Lakes Edward, George and Victoria (Uganda)

A study of Late Quaternary rift tectonics, sedimentation and palaeoclimate

Tine Lærdal

Department of Geology, University of Bergen, Norway. January 2001. ISBN 82-9958-28-0-6

Table of contents

[List of papers]

Preface

CHAPTER I
1. General introduction
2. The IDEAL project
3. Structural and sedimentological framework
4. Main results
 4.1 SUMMARY OF PAPER 1
4.2 SUMMARY OF PAPER 2
4.3 SUMMARY OF PAPER 3
4.4 SUMMARY OF PAPER 4
 5. Future work
6. References
 CHAPTER 2
"Structure and Neotectonics in the Edward and George basins, Uganda-Congo, East Africa"

CHAPTER 3
Late Quaternary sedimentation and climate in the Lakes Edward and George area, Uganda-Congo"

CHAPTER 4
SYNTHESIS; "Interaction between tectonics, climate and sedimentation in an active rift basin; the Edward-George basin, East Africa"

CHAPTER 5
"The Late Pleistocene-Holocene palaeohmnology of Lake Victoria, East Africa, based upon elemental and isotopic analyses of sedimentary organic matter"



[List of papers]
  1. Tine Lærdal and Michael Talbot. Structure and Neotectonics in the Edward and George basins, Uganda-Congo, East Africa. Paper submitted for publication in a Special Publication of the Palaeogeography, Palaeoclimatology Palaeoecology, from the Second International Congress of Limnogeology (LENNOU) held in Brest, May 1999.



  2. Tine Lærdal, Michael Talbot and James M. Russel. Late Quaternary sedimentation and climate in the Lakes Edward and George area, Uganda-Congo. Paper submitted for publication in a Special Publication of Advances in Global Research from the 2nd International IDEAL meeting held in Malawi, January 2000.



  3. Tine Lærdal and Michael Talbot. SYNTHESIS; Interaction between tectonics, climate and sedimentation in an active rift basin; the Edward-George basin, East Africa.



  4. Tine Lærdal and Michael Talbot. The Late Pleistocene-Holocene palaeohmnology of Lake Victoria, East Africa, based upon elemental and isotopic analyses of sedimentary organic matter. Journal of Paleolimnology 23: 141-164, 2000.






SUMMARY OF PAPER 1
The Edward and George basins are located in the western branch of the EARS. Both Lakes Edward and George occupy classical rift basins (half grabens). Both basins are bordered by the main boundary fault in the west and the faulted, flexural margin in the east. The area between the two lakes is dominated by structural uplift related to a high relief accommodation zone formed where the NNE-SSW trend of the Miocene rifting and NW-SE oriented basement lineaments interfere. The Kazinga Channel, connecting Lake George to Lake Edward, cuts across this uplifted area and the course of the channel has been affected by syn- and antithetic faulting. A low relief accommodation zone identified in the central parts of the Edward basin (the Kasindi Fault Zone; KFZ) appears to have formed between two oppositely facing border faults, on the flexural margin of the basin. The structural framework of the basins show many similarities with the much better known and larger basins located further south in the EARS (i.e Lakes Tanganyika and Malawi) (Reynolds, 1984; Rosendahl, 1987; Ebinger,1989). Our neotectonic period covers the last 50 ka, during which movement has been concentrated to the main boundary faults and to syn- and antithetic faults in the basins. The latter separate Holocene sediments on the lake floor (relative to the KFZ), are the main architects behind lake morphology and the meandering course of the Kazinga Channel. They also largely control river courses. The elevated areas related to the accommodation zones separate the main depo centres and may thus largely control sediment distribution within the basins.



SUMMARY OF PAPER 2
Geochemical analyses of four cores from Lake Edward provide a detailed record of climate and lake-level changes during the latest Late Pleistocene and Holocene. In addition, our record give a glimpse of lower lake levels during the Late Pleistocene. Following this low stand, lake levels rose to several metres above today's level in the Late Pleistocene-Early Holocene. Extensive, low-lying areas surrounding the lake were drowned during the high stand (Brooks and Smith, 1987; Musisi, 1991). Lake levels began to fall some time prior to 5 ka BP, as a result of a shift to drier climates and/or tectonic lowering of the Semliki outlet. This fall in lake level led to the desiccation of Lake George and was accompanied by tectonic activity in the basin.

During the Mid Holocene low stand, exposure of a >10 m fault scarp associated with the KFZ, divided the basin into a western and an eastern section and caused damming of rivers that entered the basin from the north and northeast. Two water bodies were created during this low stand. Low Lake Edward in the west, which was a closed lake and where extensive beaches formed along the eastern shore, and Lake Mweya located east of the fault zone. Lake Mweya was at a higher elevation than low Lake Edward and was apparently a well-flushed basin, receiving water from several rivers in the north and northeast and possibly draining into low Lake Edward to the west

Sediments deposited during this low stand have different geochemical characteristics, suggesting that limnological conditions in the two basins were different. Lake levels began to rise around 4.5 ka BP, reuniting the two lakes and creating modem Lake George sometime before 3.6 ka BP. This transgression caused drowning of extensive swamps and marks a change to more lacustrine conditions in the Edward and George basins. There are also indications of a climate shift around 1.7 to 1.8 ka BP, correlating with a change towards more arid climates recorded in several of the other East African lakes.



SUMMARY OF PAPER 3
Lake George is dammed by the level of Lake Edward. A fall in the level of only a few metres in the latter would be sufficient to drain the ~2.5 m deep Lake George, turning it into a mainly dry basin. Under such conditions, much of the sediment brought into the George basin would be transported across the desiccated lake floor, through the Kazinga Channel and deposited in Lake Edward, possibly sourcing a low-stand delta. Humid climate corresponds to high lake levels, drowning of low lying areas surrounding the lakes and deposition of fine-grained, organic-rich sediments throughout the basin. Lake levels in the Edward and George basins have varied throughout the Late Pleistocene and Holocene, and the overall control appears to have been climate.

The KFZ separates the Edward basin into a western and an eastern half and there is a >10 m fault scarp associated with it. In combination with lower lake levels, the fault acted as a dam to sediment and water transported into the eastern half of the basin, resulting in the formation of two separate water bodies. When this happened during the Holocene, the western lake was closed, deep and dominated by fine-grained sediment, while the eastern lake was shallow, open(?) and had more elastic and littoral material. The fault zone affects sedimentation and as a result, the distribution of potential reservoir vs source rocks in the basin differs markedly, not only relative to low- and high-stands, but also relative to syn- and antithetic faulting in the basin. Our example from the Edward-George basin illustrates the importance of understanding the combined effects of fault activity and climate change on sedimentation in continental rift basins, and further demonstrates how an intrabasinal fault zone can severely complicate an otherwise simple pattern of sediment distribution.



SUMMARY OF PAPER 4
Three piston cores from Lake Victoria were analysed for organic carbon and nitrogen content, stable isotopes (13C and 15N) and Hydrogen Index, and the data have been used to produce a detailed palaeo-environmental reconstruction of the Victoria basin over the past ca. 17.5 ka.

Two palaeosols were identified in one of the cores, indicating two separate periods of more-or-less complete desiccation of Lake Victoria during the Late Pleistocene. A thin lacustrine clay separating the two soil horizons suggests a brief return to more humid conditions. The second period of desiccation was terminated by a major lacustrine transgression ~15.2 ka BP, which flooded extensive macrophyte swamps, and which eventually led to the establishment of the present lake. The lake expanded rapidly, leading to a massive release of nutrients from the flooded landscape, stimulating very high rates of primary production. A period of deep mixing began at ca. 13.8 ka and lasted for ca. 250 years, and was probably a result of intensified winds. This event may mark the onset of the Younger Dryas in the region. The last 13.6 kyrs have only seen gradual changes in production and burial of o.m. The water column seems to have been particularly stable in the mid-Holocene.

Since the terminal Pleistocene Lake Victoria's nitrogen budget has been dominated by fixation of atmospheric nitrogen. The switch to nitrogen fixation was probably triggered by a drastic decline in dissolved nitrogen supply once the major period of transgression was over. Our data thus suggests that the main period of lake filling had ended by ca. 13.7 ka BP, by which time the lake was at or close to its present level.

The last 2000 years have seen a decline in the isotopic and elemental contrasts between the 3 core sites and a general rise in sediment accumulation rates; all are probably a result of anthropogenically induced changes to the lake and its catchment.


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