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The present-day rift morphologies and sedimentary environments of the northern (NKR) and central (CKR) segments of the Kenya Rift are characterized: 1) in the north, from 5° to 3°N, by a single half-graben rift basin that hosts today Lake Turkana; and 2) this single half-graben evolves to the south, from about 2°30'N, toward a double, fully symmetric half-graben system with to the west, the Kerio half-graben today occupied by a single, semi-perennial fluvial system, and to the east a complex half-graben basin that hosts the shallow freshwater Lake Baringo to the north, and the saline, alkaline and organic-rich Lake Bogoria to the south. Early rifting processes in this part of the Eastern Branch of the East African Rift System started during Eocene times in the region located between 3° and 2°30'N, and resulted in the development of a set of two parallel, N-S oriented half-graben basins - the Lokichar Basin to the west; the North Kerio Basin to the east - similar to the set of present-day Kerio and Baringo-Bogoria Basins. Vertical movements along the major border faults in the NKR resulted in two long-lived lacustrine environments, Lake Lokichar to the west, and the North Kerio Lake to the east. These lakes developed from Eocene under tropical climatic conditions and no volcanic activity in the region. This induced the deposition of thick high TOC shales alternating with wide fluvio-deltaic bodies that extended into the basins during periods of lake regression. Transgressive periods resulted in a single large lake occupying both the Lokichar and North Kerio Basins. Finally, these basins ended at around middle Miocene, consequently to a major change in climate and occurrence of intense volcanic activity in the area. During the Eocene period in the CKR, sedimentation of unknown type developed into the precursors of the Kerio and Baringo-Bogoria Basins. The presence at depth of sedimentary deposits is only demonstrated by geophysical data obtained in the Kerio and Baringo Basins, and by the Kimwarer and Kamego Formations, both poorly exposed in the Kerio and Baringo Basins and which are possible relicts of these early rift deposits. These basement-sourced sediments are mainly of alluvial fan and fluvial type, with brief occurrences of shallow lake environment. At the contrary of the southern NKR, abundant volcanism marked by large volumes of basaltic/phonolitic lavas resulted in the complete filling by lavas of the whole central segment from 23 to 15 Ma. Later on, major vertical movements along the Elgeyo, Saimo and Laikipia border faults contributed to create two new half-grabens prefiguring the present-day Kerio and Baringo Basins. From early to middle Miocene, large lake bodies developed in these two basins, where they are illustrated by the Tambach and Ngorora Formations both representative of large freshwater to saline lake environments with good source rocks potential. From about 8 Ma, rifting migrates from west to east, resulting in a single tectonically active half-graben that hosted a suite of lacustrine environments of which present-day Lakes Baringo and Bogoria are the remnants.

International audience; 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 (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 35 ka). 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.

International audience; Three shallow lagoons and littoral pond systems from Southern Brittany: the Lannénec pond and Grand Loc'h lagoon in Morbihan, and the Trunvel pond in Finistère, have been chosen for a multi-temporal scale (millennial, secular, and factual) study of sedimentary processes involved in shoreline dynamics. This study has so far been conducted on the Lannénec and Grand Loc'h sites, situated in an area that is particularly well exposed to western and southern oriented storms. Sedimentological and micropaleontological studies have been conducted on two cores, one from the Lannénec pond (5.93 m long, dated 5600 years BP), and the other from the Grand Loc'h (3.74 m long, dated 5500 years). This study demonstrates that in 5500 yrs, these two proximal ecosystems have registered two distinct evolutionary cycles. The Grand Loc'h lagoon shows 4 cycles of widening and constriction that at times results in full closure. For the same period, the Lannénec pond demonstrates six cycles, some of which evolve into fresh water continental lake. The differences between these two systems can be explained by a differential orientation of the respective shorelines with respect to the swell. In the case of Lannénec, major sediment transfers characterize the local dynamics during violent storms, while the Grand Loc'h shows a stable shoreline that is protected from these extreme events. Data obtained at the secular and factual scales, from in-situ measurements during storms and observation of aerial photographs and old maps, confirm the timelessness of these processes on this part of the Morbihan littoral.

International audience

International audience; Extension processes have dominated the evolution of the northern segment of the Kenya Rift from Eocene to Present-day. Several typical N-S half-graben basins have been created during two major phases of extension during Eocene times then mid-Miocene times, each phase being characterized by migration of extension processes from west to east then south to north, resulting in the development of two elongated half-grabens, i e. the Lokichar Basin of Eocene - mid-Miocene age, and the Turkana Basin of mid-Miocene - Present age. Sedimentation processes in these two rift basins have been primarily controlled by their typical half-graben morphology as well as by climatic conditions prevailing on each basin during its development, i.e. tropical humid conditions for the Lokichar Basin, and semi-arid conditions for the Turkana Basin. Large lateral clastic inputs are issued from fault-controlled watersheds, i.e. the major border faults - Lokichar Fault for the Lokichar Basin, Murua Rith-Lapur Fault for the Turkana Basin - characterized by a Precambrian basement-dominated watershed for Lokichar, and a volcanic-dominated watershed for the Turkana Basin. Tropical climate and associated vegetation prevailed in the Lake Lokichar watershed from Eocene to lower Miocene resulting in quartz-rich, well-sorted sand inputs in shallow lake waters and deltaic platforms. In the Turkana Basin, semi-arid conditions induced poorly sorted conglomerates and sandstones inputs, transported by an ephemeral lateral fluvial network and deposited in shallow depth littoral platforms. The nature of the axial clastic inputs in both lakes were mainly controlled by the fault structure at the watershed as well as by its lithology, issuing well-sorted quartz-rich sands for the Lokichar Basin, and mainly silt and mud from the volcanic-dominated far watershed for the Turkana Basin. Major differences in basin sedimentation for both the Lokichar and Turkana Basins concern the deep lake basin environment. Thick (100m's), black, organic-rich shales form 2 major sedimentary units deposited in the Lokichar Basin during 2 lake highstand periods in the Eocene and Oligocene. Organic matter resulting from freshwater algii indicates the existence at these times, of a deep freshwater lacustrine environment with well-marked anoxic conditions at depth, which is in line with high rainfall conditions at the watershed. Offshore lake sedimentation in the Turkana Basin was controlled by semi-arid climate conditions, resulting in major inputs of fine-grained sediment with poor organic content from axial and lateral origins. Organic-rich mudstones only occurred in the Turkana Basin during high lake levels that characterized the Plio-Pleistocene period. Thereby, climate conditions play a major role in rift basin sedimentation by controlling the sorting and petrography of inflowing sediments, as well as influence offshore sedimentation, where organic-rich deposits characterize deep lakes/high lake level conditions that can be linked to short or longer humid climatic conditions. Precise knowledge of such conditions may greatly help to identify lake basins favourable for hydrocarbon prospects.

International audience

We developed paleogeographic database of the African plate during MesoCenozoic times. Our objective is to quantify the paleoelevations, and then uplift variations on the continental domain. We first map the shoreline, the bayline, the type of continental systems. We also compile the paleocurrents data available on the fluvial deposits. We define the main river catchments and their divides, based on the paleocurrents data and the location of the river mouths on both lakes and seas. We integrate the boundary conditions such as plate kinematics and magmatism (a magmatism database is also compiled). We define the location of rift shoulders, highs of volcanic arcs, mountain belts, and plateau associated with flood basalts or any other magmatic activity (alkaline small province, kimberlites...). The main tectonic events responsible of relief creations were during: (1) the Late Kimmeridgian-Early Tithonian (155-150 My), (2) the intra Berriasian (145-140 My?), (3) the Austrian deformations (e.g. 130-125 My), (4) the Late Albian to Cenomanian (100-94 My), (5) the Turonian to Maastrichtian (90-65 My) with a paroxysm at the Santonian/Campanian boundary (84 My), (6) the Oligocene-Early Miocene (34-16 My). The topography of Africa during Late Jurassic to Late Cretaceous is mainly controlled by a very long wavelength deformation (x1000 km) showing a maximum in the middle part of North Africa (Mauritania to Sudan) and South Africa and a minimum along the Congo "trough" corresponding to a large lake base-level. This pattern is strongly modified during the Late Cretaceous time by the uplift of the Congo "trough" (Senonian deformations). The last period of uplift occurred during Oligocene and Early Miocene times. These very long wavelength deformations are directly related to the mantle dynamics beneath Africa.

6 p.

1 p.; In the frame of various French research programs (CNRS), we are developing a paleogeographic database (ArcGIS) of the African plate (including Arabia) during the Late Jurassic- Cretaceous. Our main objective is to characterize and to quantify, at the scale of the continent, the relief changes and associated uplifts. - The database also includes for each times step: (1) the shelfbreak, (2) the shoreline (and the type of shoreline: fluvial, wave or tidal-dominated), (3) the bayline (and the lithology of the coastal plain: amount of evaporites or coals) and (4) the type of continental sedimentary systems (lakes: deep vs. shallow, channel rivers: bed load vs. suspended load rivers, alluvial fans) and the paleocurrents data available on the fluvial deposits. - From this, we defined the main river catchments and their divides, based on the paleocurrents data and the location of the river mouths on both lakes and seas. We also defined the uncertainties on catchment limits, their endoreic or exoreic caractere, and the erosional, by-passing and depositional domains for each catchments (plus the uncertainties on the limits between the different domains). - We integrate the boundary conditions such as plate kinematics and magmatism (a magmatism activity through time database is also compiled). From this, we defined the location of rift shoulders (if rifting), of highs of volcanic arcs (if subduction) or mountain belts (if accretion or collision), and of plateau associated with flood basalts (if plumes) or any other relief due to magmatic activity (alkaline small province, kimberlites?). The main tectonic events responsible of relief creations were during: - the Late Kimmeridgian-Early Tithonian (155-150 My; Yemen-Somalian rift initiation, oceanic accretion in the Indian Sea), - the intra Berriasian (145-140 My?; birth of the central African rift system from Niger to Kenya, uplift of the Western African Craton, drowning of the Atlantic carbonate platforms and growth of Atlantic and Tethys deltas), - the Austrian deformations (130-125 My; Barremian to Early Albian: intraplate deformation with reaction of Pan-african structures in North Africa), - the Late Albian to Cenomanian (100-94 My; Ethiopia/Somalia uplift, tectonic relaxation elsewhere and continental onlap), - the Turonian to Maastrichtian (90-65 My) with a paroxysm at the Santonian/Campanian boundary (84 My: main intraplate deformation of Africa, uplift of the South African Plateau in two times: Cenomanian eastward and Campanian westward: Kalahari epeirogeny of de Wit, in press). The topography of Africa during Late Jurassic to Late Cretaceous is mainly controlled by a very long wavelength deformation (x1000 km) showing a maximum in the middle part of North Africa (Mauritania to Sudan) and South Africa and a minimum along the Congo "trough" corresponding to a large lake base-level. This pattern is strongly modified during the Late Cretaceous time by the uplift of the Congo "trough".

International audience

1 p.; We present the results of a geochemical study of Quaternary sediments from two rift basins in the East African Rift, the Makgadikgadi-Okavango-Zambezi (MOZ) basin and the Mpulungu sub-basin of Lake Tanganyika. The early initiation stage in the development of a continental rift is characterized by the development of shallow half-graben basins where nascent faults exert a primary control in the evolution of drainage catchment formation. This stage is illustrated by the present Okavango fan, located within the larger Makgadikgadi-Okavango-Zambezi basin (MOZ) in NW Botswana. The immediate successive stage concerns the development of basins where shallow lacustrine environments occur, as the result of fault growth represented by the larger MOZ basin. A more mature stage concerns the development of well-defined, actively subsiding half-graben basins, where propagation and interaction between fault segments lead to basin linkage illustrated by the southern Mpulungu sub-basin of Lake Tanganyika. The use of geochemical compositions of sediments in diagnostic diagrams has been shown to be useful in determining tectonic settings although the field defined for passive margins (PM) include a wide range of tectonic environments, from rifted continental margins of the Atlantic-ocean type, sedimentary basins near to collision orogens and inactive or extinct convergent margins. In this work, based on the major element geochemistry, we define two new fields within the PM setting being 1)alluvial fan (AF) and 2) lacustrine basin (LB).The provenance of the sediments of both the Okavango Delta and Mpulungu sub-basin sediments was determined in this study. For the Mpulungu sub-basin, our results indicate that the immediate source material is a quartzoze sedimentary rock represented by the sandstones of the Mbala formation. The ultimate source material was a felsic source represented by the basement granites. These results agree very well with the current hydrological conditions in the basin, where the Lufubu River traverses these two source rocks. For the Okavango Delta sediments, the interpretation is more complex as the sediment source includes a felsic rock source and a pyroxene and olivine rich mafic-ultramafic rock complexes which are mixed with sand and diagenetic carbonates to produce the Okavango sediments. Geochemical data was used as proxies for climate change in both the Lake Ngami (SW Okavango) and the Mpulungu sub-basin of Lake Tanganyika. The Lake Tanganyika data shows a remarkable excursion coinciding with the Younger Dryas (YD) event, reflecting the input of previously chemically weathered material into the basin due to a change of vegetation

1 p.; Littoral lakes are characteristic of the Atlantic shorelines of Brittany, western France. These lakes are generally small and shallow and are separated from the Atlantic Ocean by dune systems. In certain areas of the littoral, these lakes alternate with small rias that are known to have been open toward the sea for at least the last centuries. In Morbihan, southern Brittany, a legend indicates that during the 5th century, a princess of Wales known as Princess Ninok escaped with her vessel and escort to a major storm by mooring along the shoreline of the "Lannénec Pond", that is today a small freshwater littoral lake with no link with the sea. Cores have been obtained from this lake as well as from a nearby ria know as the "Loc'h". A 6 m long core (LA-1) was obtained from the Lannénec Pond, covering, while a 5 m long core (LO-1) was obtained from the Loc'h, both covering the last 6000 years. Sedimentological and micropaleontological studies have been conducted on these two cores, with peculiar attention concerning marine and freshwater microfauna and microflora. Foraminifer and dinoflagellate associations found in the LA-1 core clearly demonstrate that the Lannénec Pond was connected with the open ocean several times during the last 6000 years, alternating with periods of closed, freshwater lake separated from the ocean by a dune system. The chronology obtained on the LA-1 core shows that open sea conditions existed during the 5th century, thus permitting Princess Ninnok's vessel to moor along the Lannénec shoreline. On the contrary, micropaleontological data obtained for the LO-1 core show that marine conditions prevailed all along the last 6000 years, evolving from almost dry sea-swamp environment to open sea. No lacustrine conditions have been identified on this core. Alternating phases of lacustrine and open sea environments in the Lannénec Pond may relate to sea level fluctuations during the last 6000 years. Nevertheless, such sea level changes may also have affected the nearby Loc'h, thus resulting in establishing temporary lacustrine conditions in this area. Considering the present-day hydrodynamics along this part of the Morbihan shoreline, major storm events may induce huge sediment transport along the shoreline, resulting in possible damming of estuaries. Such conditions may have prevailed in the Lannénec area, while the Loc'h area appears to be preserved from such sediment transport. Storm frequency along the Morbihan coastline can be established from high resolution studies of littoral lakes.

International audience; High-resolution studies of variations in the elemental and stable carbon- and nitrogen-isotope composition of organic matter in cores from Lakes Malawi, Tanganyika, and Bosumtwi (tropical Africa) indicate an abrupt change in the wind-driven circulation of these lakes that, within the limits of available chronologies, was contemporaneous with the end of the Younger Dryas in the northern hemisphere. The change was also coincident with shifts in surface winds recorded in cores from off the west and northeast coasts of Africa. A range of other proxies indicate that these changes in wind regime were accompanied by a marked increase in precipitation in the northern tropics. Africa south of ~5°-10°S, on the other hand, initially suffered drought conditions. Together, the evidence suggests an abrupt northward translation of the African monsoon system at circa 11.5 ± 0.25 ka B.P. The data assembled here contribute to a growing body of work showing that the Younger Dryas was a major climatic excursion in tropical Africa. Furthermore, they add substance to recent suggestions that climatic events in the southern hemisphere may have played a significant role in the abrupt demise of the Younger Dryas

International audience

East African Plio-Pleistocene deposits contains archaeological sites from fluvio-lacustre environments. A number of works allow us to discuss the modifications of assemblages recovered from fluvial channel or banks. In contrast, sites retrieved from foodplain deposits are generally assumed to be well preserved and few studies have adressed site formation processes in such environments. The site of Nadung's 4 has yielded detailled data conerning typical modifications in this environnement., Les formations du Pliocène final et du Pléistocène inférieur d'Afrique de l'Est livrent des sites archéologiques dans les différents milieux des paysages fluvio-lacustres associés au rift. De nombreux travaux, expérimentaux ou actualistes, permettent d'appréhender le degré de préservation des sites retrouvés en contexte de chenal. En revanche, peu d'attention a été accordée aux plaines de débordement, réputées favorables à une bonne préservation des sites. La fouille du site de Nadung'a 4 apporte des informations détaillées pour décrire les modifications des sites dans ce type de milieu.

International audience; Les formations du Pliocène final et du Pléistocène inférieur d'Afrique de l'Est livrent des sites archéologiques dans les différents milieux des paysages fluvio-lacustres associés au rift. De nombreux travaux, expérimentaux ou actualistes, permettent d'appréhender le degré de préservation des sites retrouvés en contexte de chenal. En revanche, peu d'attention a été accordée aux plaines de débordement, réputées favorables à une bonne préservation des sites. La fouille du site de Nadung'a 4 apporte des informations détaillées pour décrire les modifications des sites dans ce type de milieu.

Associé au 20ème Colloque de Géologie Africaine, BRGM, Orléans (3 Juin 2004)

Associé au 20ème Colloque de Géologie Africaine, BRGM, Orléans (3 Juin 2004), vol. de résumés: 42

Located in the Ziway-Shala Basin of the Main Ethiopian Rift, Lake Langano is part of an asymmetric half-graben, defined by a series of north-northeast-trending faults in the tectonically active zone of the rift. A 15-m deep succession of organic homogeneous muds, silts, bioclastic sands, and pyroclastic layers was cored in 1994. The definition of a certified radiocarbon chronology on these deposits required the indispensable establishment of modern hydrological and geochemical balances. The isotopic contents of the total dissolved inorganic carbon (TDIC) of surface water clearly show the influence of a deep CO 2 rising along the main fault crossing the lake basin. The 5.8 pMC disequilibrium existing in 1994 with the atmosphere likely produces the aging of authigenic materials developing at the lake surface. However, with a mean residence time of ~15 years, this apparent 14 C aging of Lake Langano water still integrates the 14 C produced by the nuclear tests in the 1960s. Reconstructing the natural 14 C activity of the lake TDIC allows for the quantification of the deep CO 2 influence, and for the correction of AMS-14 C datings performed along the core. The correction of the AMS-14 C chronology defined on Lake Langano allows for a better understanding of paleohydrological changes at a regional scale for at least the last 12,700 cal BP.

International audience; no abstract