Garba IV and the Melka Kunture Formation. A preliminary lithostratigraphic approach

autorisation des éditeurs de l'ouvrage par email du 01/07/05The lithostratigraphic analysis of the different sections at Garba and Gombore clarify the dynamics nature of the deposition processes for the lower part of the Melka Kunture Formation. Downstream transport and accumulation are prevalent during sedimentation processes. Channel lag petrographic suites reflect the basin geology and indicate a period of global dismantling of ancient volcanic relief and important lateral contributions from both banks' tributaries to alluvial processes. Volcanic activity is the main provider for fine grains sediments: pumice derived from pyroclastites of penecontemporaneous plinian eruptions contribute massively to channel lags and to major accretion events (Sl, Sp, St facies), reworked coarse tuffs form massive beds (Sm facies) while cinereous tephras are responsible for silts deposition (Fsm, Fm facies). Resistant and opaque minerals form the matrix of bed lags and most of the sandy bases of current structures. Grain-size distribution also reflects the evolution of channels. At Gombore, six channels are superposed and partly embedded. Each of them exhibits a logical facies succession leading to complete infilling and abandonment while the main stream migrates towards SE. Archaeological layers are systematically associated with the bedforms, except for the uppermost one. The volcanic input is responsible for overloading of streams and forces channel changes. Facies previously considered as lacustrine are actually fluvial facies. The “major cut-and-fill” previously considered as major erosion phases by Chavaillon (1973) are only simple channeling processes. At Garba IV, three channels are superposed and partly embedded. Each of them exhibits a logical facies succession leading to a complete infilling and eventual abandonment. Archaeological layers are associated with the bedforms, local erosional surface or temporary abandoned channel. The volcanic input is systematically responsible for over-loading and forces channeling. Here again, facies previously considered as lacustrine are actually fluvial and the “major cut-and-fill” previously considered as important erosional phases are only simple channeling processes. Considering the paleomagnetic data and especially the brief normal polarity event recognized both at Garba in the upper part of “Tuff A” and at Gombore Ig at the top of sands above the archaeological layer (Cressier 1980), the preliminary absolute dating (Schmitt et. al. 1977) and the revisited tuff correlations, it can be concluded that both series deposited between 1.77 Ma (end of Oldoway Polarity Subzone) and 0.78 Ma (Brunhes-Matuyama limit) at most. From an archaeological perspective, Oldowan and Acheulian sites are associated with bedforms and gravel bars. These layers have been affected by currents (sieve effect, re-orientation of large pieces) and repeatedly washed and covered by abrasive sands during accretion or subjected to erosional episodes which have highly disturbed the primary organisation of artefacts. The superposition of the different facies clearly indicates a tectonic control of the sedimentation for the Melka Kunture Formation (at least for its lower terms described in this paper) during the Lower Pleistocene: the recurrent faulting (moderate subsidence of the semi-graben) along the Melka scarp which borders the Garba-Gombore area is directly responsible for this architecture. A pause in subsidence combined with head erosion through the Melka Kunture raised block allowed a return to valley incision processes and installation of valley-plain and stepped terraces fossilized by the non-welded ignimbrite episode, significantly wide-spread and thus named Kella Formation.This unit belongs clearly to the Matuyama Polarity Zone with a minimum age of 0.78 Ma. The setting of this ignimbrite seems to have fit into a topography close to the present one. After the emplacement of the non-welded ignimbrites a fault reactivation takes place with an important displacement. The meandering of the palaeo-Awash was certainly determined by the obstacle of the raised block of Melka Kunture according to the Main Ethiopian Rift evolution (see Bardin et al. in this volume). Combined head erosion and subsident faulting are responsible for the successive changes in sedimentation style such as channeling, superposition or down-cutting when volcanic input increases deposition rates and forces channeling changes. The different facies identified indicate a shallow, seasonal, low-sinuosity, braided river environment. This is indicative of a well-contrasted dry-wet seasonal climate