Albertine rift, Uganda : Deformationsedimentation-erosion relationships

International audienceLake Albert Basin is assumed to be a classical half-graben initiatedaround 10 Ma and oriented NNE-SSW, with a major northwesterlybounding fault located along the western Congolese shoreline (Ebinger,1989 ; Pickford & al., 1993).This study aims to characterize the relationships existing between deformation,erosion, and sedimentation of the rift through time by restoring(1) the timing and amplitude of vertical movements (subsidence, uplift),(2) the geometry and paleo-environmental evolution of the sedimentaryinfilling and (3) the geomorphological evolution of the surrounding areaand its associated erosion budget.Seismic data and outcrops studies suggest a much more complex historythan previously described. (1) The age model, mainly based on mammalfossils (Pickford et al., 1993 ; Van Damme and Pickford, 2003), isdebated, but the early stage of the rift is probably Early to Middle Miocene.(2) No half-graben geometry has been characterized : the infillingconsists of juxtaposed isopach tabular compartments with sharp thicknessesvariations along bounding faults, in response of either low rate extensional or combined strike-slip/extensional movements. The followingonshore-offshore evolution is proposed :- Middle Miocene (18-13Ma) to Late Miocene (7Ma) : low extension -low and slightly differential subsidence - deepening from fluvio-deltaicto deep lacustrine environments - first generation of pediments.- Late Miocene (7Ma) to Late Pliocene (3Ma) : combined strikeslip/extensional movements - homogenous and high subsidence - lacustrineclays interbedded with sandy flood-lobes - second generation ofpediments.- Late Pliocene (3Ma) to Early Pleistocene (2Ma) : Ruwenzori uplift- growth of the Ruwenzori Mountains (5000m) - high and slightly differentialsubsidence - deltaic to wave-dominated coast sandy deposits -pediments degradation by fluvial erosion.- Middle-Late Pleistocene : late regional uplift and tilting - drainage inversionand present-day scarp formation