The Lake Albert Rift (Uganda, East African Rift System): sediment budget, deformation, basin and relief evolution since 17 Ma
Abstract
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.