Sedimentologic analysis of the Kalya and Sibwesa beaches, Lake
Tanganyika
Student: Paul Masatu
Mentor: Dr. Kiram Lezzar
Introduction
Sedimentary processes in the Lake Tanganyika Rift Basin are dependent on both the tectonic processes that
occur in this segment of the East African rift system and the climate changes that have occurred in the
region over the past 12 Ma. (Tiercelin et al, 1992). Sibwesa and Kalya are situated in the northern tip of
the southern basin as the presence of Mahale mountains marks the difference between the two beaches.
Sibwesa beach is closer to the Mahale mountains and the slope from the mountains to the beach is steeper
compared to the gentle slope of the Kalya beach, situated far away from the Mahale mountains. The two
beaches experience two deferent types of wind; during the wet season the wind is blowing from the NE
direction so the Kalya beach is protected, and Sibwesa beach experiences high energy compared to the low
energy of the Kalya beach. This situation is reversed during the dry seasons.
Objectives of the study
My objectives were to study the distribution of sediments on the two beaches by determining the size and
sorting of the sediment grains. These studies may reflect sedimentation mechanisms and deposition
conditions. Also sediment studies can give an indication of climate condition.
Materials and Methods of Study
Field methods
Samples were collected from Sibwesa and Kalya beach. Each beach was divided into several sites (for
example, Site1, Site2, etc). Each site was assigned a GPS point and location on a map. At each site, a
50cm to 60cm hall was dug and measured the entire vertical stratigraphic section at each exposure,
measuring thickness and describing lithology for each distinct unit.
Laboratory analysis
Sediment sample from the field was sieved through 7-sieve size to determine grain sizes, the weight of each
sieve is measured by using a weigh pan. Data from the laboratory was fed in a computer to obtain the
graph.
Results and Observations
A description of sediment analysis on Sibwesa and Kalya beaches is shown in the graphic representation of
grain size for each beach (Figure 3 & 4); graphic mean and inclusive graphic standard deviation for
respective graphs is shown in Table 1.
Graphic Mean: MZ=Ø16+Ø50+Ø84
3
Inclusive standard deviation: ó1=Ø84-Ø16 +Ø95-Ø5 Ø=--Log2
4
6.6
Where “Ø” is phi size and “d” is the grain diameter in millimeters (Lewis and McCondie, 1994).
Sibwesa beach is dominated by coarse sand contrary to the fine sand of Kalya beach; the rocks a few
centimeters from the water surface cover most of the Kalya beach. The sediments of layer 2 from Sibwesa
Table 1
Sibwesa
Site
Layer
1
2
3
4
Graphic
mean
Inclusive graphic
Average
standard
standard deviation
deviation
Description
Average site description
1
1.06
1.065
2
-0.02
0.35
poorly sorted
3
Only
one
layer
-0.1
0.39
-0.24
0.219
1
0.55
1.12
2
0.08
1.013
3
Only
one
layer
0.07
1.02
-0.98
0.73
0.73
moderate sorted
moderate sorted
Graphic
mean
Inclusive graphic
Average
standard
Description
Average site description
standard deviation
deviation
0.6
well sorted
moderate well sorted
well sorted
0.219
very well sorted
very well sorted
poorly sorted
1.051
poorly sorted
poorly sorted
poorly sorted
Kalya
Site
1
2
3
4
5
Layer
1
2.13
1.37
2
0.83
0.72
3
-0.57
-0.61
1
2.51
0.495
2
2.24
0.61
1
2.35
0.69
2
2.29
0.1
1
2.05
1.035
1
2.07
0.83
2
1.82
0.66
3
1.83
0.65
4
0.24
1.056
poorly sorted
0.49
moderate sorted
well sorted
very well sorted
0.55
0.34
1.035
well sorted
moderate sorted
moderate sorted
very well sorted
poorly sorted
moderate well sorted
very well sorted
poorly sorted
moderate sorted
0.8
moderate sorted
moderate sorted
moderate well sorted
poorly sorted
beach are well sorted compared to the poorly sorted sediments of site. On the Kalya beach there is an
increase in the size of sediments from site 1 to site 5. A vertical stratigraphic section for each site is shown
in Figure 1.
Discussion and Conclusion
According to the observation above, Sibwesa beach is dominated by coarse sediments, which can be
explained by the steeper slope from Mahale mountains to the beach. The sediments eroded from the
mountains takes little time to be transported and abraded, thus they reach the beach while they are still
coarser. Another reason is that there is no river, which brings new inputs on the beach. The fine sediments
of Kalya beach can be explained due to the gentle slope between it and the Mahale Mountains, as sediments
eroded from the mountains has enough time to be transported and abraded. Presence of a river crossing the
Kalya beach is another source of coarse sediments deposited on the beach.
Figure 3
Site5 layer1 kalya
beach
Site5 layer2 Kalya
beach
12
20
15
8
Weight(gm )
Weight(gm )
10
6
4
10
5
2
0
0
-2
-1
1
2
3
-2
4
-1
Ggrain size(phi)
20
Weight(gm )
w eight(gm )
25
15
10
5
0
1
2
Grain size(phi)
3
4
Site5 layer4 Kalya
beach
30
-1
2
Grain size(phi)
Site5 layer 3 Kalya
beach
-2
1
3
50
45
40
35
30
25
20
15
10
5
0
-2
-1
1
2
Grain size(phi)
3
4
Figure 4
Kalya site5 layer2
100
90
100
90
80
70
60
50
40
30
20
10
0
80
%cum w eight
%cum w eight
Kalya site5 layer1
70
60
50
40
30
20
10
0
-3 -2 -1 0 1 2 3 4 5
-3 -2 -1 0 1 2 3 4 5
phi values
phi values
100
90
80
70
60
50
40
30
20
10
0
Kalya site5 layer4
%cum weight
%cum weight
Kalya site5 layer3
-3 -2 -1 0 1 2 3 4 5
phi values
100
90
80
70
60
50
40
30
20
10
0
-3 -2 -1 0 1 2 3 4 5
phi values
Acknowledgements
I would like to show my appreciation for my mentor, Dr. Kiram Lezzar, Geoff Ellis, Kamina Chororoka
and other Nyanza staff. I would also to thank my fellow Nyanza students for their help.
References
Boggs, J.R., 1994. Sedimentology and Stratigraphy. PUBLISHER
Lewis, Douglas .W& McConhie, D., 1994. Analytical Sedimentology. PUBLISHER
Tiercelin, Jean-Jacques, 1992. Sedimentation in the East African Rift Great Lakes. JOURNAL,
VOLUME, PAGES.