Plio-Pleistocene short-term lake-level fluctuations and associated onland landscapes in the West Turkana Rift Basin, Northern Kenya. Consequences on Hominids habitats and culture
Résumé
The northern end of the Kenya Rift is today occupied by Lake Turkana, the largest lake of the Eastern Branch of the East African Rift System. This lake basin is inherited from an upper Miocene rift basin known as the Turkana Basin, that began to develop as a result of a migration of rift extensive tectonics from Eocene-Oligocene times along East-West and South-North axes. At the northwest end of the Turkana Basin, the 750-m thick Hominid-bearing Nachukui Formation represents 4 Ma of the limnogeological history of the Turkana Basin, in an area where open water and littoral lacustrine deposits interfinger with alluvial fans or delta systems. These detrital bodies develop along the uplifted margin of the basin that reaches elevations of more than 1,000 m above the present-day lake level and is deeply dissected by lateral streams bringing wide quantities of coarse clastic sediments toward the lake. The lateral extension of this fringe of fan/deltaic bodies is directly controlled by climatic/tectonic induced lake-level fluctuations. Archeological investigations conducted in this region by the Mission Préhistorique au Kenya/West Turkana Archaeological Project have produced large number of artefacts and faunal assemblages, as well as Hominid remains, spanning the 2.5-0.7 Ma time period. Paleoenvironmental reconstructions based on limnogeological and paleontological data for these sites demonstrated that Hominids occupation areas were preferentially associated to places connected to stream shorelines (floodplain environment) generally close to a main lacustrine environment. Detailed sedimentological investigations conducted along the Nachukui Formation have revealed series of short-term positive or negative fluctuations in lake level. Transgressive sequences: bioclast-rich beach deposits overlapping deltaic or alluvial fan bodies, then followed by platform margin environments characterized by wide stromatolite bioherms that are in turn overlapped by green massive or laminated shales indicating open deep water conditions. Regressive sequences: stromatolite bioherms overlapped by coarse deposits from bioclast-rich sandstones to conglomerates demonstrating a downward migration of the lake shoreline or a rapid progradation of deltaic environment in response of lake level drop. Such-short terms lake-level fluctuations are linked to rapid changes in water input to the lake from the upper part of the lake watershed, mainly the Omo River to the north. Climatic changes at a regional scale may have directly influenced the localization of Hominids settlements, by restricting the surfaces of potential living areas between the basin margin and the shoreline, thus forcing the populations to migrate in less hospitable areas.