Student: Leah Morgan Mentor: Kiram Lezzar. Introduction

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1 Tectonic controls of sedimentary pathways and depocenters: Canyon conveyor belts and ridge rubbish on the Luiche River Platform Margin, Lake Tanganyika Student: Leah Morgan Mentor: Kiram Lezzar Introduction Lake Tanganyika provides the means of transport and food, and a political boundary for the countries of Burundi, Democratic Republic of the Congo, Tanzania, and Zambia. As the largest and deepest lake of the East African rift lakes, Tanganyika is home to a unique biological community with remarkably high endemicity. The dry season (May August) is characterized by strong southerly wind and wave action on the lake, while the majority of precipitation falls in two rainy periods (September November and February April) (Soreghan and Cohen 1996). Tanganyika s only outlet is the Lukuga River; over 90% of water loss is through evaporation (Coulter and Spigel 1991). Tectonic and climatic variations combine to make Tanganyika susceptible to drastic lake level changes in relatively short time spans (Cohen et al. 1997). Geological Setting Lake Tanganyika is an expression of the western branch of the East African rift system, a series of basins formed by overlapping half-grabens (Morley 1988) extending from Uganda to Malawi. Tanganyika, with an estimated age of 9 12 Ma, stretches 650 km from Burundi to Zambia, has a mean width of 50 km, a maximum depth of 1470 meters, and a sedimentary sequence of up to 6 km in thickness. (Cohen, et al, 1993). The geological basement is constructed of the Kigoma Quartzite, a Precambrian metamorphic formation with major pre-rift fault sets trending N-S and E-W. The N-S family of normal faults is the predominant fault group in Miocene rifting (Morley, 1988), while the E-W family consists of oblique dextral transform faults, resulting in offset of the N-S set (Yairi and Mizutani, 1969). Additional trends in the Kigoma area include NW-SE and ENE-WSW, also mapped by Yairi and Mizutani (1969). Study Objectives The Luiche River drains 1065km 2 of land before reaching Lake Tanganyika on a hinged margin south of Kigoma and Ujiji, Eastern Tanzania (Cohen and Palacios, 1998). Seismic lines of the Luiche platform were compiled by Scholz, et al. in However, due to the larger scope of the study, these lines are widely spaced and focused on deep water and thus do not provide a detailed analysis of the entire Luiche platform. Here, a more defined bathymetric study attempts to detect sub-lacustrine structural and sedimentological features, and to connect these offshore structures with onshore topographic and tectonic features. A previous study by Soreghan, et.al. (1999) investigated the delta and canyon system of the South Rukuru River on Lake Malawi by examining seismic and core data. The resulting geologic map shows a series of canyons projecting from the river mouth and defines various depositional facies found within the system. Soreghan also discusses the role of lake level change on depositional facies. Lake level change plays a role in the location and size of delta lobes and canyons, and can also inactivate canyons by affecting sediment load and water discharge. Methods Bathymetric data, consisting of latitude, longitude, and water depth, was collected with a Garmin GPS unit and a Raytheon echo sounder over five days on the R/V Echo and the M/V Maman Benita. This data, consisting of over 1700 data points and approximately 175 km of transects, was collected by hand in a notebook and subsequently transferred to computer. The triangular and seemingly chaotic transect pattern seen in Figure 1 was selected to avoid obliquely crossing over canyons expected to extend from the Luiche River mouth. Golden Software s Surfer was used to create a bathymetric map of the platform (Figure 1), while Microsoft Excel produced depth profiles of transect lines (Figure 3). Transects in this study focused

2 on shallow water surrounding the delta, while deep water structures were determined from seismic data of Scholz, et.al. (1997) (Figure 2) and used to complete a structural and geomorphological map of the platform (Figure 4). Bathymetric map The bathymetric map in figure 1 shows a large offshore delta system surrounded by a series of canyons and channels. Bathymetric variation appears in the delta near the river mouth but is not present in greater depths farther from shore. Also seen are three canyons to the north of the delta and two to the south. To the west, a steep slope drops to the deepest part of the basin. Bathymetry on this map is defined by dotted transect lines, not including solid seismic lines. Areas of poor data include the NW and SE corners of the map. Bathymetric and seismic profiles Bathymetric data used to create figure 1 was also used to create depth profiles of transect lines. Selected bathymetric profiles are shown in Figure 3 and can be located on Figure 1. Note the differences in scale, particularly vertical scale, among profiles. Transects 11 and 13 both cross the proximal delta in front of the Luiche River mouth. Bathymetric variation seen across the proximal delta on 13, closer to the river mouth, is greatly reduced on transect 11. Transect 8 crosses two separate sections of horst block 2 (HB2), although specific faults are absent because the transect runs N-S. Transect 16 shows very little bathymetric variation apart from a consistent rise up-platform. However, small variations show an intermediate delta and canyon. Canyons are also seen on either side of the delta in 11 and 13, and between the sections of HB2 in transect 8, with levees on the sides of the canyons. Seismic line T97 2C shows the offshore Luiche delta consisting of proximal, intermediate, and distal lobes. Stratification is seen in both the proximal and intermediate delta lobes, but is more pronounced in the intermediate lobe. The distal lobe is unstratified and chaotic. Ridges, or horst blocks, formed by normal faults are found along the eastern portion of the study area. These horst blocks are offset from SW to NE (T-97 2D, T-97 2C, transect 8). Interpretation Seismic profiles show stratification on the proximal and intermediate lobes suggesting they are formed by sediments derived directly from the river mouth. Additionally, grain size on the proximal lobe decreases downslope (William, 2002). Seismic profiles of the distal lobe, however, show a chaotic structure suggesting the lobe is canyon-derived. The bathymetric variation of transect 13 (fig.3a) could be interpreted as canyons cross-cutting the delta. However, the lack of canyons on transect 11 (fig.3b) and other transects, as well as the findings of William (2002) suggests these are paleo-canyons that are no longer active. The change in activity could be due to either a change in location of the Luiche River mouth, or to the excessive growth of the delta blocking the canyons. This rather surprising lack of canyons at the Luiche River mouth could be explained by littoral drift. The large delta at the mouth of the Luiche, and wave patterns from the south and northwest create a backstop for river sediments, which then are carried by waves and currents to a canyon or channel. The structural and geological map in Figure 4 interprets the bathymetric and seismic profiles of the Luiche River platform delta. A three-lobed delta is bordered by canyons and their levees, as sediment from the Luiche River progresses toward the deepest part of the basin. Horst blocks (HB) can be seen running N-S along the western edge of the platform, creating the divide between platform and basin. These blocks are offset by E-W dextral transform faults described on the Bangwe Peninsula by Yairi and Mizutani (1969). The offset creates space for sediments to pass and to reach the basin. Each canyon is found to have an associated on-shore stream (C1/R1; C2/R2; C3/R3; C4/R5). Although the source of C5 is not known because it is outside the study area, it is likely the canyon is part of the extensive Malagarasi River delta system.

3 Acknowledgements I would like to thank my mentor, Dr. Kiram Lezzar, for his enthusiasm and expertise in both geology and French, R.J. Hartwell for his excellent navigation and stress-busting capabilities, the crews of the M/V Maman Benita and the R/V Echo, and the National Science Foundation and WWF for funding. References Cohen, A., M. Soreghan, and C. Scholtz, Estimating the age of formation of lakes: An example from Lake Tanganyika, East African Rift system. Geology, v. 21, p Cohen, A.S., M.R. Talbot, S.M. Awramik, D.L. Dettman, P. Abell, Lake level and Paleoenvironmental history of Lake Tanganyika, Africa, as inferred from late Holocene and modern stromatolites. GSA Bulletin, v.109: no.4: p Cohen, A.S., and M. Palacios, Cruise report for the Lake Tanganyika Biodiversity Program. Subcomponent of Sedimentation Study; Cruse Period 6 26 Jan., Department of Geosciences, University of Arizona, unpublished data. Coulter, G.W., and R.H. Spigel, Hydrodynamics, in G.W. Coulter, ed., Lake Tanganyika and its life: New York, Oxford University Press, p Morley, C., Variable extension in Lake Tanganyika. Tectonics, v. 7, p Sholtz, et. al., PROBE Seismic Atlas, Duke University Soreghan, M., et.al., Coarse-grained, deep-water sedimentation along a border fault margin of Lake Malawi, Africa: Seismic stratigraphic analysis. Journal of Sedimentary Research, v. 69, No. 4, p Soreghan, M. and A.S. Cohen, Textural and compositional variability across littoral segments of Lake Tanganyika: The effect of asymmetric basin structure on sedimentation in large rift lakes. AAPG Bulletin, v.80, No.3, p William, E., Spatial relationship of grain size and coarse sediment mineralogy on the shallow Luiche delta platform and its river streams, Nyanza Project 2002 Annual Report Yairi, K. and S. Mizutani, Fault system of the Lake Tanganyika rift at the Kigoma area, western Tanzania. J. Earth Sci. v.17 p

4 8 13 Luiche River Mouth Luiche River T97-13 T97-11A T97-9 T97-7 Figure 1: Bathymetric map of study area showing transect and seismic lines (Contour interval = 20 m). For Grab Samples (GB) location, refer to figure 1 in E. William s Report, this Volume.

5 Figure 3 (a,b,c,d): Depth profiles of Luiche Platform 3a: NW transect SE -5 3b: transect 11 SE NW c: transect 8 S transect 16 E N W d:

6 Figure 2: Seismic profiles from Scholz et.al, W E 2a: T 97-2d W 2b: T 97-2c E 2c: T 97-11A N S 2d: T Distal delta N S Distal delta Levee Intermediate delta Channel or canyon (C) Horst block (HB) (Precambrian basement ridge)

7 C 2 Longitude E Bangwe Peninsula H B 1 F 3 R 1 R 2 Latitude S 4.95? Poor data H B 1 H B 1 F 3 F 3 F F 3 F F 4 C C 3 *Ujiji R 3 R 4 Luiche River Delta (swampland) 50 Luiche River 100 C 4 C ? Kilometers R 5 Distal delta Levee Intermediate delta Channel or canyon (C) R: river Horst block (HB) (Precambrian basement ridge) E-W transform fault Figure 4: Geomorphologic Map of the Luiche Platform N-S Normal fault (F)