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Project B3: Linking Source and Sink in the Rwenzori Mountains and adjacent areas — Uganda: landscape evolution and the sedimentary record of extreme uplift

During the first project phase between June 2006 and Mai 2009, four field campaigns were carried out in the western branch of the East African Rift system in the vicinity of the Rwenzori Mountains. The aim of the sedimentological investigations was to reveal on the one hand the characteristics and the conjunction of the different source and sink areas and on the other hand to gain an overall picture of the rift related sedimentation.

The Albert rift, on which concentrates the work of the Riftlink research group, consists of two rift segments being partly filled by lakes and separated by the structurally and topographically outstanding Rwenzori Mountain fault block (see project B1). Since the beginning of the rifting activity around 12.5 Ma BP, the topographically elevated parts of the rift structure, i.e. both the rift shoulders being now at heights of 1500 to 2500 masl and the extremely uplifted Rwenzori block with its highest peaks at 5100 masl, respectively, constitute the source areas for the predominantly siliciclastic synrift succession reaching a maximum thickness of 3 km in central graben positions.

Due to the equatorial climate conditions, the region is covered by dense vegetation and intense tropical pedogenesis produces a thick soil layer. The rare outcrops of the synrift sediment succession area located south of the Rwenzori Mountains along the Kazinga Channel between Lakes George and Edward, east of the central Rwenzoris at the Mobuku river, and finally northeast of the Rwenzori Mountain range in the Kisegi-Nyabusosi area and along the eastern shore of Lake Albert in Kaiso area (Fig. 1). The two southern outcrop areas only show insight into relatively young deposits of Pleistocene and Holocene age, i.e. at maximum some ten to hundred tousand years old. Some of these sites show alluvial deposits, whereas others are covered by thick volcaniclastic ashfall deposits. In contrast, the two northern outcrop areas allow much deeper insight into the synrift sequence encompassing the last 12.5 Ma. As both located in a marginal graben position where the sediment sequence (in contrast to stronger subsiding central graben areas) does not reach maximum thickness and was deposited under nearshore, shallow lacustrine, or alluvial conditions. The todays exposed position above lake level is due to late stage inversion tectonics.

 

 

Figure 1: Sketch map of the Albert rift with Rwenzori Mountains in central graben position and schematic cross sections (not to scale) of the four areas providing accessible outcrops of the synrift sediments in the Albert Rift

 

 

In the Kisegi-Nyabusosi area (Fig. 1, section 2), the almost complete middle Miocene to upper Pleistocene siliciclastic succession reaches a thickness of ca. 600m and is accessible in a suite of outcrops of different size and quality (Fig. 2). Altogether 380m were logged in detail in 20 outcrops along a NE-SW oriented stripe in offset manner throughout the sequence. The succession is mainly composed of clay, silt and sand beds. Conglomeratic layers are mainly restricted to the oldest and youngermost parts of the succession. Rare intercalations of thin carbonate beds, carbonaceous concretions and gypsum precipitations occur throughout the succession. A remarkable feature in the middle and upper part of the sequence are the frequent impregations by reddish and ochre coloured ironoxide and hydroxide impregnations. Some of these ferrugineous beds consist of accumulated pisolithic and ooidic concretions, with the latter often containing a lot of intermixed fossil fresh water mollusc shells and fish bones. The mollusc associations were investigated by Gautier (1970) and Pickford et al. (1993) and from biostratigraphic dating there resulted the division of the succession into 8 formations from Middle Miocene to Upper Pleistocene in age (Fig. 2). This stratigraphic frame was the base for our sequence stratigraphic work on the synrift sediments. The depositional environments of the beds alternate from alluvial to lacustrine with transitions, e.g. nearshore and delta deposits (Fig. 3).

 

 

Figure 2: Geological map of the Kisegi-Nyabusosi area, situated northeast of the Rwenzori Mountains. Division into stratigraphic formations from Pickford et al. 1993. The biostratigraphic dating bases on mollusc associations investigated by Gautier (1970), Pickford et al. 1993, and Van Damme & Pickford (2003).

 

 

Figure 3: Facies model of the region northeast of the northern tip of the Rwenzori Mountains at Lower Pleistocene times when a mature graben structure evolved.

 

 

Aiming to interprete the logged sections with respect to genetically related strata and the repeated reoccurrence of associated architectural elements, we followed the base level concept of Cross and Lessenger (1998). In this concept, sedimentation and erosion is controlled by changes in the ratio of accommodation to sediment supply , thus the spatiotemporal rift graben evolution can be reconstructed from changes of supply and available accomodation space (related to graben subsidence and lake level fluctuations).

The so far gained results will be complemented during a field campaign in January/February 2010 at the eastern shore of lake Albert (Fig. 1) by lithologic logging of the Pliocene to Pleistocene rift sediments of the Kaiso area which are (in contrast to the Kisegi-Nyabusosi sequence) unaffected by the extreme uplift of the Rwenzori Mountains concerning their depositional history.

Parallel to the work in the old rift sediments, we carry out investigations on recent denudation rates (of the last 10000-30000 years) on the Rwenzori Mountains, the rift shoulders and the adjacent basement plateau regions. All this data finally merges to a general picture of the changing denudational and sedimentational processes and hence landscape evolution from Middle Miocene to recent times for this so far merely explored part of the East African Rift system.