Spatial relationship of grain size and coarse sediment mineralogy on the shallow
Luiche delta platform and its river streams
Student: Emmanuel William
Mentors: Dr. Kiram Lezzar & Dr. Andrew Cohen
Introduction
Lake Tanganyika is located in Western Tanzania and is the world’s second largest lake (32,600 km2 and
1470m deep), containing 18% of the world’s fresh water. It belongs to western arm of the Tertiary East
African Rift System, and is the oldest lake with an estimated age of 9-12 Ma (Cohen et al., 1993). The
lake drains water from three main rivers: the Rusizi, Malagarasi and Luiche. These rivers are important for
terrestrial sediment supply. The study of grain size distribution and mineralogy of lake sediments and their
associated areas of drainage are important as they shed light on the provenance of sediments and the way
they are transported along and away from the shore. Based on that study, it will be possible to compare the
mineralogy and grain size distribution in the lake, as well as the sediments that are continuously supplied
by the river into the lake, and thus be able to characterize them. This study will explain how tectonic and
lake level fluctuations influence the distribution of coarse sediments along the shallow water of the Luiche
delta platform.
Methodologies
Field work involved sample collection (Maps 1, 2 and 3) that was divided into two regions: River (Maps 2
& 3) and shallow water lake sampling (Shallow waters/<70m on Map 1-Note: The bathymetry contours are
derived from Morgan’s 2002 report, this volume). All lake & river samples were collected using 2 types of
grab samplers of different size and weight. Five accessible rivers were sampled and samples were sealed in
plastic bags. Likewise, sediment sampling in shallow lake water was done using the grab sampler at
various depths, collecting samples at an interval of 10m depth along five transects (see Figure 1), which
extended offshore from about 20m to a water depth of about 110m. The distance between two successive
transects was approximately 5 km. Depth measurements were taken using a Hummingbird echosounder on
the R/V Echo (depth ranges about 0 to 200 m) and a Raytheon echo sounder on M/V Maman Benita (depth
range about 0-1500 m). The samples were preserved for water content measurements in the laboratory.
Laboratory work included sieve analysis and coarse fraction (>0.062 mm-sand) mineralogical observation.
Samples were divided into two batches, one for water content and a second for wet sieving. Approximately
10 g of sample was weighed in pre-weighed aluminum foil and the sample dryed for two days.
Approximately 20 g to 50 g of sample was wet sieved through a 0.062 mm sieve (to eliminate clays and
silts). Samples were placed into the oven for drying at 550C to obtain the dry weight of the sand fraction.
The percentage of the sand fraction was obtained using the following equation:
C-A=D
where C is the weight of dried sample with aluminum cup, A is the weight of the aluminum foil, and D is
the dry sand weight of the sample. Dry weight of the bulk sample was obtained using values of the
calculated water contents.
T2*y/T1
where T1 is the weight of the water content sample, T2 is the weight of wet sieved sample and y is the
water content for T1 in grams. This equivalent water content was subtracted from the bulk sample wet
weight to obtain bulk dry weight. To obtain the weight of the clays, sand fraction weight was subtracted
from the bulk dry weight. Finally the percentage composition of each fraction was calculated using the
mathematical relationship;
(X i / X)*100= Xi %
where Xi is the weight of an individual fraction, X is the bulk dry weight of the sample, and Xi % is the
percentage composition of each fraction.
All the data above were contoured and graphs were plotted for interpretation.
Sediment mineralogy was studied in the coarse fractions a petri dish. Different mineral types were counted
under a binocular microscope, with an aid of a counter. A spatula, brush and a spreader were used in the
examination. All the data were entered in Microsoft Excel and imported into Surfer for contouring.
Data Analysis and Interpretation
Sieve analysis
Luiche River streams samples
Sieve analysis of sand fractions of the Luiche streams shows that all sediments were sands with clay/silt
content <20% (Maps 2 and 3).
Shallow lake water environments
The variations in sand percentage decrease generally offshore (map 2). In Transect 1, the decrease is
marked by widely spaced contours (sample ranging between 30 and 60%). Transect 2 is marked by an
abrupt change in contour levels over a short distance, percentages ranging from 0.3 to 68%. In Transect 3,
the contours are closely spaced with some samples counting above 70%. Transect 4 has a sudden drop in
one of its samples, from 2.7% to 1.4% and then back to 2.7%, thus the values lie between 1% and 96% of
sand. The fifth transectalso has closely spaced contours, the values lying between 1% and 95%, with one of
the areas showing a sudden drop from 93% to 40% sand.
The silt-clay percent increases offshore, although some contour variations are recorded in each transect.
Transect 1 is marked by widely spaced contours, with its silt-clay proports lying being between 40 and
63%. Transect 2 has closer spaced contours, with proportions between 30% and 100%. Transect 3 has
closer contours with clay-silt proportions between 40 and 60%. Transect 4 (TR4) has closely spaced
%Contours with high percentages of the silt-clay (between 4% and 99%). The percentage goes up to 60’s to
ninety’s and drops from 98% to 97%. Transect 5 is characterized by a closely spaced %Contours with silt
clay proportions between 6% and 99% of the silt-clay content (Map 3).
Binocular microscopy
Luiche River streams samples
River stream mineralogy is characterized by quartz, feldspars, green minerals (? olivine or green quartz),
magnetite, mica and some calcite (evidenced by the calcareous behavior of the samples). Shells are not
present in the river. Plant debris are low in abundance (see Map 4, 5 and 6). The angularity, sorting and
sphericity of river sample are not very uniform; they are mostly poorly sorted, sub-angular, with only about
one in six samples being rounded (see Table 1).
Shallow lake water environments
From microscopic studies, feldspars, quartz, olivine (green quartz?), magnetite, and mica (biotite and
muscovite) were evident in the samples. Of all the minerals, quartz is the most abundant, followed by
feldspars, magnetite and finally micas. Each transect has a different quartz content. Transect 1 has quartz
proportions ranging between 60 and 86%, with widely spaced contours decreasing offshore. The second
transect has closely spaced contour lines, with quartz values between 87 and 80%. Here, the decrease is not
uniform, but rather drops suddenly from 24% to 8% from sample GB 16 to GB 17. Transect 3 has fairly
constant values ranging between 70 and 85%. Transect 4 has values between 20 and 86% quartz, with a
sudden quartz decrease in sample GB 38 (66% to 21% quartz). In this case, the contours are more or less
closely spaced. Transect 5 has closely packed contours with values ranging between 35 and 81% (map 3).
The feldspars in Transect 1 range between 4 and 12 % and generally increase offshore. Transect 2 has
increasing values offshore, although from GB 17 to GB 15 there is a sudden drop and then rise, in this case
the values ranging between 1 and 24%. Transect 3 feldspars range between 1 and 16%. Transect 4 has
values ranging between 3 and 29%, with the contours close and values not uniform. Transect 5 contains 10
to 18% feldspars, with contours closer and generally increasing offshore (map4).
Although the green and black minerals are present at some of the deep-water samples, they have negligible
percentages in relation to the quartz and feldspars. Organic components consisted of plant debris and shell
fragments.
The shell fragment distribution in each transect follows. Transect 1 has values between 3 and 9%,
decreasing offshore and with widely spaced contours. In the same transect, the values go to zero at 25m
depth. Transect 2 has shell fragment percents from 0 to 13%. In this transect, samples GB 13, 15, 16 and
17 contain no shell fragments, but sample GB 14 contains an ostracode shell. Transect 3 has shell
fragments ranging from 3 to 9%; Transect 4 from 1 to 24 %. In Transcet 4, the contours increase offshore.
Transect 5 contains no shell fragments (map 5).
The plant debris in each transect generally increases to about 400 m water depth, where the debris content
then begins to decrease. Transect 1 has offshore plant debris contours ranging between 1 and 20%. In
Transect 2 the debris increases offshore, although some of the samples contain sudden drops in their values,
ranging from about 3 to 72%. Transect 3 has increasing amounts of the plant matter and Transect 4 has an
plant matter ranging from 1 to 49%. Finally, Transect 5 has increasing plant debris from 1 to 34%, with
closely spaced contours.
The angularity, sorting and sphericity of the lake sediments is not very uniform. Transect 1 has well sorted,
equigranular and sub-angular grain fabric. In Transect 2, the sand is generally well-sorted, with 70%
equigranular or sub-angular, and only one percent sub-rounded. Transect 3 contains well-sorted grains,
70% equigranular, fragmented, or sub-angular. Transect 4 is characterized by well-sorted, fragmented,
equigranular and sub-angular grains. The fifth transect has grains that are mildly sorted, fragmented
equigranular and sub-angular.
Discussion
Results show that the abundance of quartz is in part due to the lithology and resistance of quartz to
weathering. Based on the Kigoma geological map by Halligan (1960) (see map 8), the rivers supplying
sediments into the lake drain various geological formations. Dove River drains amphibole gneiss with
basal conglomerates, quartzite and sub-greywacke, white quartzites and sandstones. The Mungonya River
drains red sandstones, quartzite subgrey wacke and white quartzite, sandstones with basal conglomerate
and rare shale horizons. The Balenzi River drains red sandstones, quartzite and subgrey wacke formations.
Kaseke River drains areas that have amphibole gneiss and schists, with some quartzo-feldspathic injection,
white quartzites and sandstones with basal conglomerates. The Luiche River, which is enriched with green
minerals, drains from calcareous shale, limestone, grits and conglomerates. Other formations are dolomitic
limestone, often silicified, flood basalts, and amygdaloidal lava with occasional intercalations of chert
bands. From such a wide field of drainage it is evident that the dominant mineralogy should quartz. The
mineralogical similarity between river and lake samples reveals river participation in sediment supply.
This is another line of evidence that the depositional environments are dependent on the structural setting
of the area. From map 6, it is shown that these sediments are supplied through the channels (channels 1, 2,
and 4, which intersect transects, map 1), evidenced by the presence of the green granular minerals in the
channel and river samples. This also supplies information about the provenance of the sediments in the
lake. The angularity, sphericity and sorting define the distance which particles traveled from their origin to
the environment of deposition.
In the first transect, sand decreases gently offshore, due to the change in energy involved in sediment
transportation. The second transect has seven sample s which fall in the channel; these sample have
relatively low proportions of sand compared to the levee samples, showing that the channel might be
inactive, hence the increasing proportion of sand further up the channel. Transect 3 has an increasing sand
proportion offshore, indicating that the samples are channel derived, with sample GB 65 being in the
channel and the other on the levee. This transect is located in the proximal delta. On Transect 4, the
sediments sampled in the channel appear very coarse (samples GB 37, 36 and 35). Transect 5 is
characterized by an onshore increase in the sand proportion. The sudden drop in sample GB 32 can be
attributed to a change from the channel to the levee, then back to the channel (with high values of sand), as
shown in map 6.
Clays are inversely proportional to sands in every transect. This explains why the sand increases onshore
and then decreases offshore. This is caused by unequal distribution of the energy, with areas of high clay
having low energy and those with low clay proportions having high energy (and thus not giving the clays
time to settle). The variation in sediments is also influenced by the delta shape, as the sand and clay-silt
data forms three regions of concentration. That is, the center is relatively low and the sides are higher.
Quartz values are high in allareas containing high concentrations of sand, implying that most of the sand is
composed of quartz. That points out the dominance of quartz in the area. Feldspars, on the other hand, are
the opposite of quartz; their availability is dictated by the availability of sand. This is typified by samples
with high amounts of sand having high quartz and low feldspar contents.
Shell fragments are restricted to cartain environments. These shells were very prominent near the shore,
while offshore their dominance is restricted to a specific depth. Only ostracod shells are abundant at depths
greater than 25 m. Most of the shells recorded in the samples were fragmented. Their distribution is
dictated by the structural setting of the substratum and the energy of deposition for various sediments,
similar in pattern to the sand and clay-silt distribution. Plant debris are generally increasing offshore, a
trend caused by the decrease in depositional energy in the lake. Because they weight little, they require a
minimum of energy for transportation and hence deposition. On other hand, oxygen content dictates the
availability of plant debris, as it oxidizes them to other inorganic products when they are in the oxic zone,
and they become safe below it. This situation is typified by the presence of brown coarse sediments in
shallow parts of the lake.
Conclusions
The sediment supply in the shallow water lakes is highly influenced by rivers, thus anything affecting them
would impact the system in the lake. The dominance of quartz is due to the geological formations from
which they are derived. A further control is the amount of sand available. The distribution of the sand is
dictated by the channels, though they are inactive at an increased lake level and thus depositing fine
sediments such as clays and silt. These channels contribute to an anomalous trend of the sand, silt-clay,
and related component distributions in the lake. Other controlling factors are shape of the delta and energy
of deposition of the particular sediments. The sediments distribution shows a coarsening trend towards
shore.
Acknowledgements
I acknowledge the WWF for funding this research training program, as well as the director (Prof A. Cohen)
and all other Nyanza administrative staff (Catherine O' Reilly, Ellinor Michel and others) for organizing the
program and making it very delicious and encouraging. I would ultimately like to thank my mentors
(Kiram Lezzar and Andy Cohen) for their readiness in giving me knowledge on my project and any other
experience they shared with me (R.J. Hartwell, Kamina Chororoka, and Willy Mbemba). Lastly I’m
appreciative to all my fellow Nyanza participants; students and all of those who shared a hand along with
me.
References
Cohen, A. S., 1990. Tectnostratigraphic model for sedimentation in Lake Tanganyika, Africa. AAPG Memoir’. p 137-149.
1
TR
GB12
GB49
Luiche River
GB13
GB46
GB45
TR1 (Sampling Transect 1)
GB44
GB34
TR5
GB69
TR
4
GB47
GB23
TR3
GB48
TR
2
GB1
GB5
GB60
GB33
GB43
GB59
Luiche Platform
GB24
GB50
GB58
GB57
GB51
GB56
GB55
GB52
GB53
GB54
Lake Tanganyika
Map 1: Location Map of Luiche River Delta and Offshore Platform (depth contours in
meters). Grab Sample locations are indicated by black dots along 5 transects TR1 to
TR5).