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). 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 Definition Siliceous Sand (sil-S) Grains hardly visible above water; Granular Sand (gran-S) Grains readily visible above water; < 5mm Pebble (P) Grain diameter 4- 65 mm Small Cobble (s-Cble) Diameter 65-128 mm Large Cobble (l-Cbl) Diameter 128-256 mm Small Boulder (s-Bldr) Diameter 256-500 mm Large Boulder (l-Bldr) Diameter >500 mm Rock (Rk) Precambrian outcrop (intact) Table 2. A field data form used for recording observations during the mapping of near- shore 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 Shoreline Geology Substrate GPS Unit Shape Biogenics DST Slope Others Unit Structure Vegetation Slope Others * * Approximated distance from the shoreline 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.