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Renaut R.W.,University of Saskatchewan | Owen R.B.,Hong Kong Baptist University | Jones B.,University of Alberta | Tiercelin J.-J.,CNRS Geosciences Laboratory of Rennes | And 3 more authors.
Sedimentology | Year: 2013

Travertine is present at 20% of the ca 60 hot springs that discharge on Loburu delta plain on the western margin of saline, alkaline Lake Bogoria in the Kenya Rift. Much of the travertine, which forms mounds, low terraces and pool-rim dams, is sub-fossil (relict) and undergoing erosion, but calcite-encrusted artefacts show that carbonate is actively precipitating at several springs. Most of the springs discharge alkaline (pH: 8·3 to 8·9), Na-HCO3 waters containing little Ca (<2mgl-1) at temperatures of 94 to 97·5°C. These travertines are unusual because most probably precipitated at temperatures of >80°C. The travertines are composed mainly of dendritic and platy calcite, with minor Mg-silicates, aragonite, fluorite and opaline silica. Calcite precipitation is attributed mainly to rapid CO2 degassing, which led to high-disequilibrium crystal morphologies. Stratigraphic evidence shows that the travertine formed during several stages separated by intervals of non-deposition. Radiometric ages imply that the main phase of travertine formation occurred during the late Pleistocene (ca 32 to 35ka). Periods of precipitation were influenced strongly by fluctuations in lake level, mostly under climate control, and by related changes in the depth of boiling. During relatively arid phases, meteoric recharge of ground water declines, the lake is low and becomes hypersaline, and the reduced hydrostatic pressure lowers the level of boiling in the plumbing system of the hot springs. Any carbonate precipitation then occurs below the land surface. During humid phases, the dilute meteoric recharge increases, enhancing geothermal circulation, but the rising lake waters, which become relatively dilute, flood most spring vents. Much of the aqueous Ca2+ then precipitates as lacustrine stromatolites on shallow firm substrates, including submerged older travertines. Optimal conditions for subaerial travertine precipitation at Loburu occur when the lake is at intermediate levels, and may be favoured during transitions from humid to drier conditions. © 2012 The Authors. Journal compilation © 2012 International Association of Sedimentologists. Source

Tiercelin J.-J.,French National Center for Scientific Research | Potdevin J.-L.,Lille University of Science and Technology | Thuo P.K.,National Oil Corporation of Kenya | Thuo P.K.,CNRS Oceanic Domains Laboratory | And 9 more authors.
Journal of African Earth Sciences | Year: 2012

The northern Turkana region of northwestern Kenya forms the intersection between two major rift systems in Africa, the Cretaceous-Paleogene Central African Rift System (CARS), and the eastern arm of the Paleogene-Present East African Rift System (EARS). The southern Sudanese oil-rich rift basins form part of the CARS, and their extension into the Anza Rift in northeastern Kenya makes the area of northern Turkana an important target for oil exploration. Limited past exploration activity in the area leaves the study of surface outcrops as the main avenue for understanding the reservoir potential of the fluvial deposits of these rift systems. The outcrops of these potential reservoirs, collectively referred to as " Turkana Grits" in the past, are represented on the western side of Lake Turkana by the Lapur Sandstone in the north, and by other grit formations in the central and southern parts of the basin. Isotopic age determinations on the basal parts of the " Turkana Volcanics" that overlie the Lapur Sandstone have enabled the precise dating of the upper parts of the LS at between 35 and 37. Ma, while the lower part of the formation near the contact with the underlying Precambrian basement is estimated as Upper Cretaceous (Turonian-early Campanian), based on the discovery of dinosaur and other reptilian fauna. Detailed lithological logging, coupled with subsequent petrographic and sedimentological studies, have enabled the determination of the depositional environments and the diagenetic evolution of the Lapur Sandstone. The basal and uppermost parts of the formation are interpreted as distal alluvial fan environments possibly connected to the last stages of rifting characterizing the Central African Rift System. The middle part of the Lapur Sandstone corresponds to a wide braided fluvial system that can be compared to fluvial episodes of Late Cretaceous age in the Sudan region, associated to major palaeogeographical changes in northern Africa. The nearly abrupt disappearance of the Lapur upper fan system relates to the deposition of the " Turkana Volcanics" from Late Eocene, possibly as a consequence of the emplacement of the Afar Plume at 45-35. Ma. In terms of diagenesis, the main cement material at the base of the Lapur Sandstone is calcite, whereas at the middle of the formation, hematite becomes the dominant cement, and at the topmost section, kaolin cement dominates. The diagenetic evolution of the sandstones has been favourable to the retention of adequate primary intergranular porosity and the creation of secondary intragranular dissolution porosity, mainly through feldspar dissolution, and thus preserving the reservoir potential of the Lapur Sandstone. The reservoir characteristics, such as the porosity and cementation style, of the Lapur Sandstone are comparable to those of the fluvial sandstone reservoirs of the southern Sudan oil fields and this should positively contribute to the overall petroleum potential of the northern Turkana region. Though the northern Turkana area has remained largely unexplored, it is hoped that the demonstration of the presence of reasonably good reservoir quality sandstones in the Lapur Sandstone will serve to encourage further interest in hydrocarbon exploration in the Turkana area. © 2012 Elsevier Ltd. Source

Tiercelin J.-J.,CNRS Geosciences Laboratory of Rennes | Nalpas T.,CNRS Geosciences Laboratory of Rennes | Thuo P.,National Oil Corporation of Kenya | Potdevin J.-L.,Lille University of Science and Technology
AAPG Memoir | Year: 2012

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 Cenozoic 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 sandstones 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. ©2012 by The American Association of Petroleum Geologists. Source

Cohen A.,University of Arizona | Campisano C.,Arizona State University | Arrowsmith R.,Arizona State University | Asrat A.,Addis Ababa Institute of Technology | And 59 more authors.
Scientific Drilling | Year: 2016

The role that climate and environmental history may have played in influencing human evolution has been the focus of considerable interest and controversy among paleoanthropologists for decades. Prior attempts to understand the environmental history side of this equation have centered around the study of outcrop sediments and fossils adjacent to where fossil hominins (ancestors or close relatives of modern humans) are found, or from the study of deep sea drill cores. However, outcrop sediments are often highly weathered and thus are unsuitable for some types of paleoclimatic records, and deep sea core records come from long distances away from the actual fossil and stone tool remains. The Hominin Sites and Paleolakes Drilling Project (HSPDP) was developed to address these issues. The project has focused its efforts on the eastern African Rift Valley, where much of the evidence for early hominins has been recovered.We have collected about 2 km of sediment drill core from six basins in Kenya and Ethiopia, in lake deposits immediately adjacent to important fossil hominin and archaeological sites. Collectively these cores cover in time many of the key transitions and critical intervals in human evolutionary history over the last 4 Ma, such as the earliest stone tools, the origin of our own genus Homo, and the earliest anatomically modern Homo sapiens. Here we document the initial field, physical property, and core description results of the 2012-2014 HSPDP coring campaign. © Author(s) 2016. Source

Long A.J.,Geosoft Africa Pty. Ltd. | Njuguna F.M.,National Oil Corporation of Kenya | Wanjala E.,National Oil Corporation of Kenya
73rd European Association of Geoscientists and Engineers Conference and Exhibition 2011: Unconventional Resources and the Role of Technology. Incorporating SPE EUROPEC 2011 | Year: 2011

The Lotikipi Basin is a rift basin lying within the East African Rift System, west of Lake Turkana in north west Kenya. The basin is filled with a 4km sequence of volcanic derived sediment, stacked rhyolite flows and olivine basalts, underlain by supposed reservoir potential sediments at least 500m thick. 2D Seismic imaging is observed to be degraded below the shallow volcanic sequences. Basalts are strongly absorbent of p-wave energy, so act as high impedance barriers to the seismic energy. Seismic reflectors below the basalt are therefore poorly defined. Consideration is given to gravity processing and quality control methods that can be used to review data when its description is incomplete. Other data sources and previous gravity interpretation can be employed to define a gravity model that fits the seismic interpretation and other qualitative information. Gravity modelling enables better constraint on the interpretation through thick intrusive shadow zones (using known seismic facies) and provides better estimation of the thickness of the basin's deep sediment, as well as basinal depth to Precambrian basement. Source

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