A high resolution seismic study of the Kalya horst
and platform: Long term paleoclimate drilling
site survey in central Lake Tanganyika
Students: L. Cody Helfrich, Marla Torrado,
Christine R. Gans, & Kamina Chororoka
Mentor: Kiram Lezzar
boat, and 70 meters behind the GPS antenna. The source used
was a fish-type sparker, built by S.I.G., operating with 1000
joules and delivering 4000 volts. Arrivals were recorded on
a DELPH computer between 0 and 1500 Hz frequency. Data
from channel 3 was then scaled and printed by an Oyio 612 on
12-inch thermal paper. Interpretation of the data focused on
locating terminating and crossing reflectors, as well as changes
in reflector characteristics.
Introduction
Seismic Analysis Results
Lake Tanganyika, located in the western branch of the East
African Rift, is an elongated basin which formed as a result of
regional extension (Tiercelin and Mondeguer, 1991) (Figure 1).
The lake provides the ideal setting for characterizing the complex
kinematics and sequence stratigraphy associated with an active
rift system (Cohen et al., 1993). Further, Lake Tanganyika has
the potential of containing a continuous paleoclimate record
of the southern hemisphere, on the order of several million
years. Subsidence occurs through linked and alternating half
grabens, internally faulted and separated from each other by
horsts of varying development and sediment burial (Tiercelin
and Mondeguer, 1991). Our work focuses on the central part of
Lake Tanganyika; just south of Mahali Mountains in the Kalya
region, where two opposite-polarity half grabens are separated
by a topographic high: the Moba-Kalya Horst (Rosendahl et al.,
1987) (Figure 2). A similar survey of the Kavala Island Ridge,
located on the northern side of the Mahali Mountains (Figure
1), found low stand erosional unconformities (Scholz, 2003).
This observation would make the Kavala horst too shallow to
contain a continuous paleoclimate record. The Kalya Horst,
described as a high relief accommodation zone and imaged
by earlier Nyanza Project bathymetric surveys, is believed to
be deep enough to have not been affected by low lake stands
yet high enough to be isolated from terrestrial input (Brislen,
2001; Henderson and Gans, 2000). If so, the horst potentially
provides an excellent site for long-term subtropical African
paleoclimate studies. The goal of this project was to image the
Kalya Horst eastern ridges and associated platform and deep
basin with a high-resolution sparker reflection seismic survey,
and to evaluate the data for major border and intrabasinal fault
trends, as well as to interpret seismic sequences, unconformities
and facies. Here we will discuss the preliminary results of our
structural and stratigraphic analysis in the context of site
location for a long-term paleoclimate drilling program.
Topographic and Structural Observations
Methods
Approximately thirty lines of multichannel reflection seismic
data were acquired in the Kalya Horst region of Lake
Tanganyika, during an eight day cruise aboard the M/V Maman
Benita. Information was collected in a seven line by seven line
grid of approximately 4 to 7 km spacing (see Figures 1 and 3
for location). Position, bearing and speed were monitored and
recorded by a Garmin GPS II+ system. Data were correlated
to PROBE survey lines P83.84 and P83.78, (Rosendahl et al.,
1987) which were acquired with a lower frequency, deeper
penetrating system. Seismic information was acquired using
an S.I.G. built streamer with 6 channels and 8-meter takeouts.
The first hydrophone was positioned 60 meters behind the
Shot numbers and geographic location were plotted in ArcGIS, along with water depth and fault traces, to generate a
bathymetric map of the survey area (Figure 3). Steep faults
were imaged running parallel to the shoreline, trending
NW-SE, and became less apparent in the deep basin. In the
northeastern region of our survey area channels were found
with a northeast trending direction across the East Marungu
Half Graben (Figure 3). The Kalya Horst’s eastern ridge has
an imaged length of 25 km NW-SE, and is shown as a “V”
shaped ridge. To the northwest, the ridge become less apparent,
imaged as a gentle almost buried dome, as opposed to the steep
structure imaged to the southeast (Figure 4). The maximum
depth imaged in the survey was 768 m (Kalya deep basin) and
the shallowest was 52 m (Kalya upper platform).
Axial faults trending N 140o define the geometry of the area.
The Moba-Kalya Horst is bordered by two major faults, LUF1
to the east and WMFZ to the west (Rosendahl et al., 1987).
These faults define two previously identified half grabens:
Moba Half Graben to the east and West Murungu Half Graben
to the west (Figure 2). The LUF1 border fault defines the
eastern boundary of the Kalya Horst, and accommodates the
region of maximum subsidence for the Moba Half Graben
(Rosendahl et al., 1987). Three main ridges could be identified
at the Kalya Horst eastern edge separated by alternating halfgrabens (Figure 2a). The most important and pronounced ridge
of our survey is Ridge 1, which trends SE-NW. In general, the
trace of the ridge appears to be continuous but slightly variable
in trend direction, suggesting gentle oblique geometries across
Ridge 1 (Nelson et al., 1992) (Figure 4).
No major transverse faults could be found responsible for
displacing the horst, as proposed by previous studies (Brislen,
2001; Henderson and Gans, 2000). Two parallel, left-lateral
faults, trending North 45-70o were found bisecting the
northernmost part of our survey area displacing the Kalya steep
slope fault. As a result, the Kalya steep slope fault escarpment
shows a displacement perpendicular to the trending of these
faults. No normal component was seen, allowing us to infer
almost pure strike-slip motion. Minor faults were found west
of the horst, but none on the eastern side, suggesting small,
localized deformation as part of internal fractures.
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Stratigraphic Analysis
Seismic Facies Descriptions
Seven dominant seismic facies were evident throughout the
survey based on reflector geometry, amplitude, continuity, and
frequency (Figure 5).
Seismic Facies 1 - well laminated, sub-transparent layer (SF1).
Seismic Facies 2 - well laminated, high amplitude reflectors,
high frequency (SF2).
Seismic Facies 3 - well laminated, high amplitude reflectors,
low frequency (SF3).
Seismic Facies 4 - moderately laminated, mostly chaotic, subtransparent reflectors (SF4).
Seismic Facies 5 - moderately laminated, high amplitude
reflectors (SF5).
Seismic Facies 6 - discontinuous, moderate to low amplitude
reflectors (SF6).
Seismic Facies 7 - chaotic high amplitude reflectors (SF7).
Sequence Descriptions and Interpretations
Six seismic sequences of multiple facies are described
throughout the study area. Each sequence has been defined on
the basis of seismic discontinuities, using reflector termination
and/or major facies changes. Sequence distribution is shown in
Table 1. Three major unconformities, one defining a sequence
boundary, are shown in Table 2.
Sequence A, the uppermost sequence in the survey, is comprised
of SF1. This sequence is not found everywhere throughout the
survey, pinching out in shallow water on the East Marugu
HG, and on steep ridge escarpments (Table 1). Sequence A is
interpreted as draped sediments from the most recent stages of
deposition. Though discontinuous across many lines, Sequence
A does not onlap or show any unconformable termination at the
top of Sequence B. This implies the transition is not an erosional
surface, but rather Sequence A terminates by pinching out onto
Sequence B, as a conformable transition (Figure 5).
Sequence B is composed of three units and five seismic facies.
Unit 3, the overlying unit, is formed by SF3, SF2, or SF7. The
underlying Unit 2 can be comprised of SF2 or SF7. Unit 1,
when present, is the basal unit made of SF5, or SF4 when
contained within Unit 2. (Distribution of Seismic Facies shown
in Table 1). Generally SF2 is found immediately below SF3.
Where present, SF5 underlies SF2. In three lines SF4 is seen
as a lens-shaped deposit between SF3 and a very thin layer of
SF2. SF7 is only found alone or underlying SF2, as on the horst
ridge. The transition to Sequence A is seen as a conformable
surface. In several locations Sequence B is the uppermost
layer. Sequence B is interpreted as coarser deposits. The base
of Sequence B is marked by a major erosional unconformity
truncating the top of Sequence C (Figure 5). This unconformity
can be correlated to one found in a 1996 study of the Ubwari
horst in the northern part of Lake Tanganyika (Lezzar et al.,
1996), where it was estimated to date to 193 ka for the sequence
transition.
Sequence C is formed by SF4 and is only found on the Moba
Half Graben with an apparent dip toward the Kayla Horst
(Figure 5) (Table 1). The upper boundary is typically, with the
exception of the Kalya Horst, marked by an erosional surface
over which Sequence B is unconformably deposited. The base
is marked by the onlap of Sequence C onto Sequence D.
Sequence D, formed by SF6, most commonly defines the
acoustic basement, being the lowest distinguishable sequence.
The upper boundary of this sequence is the onlap of Sequence C,
or more commonly an erosional truncation caped by Sequence
B. Sequence D shows a greater apparent dip than Sequence C
(Figure 5). When present, Sequences E and F are cut by an
erosional truncation over which Sequence D is found, or in one
case on the Kalya Horst, they are all deposited conformably.
Sequences E and F are formed by SF4 and SF6. These two
sequences are the oldest in the stratigraphic succession, and
are only imaged on the horst. The interpretation of these
sequences is unclear because of the imaging resolution at
depth, but Sequence E (SF4) appears to be onlaping Sequence
F (SF6) where the two sequences are erosionly truncated
and unconformably underlying Sequence D. On the deep
Kalya Horst however, Sequences D, E, and F appear to be
conformably deposited.
Discussion
Topographic and Structural Analysis
After analysis of the data, a more detailed description and
interpretation of the Kalya horst and platform area can be
made, expanding on previous studies of the same region
(Henderson and Gans, 2000; Brislen, 2001). Correlations are
made between line NP04-18 of this survey and PROBE Line
P83.84 (Figure 2). The major border faults imaged by the
PROBE survey, which we were able to correlate to our study,
allowed us to designate the structural environments and review
the minor faults in context. The higher resolution data from this
year’s project shows a larger number of minor near-surface
faults, and allowed us to distinguish a third ridge along the
Kalya horst. The Kalya Horst eastern ridge (R1) shows neither
signs of displacement throughout the study area, nor evidence
for an offset ridge as a product of transverse strike-slip faulting,
as was proposed by previous Kalya Horst studies (Henderson
and Gans, 2000; Brislen, 2001). There was also no evidence
for an overlap between ridges segments. Previous studies
elsewhere have shown that there are different types of rift
segment interactions, especially in a long rift structure such as
the one found at the Kalya region (Nelson et al., 1992). Ridge
R1 seems to have an oblique NW-SE but continuous northwest
trend parallel to the Mahali Mountains escarpment (Figures
3 and 4). Internal faults are found throughout the horst area;
nevertheless, they are not capable of displacing the horst.
These faults are not major faults but localized transverse faults
that affect small segments of the whole ridge structure.
At the northernmost part of the survey two parallel tracing
left-lateral srike-slip faults, trending N 45-70o, mark the
northernmost boundary of the Kalya ridge. These faults show
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little to no vertical displacement, which is indicative of lateral
movement. North of these faults lies a deeper basin (Figure
3). A displacement of the Kalya steep slope fault escarpment is
imaged south of Sibwesa Bay, which is parallel to the strike-slip
faults found. The displacement from this strike-slip fault also
appears to continue through the slope escarpment, and into the
southeast Mahali Mountains (eastern end of the Kalya beach).
The Mahali Mountains are being displaced, along the trend of
the fault, approximately 7.5 km and the slope escarpments have
an apparent displacement of ~ 4.1 km.
Stratigraphic Analysis
The major discontinuities seen between the sequences described
can be attributed to very large lake-level fluctuations. The
dipping Sequence C is generally the oldest sequence imaged.
The typically high amplitude of the reflectors in Sequence E
can be interpreted as the coarser sediments characteristic of
lake low stands (Scholz, 2003). Immediately overlying and
onlaping is Sequence C. The transparency of this sequence,
along with its onlaping unconformity, is typical of a rising
lake level (Emery and Myers, 1996). The erosional truncation
between sequence E and Sequences D and C is shown as a
very large unconformable surface almost uniformly found
around 600 ms (Figure 5, Table 2). This unconformity can be
attributed to a different low stand, which is concurrent with the
coarser low stand deposits that comprise Sequence E. A date for
this low stand from between 190 and 135 ka can be assigned
based on correlation with an erosional surface in the Northern
Lake Tanganyika record (Labeyrie, 2003; Lezzar et al, 1996).
Within Sequence E lie the lens-shaped progradational delta
deposits of lines NP04-DC and NP04-6 (Figure 6). At the top
of this delta lies a paleo-shoreline and shelf break. This low
stand at and around 480 ms records a late Pleistocene lowstand.
(Figure 6). The delta deposits are then caped with a thin layer
of coarse-grained low stand deposits. Above Sequence E the
transparent continuous layers of Sequence F are once again
indicative of a higher lake stand. Within Sequences C, D, E and
F are found evidence for at least two major regional-scale low
stand fluctuations. Similar sequences and low stands have been
observed in the Kavala Horsts region, as well as in the northern
part of Lake Tanganyika (Lezzar, 1996; Scholz, 2003).
Conclusions
The previously imaged Kalya Horst eastern side, adjacent to
the Moba Half Graben, is formed by continuous trending ridges
and is not displaced by faulting (Figures 3 and 4). However,
the northernmost extent of the Kalya Horst is bordered by two
parallel strike-slip faults. These faults displace the Kalya steep
slope escarpment, as well as the southern end of the Mahali
Mountains (Figure 3). Two major erosional unconformities
indicative of lake low stands, further supported by overlying
low stand deposits and paleodelta deposits, can be found at 480
ms and 600 ms depths respectively (Figures 5 and 6). The low
stand at 480 ms is late Pleistocene in age, and gives a low stand
water depth of 360 m below present. The unconformity at 600
ms is correlated to a similar low stand in the northern lake basin
which is dated at 192 ka before present (Lezzar et al, 1996). It
may correlate with a sea-level lowstandglacial interval (0156)
~160 ka. (Labeyrie, 2003). A third, presumably non-low stand
erosional unconformity is found at a depth of 800 ms and is the
constraining element in proposing a potential drilling site. The
proposed site is shown on Figure 3, where there are no apparent
erosional surfaces, meaning a continuous sediment package can
be retrieved (Figure 7). The proposed site is in 600 m of water
which would allow for a 200m sediment core to be retrieved by
using the GLAD 800 drill rig. A date estimated for the base of
Sequence B (118 m below the sediment/water interface) is 1.2
Ma using a sedimentation rate of 0.1 mm/yr, and 0.6 Ma using
a sedimentation rate of 0.2 mm/yr. To determine the viability of
this site, a higher density seismic study must be performed, and
correlated to piston cores taken on the proximal horst.
This study has shown that the Kalya Horst Ridge is a
continuous topographic high, neither bisected nor displaced by
major transverse faults. The northern extent of the Kalya Horst
is marked by two srike-slip faults potentially displacing the
Mahali Mountains. Further, two regional climate fluctuations
are shown as major erosional unconformities both defining and
within sequences. Finally, this study located a potential drill site
containing a record of climate change in Africa undisturbed by
tectonics or erosion for the last million years.
Acknowledgements
We would like to thank the National Science Foundation for
funding this project, and the crew of the M/V Maman Benita,
as well as C. Winston Wheeler for providing the bathymetric
model. We would like to give special thanks to the infallible
mentors that supported us throughout this project: Kiram
Lezzar and Jim Russell.
References
Brislen, M. 2001. Bathymetry ans structure of the Kalya horst
region, South basin, Lake Tanganyika. Nyanza
Project 2001 Annual Report, p.12-15.
Cohen, A., Soreghan, M., and Scholtz, C.,1993. Estimating the
age of formation of lakes: An example from Lake
Tanganyika East African Rift system. Geology 21,
414-418.
Emery, D. and Myers K. J., 1996, Sequence Stratigraphy.
Blackwell science ltd.
Gasse, F., Lédée, V., Massault, M. and Fontes, J.C. 1989.
Water-level fluctuations of Lake Tanganyika in phase
with oceanic changes during the last glaciation and
deglaciation. Nature, v.342, p.57-59.
Henderson, A., and Gans, C.,2000. Bathymetric survey and
structural models of the Kalya Ridge, South Basin,
Lake Tanganyika, Tanzania. Nyanza Project 2000
Annual Report, p.16-19.
Labeyrie, L., Cole, J., Alverson, K. and Stocker, T. 2003. The
history of climate dynamics in the late Quaternary.
Paleoclimate, global change and the future:
33-61. Springer, Berlin.
Lezzar, K.E., Tiercelin, J.J., De Batist, M., Cohen, A.S.,
Bandora, T., Van Rensbergen, P., Le Turdu, C.,
Mifundu, W., and Klerkx, J. 1996. New seismic
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Research, v.8, p.1-28.
Nelson, R.A., Patton, T.L. and Morley, C.K. 1992. RiftSegment Interaction and Its Relation to Hydrocarbon
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Rosendahl, B.R. 1987. Architecture of continental rifts with
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Scholz, C.A., King, J.W., Ellis, G.S., Swart, P.K., Stager, J.C.
and Colman, S.M. 2003. Paleolimnology of Lake
Tanganyika, East Africa, over the past 100 k yr.
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Table 1
Region
Kalya Horst
Kalya Ridge
Moba HG
E. Marungu HG
Sequences Present
Imaged on
A,B,C,D,E,F
NP04-11,13,15,18,19,24
B,D
NP04-5,8,18,19,24
A,B,C,D
NP04-00,3,5,6,8,8A,15,18,19,24
A,B,D
NP04-8A,18,24
Table 2
Unconformity
1
2
3
Time Depth Range
at and around 480
540- 680
760- 860
Imaged on
NP04-6,15
NP04-3,18,19,24
NP04-6,8,13,19
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