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 Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/136/6/693/4885968/gsjgs.136.6.0693.pdf by guest on 28 September 2021 0\ miles I 1 I I I ' I I I I I I 1 I I I' I' \ I I ' ~ I . 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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. Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/136/6/693/4885968/gsjgs.136.6.0693.pdf by guest on 28 September 2021 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).