Characterization of the near-shore substrate along the eastern shore of Lake Tanganyika at the Kigoma area, western Tanzania

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1 Characterization of the near-shore substrate along the eastern shore of Lake Tanganyika at the Kigoma area, western Tanzania Student: Patrick Nduru Gathogo Mentors: Dr. Kiram Lezzar and Dr. Andy Cohen Introduction The Kigoma area of northeastern Tanzania, East Africa, is a major port town on the eastern shore of Lake Tanganyika where extensive scientific work (mainly focused on the lake) has been going on. Findings show that the lake is unique for its physiography and biodiversity: It is the second deepest lake in the world (1470 m), and the number of invertebrates endemic to the lake is outstanding (Michel, 2001). Lake Tanganyika has come to be regarded as a modern analogue to some ancient lacustrine systems, and thus it serves as a model for some abiotic-biotic relationships that might have existed in such systems. My project seeks to contribute to such understanding by studying the relationships from a geological perspective that exist at the terrestrial-aquatic interphase along the Kigoma area shoreline. Mountain ranges extending from Burundi to the south reach the Mtanga and Kagongo area north of Kigoma Bay, where the altitude is 1,500 to1,600 m above sea level. See Map 1 for location. The shoreline north of Katongwe Point is trends NNE-SSW. Further south of Katongwe the trend changes to a composite of NNE-SSW, NNW-SSE and WNW-ESE trending lines forming major bays and headlands. Previous work (Shluter, 1997; Tiercerlin & Monderguer, 1991; Yairi & Mizutani, 1969) show that the topographical features in the Kigoma area are largely controlled by geological structures such as fractures and joints. Yairi & Mizutani (1969) studied the fracture system in the Kigoma area and identified two characteristic fracture systems, which transcend the local rock facies. The name R- (rift) direction was adopted for N-S fractures, and T- (transform) direction for the E-W fractures. These two workers also observed that the local structure of beds in the area between south of Kigoma Bay and Bangwe peninsula strike nearly E-W and dip to S. The R-direction, the workers noted, mainly compose an open fracture set concordant with the trend of the Lake Tanganyika Rift. By contrast, the T-direction is made of shear fractures with horizontal displacements that coincide with the direction of the preexisting weak lines, which might be linked to the ocean-floor spreading. They believed that the two faults systems formed simultaneously though independently. Two rock facies, both Proterozoic in age, are exposed in Kigoma area: The Kigoma quartzite of the Karagwe-Ankolean System, and the Manyovu red bed of the upper part of the Bukoban System. Locally the Kigoma quartzite consists mainly of white or gray medium- to coarse-grained orthoquartzite, and the Manyovu red bed comprises a rounded cobble to pebble conglomerate. Some sections of these rock facies are elongated subparallel to the R-direction trend suggesting that even during the Proterozoic, local sedimentary basin(s) resembled the modern Tanganyika basin (Shluter, 1997; Yairi & Mizutani, 1969). For example, Yairi & Mizutani (1969) observed that the regional structural trend of the Kigoma quartzite and the Manyovu red bed north of Kigoma is nearly N-S and almost concordant with the Tanganyika rift trend. In this extended abstract I highlight the methodology and some important observations made in my project. An attempt is also made to link some key observations with their probable geological implications where possible. Details on the observations and some important relationships are compressed within/in illustrations found elsewhere in this report, and also as part of a final digital map (in ArcView GPS 3.2a format) compiled for the project. Methodology Fieldwork involved examination, classification, quantification and mapping of some specific parameters (See Table 1) chosen in advance after consultation with my project geology mentors (Drs. Kiram Lezzar and Andy Cohen) together with the biology mentor Dr. Ellinor Michel. Essentially, all the observations were made and recorded in the field by filling out field data forms (See Table 2). Detailed description in the field was only done in a field notebook when necessary. Some basic concepts that were adopted for the project are discussed by Boggs (1995), Lewis & McConchie (1994) and Compton (1985).

2 Table 1. Definitions for near-shore substrate units at Kigoma area (Tanzania). The classification is based on the Wentworth scheme (Boggs,1995; and Lewis & McConchie, 1994) Unit Siliceous Sand (sil-s) Granular Sand (gran-s) Pebble (P) Small Cobble (s-cble) Large Cobble (l-cbl) Small Boulder (s-bldr) Large Boulder (l-bldr) Rock (Rk) Definition Grains hardly visible above water; Grains readily visible above water; < 5mm Grain diameter 4-65 mm Diameter mm Diameter mm Diameter mm Diameter >500 mm Precambrian outcrop (intact) Table 2. A field data form used for recording observations during the mapping of nearshore substrate at Kigoma. Entries for Shape and Structure are as defined by Boggs (1995). Brief descriptions on unique features were recorded under Others. Near-shore Substrate GPS Unit Shape Biogenics DST * Shoreline Geology Slope Others Unit Structure Vegetation Slope Others * Approximated distance from the shoreline

3 Mapping of the near-shore substrate was mainly done on a Zodiac inflatable boat, and only substrate within one meter of water depth was described. Paddles marked at one meter were used for confirming the depths. Where the water depth exceeded one meter, the unit at shoreline (or water level) was recorded as a substitute for the near-shore substrate type. For example, 'bedrock' was recorded as the near-shore substrate type when Proterozoic conglomerate or quartzite-sandstone occurs along the shoreline at deeper area. The term 'macrophytes' was used in place of substrate where marshy areas made the shoreline inaccessible. Distance from the shoreline to the location of the described near-shore substrate was also recorded for depth magnitude comparison. Geographical coordinates were recorded with a Garmin 45XL GPS unit at every boundary point between substrate types. Snorkels were used for enhancing visibility through the water where turbidity was high. Geology of areas just above shoreline was examined and observations recorded following the same format as with the near-shore substrate. Outcrops at the headlands and along the main beaches were examined on foot, and descriptions were made in the field notebook. However, structural parameters such as dip, strike, and bedding were not measured due to time limitation. Photographs were taken covering parts of the shoreline. A final digital map was compiled in Arcview [comprising double-striped shoreline. The inner stripe towards the lake represents the near-shore substrate units, and the outer strip shows the shoreline geology units and beaches] (See Map). Observations It was very challenging to distinguish between the two Proterozoic rock units that are exposed in the Kigoma area again, this was due to time limitation. For instance, the conglomeratic facies of the Manyovu red bed and that of the Kigoma quartzite resemble one another a lot and thus it proved difficult to tell them apart based on their lithology alone, and without knowing their stratigraphic levels. For practical application, therefore, I replace the formal stratigraphic names used for the two Proterozoic rock units with informal lithologic (descriptive) names. I adopt the names conglomerate lithologic unit (CLU) and sandstone/quartzite lithologic unit (SLU) for the conglomerate and all the sandstone-quartzite of the Proterozoic rock units respectively. Significant relationships were observed between occurrence of the CLU and SLU outcrops, and the location of the headlands and bays that define the shoreline morphology at the Kigoma area. With a few exceptions such as the Hilltop Hotel, the major headlands in the study area comprise the SLU outcrops, whereas most of the bays are associated with CLU outcrops (See Map). The mapped near-shore substrate units are generally deeper and with steeper slopes near the headlands, but shallower and gentler along the bays. Within the deeper near-shore where the shoreline units are described to be the substrate bedrock to boulder substrate units are also common. The 'macrophyte' substrate units characterize transitional areas between the deeper headland localities and the shallower areas near the bays (or beaches). Beach characteristics in the study area also are related to the proximity of the Proterozoic outcrops. These beaches can be informally grouped into four basic classes. Class A beaches are characterized by well-sorted siliceous sand with a very gentle slope, and having the northern Ujiji beach as the type. Class B beaches comprise granular sand with boulders (and bedrock) along a narrow and short-stretch area, with the type being Jacobsen s beach. Class C beaches are similar to those of Class B by having granular sand, but in addition the former have few pebbles and are formed within gentle and relatively extensive areas. Southern Bangwe beach is the type for Class C beaches. The last type of beach is Class D, which is constructed of pebbles and cobbles in a narrow but long-stretch area, and with Zungu beach as the type. Class A and C are associated with detrital sediments, Class B with SLU outcrops, and Class D with CLU outcrops. Cementation is another factor significant within the near-shore substrate, along the shoreline and in some of the beaches. Thin (15-20 cm) and discontinuous slabs of carbonate-cemented granular sand and pebble occur. The slabs that comprise fine-grained sediments are common along TAFIRI (Tanganyika Fisheries Research Institute) area beach, where variations in degree of cementation coupled with selective dissolution modifies the shape of the slabs. The coarser-grained slabs characterize the substrate south of Luichi point and east of Nondwa point (near a pebbly deltaic lobe). Similar cementation also occurs at Zungu beach involving sorted pebbles. Emerged carbonate-cemented angular cobble to pebble

7 conglomerates are also common along shorelines with angular cobble to boulder as the near-shore substrate units. Discussion and Conclusion Relationships observed in the field could help provenance the near-shore substrate units reliably. Most of the rounded pebble and cobble substrate can be traced to the CLU outcrops, the angular cobble and boulder to the SLU outcrops, and the granular and siliceous sand to detrital sediments. Likewise, the shoreline morphology may be linked to lithologic properties such degree of induration of the Proterozoic outcrops: Generally, the CLU outcrops are less indurated compared to the SLU outcrops. Bays, therefore, are prominent along the CLU, and the headlands along the SLU. Hilltop Hotel point is unique for being the only major headland within the study area that comprises CLU outcrops. This can be explained by considering the location of the headland in relation to the prevailing (and destructive) hydrodynamic forces. Lake Tanganyika s strongest waves and currents are mainly longitudinal, and deviate only slightly from N-S (Coulter, 1991). The Hilltop headland seem to be sheltered by Nondwa point to the north and Bangwe point to the south against these destructive energies. Otherwise, I propose that if Hilltop headland were not sheltered from the north and south, it would be eroded back relatively fast, as is the case at Meno hill that is sheltered only from the north. Still, caves are seen undermining the headland at the shoreline at the base of Hilltop hill. Boulder blockislands occurring near Hilltop are evidence of advanced stage of destruction when caves and arcs break through. Similar arcs and caves are also common along shorelines with CLU outcrops such as at central Katabi beach and just north of Nondwa point. The SLU outcrops seem to be more resistant to the main erosive hydrodynamic energies along the shoreline. Instead, the outcrops are divided into angular blocks by the major joint structures, which are well exposed near Jacobsen s beach. The granular sand that composes Jacobsen s beach can be traced uphill where it is seen to result from weathering of weakly indurated sandstones, partly mapped as the Manyovu red bed by Yairi & Mizutani (1969). Field observations also tie in well with the structural geology work that was done by Yairi & Mizutani (1969) and offshore seismic investigations carried out by Scholz (1997). The structural geology that controls the occurrence of the R- and T-direction fault systems in Kigoma area might explain the pattern seen with the CLU-SLU outcrops in relation to the shoreline morphology. Considering that the R- direction fault system is more prominent north of Kigoma bay, it is expected to have less N-S alternation of the CLU and SLU outcrops, and therefore less prominent headland-bay shoreline morphology. The opposite situation occurs south of Nondwa point where the T-direction fault system becomes more prominent. There the CLU-SLU outcrops alternate frequently, and the heandland-bay shoreline morphology becomes enhanced. The seismic data shows a pattern similar to what is observed with the shoreline morphology but is replicated further away from the shoreline at greater depth. Line T97-13D (See Figure xxx?) of the seismic survey depicts a topographical profile identical to mapped shoreline geology. The CLU (and bays) and SLU (and headlands) outcrops along the shoreline matches side-toside with the topographical lows and highs that are seen from the seismic line, respectively. Ideally, intersection of the R- and T-direction faults would form checker-board fault blocks, which dip in perpendicular directions (E-W or N-S). And since the CLU outcrops generally overlie the SLU outcrops, the shoreline morphology resembles a ' piano-keypad' (Lezzar, pers. com., 2001) where the T- direction fault segments result in variable vertical displacement along the shoreline. Comments and Recommendations Interesting hypothesis or models relating the substrate to the geology at the Kigoma area can be developed from findings of this project. For example, an attempt can now be made to extrapolate the extent of the main near-shore substrate types to cover deeper near-shore geology with better precision. It might also be possible to infer the palaeomorphology of the shoreline at Kigoma area after understanding the relationships observed in this project. Such ideas might also be extended to try and explain (or understand) the peculiar diversity of invertebrate that has been observed along the shores of Lake Tanganyika.

8 Regression-transgression events have been recorded in the Tanganyika basin. One of the major events includes a transgression reported by Dr. Livingstone when the shoreline was at his memorial museum at Ujiji a in the middle part of the 19 th Century, and the major regression that dropped the water level by about 600 m a few thousand years ago (Cohen, pers. com., 2001). Fluctuations in water levels in the past, coupled with geological factors such as lithology and geological structures, might have led to development of microbasins along the shore each defined by a distinct substrate type. The distinct substrates would have then localized different populations of biota, or acted as physical barriers against mixing, or even both. The idea about development of localized microbasins along Kigoma shoreline in the past could be well argued by referring back to the geological features that are still present along the shore. For example, R- direction fractures form N-S relay-ramps (Lezzar, pers. com., 2001), which are step-fault blocks that dip E and are N-S bounded by T-direction faults. Such features, when present and significant, could be topographical traps necessary for development of structurally controlled localized microbasins. Understanding of the relationships that exist at along the Kigoma area shoreline may help also to reconstruct the processes that might have taken place in other African rift basins, like the Turkana basin of the eastern rift. Basin parameters such as the shape, orientation, structural history and the absolute distance from the equator (or latitude) are much alike between the Turkana and Tanganyika lakes except for the size of the basins, for Tanganyika is much larger than Turkana. Studies show that thet Lomekwi area, a major hominid fossil locality in the Turkana basin of northern Kenya, was juxtaposed with topographical highs often during the Pliocene (Leakey et al, 2001). Some fossil invertebrates of the Turkana basin are also believed to relate to parameters (biological, chemical and physical) defining the basin (Williamson; Cohen, 1982; and Feibel, 1988), and much more can be learned from the modern relationships such as the one existing along the Kigoma area shore. However, one must exercise great caution when interpreting observations from this project because the fieldwork was carried out more or less at a reconnaissance scale. More work needs to be done in refining the geology of Kigoma area. Understanding the lithological aspect of the bedrock in this area is much more useful for understanding local geomorphology than is knowledge of the exact stratigraphic unit to which the lithologic type belongs. It is the lithology thatis related to the shoreline morphology and substrate as has been demonstrated in this project. Besides routine mapping of the Kigoma quartzite and the Manyovu red bed, therefore, more effort should be put into trying to infer the depositional facies recorded in the two Proterozoic rock units. It would be interesting also to prospect for outcrops of Cenozoicstrata in addition to the dominant recent colluvium/alluvium. Detailed mapping of the geological structures such as fractures, joints, dip and bedding is also essential. Acknowledgements I thank the various people and institutions for helping with the success of my project. My project mentors Drs. Kiram Lezzar and Andy Cohen were instrumental with project ideas. Dr. Ellinor Michel though being the biology mentor also helped a lot in the project by encouraging integration of biology and geology in my project. Thanks likewise go to my field assistants Mr. George and Mr. Pius Mweambe for their devotion. Appreciation also goes to the US National Science Foundation Grant #ATM (the Nyanza Project), and the Universities of Arizona and Utah for their financial and technical support. The governments of Tanzania and United States of America also played an important role in processing the documents that were required for the project. References Bannister, Kirsten, Morphology and Sedimentation of Kigoma Bay and Vicinity; In Nyanza Project 1998 Annual Report. Boggs, Sam Jr., Principles of Sedimentology and Stratigraphy. Prentice Hall, New Jersey, p Cohen, A. S., 1982a. Ecological and paleoecological aspects of the Rift Valley lakes of East Africa: Unpublished Ph. D. Dissertation. University of California, Davis. P Cohen, Andrew, Personal communication; Nyanza Project, Lake Tanganyika. Compton, Robert R., Geology in the Field. John Wiley & Sons, New York, p Coulter, G. W. & Spigel, R. H., Hydrodynamics In Lake Tanganyika and its Life,: Coulter, G. W. (Ed.). London: Oxford University Press

9 Feibel, C. S., Palaeoenvironments of the Koobi Fora Formation, Turkana Basin, northern Kenya. Unpublished Ph. D. Dissertation: Salt Lake City, Utah, University of Utah, p Leaky, M. G., Spoor, F., Brown, F. H., Gathogo, P. N., Kiarie, C., Leakey, L. N., & McDougall, I., New homonin genus from eastern Africa shows diverse middle Pliocene lineage: 410: Lewis, Douglas W. & McConchie, David, Analytical Sedimentology: Chapman & Hall, New York, p Lezzar, Kiram, Personal communication; Nyanza Project, Lake Tanganyika. Michel, E., Introductory Notes And Notes for Biology; Nyanza Project Parker, Sybil P., McGraw-Hill Dictionary of Geology & Mineralogy: McGraw-Hill, New York, p Scholz, C. A., Grosschel-Becker, H. & Cattaneo, P. K., Lake Tanganyika Lacustrine Carbonates and Mixed System Study: Unitversity of Miami RSMAS. Shluter, Thomas, Geology of East Africa. Gebruder Borntraeger, Berlin-Stuttgart, p Tiercerlin, Jean-Jacques & Mondeguer, Andre, The Geology of the Tanganyika Trough. In Lake Tanganyika and its Life,: Coulter, G. W. (Ed.). London: Oxford University Press Yairi, Kenji & Mizutani, Shimjiro, Fault System of the Lake Tanganyika rift at the Kigoma area, western Tanzania, Journal of Earth Sciences, Nagoya University, v. 17,

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