Abstract The northern (NKR) and central (CKR) segments of the Kenya Rift are among the most important areas of the East African rift system for hydrocarbon prospecting because they offer the oldest and longest lived sedimentary basins and they are a crossover area between Ce-nozoic and Cretaceous rifts. During the 1970s and 1980s, the interest of oil companies focused in the Turkana depression and the northeastern region of Kenya. Seismic reflection surveys and several exploration wells enabled the identification of several deeply buried basins: (1) In the NKR, three strings of north–south-oriented half grabens, the oldest known basins being of Cretaceous?–Paleogene to middle Miocene age; (2) In the CKR, two north–south half grabens, the Baringo-Bogoria Basin (Paleogene–Present), and the Kerio Basin (Paleogene–upper Miocene). All basins are filled by up to 8 km (5 mi) thick sediments of alluvial, fluviodeltaic, or lacustrine origin and volcanics of late Eocene to Neogene age. New studies have focused on reservoir and/or source rock quality in several of these basins. In terms of hydrocarbon potential, arkosic sand stones in CKR or NKR demonstrate a fair to good reservoir quality, with porosity up to 25%. Strong changes in terms of diagenetic alteration relate to deformation events or change in sediment source as a result of tectonic activity and hydrothermal fluid circulation associated with volcanism. High-quality source rocks were deposited in freshwater lake environments under a tropical climate. Such environments have been identified during the Paleogene in the NKR and lower Neogene in the CKR. The combination of reservoir and source rock characteristics results in a provisional classification of each studied basin, in terms of very high to medium potential for hydrocarbons.

Abstract A significant climate shift around 9.6 Ma has been detected from the Middle to Upper Miocene Aka Aiteputh and Namurungule Formations exposed in the Samburu Hills, northern Kenya. Around 9.6 Ma, changes in sediments are recorded from the red soil-dominated interval of the upper Aka Aiteputh Formation to the lacustrine and deltaic facies of the lower Namurungule Formation, containing open woodland/savanna mammalian fauna. These reveal a shift from a dry climate with seasonal precipitation to a climate with strong seasonality. In particular, an increase in precipitation was recorded by the predominance of lacustrine facies. This shift happened at around the same time as the intensification of the Indian summer monsoon that has been detected in the Himalayas and some of surrounding regions. There are two scenarios that could explain the increased precipitation at the beginning of the deposition of the Namurungule Formation: (1) enhanced moisture transport by the Intertropical Convergence Zone (ITCZ), synchronized with Indian summer monsoon intensification, or (2) intensification of the Indian summer monsoon itself, permitting moisture to penetrate deep into East Africa if the altitude of the rifted area was lower than it is now. Presently, the former is considered to be the more plausible explanation for the climate shift detected in the Samburu Hills.

Abstract The Ngakoringora Ridge is a large (300 m long), linear mound of limestone, dolomitized limestone, and chert that rises abruptly from the desert floor on the southwestern edge of the Lothidok Hills, west of Lake Turkana, Kenya. The origin of the ridge has been controversial. It has previously been considered either a hot-spring deposit or an uplifted holier of pre-Cenozoic marine carbonates. Interpretation has been hampered by the extensive diagenetic alteration of the rocks and the lack of identifiable fossils. A preliminary examination of the ridge morphology and the facies and fabrics of the rocks confirms a hydrothermal origin. The carbonates contain radial calcite fans, micrite-microsparite laminae, stromatolites, coated grains, peloids, intraclasts, Mn-shrubs, and other fabrics that characterize hot-spring travertines. Many of the carbonates have been partly silicified and dolomitized. The ridge is interpreted to be a fissure-ridge travertine, precipitated from thermal waters that discharged along a medial fissure. Fluids flowed laterally from five main mounds that were probably active at different times. Crystal fabrics along the ridge crest are compatible with abiotic precipitation from alkaline spring waters undergoing rapid degassing of CO 2 . Microbially influenced fabrics, including stromatolites, become more common distally. Silicified plants, filamentous microbial mats, and thin chert beds are locally present in distal slope settings. Plant silicification and chert formation may have taken place in shallow terrace pools from spring fluids undergoing cooling and evaporation. The Ngakoringora Ridge formed after the faulting and tilting that formed the Lothidok Hills, but its age is difficult to constrain. Silicification and dolomitization of the carbonates resulted from contact with hydrothermal fluids, and possibly from circulating ground water or lake water after deposition. Hydrothermal activity in rifts migrates with the evolving structural configuration. In the Kenya Rift, this is evident as a migration of hydrothermal activity toward the rift axis. Fossil spring deposits can provide much useful paleoenvironmental information even though they are of small lateral extent.