THE LAKE ALBERT RIFT (UGANDA, EAST AFRICAN RIFT SYSTEM): SEDIMENT BUDGET, DEFORMATION, BASIN AND RELIEF EVOLUTION SINCE 17 MA
Résumé
The purpose of this study was to quantify the sediment budget, from the catchment to the basin, of an
intra-continental rift: the Albertine Rift system located at the northern part of the western branch of the East
African Rift. The measurement of deposited volumes of sediments is based on the basin infilling study which
consists on both subsurface data and outcrops analysis. The main objectives were (1) to obtain an age model
based on onshore mammals biozones and (2) to reconstruct the 3D architecture of the rift using sequence
stratigraphy correlations and seismic data interpretation. Deformation evolution of the rift through times is
characterized according to seismic interpretation and to the distribution and quantification of the
accommodation for several time intervals. Two major unconformities were identified and dated at 6.2 Ma
(Uppermost Miocene) and 2.7 Ma (Pliocene-Pleistocene boundary), coeval with major subsidence and climatic
changes.
The landforms analysis is based on the characterization, relative dating (geometrical re- lationships with
volcanism) and 3D mapping of Ugandan landforms which consist of stepped planation surfaces (etchplains
and pediplains) and incised valleys.
We here proposed a seven-steps reconstruction of the deformation-erosion-sedimentation re- lationships
of the Lake Albert Basin and its catchments.
- <45 Ma (Lower-Middle Eocene): formation of laterites corresponding to the African Surface during
the very humid period of the Lower-Middle Eocene;
- 45-22 Ma (Upper Eocene to Lowermost Miocene): stripping of the African Surface in response of the
beginning of the East-African Dome uplift and formation of a pediplain as- sociated to the Atlantic Ocean base
level;
- 17-2.7 Ma (Uppermost Lower Miocene to Pliocene): initiation of the Lake Albert Basin
around 17 Ma and creation of local base levels (Lake Albert, Edward and George) on which three
pediplains tend to adapt;
° 17 Ma-6.2 Ma: ”low and diffuse deformation” stage (subsidence rate: 150-200 m/Ma;
sedimentation rate 1.3 km3/Ma between 17 and 12 Ma and 0.6 km3/Ma from 12 to 6 Ma) – depocenters
location is poorly controlled by fault;
° 6.2 Ma-2.7 Ma: rift stage 1 (subsidence rate: > 500m/Ma up to 600-800 m/Ma; sedimentation
rate: 2.4 km3/Ma) – rifting climax;
- 2.7-0.4 Ma (Lower to Middle Pleistocene): rift stage 2; uplift of the Ruwenzori Mountains and
shifting from an alluvial system to a network of bedrock river incisions (subsidence rate: 450 to 250 m/Ma;
sedimentation rate: 1.5 km3/Ma);
- 0.4-0 Ma (Middle to Upper Pleistocene): long wavelength downwarping of the Tanzanian Craton,
initiation of the Lake Victoria trough, drainage network inversion and uplift of the present-day Ugandan
escarpment.
The sediment budget is successful with, between 17 and 2.7 Ma, an excess of 16 % of up-stream
eroded material compared to the sediment volume deposited that can be explained by the chemical erosion
prevailing at this period in Central Africa. The significant (60%) opposite difference between 2.7 and 0 Ma
may be the consequence of a high sediment supply resulting from the erosion of the uplifted Ruwenzori
Mountains.