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
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 65-128 mm
Diameter 128-256 mm
Diameter 256-500 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 Slope
*
Shoreline Geology
Others
* Approximated distance from the shoreline
Unit Structure Vegetation Slope Others
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
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 Rdirection 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 Tdirection 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.
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 19th 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, Rdirection 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
#ATM96194558 (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
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Cohen, Andrew, 2001. Personal communication; Nyanza Project, Lake Tanganyika.
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