Jean-Jacques Tiercelin

New Oligocene vertebrate localities from Northern Kenya (Turkana basin)

STÉPHANE DUCROCQ,*,1 JEAN-RENAUD BOISSERIE,1 JEAN-JACQUES TIERCELIN,2 CYRILLE DELMER,3 GÉRALDINE GARCIA,1 MANTHI FREDERICK KYALO,4 MEAVE G. LEAKEY,5 LAURENT MARIVAUX,6 OLGA OTERO,1 STÉPHANE PEIGNÉ,7 PASCAL TASSY,7 and FABRICE LIHOREAU6; 1IPHEP, UMR CNRS 6046, Faculté des Sciences de Poitiers, 40 avenue du Recteur Pineau, F-86022 Poitiers cedex, France, stephane.ducrocq@univ-poitiers.fr, geraldine.garcia@univ-poitiers.fr, olga.otero@univ-poitiers.fr; jean.renaud.boisserie@univ-poititers.fr; 2UMR CNRS 6118 Géosciences Rennes, Université de Rennes 1, Campus de Beaulieu, Bât. 15, F-35042 Rennes cedex, France, jean-jacques.tiercelin@ univ-rennes1.fr; 3Palaeontology Department, Natural History Museum, Cromwell Road, London SW7 5BD, United Kingdom; 4Palaeontology Section, NMK HQ Museum Hill, P.O. Box 40658, 00100, Nairobi, Kenya, fkmanthi@museums.or.ke; 5Stony Brook University, Turkana Basin Institute, N511 Social and Behavioral Sciences, Stony Brook, New York 11794-4364, U.S.A, meave.leakey@stonybrook.edu; 6ISEM, UMR CNRS 5554, Université Montpellier II, c.c. 064, Place Eugène Bataillon, F-34095 Montpellier cedex 5, France, Laurent.Marivaux@univ-montp2.fr, fabrice.lihoreau@univ-montp2.fr; 7UMR CNRS 7207, Muséum national d’Histoire naturelle, 8, rue Buffon CP 38, F-75231 Paris cedex 05, France, peigne@mnhn.fr; ptassy@mnhn.fr

Hydrocarbon Prospectivity in Mesozoic and Early–Middle Cenozoic Rift Basins of Central and Northern Kenya, Eastern Africa

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 Term), 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.

Evolution of a Coastal Beach/Barrier/Marsh System in Response to Sea Level Rise, Storm Events and Human Impacts: A Case Study of Trunvel Marsh, Western Brittany

The evolution of coastal sites such as beach/barrier/marsh systems is known to be strongly forced by sea level rise and controlled by storms, sediment input and human impacts. The relative weight of each may vary in time. However, it is difficult to determine the relative importance of these forcing controls and, therefore, how coastal systems evolve through time. In order to study this evolution we have selected the case study of Trunvel marsh, western Brittany, France, which is directly exposed to the most violent storms and has been extensively depleted of sediment during and since WW2. The relative balance of anthropogenic and meteorological controls and relative sea level rise is compared. Sediment cores have been obtained from within the marsh, cross sections of the barrier have been studied and air photos and old maps have been analysed. From 4000 BP to recent times the system has behaved in a simple way: the beach and the barrier accumulated sand and gravel, seeming to migrate inland with relative sea level rise and the marsh was alternatively eroded by the local river or fed by aeolian drifted sands. Very occasional storms may have breached the barrier and temporarily invaded (flooded) the marsh. Conversely, large events of river discharge may have breached the barrier, although there appears to be some natural resilience and the barrier rebuilds itself after each storm and the marsh is, once again isolated from the sea. At the beginning of the Roman period land use change appears to have modified the river discharge, following which the marsh seems to have been in its natural condition again until WW2, although some dykes were built and channels excavated. During WW2 the gravel was almost totally removed and used for concrete to build fortifications along the coast. After WW2, the system was totally controlled by management practices, the aim of which was to recreate a “natural” environment so that today this is a “human made natural landscape” and is now classified as a nature reserve. The barrier is no longer able to withstand storms and the river discharge does not always reach the sea. Therefore human management of water level in the marsh is today the main morpho-dynamic control for the whole system.