Rift structure in southern Ethiopia

Tectonophysics, 46 (1978) 159-173 0 Elsevier Scientific Publishing Company, Amsterdam -Printed

159 in The Netherlands

RIFT STRUCTURE IN SOUTHERN ETHIOPIA *

JOHN M. MOORE JR. 1 and A. DAVIDSON

’

’ Carleton University, Ottawa (Canada) 2 Geological Survey of Canada, Ottawa (Canada) (Received May 16, 1977; accepted for publication September

5, 1977)

ABSTRACT Moore, Jr., J.M. and Davidson, A., 1978. physics, 46: 159-173.

Rift structure

in southern Ethiopia.

Tectono-

The Main Ethiopian Rift, extending southward from the Afar triangle, is not continuous with the Gregory Rift in Kenya, but attenuates in southern Ethiopia. Here the Stefanie and Lake Rudolf rifts dominate and connect directly with the Kenya rift system. These rifts are terminated northward by northeast-striking normal fault systems and by diminishing throws on the boundary faults. In particular, the Gofa basin-and-range province interrupts the Stefanie Rift, and the entire system loses its expression north of an east-northeast fault set on strike with the southern boundary of the Afar depression. Estimates of fault movements are based on displacement of the sub-planar, pre-Tertiary surface eroded on the basement gneisses. The Main Ethiopian and Stefanie rifts are relatively symmetrical and lie at the crests of basement arches; gneisses are exposed at 2750 m a.s.1. west of Lake Chamo, and structural relief is at least 2 km. The other rifts are chiefly half-graben; they and the north-trending systems are composed dominantly of westerly-tilted blocks with fault throws reaching at least 3 km. Thick alkali basalt and trachyte-pantellerite effusions vary in age from Mid-Eocene to Mid-Miocene. Mafic and salic successions are in part contemporaneous and predate major rifting, which occurred between Late Miocene and Mid-Pliocene time, with diminished movements to the present. Rifting is evident over a width in excess of 300 km in southern Ethiopia. Although the strikes of individual fault sets can be locally related to basement foliation trends, the gross alignments of the major rift valleys must stem from deeper-seated control.

INTRODUCTION

In the years since J.W. Gregory coined the term “rift valley” and described the East African rifts (1921)) a large fund of data has accumulated concerning this fault system and its associated volcanism in East Africa and, * Paper presented to the 25th International Geological Congress, Sydney, Australia, 1976, by permission of the Canadian International Development Agency and of the Director, Geological Survey of Ethiopia.

160

recently, in central and northern Ethiopia around its “triple junction” with the Red Sea and the Gulf of Aden (Fig. 1). The pivotal nature of the Afar triangle in the New Global Tectonics has resulted in a rash of new studies, as well as summaries of previous and current knowledge (Falcon et al., 1970; Baker et al., 1972; Pilger and Riisler, 1975). Compilation of previously published maps of the rift system (Fig. 2) reveals that the projection of the Gregory Rift of Kenya into the Main Ethiopia Rift is not st~ghtforward; the two systems have an apparent en Cchelon relation, and although ERTS-1 images of the intervening region were interpreted by Mohr (1974), no satisfactory geologic maps have been available. In 1972 a regional geological and geochemical survey, the Omo River Project, was initiated under bilateral agreement between the governments of Ethiopia and Canada. The survey area was chosen to include all of the unmapped Precambrian-Lower Paleozoic crystalline basement of southwest Ethiopia, and the principal goal was to evaluate the metallic mineral resource potential of the region. Fortuitously, however, the area also included the more

0

INDIAN OCEAN IOS 5OE

Fig. 1. The rift system and Baker et al., 1972).

in east and northeast Africa (modified from Falcon et al., 1970 The Omo River Project area in southwest Ethiopia is outlined.

161

‘> ,’

^._a.._ ‘.

ETHIOPIAN

36 40 Fig. 2. Ethiopian and East African rifts (compiled from Baker et al., 1972) in relation to the Omo River Project map-area.

above-mentioned portion of the East African rift system (Fig. 2), and thick successions of Tertiary volcanic rocks. Semi-controlled air photo mosaics at 1 : 50,000 scale were prepared for the entire Project area. During a total of twelve months’ field work, 80,000 square kilometres were surveyed with the

162

nearly continuous support of two helicopters, by a team of Ethiopian and Canadian geologists. Helicopter traverses at four-kilometre intervals, with periodic landings, and supported by aerial observation and photo-interpretation, formed the main basis of the geological maps. More detailed “followon complex basement rocks. Accurate 1 : up” work was concentrated 250,000 scale topographic maps became available only after completion of field work, but good elevation control was maintained throughout by helicopter altimeter. Preliminary reports have been published (Davidson et al., 1973, 1976) with geological maps at 1 : 250,000; these are available from the Geological Surveys of Ethiopia and Canada. REGIONAL

TOPOGRAPHY

AND RIFT STRUCTURE

Figure 3 depicts the fault pattern in the Project map-area and the adjacent southwestern margin of the Ethiopian plateau. The region is one of extreme topography; elevations range from 3568 m a.s.1. in the Gemu highland to 375 m a.s.1. at Lake Rudolf. There is close correlation between topography and rifting in all but the northwestern part of the area. Major rifts are, from east to west: Stefanie, Lake Rudolf (Usno branch), Lake Rudolf (Omo branch) and Kibish (Fig. 3). Two important rift valleys, the Main Ethiopian and Ririba rifts (Mohr, 1962,1974), lie at the east margin of the Project area (see cross-sections, Fig. 5). The Main Ethiopian Rift is a pronounced feature in the vicinity of Lake Chamo; southwards, it disappears in the Konso upland north of the Sagan River. The Ririba Rift is a shallow feature, but with important associated Quatemary volcanism; it lies south of, and on strike with, the Amaro Horst (Levitte et al., 1974), which is immediately east of Lake Chamo. The Ririba Rift is separated from the Stefanie Rift by the Boran uplift of Tertiary volcanic rocks. The Stefanie Rift is the most striking feature of the map, with serrated margins on both east and west sides along faults that yield steep walls up to 1500 m high. Northwards, this graben is interrupted successively by the Woito Horst and by the Gofa basin-and-range province (Fig. 3), a gently arcuate system of predominantly northeast-trending blocks tilted toward the northwest. West of the Stefanie Rift is the Hamar Horst, a huge tilted basement block, the surface of which dips to the west-southwest beneath a veneer of basalt and the Pliocene to Recent beds of the Lake Rudolf-Lower Omo River depression. The basin-and-range province culminates in the Ari highland; its expression diminishes in both directions along strike. The Lake Rudolf Rift is a broad half-graben, broken into two branches northwards by the Nkalabong Horst, a west-tilted block of salic lavas and tuffs mantled by the Pliocene Mursi basalt (De Heinzelin et al., 1971). The rift is bounded sharply on the west by major north- and northeast-striking faults along the east sides of the Ilibai Horst and the Shasha complex (see Fig. 6). The Labur Range to the south in Kenya (Walsh and Dodson, 1969) is bounded by a fault of the same set. Within the Lake Rudolf graben, beds of

Stippling

34*E

on elewted

side

36%

.. 37 L’E

Fig, 3. Fault pattern in southwest Ethiopia {Baker et al., 1972; Davidson et aI., 197; 1976). Faults between &X000% 3?@301E and the Urns River Project area are inferred fro; topo@aphy, aerial observations, and in part reproduced from Baker et al, (1972, Fig. 14, p. 322, Letters denote structural and topographic units: A = Ari highland; 3 = Boran upIX%; C = Chebera highland; G = Gurraferda plateau; Ge = Gemu Horst; Go = Gofa basinand-range province; H = Hamar Horst; I= Bib& Horst; J = Jeba plateau; K = Konso upland; M = Maji highland; N = NkaIabong Horst; S = Surma Horst ; T = Tishena highland; W = W&to Horst. tie Plio-PMstocene Omo Group, fButzer, 1972; De Neinzelin et aI., 1971) have been broken by numerous minor felts with similar orientations. Strong linear trends in the lower Omo and Usno Rivers are probably Mated to the

164

strike of west-tilted blocks, rather than being localized by faults. The Ki&ish Rift is a subordinate half-graben west of the Ilibai Horst and is

separated by a steep scarp from the Surma Horst. Unlike most of the major rifted blocks in the region, the Surma Ho& trends northwesterly, in close parallelism to the structural grain of its underlying gneisses. The Kibish and Surma blocks are in part terminated along strike by northeast-striking normal faults which do not, however, influence the map pattern as strongly as to the east. To the north, in the Gimera ranges (Fig. 5) important northeaststriking normal faults again appear, bounding southeast-tilted blocks. North of 7”N in the Project area, the fault pattern in Tertiary rocks is dominantly east-northeast in strike, as in the narrow graben that passes through the Gurraferda plateau. This system strikes into the Jimma area, where it is well displayed in the Gojeb River valley (Fig. 3). Associated with it are major effusions of Quatemary basalt (see Fig. 6). FAULT D~SP~ACEMENTS

Fig. 4 shows the extensive exposure of the unconformity separating crystalline basement from Tertiary volcanic rocks. Throughout most of the region this surface is planar, with only a few me&es of local relief. It is mantled by 5-10 m of ferruginous grit, silicified at the top, which is taken to mark a single, welldeveloped erosion surface. This rock unit is missing on the southern parts of the Hamar Horst and the Boran uplift, and on the northwest part of the Surma Horst. It is also absent beneath the basalt%of the Makonnen and Gurraferda plateaux (Fig. 5). In these localities too, however, the basement surface is relatively regular and synvolcanic fault displacements do not exceed a few tens of metres. Basal lavas are everywhere within a few degrees of conformity with the erosional grit, and no large or persistent angular unconformities were observed within the plateau volcanics. It is therefore unlikely that large vertical movements predated or accomp~ied effusion of the major volcanic successions. The pre-Tertiary laterized surface is a datum permitting the determination of throw on many of the rift faults. The absence of map-scale displacement of steeply dipping basement rock units demonstrates that no major horizontal movements took place, at least on faults cutting basement structural grain. Absence of significant displacement of the unconformity at the reentrant junctions of faults in upthrown blocks further demonstrates that the serrated pattern of rift margins does not derive from lateral offsets of normal faults by Tashkent faults. Rather, the net d~placement vector along such boundaries must be steeply inclined. The cross-sections in Fig. 5 are in part schematic, but serve to depict the arrangement of fault blocks, their Tertiary volcanic cover, and the relative magnitudes of fault displacements. An absolute minimum estimate of subsidence for the Stefanie Rift of 1 km is given by the difference in elevation of the unconformity under correlative basalt outliers on the Hamar Horst close

Alluvium

Recent

volcanics

Pliocene,

Omo

Mid-Tertiary

i

.** id La++ .‘**I* .*

+

Permian

Group

volcanics

sediments ‘q\,!

Crystalline

basement 36*E

34.E

Fig. 4. Geology

of the

area

of Fig. 3 (Davidson

\‘,

J4’N 37PE

et al., 1973, 1976).

to the rift wall at 1700 m, and near the rift floor at 700 m. Actual displacement certainly exceeds 1650 m, the difference between the elevation of the highest peaks of the horst, formed in basement rocks (2300 m), and the alluvium of the adjacent rift floor. Basement is exposed in the north end of the Stefanie Rift floor, at the east edge of the Woito Horst, where displacement on the main boundary fault is more than 740 m. Displacements of faults in the Gofa basin-and-range province, where the Tertiary lams are tilted as much as 40”, are less certain; for example, the fault that bounds the Beto

Crydailme

basement

Sandstone (Permian- Crefacwusl

RUDOLF

Fig. 5. Geologic cross-sections through Fig. 4. Partly schematic; vertical exaggeration approximately Rift.

liEi f

Mmcene)

Early flood bosolls (Eocene- Lower Miocene)

tLower-Middle

bosolfs (MIddIe Mmcenel

bosolF lUpper Miocene1

Sohc volcan~cs

Ploieau

Rdjej

Quolernary

3

L

STEFANIE RIFT

threefold. M.E.R. = Main Ethiopian

GOFA BASIN AND RANGE ~RQVINCE

167

valley may have a throw as great as 4 km (see Fig. 5). The fault along the upper Mwi River (Fig. 3), one of the major breaks bounding the Omo rift branch, has vertical movement of at least 500 m; total subsidence must exceed 1 km. Correlative basalts in the floor of the Kibish Rift and on the Surma Horst demonstrate 850 m net displacement. The east-trending graben through the Gurraferda plateau underwent 400 m of subsidence along its north boundary fault. The basement reaches its highest elevation, 2750 m, in the Gemu Horst west of Lake Chamo, where it lies in symmetrical relation to basement in the Amaro Hors& exposed at about the same elevation on the other side of the Main Ethiopian Rift east of Lake Chamo (Levitte et al., 1974). Gneisses form the highest peak, 2739 m, south of the Ari highland (Fig. 3), and nearby the unconformity is preserved down-dip at 2500 m. The Stefanie graben, like the Main Ethiopian Rift, lies in the center of a basement arch; under the Konso upland another, gentle arch is indicated. West of the Lake Rudolf Rift, volcanic rocks of the Tishena-Chebera highland form a broad trough within a major horst bounded by antithetic fault systems (Fig. 5), and truncated to the southwest by the Surma Horst. Variation in the amount of displacement along the strike of individual faults and fault sets accounts for the disappearance of some major rift features. For example, at the southern end of the Main Ethiopian Rift the basement surface rises from below 800 m near Lake Chamo to 1700 m in the Konso upland. West of this rift, its elevation fullssouthwards from 2750 m to 1600 m, so that the net movement sense on the faults defining the western boundary reverses in a distance of some 30 km. Similar on-strike reversal is observed along the eastern boundary of the Surma Horst, but there with basement rising northwards along the east side of the fault. This kind of rotational movement about poles to fault planes appears to account in part for the southwestward termination of the Gofa basin-and-range province, and the northward attenuation of the Lake Rudolf Rift. At the west edge of the Makonnen plateau near 8”N, the basement surface stands at 2000 m; 30 km to the west on the Gok plain it has dropped to 600 m (Fig. 5). It is not certain whether the intervening north-trending escarpment is erosional or derived by faulting, because the basalt cover is not continuous between these two localities. However, the similarity in age of the two basalts (D.C. Rex, personal communication, 1976) and the lack of significant local relief on the basement at either locality suggest that the plateau margin is faulted, and therefore that the influence of rifting extends to the westernmost edge of the plateau at this latitude. VOLCANISM

AND THE AGE OF RIFTING

Fig. 6 summarizes the distribution of Tertiary and Quatemary volcanic units. Thick successions of alkali-olivine fissure basalt flows, reaching 3 km, occupy the Gemu, Ari and Maji highlands and the Boran uplift (Fig. 3).

168

_

Alluvium Recent hlursi

basalts basalt

Fedjej/Bulal

basalts

Gok/Makonnen Salic

volcanic

Early

flood

Central

rocks basalts

volcanic

complexes

Subsidiary

centres,

Phonolite

flows

Cones

34.E

basalt

plugs

or plugs

and calderas

36*E

37i’t

Fig. 6. Distribution of volcanic rocks in the area of Fig. 3. Arrows show dip direction of flow layering. Letters denote igneous complexes: B = Badda Ichini; D = Dime; I = Ilibai; J = Jimma; K = Korath Range; M = Mago; S = Shasha; 2’ = Tepi shield.

These are interspersed with large accumulations of pantelleritic rhyolite and trachyte, comprising flows, breccias, ignimbrites, and related shallow intrusions. The pantellerites are related to centers such as Badda Ichini, Mago, Dime, Shasha and Ilibai (Fig. 6). Although these successions include northstriking dike swarms, they are not spatially related to rifts, and are not superimposed on the basalts; rather, they are of comparable age. For example,

169

rhyolites south of Mount Shasha are separated from the basement by only a few metres of basalt and are intercalated with, and overlain by, thick basalts in turn capped by salic effusives from another center lying to the west. Potassium-argon dating of volcanic rocks by D.C. Rex, Leeds University, is in progress. Preliminary determinations suggest that olivine- and augitephyric basalts south of the Akobo River near the Sudan border are Mid-Eocene, and are among the oldest Ethiopian lavas dated by recent techniques. The basal flows of plateau successions are early to Mid-Oligocene, followed by Late Oligocene basalts of the Gurraferda and Makonnen plateaux and of the Gok plain. All of these successions were deposited on planar erosion surfaces before the onset of significant rifting. An angular unconformity of as much as lo”, however, may be present between the basalt that caps the Gurraferda plateau and the succession of intercalated basalts and trachytes in tilted blocks to the east. The Fedjej basalt (Fig. 6) conformably underlies tuffs dated at 16-17 Ma (Fitch et al., 1974); it mantles the south end of the Hamar Horst and lies on a less regular surface, with up to 20 m of sandstones beneath. It clearly predates, however, the major fault movements along the Stefanie Rift. The oldest dated unit demonstrably postdating major block faulting is the Mursi basalt of the Omo Group, extruded at 4 Ma, which covers an erosion surface developed on tilted Miocene rhyolites of the Nkalabong Horst, and is intercalated with sediments related to ancestral Lake Rudolf (Butzer, 1971). Nephelinic rocks, mainly in small phonolite intrusions, are found in the Boran uplift, east of the Woito Horst, on the Hamar Horst, and, chiefly, west of the Surma Horst near the Sudan border where nepheline-bearing lavas also occur. In the last locality, which lies north of important Mid-Tertiary alkalic centers in Uganda (King, 1965) and in northwest Kenya (Walsh and Dodson, 1969)) some of these intrusions are disposed along north-trending lineaments parallel to rift faults. These rocks are younger than the surrounding plateau basalts and pantellerites; east of the Woito Horst they are dated as Late Miocene, as are phonolites in northwest Kenya (Reilly et al., 1966). At least two major Quaternary basalt effusions are related to east-northeast-trending faults (Fig. 6): the Tepi shield and an analogous feature to the east-northeast near Jimma. These were evidently built from fissure eruptions and are surmounted by lines of cones and collapse craters. Quatemary basanites and tephrites of the Korath Range (Brown and Carmichael, 1969), and basaltic cones along the Stefanie and Ririba rifts (Fig. 6), are clearly related to rifting; the Korath lavas were extruded between the deposition of members III and IVA of the Kibish Formation (Butzer and Thurber, 1969). The Korath Range lies on the northward projection of the major rift fault bounding the Labur Range in Kenya. In summary, flood basalt effusion began in Mid-Eocene time, and thick, more centralized salic accumulations began probably in the Late Eocene, and certainly before the Late Oligocene. Although some tilting is evident by that time, major rifting did not predate the Late Miocene. The present pattern of

170

rifts and uplifts was well established by the Pliocene. There is evidence of at least three erosion surfaces: a widespread pre-Tertiary laterized surface, a pre-Mid-Oligocene bevelling under the basalts of the Gurraferda and probably the Makonnen plateaux, and a Mio-Pliocene surface postdating major block movements and antedating the Omo Group. Red elastic sediments, eroded to badlands, lie on roughly planar surfaces in the northern parts of the Hamar Horst, the Boran uplift and the Konso upland; these may reflect the Oligocene planation. Block movements continue to the present time. Although the Pleistocene Kibish Formation was thought to postdate all rifting (Butzer and Thurber, 1969), beaches of ancestral Lake Rudolf formed on these sediments (Kibish IV) are displaced by Noah-striking faults downdropp~ to the east. Along the east boundary of the Woito Horst, the Woito River has, probably within the last twenty years, been shifted out of its meandering course by westward tilting of the graben floor, and now runs a straight course along the boundary fault. REGIONAL RELATIONSH~S

In southern Ethiopia the East African rift zone occupies a width of at least 300 km and includes three major rift valleys and a basin-and-range terrain (Fig, 7). The Stefanie Rift is the projection into Ethiopia of the Gregory Rift (Baker et al., 1972, p. 29). The Lake Rudolf Rift branches from the rift system in Kenya near the south end of that lake. The en &chelon arrangement of the component rifts does not result from transcurrent movements; the “Sagan offset” suggested by Mohr (Baker et al., 1972, p. 31) is illusory. Rifts terminate where they meet transecting normal fault systems, or by virtue of diminishing movement on the component faults. The Gofa basinand-range province has a similar pattern, in miniature, to the whole rift system in central Tanzania (Fig. 1). It attenuates to the southwest, and northeastwards becomes tangential to the west boundary of the Main Ethiopian Rift. It is interesting to note that the entire north-south rift system is relatively obscure north of the east-northeast fault set which commences west of the Tepi shield and passes south of Addis Ababa to merge with the southern boundary of the Afar Triangle. Projection of the west and south edges of the Triangle divides Ethiopia into four domains, of which the northeast and southwest domains are characterized by north-south normal faults; the remaining two are relatively free of rifting. It may be asked what relationship exists between the present rift pattern, deeper crustal structure and mantle structure. Although structural detail in the basement rocks is not everywhere reflected in fault patterns, there is locally a strong tendency for rift faults to be either parallel or normal to planar structures in the gneisses; for example, the fault pattern on the Surma Horst is closely parallel to the foliation trends and to concordant mylonite zones. The arcuate pattern of the Gofa basin-and-range terrain and the ser-

4(

Fig. 7. Fig. 2 with fault pattern in the Project area added.

rated edge of the Stefanie Rift faithfully reflect variation in the normals to trends of linear or planar structures, The overall northerly and northeasterly trends of the principal rifts, however, are not consistent with any clearly developed shallow structure and must therefore reflect deeper control. The chronology of volc~o-~ctonic activity and quiescence suggested by

172

preliminary age data is consistent with that indicated for central and northern Ethiopia by Mohr and others (Baker et al., 1972) except that both basaltic and rhyolitic eruptions commenced earlier than to the north. The early effusions also antedate lavas to the south in Kenya, dated between 32 and 12 Ma (Reilly et al., 1966). Additionally, there is major overlap in time of mafic and salic volcanism in southern Ethiopia. The early pantelleritic rocks have largely escaped attention, as effort has been concentrated on PlioPleistocene central volcanoes of the Main Ethiopian Rift. It appears that the Gurraferda basalts correspond more closely in age to the central Ethiopian “Trap Series” than do the earlier plateau basalts. Fissure eruptions of basalt, and more centralized salic effusions, must have been produced side by side and localized by broad depressions and mutual interference. Arching and rifting followed, probably as concurrent processes. The amount of extension involved need not have been great, and the continuity of continental crust remained essentially unbroken. Relatively little magma was generated once rifting had begun. Although the concept of the East African rift system as a “failed arm” of the Afar Triple Junction may well be valid, the relations between magma genesis, uplift and faulting require to be better understood before a satisfactory tectonic synthesis for the region can be achieved. ACKNOWLEDGEMENTS

The data from which this paper is drawn derive from the efforts of a team comprising Alemu Shiferaw, John C. Davies, Aberra Degeffu, Alemayehu Guyassa, Alemayehu Wolde Rufael, Muluneh Gelatta, Eyob Gebre Leul, Mengesha Teffera, Negist Hintsa and the authors. We are indebted to the Geological Survey of Ethiopia and to the Canadian International Development Agency for support of the Omo River Project, and to the Geological Survey of Canada for supporting facilities in Canada. Lesley Anne Whitmore assisted with the draughting of the illustrations. REFERENCES Baker, B.H., Mohr, P.A. and Williams, L.A.J., 1972. Geology of the Eastern Rift System of Africa. Geol. Sot. Am., Spec. Pap., 136: 67 pp. Brown, F.H. and Carmichael, I.S.E., 1969. Quaternary volcanoes of the Lake Rudolf region, I. The Korath Range. Lithos, 2: 239-260. Butzer, K.W., 1971. Recent history of an Ethiopian delta. Univ. Chicago, Dep. Geogr. Res. Pap., 136: 184 pp. Butzer, K.W. and Thurber, D.L., 1969. Some Late Cenozoic sedimentary formations of the Lower OmoBasin. Nature, 222: 1138-1143. Davidson, A., Alemu Shiferaw, Eyob G. Leul, Davies, J.C., Moore, J.M., Aberra Degeffu, Alemayehu Guyassa, Alemayehu W. Rufael and Muluneh Gelatta, 1973 . Preliminary report on the geology and geochemistry of parts of Sidamo, Gemu Gofa, and Kefa Provinces, Ethiopia. Imp. Ethiop. Gov., Ministry of Mines, Omo River Project, Prelim. Rep., 1: 21 pp.

173 Davidson, A., Alemu Shiferaw, Davies, J.C., Moore, J.M., Mengesha Teferra, Aberra Degeffu, Alemayehu W. Rufael, Muluneh Gelatta and Negist Hintsa, 1976. Preliminary report on the geology and geochemistry of parts of Gemu Gofa, Kefa, and Ilubabor Provinces, Ethiopia. Imp. Ethiop. Gov., Ministry of Mines and Power, Omo River Project, Prelim. Rep., 2: 28 pp. De Heinzelin, J., Brown, F.J. and Howell, F.C., 1971. Pliocene/Pleistocene formations in the Lower Omo Basin, southern Ethiopia. Quaternaria, 13: 247-268. Falcon, N.L., Gass, I.G., Girdler, R.W. and Laughton, A.S. (Editors), 1970. A discussion on the structure and evolution of the Red Sea, Gulf of Aden and Ethiopia Rift junction. Phil. Trans. R. Sot. London, Ser. A, 267: l-417. Fitch, F.J., Findlater, I.C., Watkins, R.T. and Miller, J.A., 1974. Dating of the rock succession containing fossil hominids at East Rudolf, Kenya. Nature, 251: 213-215. Gregory, J.W., 1921. The Rift Valleys and Geology of East Africa. Seeley, Service, London, 479 pp. King, B.C., 1965. Petrogenesis of the alkaline igneous rock suites of the volcanic and intrusive centres of eastern Uganda. J. Petrol., 6: 67-100. Levitte, D., Columba, J. and Mohr, P., 1974. Reconnaissance geology of the Amaro Horst, southern Ethiopian Rift. Geol. Sot. Am. Bull., 85: 417-422. Mohr, P.A., 1974. Mapping of the major structures of the African Rift System. Smithson. Astrophys. Obs., Spec. Rep., 361: 70 pp. Mohr, P.A., Mapping of the major structures of the African Rift System. Smithson. Astrophys. Obs., Spec. Rep., 361: 70 pp. Pilger, A. and Riisler, A. (Editors), 1975. Afar Depression of Ethiopia. Inter-Union Comm. Geodynamics Sci. Rep., 14 - Schweizerbart, Stuttgart, 416 pp. Reilly, T.A., Mussett, A.E., Raja, P.K.S., Grasty, R.L. and Walsh, J., 1966. Age and polarity of the Turkana lavas, northwest Kenya. Nature, 210: 1145-1146. Walsh, J. and Dodson, R.G., 1969. Geology of northern Turkana. Geol. Surv. Kenya, Rep., 82: 42 pp.