/'/YT/ --~~//// ' "/' "~''""-" ...... "..... I / ~ /.1../_l..-/..i-/,.Z-./-'Z l..z z.z.," /

-~/ZL.L/.A.L//////_A~/~/_.~/f.Z,/_.~/"~/'~//J/-"/"~0t ...... , ______10kin -

FIG. 6. The Saimo Fault, a north-south profile showing the vertical displacements on the base of the Tiim phonolites (14.5 Ma) (T) and the Kabarnet trachytes (7 Ma) (K).

result of major faulting and uplift between 2 Ma and remnants of the Kapthurin beds and the youngest 0.5 Ma. These movements mainly accentuated existing lavas, less than 0.5 Ma old, rest on both surfaces and structures. Post-Chemeron beds (c. 2 Ma) movement are not tilted or faulted in the foothills region. Further on the Saimo Fault has amounted to more than east around Lake Baringo the rocks are cut by small 1000 m. It appears that the dip-slope of the Kamasia vertical faults and open fissures, probably of Recent was largely formed at this time. Part of the evidence age. This 'grid' faulting is found along most of the for this comes from relatively recent drainage modifi- axial graben of the rift. cations where an earlier NW drainage has been be- headed by shorter streams with steeper gradients Mechanisms of faulting draining westwards down the tectonic slope. In the eastern foothills a period of planation followed the Structural cross-sections of the Kamasia area (Fig. faulting and a sloping planar surface, at between 5) show that most of the faults have a nearly right- 1150 m and 1110 m, was formed. Later rejuvenation angle relationship with the strata. This suggests that has produced a youdger surface in the north at about the structures developed by block-tilting, that is, the 875m. Dating of these surfaces is difficult and rotation of both the faults and intervening blocks from

E W DIP-SLOPE MAIN EASTERN FOOTHILLS RANGE ! ! ! ! \ .

'°° 1 ,):

O-J k ...... j

FIG. 7. Tectonic control of sediment thicknesses across the Saimo Fault.

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m Z Z 0 "4 ~-

v !- >. > "r o°= tj O3

I-- z~_ I-. 0 .- >. z t~ t~ 0 I-

0 {/ ,.... a:_

-r /. o

t.u I

ts~ E

\ / ! o E L

/ I .,.., !

~d '( t ~2 Z ¢1'• / i~:/

.~.

t I / E 0 0

1 ...... I

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W E ELGEYO KAMASIA LAIKIPIA

" ///////////,/......

(a) CIRCA 10Ma MAXIMUM SPREAD OF MIOCENE PHONOLITES (Faulting hypothetical)

(b)CIRCA 5Ma AFTER Ist MAJOR FAULTING AND ERUPTION OF LOWER PLIOCENE LAVAS

(C) PRESENT AFTER 2nd MAJOR FAULTING AND ERUPTION OF QUATERNARY VOLCANICS

0 10 20 30 km I , I I j

Vertical exaggeration = X 3 QUATERNARY ~ MIOCENE PHONOLITES , [ METAMORPHIC BASEMENT

...., ...... A PLIOCENE ...... MIOCENE BASALTS ...... "" (east side of rift )

FIG. 9. Cross-sections showing the structural evolution of the Kenya Rift at about 0°30'N.

an initially vertical orientation. However, it must also with accompanying increasing dip of the clay surface, be remembered that in the third dimension the blocks as the displacements increased. The faults all tended are folded on E-W axes between en echelon faults. to throw in the same direction in any one model, There is no evidence that large unfaulted anticlines which is the result of the unidirectional stress applied. ever existed in the area and it appears that faulting The latter effect is a common feature of many tilt- was coeval with the beginning of relative uplift and block provinces (Cox 1970). It seems to have been the that the flanks of the developing arch were progres- pattern of the early rift-faulting in the central Kenya sively steepened by fault block-tilting. It is highly area at about 7 Ma where the eastern side of the rift probable that the complex faulting in the Cainozoic was a monocline and the major faults of the Elgeyo rocks is related to larger fractures in the basement; and Kamasia were antithetic to the flexure (Baker hence perhaps explaining the contrast between the 1965). More recent model experiments (Freund & single fracture of the Elgeyo Fault and the relative Merzer 1976), using a bilateral tensional stress field, complexity of the Kamasia area where the Cainozoic produced symmetric intersecting and en echelon fault sequence is considerably thicker and contains a greater patterns and tilted fault-blocks similar to those of the proportion of relatively incompetent sediments. Kamasia area. The tectonic pattern of the Kamasia of folded and The block-tilting model provides a reasonable ex- tilted blocks and en echelon faults is similar to that planation of many of the Kamasia structures including produced experimentally by Bain & Beebe (1954) who the symmetrical folding on axes perpendicular to the mounted a clay-cake on the planar surface of an faults and the right-angle relationship between the extensible rubber sheet, the clay being contained in strata and fault planes. It is thus proposed that the two half-boxes. The tensional stress applied produced majority of the faults were initiated as vertical frac- a set of en echelon faults the dips of which decreased, tures in horizontal rock sequences and then rotated by

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progressive movement of the faults and intervening major fault movements at 7 Ma (King & Chapman blocks. 1972). The possibility of a discordance between the structures in the basement and the Cainozoic rocks points Crustal extension to a major problem concerning the origin of the structures: to what extent are they major structures Estimates of crustal extension across the Kenya Rift cutting most of the upper crust or are they merely vary from 30 km, based on geophysical evidence and superficial in nature? A dislocatiofl the size of the the reconstructed movements of lithospheric plates Saimo Fault clearly falls into the former category and (McKenzie et al. 1970) to about 5 km, based on since this fault is geometrically compatible with an geological evidence (Baker & Wohlenberg 1971). The origin by relative tensional stress perpendicular to the last figure was calculated from the extension due to rift, with a secondary compressive component, it normal faulting by non-rotational dip-slip movement seems justified to consider that the stress-field that on fault planes with a mean dip of 63 °. If, however, produced this structure operated to a considerable the faulting is of tilted-block type, as suggested for the depth (10-15 km) in the crust. Kamasia area, then the extension is only half that Freund & Merzer (1976) showed, on the basis of produced by non-rotational movement. Using the their model experiments, that the stress field can tilted-block model it is estimated that extension across change across a graben structure from that described the Elgeyo and Kamasia structures is about 1 km, and by Anderson (1951) to produce normal faults on the for the whole rift at this latitude, 2 km to 2.5 km. flanks to one within the graben wedge where the These figures are minima owing to the ccmcealment of maximum compressive stress is no longer vertical but older faults beneath younger rocks, but this is the area is sub-horizontal and parallel to the rift. This then is where the old structures are best exposed and the the same as Anderson's system for strike-slip faults; figures are unlikely to be more than 50 per cent too but, because cr3 is considerably less than ~rl and or2, low. The tectonic pattern of some areas, for example there is only a small component of strike-slip move- the horst and graben structures along the crest of the ment which is partly taken up by folding. In addition, Kamasia at around 0°30'N, can be explained as the the angle between the conjugateshear planes would result of vertical movements alone, although this is be small (20-40 ° ) and one set may predominate within inadequate to account for the extension across the a particular area. The two predominant fault trends whole rift. and the secondary compressive features in the Kamasia area are compatible with this model. Gravity evidence for the structure of the rift Rdationship o| faulting to vokanidty Most gravity surveys across the Kenya Rift (Searle Few major volcanic centres occur in the Kamasia 1970, Khan & Mansfield 1971, Fairhead 1976) have area, compared to those parts of the Kenya Rift revealed a positive anomaly along the axis of the rift further north and south, but the area as a whole shows which is superimposed on a broader negative anomaly a general correlation between volcanism and close- associated with the rift as a whole. This pattern has spaced faulting, both having migrated progressively been interpreted as the effect of a body of low-density towards the rift centre with time. This feature has asthenosphere material which has replaced the upper been explained by the progressive narrowing of the mantle and locally penetrated the crust. All the mod- width and decrease of depth of intrusions beneath the els proposed to explain these data attribute the nega- rift (Baker & Wohlenberg 1971). The thesis that tive anomaly to the negative density contrast between major faulting episodes terminate periods'of increased asthenopheric material and the normal upper mantle, volcanicity, notably two main phases of salic while the superimposed positive anomaly is explained volcanism--the upper Miocene phonolites and Plio- by the penetration of a narrow body of the mantle- Pleistocene trachytic tufts and ignimbrites--has been derived intrusive material to within a few kilometres proposed by several authors (Kent 1944, Baker & of the surface. In that of Khan & Mansfield (1971) the Wohlenberg 1971), but does not stand up to detailed upper surface of the intrusive body rises to only 18- investigations. In the Elgeyo-Kamasia area the acme 20 km below the rift floor, much deeper than in the of phonolite eruptions (14-12 Ma) preceded the first other models. This result is chiefly due to the much rift faulting by some 5 Ma; likewise there is no appar- broader positive anomaly in the northern part of the ent correlation between increased volcanism and tec- rift, where this survey was undertaken. It is probable tonism in the late Pliocene-Pleistocene. Indeed the that part of this anomaly in this area may lie in the only possible relationship seems to be the change in presence of considerable volumes of Miocene and magma type, from dominantly phonolite to alkali Pliocene basic lavas, Kaparaina basalts in the west and basalt-trachyte volcanism, which followed the first Samburu basalts in the east (Carney 1972), that lie

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beneath the floor of the rift between the Kamasia and stood. This work shows that the Miocene sequence in Laikipia escarpments. the Kamasia covers a greater time range and has a A second feature of the gravity anomaly in the area much greater thickness than that outside the rift and is the large negative anomaly in the southern part of must have formed in a downwarped depression ap- the Kerio Valley (Khan & Mansfield 1971). This can proximately along the site of the later, more clearly be attributed to the thick accumulation of sediments, defined, fault-bounded rift. In other areas of the ranging from Miocene to Recent in age, on the Kenya Rift, notably those south of Nakuru, the rift downthrown side of the Elgeyo Fault. floor is covered with young Plio-Pleistocene volcanics A detailed re-interpretation of the residual Bouguer and sediments and the Miocene rocks are confined to anomaly profile across the Elgeyo-Kamasia area of the thin sequences on the shoulders. Northwards from the rift (Fairhead 1976) shows a 'high' along the eastern Kamasia the thick Mioceno phonolites and sediments edge of the Uasin Gishu plateau (interpreted as due to are largely replaced by Miocene basalt sequences in relatively high density pyroxene-garnet gneisses in the the broad Turkana depression. The first major rift- basement) and a 3+ km thickness of low density sedi- faulting episode recognised in the Kamasia area at ments in the southern part of the Kerio Valley, giving 7 Ma is an important event in the northern part of the a maximum negative value in excess of -300 g.u. This Kenya Rift, but is difficult to correlate within the anomaly is asymmetric and the less steep eastern limb southern part owing to the lack of a continuous se- includes most of the Kamasia area, confirming the quence from the rift shoulders into the floor. The later great thickness of relatively low density Miocene lax,as history of the area with a predominance of alkali and sediments in that area. The axial positive anomaly basalt-trachyte volcanism and important uplift and rift lies east of the Kamasia and consists of three distinct faulting in the late Pliocene-early Pleistocene confin- 'peaks', one of which coincides with the outcrop of ing subsequent activity to a narrow axial graben, is Kaparaina basalts and the Kaparaina dyke swarm. comparable to that of most parts of the Kenya Rift. Fairhead (op. cir.) interprets the axial high as the result of both basic lava infilling and the presence of a high density intrusion some 6 km wide extending to within ACKNOWLEDGEMENTS. The writers are indebted to Professor 1 km of the surface. B. C. King who supervised the work and critically read the manuscript. The late Professor W. W. Bishop, Dr L. A. J. Williams, Dr N. J. Snelling, Dr M. P. McClenaghan and other The Kamasia area in the members of the East African Geological Research Unit team setting of the Kenya Rift are thanked for their help and discussion. The project was supported by the Ministry of Overseas Development, the Government of the Republic of Kenya and the Natural The Kamasia area is important in understanding the Environment Research Council. history and development of the Kenya Rift in that NERC Research Studentships were awarded to GRC and older sequences and structures within the confines of JEM. SJL received the William Gilles Research Fellowship the rift can be studied and compared with those of the from London University. This paper is published by permis- rift shoulder, enabling the early history to be under- sion of the Director, Institute of Geological Sciences.

References

ANDERSON, E. M. 1951. The Dynamics of Faulting. Oliver & BISHOP, W. W. • CHAPMAN, G. R. 1970. Early Pliocene Boyd, Edinburgh, 206pp. sediments and fossils from the northern Kenya Rift BAILEY WILLIS 1936. East African,Plateaus and Rift Valleys. Valley. Nature, Lond. 226, 914-18. Carnegie lnstn, Wash. , CHAPMAN, G. R., HILL, A. P. & MILLER, J. A. 1971. BAIN, G. W. & BEEBE, J. H. 1954. Scale model reproduction Succession of Cainozoic vertebrate assemblages from the of tension faults. Am. J. Sci. 252, 745-54. northern Kenya Rift Valley. Nature, Lond. 233, 389-94. BAKER, B. H. 1965. An outline of the geology of the Kenya & PICKFORD, M. H. L. 1975. Geology, fauna and Rift Valley. In: Report of the geology and geophysics of palaeoenvironments of the Ngorora Formation, Kenya the East African Rift System. UMC/UNESCO Seminar Rift Valley. Nature, Lond. 254, 185-92. 1965, Nairobi. CAHEN, L. & SNELLING, N. J. 1966. The Geochronology of , MOHR, P. A. & WILLIAMS, L. A. J. 1972. The geology Equatorial Africa. Amsterdam, North Holland, 195pp. of the Eastern Rift System of Africa. Spec. Pap. geol. CARNEY, J. N. 1972. The geology of the area east of Lake Soc. Am. 136, 67pp. Baringo, Kenya Rift Valley. Thesis, Ph.D., Univ. of , WILLIAMS, L. A. J., MILLER, J. A. & FITCH, F. J. 1971. Lond. (Unpubl.). Sequence and geochronology of the Kenya Rift Voi- CHAPMAN, G. R. & BROOK, M. in press. Chronostratigraphy canics. Tectonophysics 11, 191-215. of the Baringo Basin. In: BISHOP, W. W. (ed.) The & WOHLENBERG, J. 1971. Structure and evolution of Geological Background to Fossil Man. Spec. Publ. the Kenya Rift Valley. Nature, Lond. 229, 538-42. geol. Soc. Lond. 6.

Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/135/3/265/4885560/gsjgs.135.3.0265.pdf by guest on 27 September 2021 Structural evolution of Kenya Rift 281

Cox, K. G. 1970. Tectonics and evolution of the Karroo MCCLENAGHAN, M. P. 1971. The geology of the Ribkwo area, Period and their bearing on the postulated fragmentation Baringo District, Kenya. Thesis, Ph.D., Univ. of London. of Gondwanaland. In: CLIFFORD, T. N. & GASS, I. G. (Unpubl.). (eds.) African Magnetism & Tectonics. Oliver & Boyd, MCKENZIE, D. P., DAVIES, D. & MOLNAR, P. 1970. Plate Edinburgh, 211-35. tectonics of the Red Sea and East Africa. Nature, Lond. FAUn-mAD, J. D. 1976. The structure of the lithosphere 226, 243. bene~ith the Eastern Rift, East Africa, deduced from MURRAY-HUGHES, R. 1933. Notes of the geological succes- gravity studies. Tectonophysics 30, 269-98. sion, tectonics and economic geology of the western half Fm~UND, R. & MERZER, A. M. 1976. The formation of Rift of Kenya Colony. R~p. geol. Surv. Kenya 3, 8 pp. valleys and their zig-zag fault patterns. Geol. Mag. 113, PRIOR, G. T. 1903. Contributions to the petrology of British 561-8. East Africa. Min. "Mag. 13, 228-63. FUCHS, V.. E. 1950. Pleistocene events in the Baringo Basin, ROBSON, D. A. 1971. The structure of the Gulf of Suez Kenya Colony. Geol. Mag. 87, 148-74. (Clysmic) Rift with special reference to the eastern side. GREGORY, J. W. 1921. The rift valley and geology of East Jl geol. Soc. Lond. 127, 247-76. Africa. Seeley Service'& Co. Ltd., 479 pp. SEARLE, R. C. 1970. Evidence from gravity anomalies for the KENT, P. E. 1944. The age and tectonic relationships of East thinning of lithosphere beneath the rift valley in Kenya. African volcanic rocks. Geol. Mag. 81, 15-27. Geophys. J. R. astr. Soc. 21, 13-31. KHAN, M. A. & MANSFIELD, J. 1971. Gravity measurements SHACKLETON, R. M. 1951. A contribution to the geology of in the Gregory Rift. Nature, Phys. Sci. 229, 72-5. the Kavirondo Rift Valley. Jl geol. Soc. Lond. 106, KING, B. C. 1970. Volcanicity and rift tectonics in East 345-92. Africa. In: CLIFFORD, T. N. & GASS, I. G. (eds) African SOLOMON, J. D. 1931. In: LEAKEY, L. S. B. The Stone Age Magmatism & Tectonics. Oliver & Boyd, Edinburgh, Cultures of Kenya Colony. Cambridge Univ. Press, 263-83. 283 pp. & CHAPMAN, G. R. 1972. Volcanism of the Kenya Rift T;~LLON, P. 1976. The stratigraphy, palaeoenvironments and Valley. Phil. Trans. R. Soc. A271, 185-208. geomorphology of the .Pleistocene Kapthurin Formation, LEAr~Y, M. D. 1969. An Acheulian industry with prepared Kenya. Thesis, Ph.D., Univ. of London. (Unpubl.). core techniques and the discovery of a contemporary THOMPSON, J. 1884. Through Masailand. Samson Low, Lon- hominid mandible at Lake Baringo, Kenya. Proc. prehist. don, 364 pp. Soc., 35, 48-76. WALSH, J. 1969. Geology of the Eldama Ravine Kabarnet LIPPARD, S. J. 1973. The petrology of phonolites from the area. Rep. geol. Surv. Kenya 84, 44 pp. Kenya Rift. Lithos 6, 217-34. WEBB, P. K. & WEAVER, S. D. 1975. Trachyte shield vol- MARTYN, J. E. & TOBIAS, P. V. 1967. Pleistocene deposits canoes: a new volcanic form from South Turkana, and new fossil localities in Kenya. Nature, Lond. 215, Kenya. Bull. volcan. 39, 1-19. 476-80. WILLIAMS, L. A. J. 1969. Volcanic associations in the Greg- MCCALL, G. J. H., BAKER, B. H. & WALSH, J. 1967. Late ory Rift Valley, East Africa. Nature, Lond. 224, 61-4. Tertiary and Quaternary sediments of the Kenya Rift 1970. The volcanics of the Gregory Rift Valley, East Valley. In: BISHOP, W. W. & CLARK, J. D. (eds.) Back- Africa. Bull. volcan. 34, 439-65. ground to evolution in Africa. UnN. of Chicago Press.

Received 15 February 1977; read 12 October 1977; revised typescript received 13 October 1977. Gm~GORY R. CHAPMAN, Institute of Geological Sciences, Exhibition Road, London SW7 2DE. STEPHEN J. LIPPARD, Department of Geology, Bedford College, Regents Park, London NW1 4NS. JOHN E. MARTYN, 22 McLintock Way, Karrinyup, West Australia 6018.

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