A «SOURCE TO SINK» APPROACH OF THE LAKE ALBERT RIFT (UGANDA, EAST AFRICAN RIFT SYSTEM): RELATIONSHIPS BETWEEN SEDIMENTARY BASIN AND UPSTREAM LANDFORMS
Résumé
This study is based on a coupled basin infilling study and a landforms analysis of the Lake
Albert Rift located at the northern part of the western branch of the East African Rift.
The basin infilling study is based on both subsurface data and outcrops analysis. The
objective was to (1) actualize age model, (2) to reconstruct the 3D architecture of the rift using
sequence stratigraphy correlations and seismic data interpretation, (3) to characterize the deformation
and its changes through times and (4) to quantify the accommodation for several
time intervals. The infilling essentially consists of isopach fault-bounded units composed of
lacustrine deposits wherein were characterized two major unconformities 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 and relative dating of Ugandan
landforms which consist of stepped planation surfaces and incised valleys.
We here proposed a seven-steps reconstruction of the deformation-erosion-sedimentation
relationships of the Lake Albert Basin and its catchments:
- 55-45 Ma: formation of laterites corresponding to the African Surface during the very
humid period of the Lower-Middle Eocene;
- 45-22: stripping of the African Surface in response of the beginning of the East-African
Dome uplift and formation of a pediplain which associated base level is the Atlantic
Ocean;
- 17-2.5 Ma: 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;
- 18—16 Ma to 6.2 Ma: «Flexural» 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 (southern part of Lake Albert Basin) poorly controlled by fault;
- 6.2 Ma to 2.5 Ma: Rift stage 1 (subsidence rate: > 500m/Ma up to 600-800 m/Ma; sedimentation
rate: 2.4 km3/Ma) – Rifting climax;
- 2.5-0.4 Ma: uplift of the Ruwenzori Mountains and shifting from an alluvial system to a
network of bedrock river incision – Rift Stage 2 (subsidence rate: 450 to 250 m/Ma; sedimentation
rate: 1.5 km3/Ma);
- 0.4-0 Ma: long wavelength downwarping of the Tanzanian Craton, initiation of the Lake
Victoria trough, drainage network inversion and uplift of the present-day Ugandan escarpment
(normal faulting motion of the border faults) with formation of perched valleys
associated to the Lower Pleistocene (2.5-0.4 Ma) rivers network.
The sediment budget is successful with, between 17 and 2.5 Ma, an excess of 16 % of
upstream 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.