New Age Determinations and the Geology of the Kenya Rift-Kavirondo Rift Junction, W Kenya

Home , Kerio River, Menengai, Monocline, Perkerra River

J. geol. Soc. London, Vol. 136, 1979, pp. 693-704, 4 figs.. 1 table. Printed in Northern Ireland. New age determinations and the geology of the Kenya Rift-Kavirondo Rift junction, W Kenya

W. B. Jones & S. J. Lippard

SUMMARY: The area of the triple-rift junction in W Kenya, between the E-W Kavirondo Rift, the N-S Northern Kenya Rift and the NW-SE Central Kenya Rift, has been the site of continuous volcanic and tectonic activity sincethe Middle Miocene. From about20 to 7 Ma the area was dominated by the accumulation of a thick pile of nephelinitic and phonolitic volcanics in the Timboroa area. From 7 Ma to the present day, apart from local eruptionsof basanite in the W, the volcanic and tectonic activityhas been progressively concentrated towards the axial zone of the Kenya Rift with alkali basalt-trachyte sequences predominating. Duringthis period the Kavirondo Rift was inactive. The stratigraphic sequencein the area has been dated by 19 new K-Ar age determinations. These enable the absolute andrelative ages of the Tinderet, Timboroa, Kapkut, Londiani and Kilombe volcanoes tobe established for the first time. Topographical and structural considerationsshow that the Kavirondo Rift largely fades out 50 km W of the Kenya Rift and is barely perceptibleat its edge. The W wall of the Kenya Rift adjacent to the Kavirondo Riftis a monocline and is very similar to the South Turkana region. Although the area is adjacent to major rift-boundingfaults there is little faulting within it, the tectonic pattern being dominated by monoclines and volcano-tectonic structures.

The area is located on the Equator between 35 the Kavirondo Gulf (Fig. 2). From there it is a well- and 36"E and covers about 8000 km2 in W Kenya marked graben between a steep northern wall, the at the junction of the N-S to NW-SE trending cen- Nyando Escarpment with the Uasin Gishu Plateau tral part of the Kenya Rift (also known as the beyond, and a more gentle southern margin rising Gregory or EasternRift) and theE-W to ENE-WSW towards the Kericho Plateau. The rift flooris at about trending branch Kavirondo Rift (Fig. 1). This is an 1300 m and the shoulders 1900 m in this section. elevated region, standing over 2000 m O.D. for the Further E the eastern end of the Kavirondo Rift is most part, and almost entirely underlain by Tertiary terminated by the mountainous area of Tinderet volcanic rocks. The geology of the area was first (2649 m) and a wide area of higher ground closer to described by Gregory (1921) andWillis (1936). Subse- the Kenya Rift hereinafter referred to as the Equator quent studies concentrated on the fossiliferous Highlands which includes the mountain masses of Miocene beds and the structure and evolutionof the Kapkut (2788 m) in the N, Londiani (3010m) in the S Kavirondo Rift (Kent 1944; Shackleton 1951). Sys- and the broad upland region around Timboroa tematic geological mapping of the region was carried (2897 m). out by the Kenya Geological Surveyin the 1950s and N of about O"15'N the western side of the Kenya 1960s (Binge 1962; Jennings 1964, 1971; McCall Rift is formedby the Elgeyo Escarpment,a steep fault 1967 and Walsh 1969). The present work was under- scarp rising 1700 m above the rift floor. E of the taken as part of the mapping programme of the Elgeyo, and separated from it by the Kerio Valley,is East African Geological Research Unit mainly in an the N-S range of the Kamasia Hills which at their attempt to reconcile differences on someof the earlier highest point (2500 m) reach a similar height to the maps. Uncertainty as to the ages of the Tinderet, top of the Elgeyo. At around O"15'N the Kamasia Timboroa and Londiani volcanoes is apparent from Hills are joined to the southeastern partof the Uasin recent literature on the general volcanic stratigraphy Gishu Plateau by the mountain mass of Kapkut of the Kenya Rift (Williams 1970; Baker et al. 1971). around the headof the Kerio Valley. The steepN face In addition several new and important centres have of Kapkut marks the northern limit of the Equator been discovered. Highlands. The age determinations were carried out by K-Ar S of O"15'N the trend of the Kenya Rift is NW-SE analyses of whole rock samples using the method as compared toN-S to NNE-SSW further N. The SW described in Chapman & Brook (1978). margin is the relatively gently sloping Mau Escarp- ment which rises to the Mau Highlands (3100 m) beyond. N of the Mau there is a continuous ridge of Morphology of the rifts high ground extending to Londiani and thence to the Timboroa area. Hence the highland barrier completely The Kavirondo Rift extends as Wfar as Lake Victoria separates the two rifts. Looking W from the floor of where its floor is covered by an extension of the lake, the Kenya Rift just S of the Equator there is no

0016-7649/79/1100-0693$02.00@ 1979 The Geological Society

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FIG. 1. Geological map of the Equator area. 1, Quaternary alluvium; 2, Menengai volcanics; 3, Hannington Trachyphonolite; 4, Londiani and Kilombe volcanics; 5, Eldama Ravine and Mau Tuff; 6, Kapkut volcanics; 7, Tinderet volcanics; 8, Upper Timboroa volcanics (12-8 Ma); 9, Plateau Phonolite; 10, Koru Beds and lower Timboroa volcanics (20-12Ma); 11, Middle Miocene lavas and sediments at the head of the Kerio Valley; 12, Precambrian basement; A, Equator Monocline; B, Mau Monocline; C, Kedowa Monocline.

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Geology of the Kenya Rift-Kauirondo Rift junction 695

W - Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/136/6/693/4885968/gsjgs.136.6.0693.pdf by guest on 28 September 2021 696 W. B. Jones & S. J. Lippard indication of a second rift, although the western top of this sequence have been dated at 15 Ma (Baker escarpment is only about 700 mhigh here. et al. 1971). The Kavirondo Rift thus ceases to have any topog- The Plateau Phonolites, dated at14-12 Ma, cover a raphic expression at Tinderet, about 50 km W of the large area W of the Gregory Rift and are believed to extrapolated junction with the Kenya Rift. The Lon- have flowed westward from sources within the rift diani Plain, between Timboroa and Londiani, is the (Lippard 1972). N of Tinderet they are called the only relatively low-lying area on the continuation of Uasin Gishu Phonolite and lie on the basement sur- the trend of the Kavirondo Rift. face. S of Tinderet they are the Kericho Phonolite, which flowed around the southern flanks of a large Stratigraphy pyroclastic cone situated approximately where Tin- deret is now (Binge 1962). A Plateau Phonolite outlier on the N side of the Kavirondo Gulf is known as the Precambriau basement Kisumu Phonolite (Saggerson 1952). At the W end of the Kavirondo Rift a highly The Tertiary rocksof the area rest upon schists and complex area of mainly Lower to Middle Miocene gneisses of the Mozambique Belt which are considerednephelinite-phonolite-carbonatite volcanics, plutonic to be 475-650 Ma old (Cahen& Snelling 1966). These rocks and sediments occurs between the S shore of outcrop along the Elgeyo Escarpment, in the NyandoLake Victoria and the edge of the rift-Gwasi Hills, Escarpment and on the floorof the Kavirondo Rift SW Kisingiri, Ruri Hills, Homa Mt. (Saggerson 1952; of Tinderet. The foliations are generally orientedN-S McCall 1958; Kinget al. 1972). TheKisingiri volcano, but vary locally from 320-040". a giant composite cone similarin some respects to the Some 40 km W of Tinderet these rocks are in later Timboroa structure,is linked with Mt. Elgon and contact with Archaean rocksof the Tanganyika Shield Napak as the southern member of a chain of 21-23 (Cahen & Snelling 1966). The boundary runs NNW Ma nephelinite volcanoes lying N-S along the Kenya- towards Mt. Elgon along the Nandi Fault Zone (San- Uganda border. The activityin this region is generally ders 1965). The Kavirondo Rift runs almost perpen- older than that at theE end of the rift, although some dicular across the border of the shield and extends age determinations indicate possible local continued some 80 km within it, while the northern Kenya Rift activity into the Plio-Pleistocene (Kinget al. 1972). runs nearly parallel to the shield 80-100 km fromit.

MiadleMiocene volcanics and sedments Upper Miocene volcanics (20-12 Ma) Eruption of pyroclastic rocks at the E end of the The earliest Tertiary rocks at the eastern endof the Kavirondo Rift continued after the emplacement of Kavirondo Rift Valley are the Koru Beds (Shackleton the Uasin Gishu-Kericho Phonolites, producing coarse 1951), a series of shales and limestones with biotite- nephelinitic agglomerates (Binge 1962). These pass bearing tuffs, sometimes carbonatitic. These contain upwards into a series of phonolitic nephelinite and Miocene fossils at Koru, S of Tinderet, and Songhor, subordinate basanite flows which are exposed NW of NW of Tinderet, at bothof which sites they have been Timboroa, where they are subordinate to interbedded dated at 19-20 Ma (Bishop et al. 1969). Shackleton tuffs (Jennings 1964) andSW of Londiani Town where (1951) showed that at this time there was no fault they form a tuff-free succession 300 m thick. New age scarp along the line of the present Nyando Escarp- determinations on these lavas (Table 1) support earlier ment and that the Koru Beds were depositedin shal- results (Baker et al. 1971) which indicated that they low lakes on a basement surfaceof broad depressions were erupted between 12 and 9 Ma. and remnant hills, bearing little relation to the later The phonolitic nephelinites are overlain by phono- developed rift structures. These sediments pass up- lite flows known as the Timboroa Phonolite which wards into nephelinitic agglomerates and tuffs, up to form the Timboroa Highlands and outcrop along the 350 m thick, which outcrop all round theW and S of W edge of the Londiani Plain. Age determinations Tinderet. At Fort Ternan,S of Tinderet, a fossiliferous (Table 1) show that these lavas were erupted between site in the agglomerates has been dated at 14Ma 9 and 8 Ma. They are the age equivalentsof the Ewalel (Bishop et al. 1969). phonolites in the Kamasia Hills to theN (Chapman et At the southern endof the Elgeyo Escarpment the al. 1978), which have given several ages in the range Middle Miocene is represented by some 700 m of 9-7Ma (Chapman & Brook 1978). Lippard (1973) sediments, basic agglomerates and phonolites (Lippard suggested that the Ewalel Phonolite was derived from 1972). Sediments at the base (Kimwarer Beds, Tam-a centre underneath the Kapkut Highlands. This bach Beds) were locally deposited on a highly irregu- centre is probably also the source of the Timboroa lar, deeply incised terrain carved into the upwarped Phonolite, which, around Londiani Town, is seen to margin of the proto-Kenya Rift. The phonolites at thehave flowed from the N.

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Geology of the Kenya Rift-Kauirondo Rift junction 697

The Timboroa Phonolite flows are overlain by the 'Timboroa Assemblage' since they all seem to have Makutano Tuff, a series of phonolitic tuffs including been derived from centres around Timboroa. Previ- ignimbrites, which appear to have infilled the lower ously the nephelinitic tuffs andthe phonolitic nepheli- part of a mountainous topography in the phonolites. nites have been included with the Tinderet Basanite The greatest thicknessof tuffs, about 250 m, is seen on (see below) as the 'Tinderet Volcano' (Binge 1962; the N edge of the Mau Highlands SE of Londiani Baker et al. 1971) while the Timboroa Phonolite Town where they are intruded by a phonolite plug, outcrops on the Londiani Plain have been regarded as Mt. Blackett. A thin veneerof phonolitic tuff extends derived from the Londiani volcano (Jennings 1971). It onto the Mau Highlands. A welded tuff SE of Tim- is proposed here that the name Tinderet be restricted boroa gave a dataof 7.9 Ma and a phonolite specimen to the Upper Miocene basanites that form that moun- from Mt. Blackett gave 7.6 Ma (Table1). tain. Londiani is a structurally separate Upper All the Miocene volcanic rocks at theE end of the Pliocene trachyte volcano (see below). Kavirondo Rift described so far, except for the Uasin The Timboroa assemblagewas probably not derived Gishu and Kericho Phonolites, are included in the from a single centre, nor formed a clearly defined TABLE1: New K-Ar age determinations of volcanic rocks from the Kenya Rift-Kavirondo Rift junction in W Kenya

~~ ~ ~~ ~~~ ~~~~ Longitude1 Sample No. Locality latitude Formation RMk type Age (Ma)

1414 Menengai 36"07'E Menengai Trachyte 0.33*0.01 OO"12'S 141369 SE of Kilombe 35"52'E (Kilombe?) Trachy- 1.7k0.05 OO"09'S phonolite 1411 Kilombe 35"50E Kilombe Trachyte 1.9k0.15 OO"O3'S 141769 Londiani 35"45'E Londiani Trachyte 3.1k0.1 Mountain OO"O5'S 141576 Matebei 35"5 1'E Matebei Trachyte 3.6*0.1 OO"08'N 141570 Perkerra 35"45'E Eldama Ravine Welded tuff 4.3*0.1 OO"04'N Tuff 141125 Poror 35"43'E Eldama Ravine Welded tuff 4.3 f 0.1 OO"04'N Tuff 141903 Limutet 35"31'E Londiani Basanite 5.4k0.2 OO"O4'S Plain Basanite 141958 S of Kedowa 35'36'E Mau Tuff Welded tuff 5.8 * 0.2 OO"12'S 141843 E of Londiani 35"38'E Mau Tuff Welded tuff 6.0 * 0.2 OO"O8'S 91364 Sirwa 35"46'E Kapkut Trachyte 6.6 * 0.2 00"lO'N 141678 Perkerra 35"44'E Kapkut Mugearite 7.6k0.2 On"O6'N 141955 Mt. Blackett 35'37'E Phonolite 7.6* 0.2 QO"O6'S (intrusive) 141898 Mt. Blackett 35"37'E Timboroa Welded tuff 7.9* 0.2 OO"O6'S (Makutano Tuff) 141949 W of Londiani 35"33'E Timboroa Phonolite 8.9k0.2 Mountain OO"08'S Phonolite 14/83 E of Timboroa 35"35'E Timboroa Phonolite 9.4 * 0.3 OO"02'N Phonolite 91459 Burnt Forest 35'27'E Lumbwa Phonolitic 9.4* 0.6 OO"12'N Phonolite nephelinite 141957 E of Kedowa 35"36'E Lumbwa Phonolitic 12.4*0.3 OO"11'S Phonolitic nephelinite Nephelinite 141924 SW of 35"34'E Kericho Phonolite 11.7zk0.3 Mt. Blackett OO"07'S Phonolite

All determinations carried out on whole rock samples. Details of the method are given in Chapman & Brook (1977). Constants used: Ag = 4.72 X 10-l' yr-' A, = 0.584~10-"yr-' K4'/K = 0.0119 atomic%

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698 W. B. Jones & S. J. Lippard

Timboroa volcano. The present outcropssuggest that eruption of the rocks of the Timboroa Assemblage the nephelinite agglomerates, phonolites and finally about 7 Ma ago. phonolitic tuffs were erupted from a series of centres SE of Londiani Town 3 faults can be seen each displaced successively eastwards. Nevertheless downthrowing the Makutano Tuff to the SW, the 2 these rocks do form a discrete group. Their total easternmost having throws of about 100 m each, but thickness and volume can only be approximated be- not apparently affecting the overlying Mau Tuff of cause of the lack of relief, both structural and topog- 6 Ma (see below). These faults are thus also c. raphic, and generally poor exposure over the 7 Ma old. 3000 km2 outcrop. Sections from the Mtetei River andIn this area thereis therefore an important periodof from the Kerio Valley to the Timboroa Highlands faulting at7 Ma which correlates with the major fault- suggest thicknesses of at least 1500 m for this part. ing in the Kamasia Hills at this date. Like the latter it Assuming an average of only 500 m, the total volume coincides with a change in the lavas from strongly would be 1500 km3. It is not known how muchof the undersaturated to mildly undersaturated types. assemblage is hidden under younger rocks to theE. The gravity survey of McCall (1967) showed an ellipti- cal negative Bouguer anomaly, elongatedNW-SE be- Late Miocene-Pliocene volcanics tween Londiani and Timboroa. At its minimum this gives the lowest gravity values in W Kenya and indi- In the Kapkut areaa succession of basic and inter- cates a thickness of low density volcanics and sedi- mediate lavas, oneof which has been dated at 7.6 Ma ments probably as much as3 km, most of which must (Table 1) is interbedded with the upper membersof be attributed to the Timboroa Assemblagewhich out- the Makutano Tuff. These lavas are the lateral equi- crops widely is this area. valents of the Eron Basaltof the central Kamasia Hills area (Chapman ef al. 1978). They are overlain by massive trachyte lavas which make up the bulkof the The 7 Ma faulting episode Kapkut volcano. The trachytes are at least m600 thick in the central area, thinning to 100-150 m on the A major period of faulting occurred in the Kamasia flanks, and cover an area of about 250 km2. This Hills at about 7 Ma (King & Chapman 1972) volcano is similar in form to the trachyte shield vol- downthrowing the floor of the Kenya Rift Valley and canoes in South Turkana in theN part of the Kenya backtilting large blocksof phonolite away fromit. This Rift described by Webb & Weaver (1975). The age faulting coincided witha change in the compositionof determination on a Kapkut Trachyte shows that the the lavas from basanite-phonolite to basalt-trachyte. volcano formed at about6-7 Ma (Table 1). The Kap- This event can be recognized over muchof the north- kut Trachyte is approximately equivalent in age to the ern Kenya Rift. Kabarnet Trachyte in the Kamasia Hills (Bakeref al. The Nyando Fault, on theN side of the Kavirondo 1971; King & Chapman 1972). The eastern edgeof Rift, branches atits E end into two faultswhich curve the Kapkut volcano was downwarped into the rift round to the NE and then back to the E. Erosion valley and antithetically faulted before the eruptionof along these fault lines has produced thevalleys of the the overlying Eldama Ravine Tuff. Kipkurere and Mtetei rivers which cut across the The Tinderet volcano was built upby eruptions of regional dip. This faultingis later than the Koru Beds abundantly olivine and augite phyric basanite, datedat (Shackleton 1951). Jennings stated that the faultingis 5.5-6 Ma (Baker et al. 1971). The final flows also later than mostof the tuffs and his map implies that it contain abundant plagioclase phenocrysts. These lavas is later than the phonolitic nephelinites (Jennings form a cone-shaped pile resting on the Middle 1964). He also stated that the Tinderet Basanite, nowMiocene tuffs at theE end of the Kavirondo Rift. The known to be about5.5-6 Ma, laps against the footof topography and drainage of the volcano strongly sug- the Mtetei Fault scarp. This shows that the faulting gests a caldera 4 km in diameter at the summit anda occurred between 9 and 6 Ma and can be tentatively semicircular fault on the northern flanks downthrow- correlated with the7 Ma faultingin the Kamasia Hills. ing the higher part of the volcano to the S (Fig. 1). The Londiani Plain is bordered on the N and NE Further E on the Londiani Plain there are local oc- sides by an inward-facing escarpment. This may be currences of basanite lava at Limutet and Lessotet interpreted as a fault scarp of semicircular form. The (omitted from Fig. 1 for simplicity). An age determi- fault scarp is cut in, and therefore the faultis younger nation on lava from Limutet (Table 1) suggests that than, the Makutano Tuff, while in places outliers of they are contemporaneous with the Tinderet lavas the Eldama RavineTuff (about 4 Ma, see below) are although this age seems too old on geomorphological banked up against it. This means that the faulting grounds. occurred between 7.5 and 4Ma and we suggest that The Kapkut trachytes are overlain unconformably the Londiani Plain is a volcano-tectonic depression by the Eldama RavineTuff (Walsh 1969; Jones 1975) formed by subsidence along a fault at the endof the from which two dates of 4.3 Ma have been obtained

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Geology of the KenyaRift-Kauirondo Rift juncrion 699

(Table 1). The unconformity and up 300 to m of tuffs are 90 km2 in area. The caldera subsidence probably ac- exposed in a deep gorge along the Perkerra River. companied the eruption of the Lower Menengai Tuff These tuffs are trachytic ignimbrites, about half of (Jones 1975), a succession of pumiceous tuffs capped them welded, and airfall tuffs with intercalated basalt by an unwelded ignimbrite outcropping as far as flows near the top. Theignimbrites and lavas flowed Kilombe in the NW. The caldera has been largely from the E (see below). In the E part of the outcrop filled by younger well preserved trachyte lavas, and a there are abundanttuff dykes cutting the upper partof recent eruption, probably originating on the high the succession but these are all unwelded and cannot ground N of the caldera, has blanketed the Rongai be considered to be the source of the ignimbrites Plain, S of Kilombe, with a thin layerof black ash and (Jones 1975). pumice, the Upper MenengaiTuff (Jones 1975). A data On the Mau Highlands the MakutanoTuff is over- of 0.33 Ma was obtained from a boulder of trachyte lain by the MauTuff (Jones 1975), a series of trachytic derived from the calderawall (Table 1). tuffs very similar to the Eldama RavineTuff. Dates of Along the foot of the high ground E of Londiani 5.8 and 6.0 Ma have been determined for weldedtuffs and Kilombe a strip of red conglomerates and clayey of this formation (Table 1). The term ‘Mau Tuff’ has sands, the Esageri Beds (Jones1975), occurs between been used in the past for any tuffs from the Mau the Lower and Upper Menengai Tuffs. These beds Highlands but we would restrict itto trachytic tuffs of continue SE along the lower slopes of the Mau Es- about 6 Ma. carpment and are similar to the Kerio Valley Beds The Eldama Ravine Tuff is overlain in a broadly (Lippard 1972) and the Kapthurin Beds (Martyn conformable succession by the Saos Mugearite, a 1969). series of hawaiites passing up into mugearites, exposed E of Eldama Ravine. These flows are conformably overlain by trachyte lavas which built up thevolcanoes Structure Londiani and Kilombe. There is a tendency for the early flows to be aphyric and later ones to be increas-Fault trends ingly feldspar-phyric, a trend also notedin the Kapkut trachytes. Both volcanoes have well developed cal- Despite being located at the junctionof 3 rift struc- deras and N-trending rift zones, while Londiani also tures, faulting is relatively unimportant in this area, has a SE rift zone (Jones 1975; McCall 1964). Ages of particularly when compared to sections of the Kenya 3.1 and 1.9 Ma were obtained from rocks from Lon- Rift further N and S. Two major faults, the Elgeyo diani and Kilombe respectively (Table1). Estimates of fault on the W side of the N Kenya Rift and the the volumes of these volcanoes are: Saos Mugearite Nyando fault on the N side of the Kavirondo Rift, 20 km3, Londiani Trachyte 60 km3, and Kilombe begin only at its margins. Trachyte 15 km3. The faults at the Kenya-Kavirondo Rift junction To the NE of the area there is a complex series of show 3 distinct trends: N-S, NE-SW and NW-SE. volcanics, mostly erupted from local centres, spanning SW of Londiani Town all 3 may be seen affecting the the late Pliocene and early Pleistocene. These includerocks of the Timboroa Assemblage. N-S faulting oc- the trachyte domesof Matebei, dated at3.6 Ma (Table curs in the rift zones N of Londiani and Kilombe l),and Kiborit and the small trachyte centre Gobatof calderas while NW-SE faulting forms the rift zone on about 2 Ma (Griffiths 1977). Trachyphonolite lavas the SE side of Londiani Mountain. Both N-S and outcrop E of Kilombe, one (from just SE of that NW-SE trends are shown by the tuff dykes and small volcano) being dated at 1.7 Ma (Table 1). Further E, faults in the Eldama RavineTuff. between Menengai and Lake Baringo, the floorof the The N-S faulting, occurring throughout the area but rift valley is covered by extensive trachyphonolite especially in the NE, is a continuation of the Kamasia flows formerly called the Dispei-Lake Hannington Hills structures (Chapman et al. 1978) and is the Phonolite (McCall 1967) but now renamed the Han- direction of the N Kenya Rift between the Equator nington Trachyphonolite (Griffiths 1977). Most of the and Lake Rudolf. TheNW-SE trend of the S and E is Hannington lavas appear to be less than 1 Ma old the trend of faulting in the Mau Highlands (Jennings (Griffiths 1977). In the centre of the rift they are 1971) and follows the directionof the floor of the rift affected by closely spaced grid faulting of Pleistocene valley between Menengai and Longonot. TheNE-SW age (McCall 1967). trend, whose most northerly representative is the gorge of the Perkerra river, is characteristic of the S Menengai volcano side of the Kavirondo Rift (Binge 1962). This area then is the meeting point of 3 structural provinces The Menengai volcano (McCall 1957, 1967) lies in characterized by different fault trends. A similar con- the centre of the Kenya Rift Valley, justN of Nakuru. clusion has been reached by King (1970, 1978), who Here the building of a trachyte shield volcano was noted that the ‘bend’ in the Kenya Rift direction at followed by piecemeal subsidence to producea caldera about the Equator is determined by a change in the

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700 W. B. Jones & S. J. Lippard

EAST END EAST RIFT OF KAVIRONDO KENYA RIFT 0 r Menengai 0 1

Kilombe @ @ 2 c Londiani @ 3 Eldama 4 Ravine@ 5

Mau Q 6 Kapkut 6 7 @, 0 B / 9 8 / / lo Ma Timboroa / l1

assemblage M090 l? Kericho Uasin Gishu Q, / 13 Phono'i'es MB B phonolites / a / 14 / 15

I8 Q BAKER G 1971 Koru beds 19 @ BISHOP e* 1969 c, 20

FIG.3. Age determinations plotted against time and longitude. Vertical line through each data point is the error on the age determination.

predominant fault trends which partially overlap one there is a decline in the influence of the Nyanzian another. NE-SW trend and a rise in the importanceof the N-S The influence of structures in the basement on the and NW-SE trends characteristic of the Mozambique trends of structures in the rift valleys has long been a Belt. Whereas the 3 fault trends of the rift junction controversial issue (Shackleton 1951; Binge 1962; area all have their counterparts in the underlying McConnell 1972), but in this area the two are clearlybasement, it is noteworthy that the E-W strike of the related. In the Elgeyo Escarpment the trend of the Nyando Fault does not seem to be related to any steeply dipping foliation in the Mozambique Belt basement structure. rocks is usually slightly E of N (Lippard 1972) and this has clearly controlled the trend of the Elgeyo Fault Monoclines and perhaps the N Kenya Riftin general. The NW-SE trend of the central Kenya RiftValley is shownby the The most important structure in the areais a broad Mozambique Belt rocks in the Elgeyo Escarpment downwarp called the 'Equator Monocline' (Lippard between O"32'N and O"22'N (Lippard 1972) and Wof 1972; Jones 1975) which forms the W edgeof the the outcrop of the Uasin Gishu Phonolite between Kenya Rift Valley at the Equator. Theof hinge the mono- O"05'N and O"20'N (Jennings 1964). cline runs approximately N-S just to the Wof Londiani Around Koru, at the SW foot of Tinderet, and in Mountain (Fig. 1) and separates flat-lying Timboroa the Sotik area SWof Kericho, the Mozambique gneis- volcanics from E-tilted younger rocks to the E. ses are foliated NE-SW (Binge 1962). This trend is The structure can be traced NNE into the Kapkut also the trend of isoclinal folding and shearing in the area where the E part of the volcano is affected. The Nyanzian rocks around Sotik (Binge 1962; Schoeman oldest rocks exposed on the Equator Monocline are 1949). It may be that the rocks of the Mozambique the hawaiites of about 7 Ma underlying the Kapkut Belt adjacent to the Nyanzian have inherited the Trachyte. A plot of the dip of formations affected by pre-existing Nyanzian trend and this trend has in- the monocline against their age (Fig.4) shows that the fluenced in places the faulting in the Tertiary cover. In structure has been developing since at least the late going E from the Kavirondo Rift to the Kenya Rift Miocene at an average rate of about 1" per Ma.

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Geology of the Kenya Rift-Kauirondo Rift junction 701

At its northern end, on the E side of Kapkut, the X Lower MenengalTuff Or Equator Monocline curves round to trendNE-SW and is antithetically faulted. One of these faults has controlled the course of the Perkerra Gorge. Atits S end the Equator Monocline passes into the Mau Monoc- X Kilombe Trochyte line which trends NW-SE and forms the SW marginof the central part of the Kenya Rift Valley. It can be 2t X LondianiTrachyte seen SE of Londiani Town that the Mau Monoclineis -K also antithetically faulted. Another structure, the 3t Kedowa Monocline, trends NE-SW and separates the Eldam;uf2avine X ] Mau Highlands from the Londiani Plain. Cox (1970) showed that antithetically faulted monoclines are a common feature of the Karroo rift 5 structures and presented a hypothesis to explain them 4l which equally well fits these features in Kenya. Ten- sion in continental crust may lead to necking, i.e. the 7- Hawaiite Kopkut X development of a rift valley flanked by monoclines. Normal tensional faulting will then occur preferably of I I I l 1 I I 1 on fault planes dipping outwards from the centre 12345678the rift, since this gives greater horizontal displace- Dipo ments for a given movement on the fault plane. FIG.4. Plot of dip of formation against age across the Equator Monocline showing an average rate of Comparison of the Kavirondo and Kenya movement of about l"/Ma. Rifts Although this moncline appears to have been continu- W of the rift junction the central section of the ally active, a number of observations show that the Kavirondo Rift is almost devoid of Tertiary volcanic topographic slope has been downhill fromE to W for cover, whereas at its W end volcanic activityis largely much and probably most of the time of its develop- restricted to the Miocene (King et al. 1972). The ment. The Eldama RavineTuff is thicker in the E and Kavirondo Rift is limited on the N side by the E-W its ignimbrites become more welded in this direction. Nyando Fault. On the S side it is defined by the An intercalated basalt flow, the Theloi Basalt, has its NE-SW Kedowa Monocline SE of Tinderet, the NE- source in a dyke exposed in the Perkerra Gorge and SW Sondu Flexure (Binge 1962) SWof Tinderet (set flowed westward until it stopped and was ponded on en echelon to the Kedowa Monocline) and the E-W the E side of a ridge of Kapkut Trachyte. The trachyte Kendu Fault S of Kisumu (Saggerson 1952). W of lavas of both Londiani and Kilombeflowed a long way Kisumu there is an abrupt changeof direction and the W and N from their sources but only for a short rift continues to the SW between the Kaniamwia Fault distance to the E. The NW direction was blocked at on the SE and the Mfanganu Fault on the NW this time by the upstanding mass of the Kapkut vol- (McCall 1958). As explained above, the faulting at the cano. The ignimbrite of the Lower MenengaiTuff can E end of the Nyando Fault occurred during the period be traced onto the slopes of the rift margin (Jones 9-6 Ma. The W end of the Nyando Fault cuts the 1975) although it was derived from the centre of the Kisumu Phonolite (Shackleton 1951), correlated with rift. These observations show that since at least4.5 Ma the Uasin Gishu and Kericho Phonolites, and so is the land surface at the Equator has normally sloped 12 Ma old or younger. The Kedowa Monocline affects downhill W along the direction of the Kavirondo Rift the Kericho Phonolite and the overlying phonolitic and N along the North Kenya Rift. The presentE tilt nephelinite, making it younger than 9 Ma. McCall of the margin of the Kenya Rift Valley at the Equator (1958) considered the Kaniamwia Fault to be Upper probably came about onlyvery recently and the Esa- Miocene. It can be concluded that the Kavirondo Rift geri Beds may have been formedby the rapid erosion was formed during the Upper Miocene and that there of the new escarpment. Deposition of volcanic rocks has been very little volcanic or tectonic activity along must have been more rapid on the riftvalley floor than it since that time. This is in complete contrast to the on the flanks and this has generally approximately adjacent parts of the Kenya Rift where volcanism and balanced the subsidence of the floor. For most of the faulting continued into the Pleistocene (McCall 1967; Pliocene and Pleistocene deposition has kept aheadof Thompson & Dodson 1963). subsidence making the rift floor higher than the area The Kavirondo Rift is c. 35 km wide between the of its present W flank. However, at the moment depos-Nyando and Kendu Fault scarps and c. 30 km wide ition is losing and the result is the rift valley with between the Kaniamwia and Mfanganu Faults, upraised shoulders that we see now. whereas the Kenya Rift is usually c. 70 km wide.

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702 W. B. Jones & S. J. Lippard Similarly the faultingof the Kavirondo Riftis less than The evidence thus shows that the Kavirondo Rift in the Kenya Rift. The displacementof the E end of largely disappears W of the Kenya Rift but there is the Nyando Fault is about 300m (Shackleton 1951) some suggestion that it continues in a very reduced and in its central part the Nyando Fault scarp reachesform right up to the edgeof the latter. 700 m. The greatest displacementon the Kaniamwia Shackleton (1951) pointed out that, unlike the Fault is over 500 m (McCall 1958). On the other hand, Kenya Rift, the Kavirondo Rift showsno uplift of its in the Kenya Rift Valley displacements of about shoulders and considered it to be merely a ‘lag’ struc- 3000 m are attained on the Elgeyo and Kamasia ture on the uplifted flanks of the main Kenya Rift. Faults (King & Chapman 1972). Our work supports this idea. Thefaulting and volcan- Both rift valleys show negative gravity anomalies.A ism of the Kavirondo Rift is largely confined to the gravity profile across the Kavirondo Rift along long. Miocene, which was a period of major uplift of the 35”12’E (Fairhead 1976) shows a negative Bouguer Kenya Dome (Saggerson & Baker 1965). anomaly of about 400 g.u. 70-80 km across, wider Burke & Whiteman (1973) suggested that the than the rift and centred over the S part. However, Kavirondo-Kenya Rift junction is an example of a there is no axial positive anomaly.A positive anomaly continental ‘rrr’ type triple junction with the N and in the centre of the Kenya Rift (McCall 1967) has central sections of the Kenya Rift as spreading axes been attributed to crustal attenuation and the intru- and the Kavirondo Rift asa third dead or failed arm. sion of a mantle diapir (Fairhead & Girdler 1972). They suggested that the rift faulting and volcanic The absence of such a positive anomaly in the activity were consequenton the updoming of the area Kavirondo Rift suggests that it does not contain a and related to a ‘hot spot’ in the underlying mantle. similar mantle intrusion. However, several facts are not compatible with this hypothesis: Relationship of the Kavirondo and 1. The two rifts have different structures. The Kenya Rifts Kavirondo Rift is on a smaller scale and shows The Kavirondo Rift disappears as a topographic no evidence for an axial mantle intrusion or for feature 50 km W of the Kenya Rift. The gapis filled dilation. by the Equator Highlands,a volcanic accumulation at 2. The junction area does not coincide with the similar heights to the Uasin Gishu and Kericho crest of the Kenya Dome.The culmination in the Plateaux to the N and S respectively. If the Tinderet rift floor is to the S, between Lakes Nakuru and volcano were removed the Kavirondo Rift would con- Naivasha. In fact, King (1978) pointed out that tinue another 20 km to theE. Similarly removal of the the domeis largely the resultof volcanic accumu- Londiani volcano would leave the W wall of the lation anda N-S section along the rift flanks does Kenya Rift about 700 m high at the Equator, lower not show any domingof the basement surface. than in adjacent areas to the N and S. 3. The tworifts do not havea marked junction. The The Nyando Fault on the N side of the Kavirondo Kavirondo Rift largely fades out50 km W of, and Rift splays into two and dies inout the Timboroa area, is barely perceptible at, the edge of the Kenya where it seems to be involved in the faulting as- Rift. sociated with the Timboroa Assemblage. The faulting of the Kavirondo Rift thus disappears30 km W of the Kenya Rift. Comparison of the Kenya-Kavirondo Rift For most of the Plio-Pleistocene there was no W junction with South Turkana wall to the Kenya Rift Valley at the Equator and lavas and ash flows erupted in the main rift flowedW into At its N end the Equator Monoclineis replaced by the extrapolated extensionof the Kavirondo Rift. This the Kamasia Fault system. N of the Kamasia Fault in lack of a rift walldoes not seem to have beenconfined South Turkana there is another monocline which is to the Equator Highlands area. Further S the Mau very similar to the Equator Monoclinein that along it Tuffs (6 Ma) can be traced onto the Mau Highlands.In there is a cluster of Pliocene trachyte volcanoes (Webb fact the Pliocene ignimbrite eruptions seem to have & Weaver 1975) with the remnantsof a large Miocene completely infilled the central part of the Kenya Rift phonolite volcano, Tiati, to the W. This monocline (King 1978), so the absence of a rift wall in the also seems to have been continuously active during the Equator area cannot be taken as evidence fora rift Plio-Pleistocene, with older rocks being progressively junction. However the shapeof the Londiani Trachyte more tilted. It is noteworthy that along the Kamasia outcrop suggests that at 3 Ma the Kapkut Highlands Fault there are no central volcanoes, while the mono- and Mau Highlands were relatively upstanding areas. clines into which it passes at both ends have several. It The former might merely reflect the shape of the may be that conditions are more favourable for Kapkut volcano but the latter implies movement of magma to be generated and/or to reach the surface the Mau and Kedowa Monoclines. along a monocline than along the lineof a major fault.

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Geology of the KenyaRift-Kavirondo Rift junction 703

Another similarity between the two areasis that, in Turkana and the similarity of this area to the both, cycles of volcanic activity can be made out. In Kavirondo-Kenya Rift junction shows that the struc- South Turkana the cycle begins with the eruption of ture of the latter is explicable without reference to basic lavas to form a plateau which is overlain by interfering rifts. trachyte lavas forming a central volcano. This is followed by a period of eastward downwarping and erosion before the eruptionof basalts at the beginningof ACKNOWLEDGMENTS. Professor B.C. King supervised the the next cycle (Truckle 1977). In the rift junction area work and read an early version of the manuscript. The the basalts, hawaiites, mugearites and trachytes of research project was financed by the Ministry of Overseas Development. the Government of the Republic of Kenya and Kapkut constitute one such cycle separated by a the Natural Environment Research Council (NERC). WBJ period of erosion from the Eldama Ravine Tuff with received a NERC research fellowship and SJL the William its intercalated basalts, the Saos Mugearite and the Gilles Research Fellowship of the University of London. We trachytes of Londiani and Kilombe forming another. thank Dr N. J. Snelling for providing analytical facilities, and There is no suggestion of a rift junction in South Mrs M. Brook for carrying out some of the analyses. References

BAKER, B.H., WILLIAMS, L.A. J., MILLER,J. A. & FITCH,F. KENT, P.E. 1944. The Miocene beds of the Kavirondo Rift. J. 1971. Sequence and geochronology of the Kenya Rift Q. J. geol. Soc. London, 100, 85-1 18. volcanics. Tectonophysics, 11, 191-215. KING, B. C. 1970. Volcanicity and rift tectonics in East BINGE,F. W. 1962. Geology of the Kericho area. Rep. geol. Africa. In: CLIFFORD,T. N. & GASS,I. G. (eds.) African Sum. Kenya, 50. rnagmatism and tectonics, 263-83. Oliver and Boyd, BISHOP, W. W., MILLER,J. A. & FITCH, F. J. 1969. New Edinburgh. potassium argon age determinations relevant to the - 1978. Structural and volcanic evolutionof the Gregory Miocene fossil mammal sequence in East Africa. Am. J. Rift Valley. In: BISHOP, W. W. (ed.)Geological back- Sci. 267, 669-99. ground to fossil man, 29-54. Geol. Soc. London. BURKE, K. & WHITEMAN,A. J. 1973. Uplift, rifting and -& CHAPMAN,G. R. 1972. Volcanism of the Kenya rift break-up of Africa. In: TARLING,D. H. & RUNCORN,S. Valley. Philos.Trans. R. Soc. London, A271, 185-208. K. (eds.) Implications of continental drift to theearth -, LE BAS, M.J. & SUTHERLAND,D. S. F-972. The history sciences 2, 735-55. Academic Press, London. of the alkaline volcanoes and intrusive complexes of CAHEN,L. & SNELLING, N.J. 1966. The geochronology of eastern Uganda and western Kenya. J. geol. Soc. Lon- equatorialAfrica. North-Holland Publishing Company, don, 128, 173-205. Amsterdam. LIPPARD,S. J. 1972. The stratigraphy and structureof the CHAPMAN, R.G. & BROOK,M. 1978. Chronostratigraphy of Elgeyo Escarpment, southern Kamasia Hills and adjoin- the Baringo Basin, Kenya. In: BISHOP, W. W. (ed.) ingregions, Kenya RiftValley. Thesis, Ph.D., Univ. Geological background to fossil man, 207-23. Geol. Soc. London (unpubl.). London. - 1973. Plateau Phonolite lava flows, Kenya. Geol. Mag. -, LIPPARD,S. J. & MARTYN,J. E. 1978. The stratigraphy 110, 543-49. and structure of the Kamasia Range, Kenya Rift Valley. MARTYN,J. E. 1969. Thegeological history of thecountry J. geol. Soc. London, 135, 265-81. between Lake Baringo and the KerioRiver, Baringo Cox, K. G. 1970. Tectonics and volcanism of the Karroo District, Kenya. Thesis, Ph.D., Univ. London (unpubl.). Period and their bearing on the postulated fragmentation MCCALL, G. J. H. 1957. Menengai caldera, Kenya Colony. of Gondwanaland. In: CLIFFORD, T. N. & GASS, I. G. 20th Int. Geol. Congress, Sec. 1, 1, 55-69. (eds.) Africanrnagmatism and tectonics, 211-36. Oliver - 1958. Geology of the Gwasi area. Rep. geol. Surv. and Boyd, Edinburgh. Kenya, 45. FAIRHEAD,J. D. 1976. The structure of the lithosphere - 1964. Kilombe caldera, Kenya.Proc. Geol. Assoc. Lon- beneath the Eastern Rift, East Africa, deduced from don, 75, 563-72. gravity studies. Tectonophysics, 30, 269-98. -1967. Geology of the Nakuru-Thompson’s Falls-Lake - & GIRDLER,R. W. 1972. The seismicity of the East Hannington area. Rep. geol. Suru. Kenya, 78. African rift system. Tectonophysics, 15, 115-22. MCCONNELL,R. B. 1972. Geological development of the rift GREGORY,J. W. 1921. The rift valleys and geology of East system of Eastern Africa. Bull. geol. Soc. Am. 83, 2549- Africa. Seeley Service, London. 72. GRIFFITHS,P. S. 1977. The geology ofthe area around Lake SAGGERSON,E. P. 1952. Geology of the Kisumu district. Hannington and the Perkerra River, Rift Valley Province, Rep. geol. Suru. Kenya, 21. Kenya. Thesis, Ph.D., Univ. London (unpubl.). -& BAKER,B. H. 1965. Post-Jurassic erosion surfacesin JENNINGS,D. J. 1964. Geology of the Kapsabet-Plateau area. eastern Kenya and their deformation in relation to rift Rep. geol. Surv. Kenya, 63. structures. Q. J. geol. Soc. London, 121, 51-72. -1971. Geology of the Molo area. Rep. geol. Surv. Kenya, SANDERS,L. D. 1965. Geology of the contact between the 86. Nyanza Shield and the Mozambique Belt in western JONES, W. B. 1975.The geology of the Londiani area of the Kenya. Bull. geol. Sum. Kenya, 7. Kenya RiftValley. Thesis, Ph.D., Univ. London (un- SCHOEMAN,J. J. 1949. Geology of the Sotik district. Rep. publ.). geol. Suru. Kenya, 16.

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704 W. B. Jones & S. J. Lippard

SHACKLETON,R. M. 1951. A contribution to the geology of WEBB, P. K. & WEAVER,S. D. 1975. Trachyte shield vol- the Kavirondo Rift Valley. Q. L geol. Soc. London, 106, canoes: a new volcanic form from South Turkana, 345-88. Kenya.345-88. Bull. Volcanol. 39, 1-19. THOMPSON,A. 0. & DODSON,R. G. 1963. Geology of the WILLIAMS, L. A. J. 1970. The volcanics of the Gregory Rift Naivasha area. Rep. geol. Sum. Kenya, 55. Valley, East Africa. Bull. Volcanol. 34, 439-65. TRUCKLE,P. H. 1977. The geology of the area to the south of WILLIS, B. 1936. East African plateaux and rift valleys. Car- Lokori, South Turkana, Kenya. Thesis, Ph.D., Univ. negie Institute, Washington. London (unpubl.). WALSH, J. 1969. Geology of the Eldama Ravine-Kabarnet area. Rep. geol. Sum. Kenya, 82.

Received 11 June 1979. W. B. JONES,Department of Geography and Geology, West London Institute of Higher Education, Isleworth, Middlesex TW7 5DU. S. J. LIPPARD, Departmentof Earth Sciences, Open University, Milton Keynes, Bucks. MK7 6AA.

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