Albertine rift, Uganda : Deformationsedimentation- erosion relationships
Résumé
Lake Albert Basin is assumed to be a classical half-graben initiated
around 10 Ma and oriented NNE-SSW, with a major northwesterly
bounding 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 sedimentary
infilling and (3) the geomorphological evolution of the surrounding area
and its associated erosion budget.
Seismic data and outcrops studies suggest a much more complex history
than previously described. (1) The age model, mainly based on mammal
fossils (Pickford et al., 1993 ; Van Damme and Pickford, 2003), is
debated, but the early stage of the rift is probably Early to Middle Miocene.
(2) No half-graben geometry has been characterized : the infilling
consists of juxtaposed isopach tabular compartments with sharp thicknesses
variations along bounding faults, in response of either low rate extensional or combined strike-slip/extensional movements. The following
onshore-offshore evolution is proposed :
- Middle Miocene (18-13Ma) to Late Miocene (7Ma) : low extension -
low and slightly differential subsidence - deepening from fluvio-deltaic
to deep lacustrine environments - first generation of pediments.
- Late Miocene (7Ma) to Late Pliocene (3Ma) : combined strikeslip/
extensional movements - homogenous and high subsidence - lacustrine
clays interbedded with sandy flood-lobes - second generation of
pediments.
- Late Pliocene (3Ma) to Early Pleistocene (2Ma) : Ruwenzori uplift
- growth of the Ruwenzori Mountains (5000m) - high and slightly differential
subsidence - deltaic to wave-dominated coast sandy deposits -
pediments degradation by fluvial erosion.
- Middle-Late Pleistocene : late regional uplift and tilting - drainage inversion
and present-day scarp formation.