Electronic Material Properties - und Geowissenschaften ...

Rift dynamics, uplift and climate change in
Equatorial Africa:
Sybille Roller, Matthias Hinderer, Jens Hornung
Situated in the central western branch of the East African Rift system, the Rwenzori
Mountains and adjacent rift flanks are the highest uplifted rift flanks on earth. Their
extreme uplift may have significantly contributed to the aridisation of East Africa since
the Late Miocene.
Within the DFG research Unit (Forschergruppe “RIFTLINK” with University of Frankfurt,
University of Mainz and other institutions) we are running the sedimentological part
which is titled:
Linking source and sink in the Rwenzori Mountains and adjacent rift basins,
Uganda: landscape evolution and the sedimentary record of extreme uplift
The goal of the subproject is to study the erosional denudation of the Rwenzori
Mountains since their extreme uplift in the Late Miocene. This comprises both, analysis
of the erosional features and landscape evolution, as well as the sedimentary record in
the adjacent rift basins. The working programme for the running time of the project is
based on three investigation fields: 1.) “Source region”: Erosion and fluvial transport, 2.)
“Sink region”: Basin analysis, 3.) “Modelling”: Numerical simulations of landscape
evolution including basin fill history.
Prevailing questions concern the close connection between the extreme uplift and the
erosive unloading and the chronological relation of both processes. Therefore, we try to
understand the role of endogenic processes, the role of climate, and the possibility of a
dynamic balance between uplift and denudation. We intend to develop a timestratigraphic
frame from the Late Miocene to Present. Moreover we will calculate
denudation rates, sediment efflux, and interpret the controlling factors of both
processes.
Dense tropical vegetation and thick pedogenesis to a certain extent hamper continuous
and area-wide sedimentological investigations, but some scarcely available outcrops
provide insights both in the lake and river sediments and into several of the uplift
induced alluvial fan deposits, respectively, and admit some relative age assumptions, as
well as examinations of lithology, structure, and facies-type. Ages of younger alluvial
sediments are determined using Optically Stimulated Luminescence (OSL) dating.
Erosion rates are evaluated by measuring the concentration of in-situ produced
Terrigenic Cosmogenic Nuclides (TCN; e.g. 10 Be) in the quartz grains of river sands.
The results will then be integrated into an evolutionary model of erosion, sediment flux,
and rift basin fill for the central western branch of the East African Rift.
A first field campaign was carried out in August 2006 to get an idea about the quantity
and quality of the loosely distributed outcrops. For a complete data set of erosion rates,
quartz-bearing river sand was sampled in 14 prominent rivers all around the Rwenzori
Mountains on Ugandan territory. In the meantime the material entered its complex
preparation process for concentration measurement of 10 Be in the accelerator mass
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