Proceedings ARGeo 2 conference Entebbe, Uganda, 24-26 November 2008 Geology and Geothermal Resource Utilization Options in the Arus- Lake Bogoria Prospect, Northern Kenya Rift John Lagat, Kenya Electricity Generating Company Ltd, Olkaria Geothermal Project, P. O. Box 785, Naivasha 20117, Kenya [email protected] ABSTRACT area was carried to determine the potential and propose how best the resource could be Arus-Lake Bogoria geothermal prospect is utilized. located in the northern sector of the Kenyan Ethiopia Rift. Geothermal phenomena in form of a d n a a g i l U fumaroles, hot springs, steam jets, geysers, a KENYA L. Turkana m o sulphur deposits and high geothermal gradient S Nairobi expressed by anomalously hot ground and anomalous groundwater manifest the area. Tanzania Mombasa Detailed surface exploration surveys carried Barrier out in the prospect indicate presence of low to Namarunu 2oN intermediate temperature fracture controlled Emuruangogolak geothermal system(s) in the area. Silali Paka These low to medium enthalpy resources can Korosi be utilized in direct uses and electric power L. Baringo generation using binary-cycle power plants. L. Bogoria 0o The direct use applications that can be realized Nyanza Rift Menengai L. Nakuru in the area include tourism, spa pools, L. Elmenteita Eburru greenhouse heating, agricultural produce L. Naivasha Olkaria volcanic Longonot drying and industrial processes. The direct use complex application currently available in area is Suswa LEGEND tourism and a spa pool at one of the tourist Central volcanoes Lakes L. Magadi hotel. Advances in binary-cycle power and 2oS Faults submersible pump technologies have made L. Natron electric power generation in low enthalpy Project area 0 km 50 100 Ol'Doinyo Lengai o areas commercial. 36 E 38oE Figure 1: Map of the Kenya rift Keywords: Kenya Rift, Geology, Geothermal, showing the showing the location of and Utilization. the Arus-Lake Bogoria prospect. 1. INTRODUCTION 2. OBJECTIVES OF THE STUDY Arus-Lake Bogoria prospect is located within the Kenya Rift valley immediately south of The main objectives of the Arus-Lake Lake Baringo prospect and north of Menengai Bogoria geothermal surface exploration prospect (Figure 1). Geothermal phenomena project were to: namely fumaroles, hot springs, steam jets, geysers, sulphur deposits and altered grounds Study volcanological evolution of the manifest the area. They occur mainly at the area with a view to understanding the shores of Lake Bogoria and at Arus area. possible heat sources for the Detailed surface exploration in the prospect geothermal system(s). 1 Lagat J. K. Study structures essential for occurrence continental divergent zone, a zone where of the geothermal system(s). spreading occurs resulting to the thinning Determine the size and extent of the of the crust hence eruption of lavas and geothermal system, heat source and associated volcanic activities. hydrogeological controls of the system(s). Determine whether the prospect is suitable The development of the Kenya Rift Valley for further exploration and if so, started during early Miocene (14-23 Ma recommend site for exploration by BP) with volcanism in Turkana followed drilling. by activity southwards. The faulting that Recommend how best the geothermal accompanied rifting occurred in several resource can be utilized. stages starting with faulting on the western side accompanied by basaltic and 3. PREVIOUS WORK phonolitic volcanism on the crust of uplift (Baker et al. 1987). Major faulting on both The first detailed geological mapping in this the eastern and the western flanks was sector of the Kenya rift was done in the late accompanied by volcanic eruptions that 1960’s by the Kenya Geological Survey. resulted in the formation the graben McCall (1967) described the geology of the structure about 5 million BP. This was Nakuru-Lake Hannington areas while Walsh followed by fissure eruptions in the axis of (1969) and Jennings (1971) described the the rift to form flood lavas. geology of the Eldama Ravine-Kabarnet area. In their work they gave detailed description of During the last 2 million years, volcanic the volcanic rocks and the sediments and were activities become more intense within the able to draw up a fairly detailed history of the axis of the rift. During this time, large tectonism. They also gave a detailed shield volcanoes, most of which are description of the surface manifestations found geothermal prospects, developed in the axis in the area between Arus and Lake Bogoria. of the rift. These Quaternary to Recent Glover (1972) colleted a large number of volcanoes include Suswa, Longonot, water and steam samples from Lake Bogoria Olkaria, Eburru, Menengai, Korosi, Paka, area and concluded from his analytical data Silali, Emuruangogolak, and Barrier that a relatively large and homogeneous body Complex. Other geothermal prospects, of of warm water lies under much of the area. which Arus and Lake Bogoria prospects Griffiths (1977) in his dissertation did a are, occur between these central volcanoes comparative trace element study of the (Omenda et al., 2001). volcanic products in the area 3.1 Surface Geology of Arus Lake Geological work in the area with bias in Bogoria geothermal exploration was carried out by Geotermica Italiana Srl (1987) in their The upper Plio-Pleistocene volcanism of Menengai-Lake Bogoria reconnaissance the rift floor in the area between Arus and report, but dwelt mainly on Menengai caldera Lake Bogoria is characterized by large and did not discuss geological and structural volumes of evolved lavas that consist set-up for development of the extensive mostly of peralkaline trachyte, anomalous geothermal manifestations in the trachyphonolite and phonolite. Small Arus Lake Bogoria areas. outcrops of basaltic lavas occur in isolated areas within the prospect. The northern 4. GEOLOGY sector is, however, dominated by fluvial and alluvial deposits. The geological map 4.1 Regional Geological Setup showing all the outcrops encountered in the The Kenya Rift (Figure 1). is part eastern arm area is presented in Figure 3. of the Great East African system that extends from the gulf of Eden in the north to Beira, Mozambique in the south. It is part of a 2 Lagat J. K. trending N-S are fractures with a broadly meridional trend interact with the NNE trending faults in the eastern rift shoulder, but are the dominant structural elements of the rift floor, notably the grid-faulted platform west of Lake Bogoria and the fissure system. The same structures are aligned to the vents and the cinder cones of Korosi and Paka volcanoes further north of the prospect area. Figure 2: The geological map of the Arus-Lake Bogoria geothermal prospect 4.2 Structures in the Arus-Lake Bogoria prospect The main structural features in the Arus and Lake Bogoria areas include; the eastern rift Figure 3: The structural map of the flank, the rift proper, NW, NNE and N-S Arus-Lake Bogoria prospect. trending faults and fractures and the Marigat and Loboi lineaments (Figure 3). Marigat lineament detected on special thematic mapping Landsat imagery is one The most prominent of the NW trending faults of the minor swarm, which affects the is the line of Sattima-Aberdares and Marmanet Loboi Silts deposited between Lake Faults. Its complement to the north Baringo and Lake Bogoria. The NW comprising the Lariak-North Arabel and other trending faults, which comprise the shorter minor faults forming a belt of Sattima-Aberdares, and Marmanet Faults discontinuous fractures. Progressively coincide with the major NW shear zones, towards the northwest, both fault zones display which are considered to reflect reactivation an en echelon displacement to the west. The of the Precambrian Mozambiquan belt NNE trending faults form a dense swarm of 8- formation. The important structures are the 10 km, which correspond, generally to much Ol Arabel and Loboi/Marmanet lineaments of the eastern shoulder of the rift. In the north (Figure 3). The interaction between these the margin of these NNE trending is clearly faults and the NNE ones has created an demarcated by Laikipia Monocline clearly intensely sheared/fractured zone (Box fault defined by dip variations and the Loriki systems according to Hackman, 1988) to Escarpment (Figure 3). To the south the northeast of Lake Bogoria at the interaction with faults trending N-S and NW- Laikipia Escarpment. The shearing has SE enhances the composite effect of sigmoidal resulted some of the Lake Bogoria elements, which contribute to the flexure in phonolites develop fissility (slaty) texture, the rift structure south of Lake Bogoria. Faults which trend in the same direction. 3 Lagat J. K. Recent faults affecting Loiminangge beds every few minutes (Figure 5). There are south east of Lake Baringo, slightly northeast other numerous spouting hot springs in of the prospect area where west facing scarps more or less continuous activity at up to 6 m high displace pre-existing piedmont Kwaibeipei and a few kilometers to the fans can be recognized. The Marigat north occurring in a recognizable linear lineament is also a very recent structure arrangement along fault lines. Most of the affecting the alluvial plains between Lake occurrences are characterized by bare Baringo and Lake Bogoria (Loboi plains, ground (scarce vegetation). suggest rejuvenation of buried Proterozoic structures in response to neotectonic regime. Warm springs are common around Lake Continuation of tectonic activity in the Bogoria and within the prospect. The most prospect area has been demonstrated by the important ones include Emsos, Ndolaita 28th January 1928 (Tillotson, 1938), and Loboi (Figure 5). The hot springs at Magnitude 7.1 earthquake with its epicenter at Emsos are associated with the Emsos fault Lariak forest at the foot of Laikipia where they discharge at 37°C. The Ndolaita Escarpment (Figure 4). An elongate about 28 springs occur to the west of Lake Bogoria km curvilinear fissure appeared at the foot of at Maji Moto where they bubble up in the the Laikipia Escarpment along the line of rocky bed of the Ndolaita River at Maji Marmanet Fault and at the Lake Bogoria Moto.