Research Repository :: Browsing by Author "Mbede, Evelyne I."

Browsing by Author "Mbede, Evelyne I."

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The ~4-ka Rungwe Pumice (South-Western Tanzania): A wind-Still Plinian Eruption
(2011-05) Fontijn, Karen; Ernst, Gerald G. J.; Bonadonna, Costanza; Elburg, Marlina A.; Mbede, Evelyne I.; Jacobs, Patric
The ~4-ka trachytic Rungwe Pumice (RP) deposit from Rungwe Volcano in South-Western Tanzania is the first Plinian-style deposit from an African volcano to be closely documented focusing on its physical characterization. The RP is a mostly massive fall deposit with an inversely graded base. Empirical models suggest a maximum eruption column height HT of 30.5–35 km with an associated peak mass discharge rate of 2.8–4.8 × 108 kg/s. Analytical calculations result in HT values of 33 ± 4 km (inversion of TEPHRA2 model on grain size data) corresponding to mass discharge ranging from 2.3 to 6.0 × 108 kg/s. Lake-core data allow extrapolation of the deposit thinning trend far beyond onland exposures. Empirical fitting of thickness data yields volume estimates between 3.2 and 5.8 km3 (corresponding to an erupted mass of 1.1–2.0 × 1012 kg), whereas analytical derivation yields an erupted mass of 1.1 × 1012 kg (inversion of TEPHRA2 model). Modelling and dispersal maps are consistent with nearly no-wind conditions during the eruption. The plume corner is estimated to have been ca. 11–12 km from the vent. After an opening phase with gradually increasing intensity, a high discharge rate was maintained throughout the eruption, without fountain collapse as is evidenced by a lack of pyroclastic density current deposits.
The Coastal Basins of Somalia, Kenya and Tanzania
(2007-09) Mbede, Evelyne I.; Dualeh, A.
This chapter describes the coastal basins of Somalia, Kenya, and Tanzania. The western Indian Ocean seaboard is an Atlantic type of continental margin; hence, the sedimentary basins involved are typical pull-apart basins of Klemme. However, the margin has been subjected to transform movements through late Jurassic to late Cretaceous times when Madagascar was moving southward relative to Africa. The margin seems to have been an emergent and stable block during the whole of Palaeozoic time. Sedimentation starts on top of a peneplaned Precambrian basement surface made up of highly metamorphosed rocks. The first epeirogenic movement to affect the region formed a series of intersecting basins separated by structural highs. The latter include the Bur-Acaba uplift of southern Somalia and the Nogal and Hargesya-Ergavo uplifts of northern Somalia. The existence of thick Pre-Jurassic sediments in the axial part of Luug-Mandera Basin is indicated by the discrepancy between section inferred from geophysical data and those measured in the penetrated sections.
Correlation of Nonmarine Cretaceous Strata of Africa and the Middle East
(2004-11) Mateer, Niall J.; Wycisk, Peter; Jacobs, Louis L.; Brunet, Michel; Luger, Peter; Arush, Mohammed A.; Hendriks, Frits; Weissbrod, Tuvia; Gvirtzman, Gedalishu; Mbede, Evelyne I.; Dina, Alphonse; Moody, Richard T.J.; Weigelt, Gunter; El-Nakhal, Hamed A.; Hell, Joseph; Stets, Johannes
Nonmarine Cretaceous rocks of mainly Early to mid-Cretaceous age are found widely scattered throughout the African continent, including Madagascar and the Middle East. Correlation of these rocks between regions has been attempted in the northern part of Africa, but the less frequent outcrops south of the Sahara are poorly understood and correlations are very tentative. It is premature to attempt a continent-wide correlation scheme, but inter-regional correlations are presented to understand better the nonmarine Cretaceous throughout Africa and the Middle East. The Saharan region is dominated by nonmarine “Continentale Intercalaire” and “Nubian Sandstone” clastics of Early to mid-Cretaceous age, and in Egypt, locally Late Cretaceous. The rocks labelled “Nubian Sandstone” can be correlated into the Arabian Peninsular, where it was deposited around the Arabian craton prior to inundation by Tethyan transgressions. The opening of the South Atlantic invoked tectonic stresses forming pull-apart basins in West Africa, and exploited the Pan-African shear zone cutting central Africa forming basins from Nigeria to southern Sudan. Intracratonic basins in northern Sudan and Egypt resulted from tectonic stresses associated with the opening of the Red Sea and the northward convergence of Africa into the European continent. Nonmarine Cretaceous deposits in central and inland south-eastern Africa outcrop sporadically and are associated with regional extensional tectonics related to the separation of Madagascar. Coastal basins of Mozambique and South Africa containing marginal nonmarine Cretaceous facies appear to be associated with the separation of Madagascar and the Falkland Plateau from Africa. The ages of nonmarine Cretaceous strata in Africa and the Middle East are often imprecisely known, although recent palynostratigraphic results have improved precision. Vertebrate and megaplant fossils are found throughout the continent, but their biostratigraphic value is rather general. Nonmarine sedimentation in Africa and the Middle East is predominantly pre-Cenomanian, with some Upper Cretaceous rocks occurring in Egypt, Somalia, Yemen, Nigeria and Madagascar during regressive episodes.
Fundamental Changes in the Activity of the Natrocarbonatite Volcano Oldoinyo Lengai, Tanzania
(2010-05) Kervyn, Matthieu; Ernst, Gerald G. J.; Keller, Jörg; Vaughan, R. Greg; Klaudius, Jurgis; Pradal, Evelyne; Belton, F.; Mattsson, Hannes B.; Mbede, Evelyne I.; Jacobs, Patric
On September 4, 2007, after 25 years of effusive natrocarbonatite eruptions, the eruptive activity of Oldoinyo Lengai (OL), N Tanzania, changed abruptly to episodic explosive eruptions. This transition was preceded by a voluminous lava eruption in March 2006, a year of quiescence, resumption of natrocarbonatite eruptions in June 2007, and a volcano-tectonic earthquake swarm in July 2007. Despite the lack of ground-based monitoring, the evolution in OL eruption dynamics is documented based on the available field observations, ASTER and MODIS satellite images, and almost-daily photos provided by local pilots. Satellite data enabled identification of a phase of voluminous lava effusion in the 2 weeks prior to the onset of explosive eruptions. After the onset, the activity varied from 100 m high ash jets to 2–15 km high violent, steady or unsteady, eruption columns dispersing ash to 100 km distance. The explosive eruptions built up a ∼400 m wide, ∼75 m high intra-crater pyroclastic cone. Time series data for eruption column height show distinct peaks at the end of September 2007 and February 2008, the latter being associated with the first pyroclastic flows to be documented at OL. Chemical analyses of the erupted products, presented in a companion paper (Keller et al. 2010), show that the 2007–2008 explosive eruptions are associated with an undersaturated carbonated silicate melt. This new phase of explosive eruptions provides constraints on the factors causing the transition from natrocarbonatite effusive eruptions to explosive eruptions of carbonated nephelinite magma, observed repetitively in the last 100 years at OL.
Holocene Explosive Eruptions in the Rungwe Volcanic Province, Tanzania
(2010-07) Fontijn, Karen; Ernst, Gerald G. J.; Elburg, Marlina A.; Williamson, David; Abdallah, Edista; Kwelwa, Shimba; Mbede, Evelyne I.; Jacobs, Patric
The Holocene explosive eruptive record of Rungwe and Ngozi volcanoes of the Rungwe Volcanic Province in Tanzania was reconstructed based on detailed stratigraphic field evidence combined with whole-rock major and trace element analyses of tephra samples. This reconstruction is supported by 25 new radiocarbon dates on palaeosols that provide additional constraints on the Holocene tephro-chronostratigraphy. We show evidence of two catastrophic Ngozi eruptions and five Rungwe pumice fallout deposits, and also identify several more intercalated poorly preserved pumice and ash deposits. The Ngozi eruptions probably played a role in shaping the present-day caldera. The Rungwe record includes a ca. 2.2 km3 deposit of a Plinian-style eruption dated at ca. 4 ka, a sub-Plinian one at ca. 2 ka and at least three additional smaller-scale fallout deposits. The Rungwe explosive eruptive record shows that the volcano has been frequently active in its late Holocene past. This study highlights the need for volcanic monitoring in the region and presents herewith the first basis of future volcanic hazard assessment.
A Review of the Hydrocarbon Potential of Kenya
(2003-04) Mbede, Evelyne I.
The geologic history and stratigraphy of Kenya is represented and discussed. Sedimentary units are more than 12,000 m in deeper parts of basins. Sedimentation rates were high throughout the late Paleozoic and Mesozoic. The whole sedimentary sequence in Kenya have been tectonically classified into four basins, namely Mandera Basin in northeast Kenya, Lamu Embayment along the cost, and the Anza Graben and Gregory rift basins inland (BEICIP 1984). Each of these have been affected by rifting at one time or another. Source rocks, reservoir rocks, traps and caprocks are not very well studied but from the geology they are expected to be well developed. More than 25 wells have been drilled; although no commercial discovery had been reported so far there still is a chance of a find in Kenya. The Anza Graben, a Cretaceous/Tertiary rift basin, is the basin recommended in this paper. However, other basins need further study.
The Rungwe Volcanic Province, Tanzania – A Volcanological Review
(2011-11) Fontijn, Karen; Williamson, David; Mbede, Evelyne I.; Ernst, Gerald G. J.
The Rungwe Volcanic Province in SW Tanzania is a densely populated area that is considered volcanically active. As part of the East African Rift System, a significant control of tectonic activity seems to exist on the location and also potential destabilization of volcanic edifices. Three large volcanoes, Ngozi, Rungwe, and Kyejo, dominate the landscape and all show contrasting eruptive behaviour in the recent geological past. Kyejo volcano is a flow-dominated volcano that had a historic lava flow eruption. Lake sediment cores, drilled in Lakes Malawi, Masoko, Rukwa, and Tanganyika, provide a record of frequent explosive eruptions in the last few tens of thousands of years. In combination with on-land stratigraphic observations, they constrain the minimum eruptive frequency of especially Rungwe and Ngozi volcanoes. Both volcanoes had Plinian-style eruptions in the Holocene. The most striking documented Rungwe eruption, the ca. 4 ka Rungwe Pumice, is a rare case of a Plinian eruption in near-wind-free conditions. Furthermore, the Rungwe Pumice, just like any other Rungwe tephra deposit, does not show any evidence of pyroclastic density current deposits. Apart from explosive eruptions at a range of scales happening every few hundred years at Rungwe, the volcano also experienced at least two sector collapse events generating debris avalanches. All existing evidence shows that the Rungwe Volcanic Province is prone to future significant explosive eruptions. To further assess, quantify and mitigate volcanic hazard risks, extensive and systematic multidisciplinary geological research, and both volcanic and tectonic monitoring are needed.
The Sedimentary Basins of Tanzania - Reviewed
(2003-04) Mbede, Evelyne I.
The sedimentary basins of Tanzania have been classified into four morphotectonic groups: the coastal basin, the Karoo rift basins, basins found within the present East African rift valley and the cratonic sag basins. Except for the cratonic sag basins, each of these basin group has been affected by rifting at one time or another. The geology of each basin is discussed, structural evolution is evaluated and the prospectivity is thence looked into. Coal is exploited at Songwe-Kiwira coalfield and is found in potentially economic quantities in other Karoo basins. Prospecting for hydrocarbon resources has been going on since the 50s. Gas has been discovered in Songosongo and Mnazi bay fields, uneconomical quantities of oil have also been reported in Songosongo. Being basically rift basins which have reached different stages of development, source rocks normally associated with Initial-rifting, synrifting as well as post-rifting processes are probably well developed. Reservoir rocks, traps and cap rocks are normally not rare in such tectonic environments. Thermal gradients associated with the rifting stage are normaly high to effect maturation of source rocks even at low sedimentary thicknesses. Studies done so far are still inconclusive, because while testing has mainly been focused on structural traps stratigraphic traps seems to be more
A Semi-automated Algorithm for Monitoring Small Scale Thermal Activity at Oldoinyo Lengai Volcano, Tanzania
(2006-09) Kervyn, M.; Belton, F.; Mbede, Evelyne I.; Jacobs, P.; Ernst, Gerald G. J.; Harris, A.J.L.
Active and potentially hazardous volcanoes in East Africa (Tanzania, Kenya, Ethiopia) are not currently monitored on a regular basis. Among the most active ones, Oldoinyo Lengai has been almost continuously active since 1983. Activity has been confined to small-scale effusive and explosive eruptions of low temperature (~550°C) natrocarbonatite within the summit crater. Historical eruptions’ accounts illustrate that Oldoinyo Lengai also repeatedly displays much more explosive (i.e. violent strombolian or sub-plinian style) eruptions of silicate magma. Daily MODIS images offer a low-cost route to monitor volcanic thermal activity. Existing methods applied on a worldwide scale, such as the MODVOLC algorithm developed by the University of Hawaii, are insufficiently sensitive to monitor small-scale thermal anomalies. MODLEN is an adapted semi-automated algorithm, using MatLab, which allows daily record of the thermal emission at Lengai and the identification of high intensity activity. MODLEN is calibrated and validated using the activity reports from field visits. Despite the small-scale and low temperature of the eruptive activity at Lengai, MODLEN is able to detect all periods of increased activity observed in the field. A 5-year time-series dataset, resulting from analysis of more than 2500 night-time MODIS scenes, has been acquired as a baseline for future near-real time monitoring. It provides insights on the factors controlling the temporal distribution of high intensity activity events. Improvement of the method, to avoid false detection due to partial cloud coverage and detection of the full extent of thermal alerts are also discussed. Although recent activity has been moderate at Lengai, a more intense explosive eruption is overdue. Monitoring is needed to try and anticipate the transition to more hazardous eruption styles and to gain insights into the controls on eruption intensity at Lengai.
Subsidence History of the Rukwa Rift in SW Tanzania Analysed from Ivuna Well,
(2000) Mbede, Evelyne I.
The variation in subsidence rate during rift basin development is a good indication for the Geodynamic history of a sedimentary basin. The sedimentary section of Ivuna Well is herein used to explain the structural evolution of Rukwa Basin within the Western Rift of the East African Rift System. The sedimentary record of Ivuna Well is extracted from published information. The effects of sedimentary load, corrected for compaction and variation in water depth, and lake-level have been removed to obtain the "tectonic subsidence." Curves show two phases of accelerated subsidence related to the fault controlled rifting phases: The Karoo rifting and the Late Cenozoic rifting. Though several phases of rifting are proposed within Karoo time in eastern and southern Africa, it is difficult, with the present information from Ivuna well, to infer them. But the change of gradient of the Geohistory plots within the Karoo section does suggest at least variations of sedimentation rates. The Karoo rifting phase is followed by a steady subsidence which resulted from thermal contraction of the lithosphere thinned during Karoo crustal and lithospheric stretching, while Late Cenozoic rifting is still young at its initial phase of rifting (t = 0).
Tectonic Development of the Northern Tanzanian Sector of the East African Rift System
(1997-01) Foster, A.; Ebinger, Cynthia; Mbede, Evelyne I.; Rex, D.
The Eastern Branch of the East African Rift System diverges from a single, c. 50 km wide rift in southern Kenya to a c. 200 km wide zone in northern Tanzania, where it is comprised of three distinct rifts with different orientations. The western part of this zone contains two rift branches: the Natron-Man yara-Balangida and Eyasi-Wembere rifts. Each rift contains individual basins that are defined here on the basis of structural and geophysical interpretations. These basins are shallow (<3km) and total extension across the bounding faults is small. New K/Ar age determinations on basalts from the western rift basins show that volcanism and sedimentation began in the area at c. 5 Ma. Major fault escarpments were present by c. 3 Ma and the present-day rift escarpments developed later than c. 1.2 Ma. Pre-rift volcanism produced large shield volcanoes of a basalt-trachyte-phonolite association that now lie on the rift flanks. Volcanism after the main phase of rift faulting produced volatile- and alkali-rich explosive centres which are active today, and have no equivalent in southern Kenya. The change in morphology of the Eastern Branch of the East African Rift System, and the style of volcanism in northern Tanzania, may be the result of the transition from the rifting of Proterozoic Mozambique Belt lithosphere to the rifting of cratonic Archaean lithosphere.
Thermal Remote Sensing of the Low‐Intensity Carbonatite Volcanism of Oldoinyo Lengai, Tanzania
(2008-10) Kervyn, M.; Ernst, Gerald G. J.; Harris, A. J. L.; Belton, F.; Mbede, Evelyne I.; Jacobs, P.
Although Tanzania, Kenya and Ethiopia contain a number of active and potentially hazardous volcanoes, none of them are routinely or continuously monitored. Of these, Tanzania's Oldoinyo Lengai (OL) has been active almost continuously over the past two decades (since 1983). Recent activity has been confined to small‐scale effusive and explosive eruptions of natrocarbonatite within the summit crater, with lava flows occasionally overflowing the crater rim and extending onto the volcano flanks. The automated MODVOLC algorithm falls short of detecting all thermal anomalies within OL's crater. The sensitivity of the algorithm is insufficient to detect anomalies of the size and magnitude presented by those at OL. We explore how Moderate Resolution Imaging Spectroradiometer (MODIS) infrared (IR) bands can still be used to monitor activity. We cross‐verify our observations against field reports and higher resolution satellite images (ASTER, Landsat ETM+). Despite the limited extent and low temperature (∼585°C) of natrocarbonatite lavas, relative variations in eruption intensity and periods of increased activity alternating with periods of reduced or no detectable activity can be observed using the MODLEN algorithm. Although activity in the past two decades has been moderate at OL, a more intense explosive eruption is overdue and there is a need for routine monitoring in the future. Our work makes a case for low‐cost thermal IR monitoring as an essential component of such a monitoring programme at several Tanzanian, Kenyan and Ethiopian volcanoes. The approach presented here is already available for routine use.

Browsing by Author "Mbede, Evelyne I."

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