Quaternary International 290-291 (2013) 57e67
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Quaternary International
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Basin morphology and seismic stratigraphy of Lake Kotokel, Baikal region, Russia
Yongzhan Zhang a, Bernd Wünnemann a, b, *, Elena V. Bezrukova c, Egor V. Ivanov c,
Alexander A. Shchetnikov d, Danis Nourgaliev e, Olga V. Levina c
a
School of Geographic and Oceanographic Sciences, Nanjing University, 21 Hankou Road, Jiangsu 210093, Nanjing, PR China
Institute of Geographical Sciences, Freie Universitaet Berlin, Malteserstr. 74-100, 12249 Berlin, Germany
c
Institute of Geochemistry, Siberian Branch Russian Academy of Sciences, Favorsky Str. 1a, Irkutsk, 664033, Russia
d
Institute of the Earth’s Crust, Siberian Branch Russian Academy of Sciences, Lermontov Str. 1a, Irkutsk, 664033, Russia
e
Faculty of Geology, Kazan State University, Lenin Str. 18, RU-420008 Kazan, Russia
b
a r t i c l e i n f o
a b s t r a c t
Article history:
Available online 3 December 2012
The basin of Lake Kotokel, located along the eastern shore of Lake Baikal, Russia, has attracted several
scientific projects to investigate the climate, vegetation and lake history throughout the Late Pleistocene
and Holocene. However, little was known about its basin structure and sediment architecture. Echo
sounding and 3.5 kHz single frequency sub-bottom profiling were used to decipher the basin
morphology and seismic stratigraphy to a depth of approximately 50 m. The bathymetric map shows
a very shallow lake of 4 m mean water depth and an almost flat lake bottom. A distinct elongated smallsized depression of up to 12 m water depth between the north-western coast and a small island
developed along an NW-SE-oriented fault line. A total of 46 km of seismic profiles crossing the lake along
12 transects shows that the bottom sediments consist of three different facies, which accords to previously analyzed core sequences. Several distortions of sediment layers at various sites indicate tectonically
induced impact, which resulted in up to 3 m vertical offsets of sediment packages at local sites. The
offsets indicate a probably still active fault along the western shoreline of the lake. Soft gyttja of the
upper 6 m does not show distortions and may have obscured potential younger tectonic activity. The
sediments date to the Late Pleistocene. Small updoming features along the boundary between layers I
and II may be assigned to degassing processes or to seismic activity. River channel fills along the northeastern coast are indicative of a lower lake level prior to 15 ka BP. The sediment stratigraphy indicates
that suitable coring sites for paleoclimate studies are only located in the southern part of the basin where
almost undisturbed sediments can be expected.
Ó 2012 Elsevier Ltd and INQUA. All rights reserved.
1. Introduction
Lake basins are of particular importance for paleoclimate
studies, as they are local sinks for a variety of sedimentary transport
processes which contribute to the final deposition of material
within a lake. The geological setting and morphology of a given lake
basin and its catchment, local to regional climate conditions and
human activity influence such dynamic processes significantly (e.g.
Wünnemann et al., 2010). They even may amplify site-specific
depositional patterns, if abrupt tectonic events, human-controlled
landscape development or extreme climate events are involved.
* Corresponding author. School of Geographic and Oceanographic Sciences,
Nanjing University, 21 Hankou Road, Jiangsu 210093, Nanjing, PR China.
E-mail
addresses:
bwuenne@nju.edu.cn,
wuenne@zedat.fu-berlin.de
(B. Wünnemann).
1040-6182/$ e see front matter Ó 2012 Elsevier Ltd and INQUA. All rights reserved.
http://dx.doi.org/10.1016/j.quaint.2012.11.029
As a consequence, sediment architecture within a lake can vary
through space and time, as demonstrated by seismic surveys in
different lake basins in China and other parts of the world (e.g.
Lister et al., 1991; Vanneste et al., 2001; Colman et al., 2002;
Colman, 2006; Dietze et al., 2010; Daut et al., 2010).
Lake records are widely applied to infer the lake’s history and
related influencing factors by identifying various proxy data obtained from drilled sediment cores. They are preferably assigned to
climate change throughout the Late Quaternary. Many of the reported results refer to a single sediment record, assuming that the
obtained data reflect the general depositional conditions within the
system caused by climate impact. However, little is known how
strongly the basin morphology and the spatial variety in deposition
may have overprinted the site-specific sediment composition, thus
influencing the interpretation of records.
During recent decades, many lake records from the Lake Baikal
region in southern Siberia were investigated for paleoenvironmental
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Y. Zhang et al. / Quaternary International 290-291 (2013) 57e67
and paleoclimate purposes. They reveal millennial-scale oscillations
of the regional climate over several millions of years, particularly
during the Late Pleistocene and Holocene (e.g. Colman et al., 1996;
BDP-Members, 1997, 2005; Karabanov et al., 1998; Prokopenko et al.,
2001; Kashiwaya, 2003; Prokopenko and Williams, 2004; Demske
et al., 2005; Tarasov et al., 1994, 2005, 2007; Bezrukova et al., 2005;
Swann et al., 2005). Krivonogov et al. (2004) provided a critical
comparison of numerical-dated sediments around Lake Baikal and
used the chronological framework for reconstructing Late Pleistocene and Holocene vegetation changes.
Lake research at the relatively small Lake Kotokel, located near
Lake Baikal, became an important issue since two records from the
southern basin of the lake and records from peat bogs around the
lake reveal dramatic oscillations in vegetation history, climate and
lake-internal bio-productivity (diatoms) throughout the last ca. 48
ka BP (Bezrukova et al., 2008, 2010; Shichi et al., 2009; Tarasov
et al., 2009). The two sediment cores KTK1 and KTK 2, recovered
about 100 m separated from each other, show similar sediment
composition, but with some differences in the thickness and time
intervals of the individual sediment sequences. None of the
retrieved cores reached the bottom sediments of the lake. To date,
it remains unclear whether a further core could recover even older
sediments and might help to solve the problems of chronology and
different thickness of lithological sequences. Some of the differences between the existing cores may be also attributed to
unknown specific sedimentary conditions at the respective sites
which could have influenced their stratigraphic order.
The application of seismic stratigraphy in lake basins is
considered to reflect the depositional conditions through time and
thus allows the reconstruction of syn- and post-depositional
processes, lake-level fluctuations and/or neotectonic impact
(Niessen et al., 1999; Colman et al., 2002; D’Agostino et al., 2002;
Schnellmann et al., 2002; Brooks et al., 2005; Anselmetti et al.,
2006; Colman, 2006; Hofmann et al., 2006; Beres et al., 2008;
Wagner et al., 2008; Oberhänsli et al., 2011). Therefore, the major
objective was to identify the basin morphology and depositional
conditions of Lake Kotokel, potential distortions of sediments, and
to evaluate the applicability of drill locations for new coring
activities.
2. Study site
Lake Kotokel (52 490 N, 108 090 E; 453 m a.s.l, according to SRTM
data; 458 m a.s.l. according to Russian topographic maps) is a small
and shallow tectonic freshwater lake located close to the eastern
coastline of Lake Baikal (Fig. 1). The lake basin of Cenozoic age
(Florensov, 1960) is separated from Lake Baikal by a SW-NE
stretching ridge (maximum height: 729 m a.s.l.). The shortest
distance between the lakes is about 2 km. The Ulan Burgasy Ridge
with elevations >2000 m forms the boundary to the east
(Bezrukova et al., 2010). A 2 km2 rocky island is located in the
north-western part of the lake. It belongs to the pre-Baikalian
crystalline complex (Galaziy, 1993). The closest distance to the
main land is ca. 600 m. The area of the catchment is calculated as
187 km2, of which the lake occupies 64 km2 (excluding the island).
The Kotokel rift basin is characterized by active recent geodynamics (Ufimtsev et al., 1998; Ten Brink and Taylor, 2002;
Shchetnikov, 2007). It is surrounded by pre-Baikalian folded
complexes of metamorphic rocks and granitoides. Cenozoic
formations occur in the southwestern and north-eastern part of the
basin catchment. According to Geological map 1:200,000, sheet N49-XXV (Davidov, 1974) several SW-NE striking active faults cross
the lake area and its island and thus should have influenced the
basin morphology. However, according to the Baikal Atlas, scale
1:4000.000 (Galaziy, 1993), only a SW-NE directed major fault
touches the eastern part of the lake.
The vegetation in the area is a mixture of boreal coniferous and
deciduous forests with wetland communities including sphagnum
bogs at flat near-shore locations on the southern side of the lake
(Shichi et al., 2009). The densely vegetated area around the lake
may be also responsible for considerable input of organic matter
and nutrients throughout the year, resulting in brownish water
with high amounts of suspended particles and leading to a very low
transparency of the water, as observed during the field survey.
3. Methods
As a detailed bathymetry of Lake Kotokel was lacking,
a Hummingbird echo sounder with coupled GPS system and double
frequency sonar signal transmission (80/200 kHz) was used. In
total, more than 2000 measurements were conducted following
transects according to Fig. 2. Several hundred data points from the
seismic surveys were also included and enabled comparison of
measured water depth between both systems along parallel transects. However, due to the very soft gyttja sediments at the interface between lake water and settled sediments, the measured
water depth differed between 50 and 100 cm from manually
controlled measure points, depending on the sediment density at
selected sites. The collected data points included the geographic
position, water temperature and water depth. They were exported
to MS Excel files for further processing by ArcGIS 9.3 software.
Design of the bathymetric map used the Raster-to-Topo function
which produced the best and most reliable contour map compared
with other procedures, e.g. spline or kriging. All elevation data are
based on SRTM elevation models which differ from the reported
elevations by about 5 m (Shichi et al., 2009; Bezrukova et al., 2010).
A geophysical survey was applied to reveal the shape of the lake
basin and to penetrate the sub-bottom sedimentary strata, using
a 3.5 kHz GeoPulse sub-bottom profiler, GeoAcoustics Ltd., UK,
coupled with a high accuracy GPS (Trimble GPS SPS351). The subbottom structures are delineated using reflections from a selectable single frequency multi-cycle high power signal, transmitted
from 4 transducers mounted on a towed platform.
Field exploration at Lake Kotokel in May 2011 used the following
main parameters: signal duration time 100 ms, sampling rate 60 ms,
transmitter output 10 kW, 3.5 kHz, receiver low pass and high pass
filter as 300 Hze7 kHz, gain as 6 or 9 dB, with TVG control. The
penetration velocity of the transmitted signal was calculated to
ca. 1500 m s1. Twelve tracks with a total length of more than
46 km were obtained (Fig. 1, Table 1).
Table 1
Seismic profiles from Lake Kotokel, conducted in May 2011.
No
Name
1
S110522-1
2
S110522-2
3
S110522-3
4
S110522-4
5
S110522-5
6
S110522-6
Total length
Length (m)
No
Name
Length (m)
2400
676
197
566
3586
3410
7
8
9
10
11
12
S110522-7
S110522-8
S110523-1
S110523-2
S110523-3
S110523-4
1806
987
11,189
5834
4056
11,687
46,394
The entire collected profiles were played back in the workstation to improve quality by using gradient filter, digital low and high
pass filters, gamma correction, and TVG gain control with poly fit
function. The playback profiles were compressed in the horizontal
axis and then changed to JPG files, processed with threshold control
and interpreted with the current software Photoshop CS4 and
MapInfo.
Y. Zhang et al. / Quaternary International 290-291 (2013) 57e67
59
Fig. 1. Overview of lake Kotokel and its catchment in lake Baikal region. A: Overview Russia; B: Lake Baikal with associated major faults after Shchetnikov et al. (2012), modified; C:
Relief, morphology and bathymetry of the Kotokel basin with locations of the drill cores KTK 1 and 2, seismic tracks and locations of the figures. Red lines: Seismic tracks presented
in figures; black dotted lines: Seismic tracks not shown and lake catchment; boxes display the location of the figures. Thin arrows mark the direction of seismic tracks in the
respective figures.
4. Results and discussion
4.1. Basin morphology
Most of the relatively small-sized Kotokel Basin is occupied by
the lake. Its catchment shape is dominated by steep slopes of up to
28% (w13 inclination). Fringes of alluvial plains between the ridge
and the lake shore are very small or even absent (Fig. 1), indicating
limited sediment transport and deposition along the slopes.
However, a larger flat area of alluvial/fluvial deposits at the northeastern side of the basin has been formed by fluvial activity along
the Istok and Kotochik Rivers. The slope of this plain to north and
towards the lake is about 2.7%, without a characteristic fan shape.
The plain may be associated with floodplain development. The
rivers are the only perennial drainages within the catchment which
partly (seasonally) drain to the lake. Due to low velocity flow, their
transport capacity is restricted to suspended load. A further small
perennial creek is located at the southern side of the lake. All other
valleys facing towards the lake provide water along small drainages
only during rainy seasons.
As mentioned by Shichi et al. (2009), the southern side of the
lake basin is bordered by an 8 m high fossil bluff, composed of
fluvio-lacustrine sand covered by eaolian (dune) deposits. Its
location between peatland on both sides indicates the formation of
a sand bar during a former highstand of the lake. Several shorelines
around the lake perimeter of up to 1.5e2 m above the present lake
level (2011) are evidence of fluctuating levels during the past
decades as reported by the local people. A 2-m increase in water
level would reach the foot of the bluff. Traces of higher lake stands
have not been found, and might be impossible as the lake would
flow out towards Lake Baikal via the Kotochik River if the level
exceeded 456 m a.s.l.
According to the bathymetric map (Fig. 1C), a morphological
separation of the lake into two sub-basins (Shichi et al., 2009) is
unlikely. Both the southern and northern part are similar in basin
morphology, comprising an almost flat lake floor with mean water
depth of ca. 4 m. However, the lake floor in the northern part
inclines by about 1.7e1.8% to the west, reaching 5 m water depth
close to the island (Fig. 1C). As this slight dip corresponds with the
alluvial plain at the north-eastern shore of the lake, it is assumed
that this plain formerly extended for about 2 km into the lake
during a lake low stand. Hence, lake deposits there should be
related to fluvial transport activity, implying coarser sediments and
higher accumulation rates than in the central parts.
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Y. Zhang et al. / Quaternary International 290-291 (2013) 57e67
Fig. 2. Seismic stratigraphy along the transect S110523-1 with lithology and chronology of core KTK 2 (after Bezrukova et al., 2010). A: Original seismic profile with marked area for
detail 1; B: Continuation of profile with marked areas for details 2 and 3; C: Stratigraphy and chronology of core KTK 2.
The previously reported “complicated bottom relief” (Shichi et al.,
2009, p. 99) of the northern sub-basin with a maximum water depth
of 14 m can be only partly confirmed. The bathymetry map (Fig. 1C)
shows an important elongated depression in the western part of the
basin between the main land and the island. Maximum measured
water depth was here close to 12 m (441 m a.s.l.). This comparatively
deep channel with symmetrically steep flanks consists of three isolated depressions, separated by barriers. They are definitely not
formed by fluvial processes but probably by tectonic impact, as
several faults were also detected in the seismic profiles (see below).
The depressions terminate in the center of the lake (Fig. 1C) and pass
into a very flat lake floor towards south, not exceeding 4 m water
depth. The center of this flat area was selected for coring of two
sediment cores KTK 1 and 2, which have been recently analyzed (e.g.
Tarasov et al., 2009; Bezrukova et al., 2010).
4.2. Seismic stratigraphy
From the 12 seismic profiles, comprising in total more than
46 km length, the most representative ones were selected to
demonstrate the general sediment stratigraphy within the lake and
its architecture. These transects cross the lake from southwest to
northeast, passing the central parts of the basin and the drill
locations of KTK 1 and 2 (S11023-1: track A; S110523-2: track B,
Figs. 1e3). A second transect surrounds the island and continues
SW along the western coastline of the lake (S110523-4: track C,
Figs. 4e6).
According to the profiles, three different units of acoustic
reflections could be identified, developed in the upper 20e25 m
below the lake surface. They are assigned to Units IeIII in
descending order from the top of the lake sediments to the bottom.
Sediments in Unit I always appear as reflections with low
impedance contrast (low density), involving partly parallel to subparallel reflections and diffuse backscatter signals (e.g. Fig. 2) which
may be most likely assigned to acoustic artifacts due to very
shallow lake. This unit indicates an up to 8 m-thick layer of uniform
sediments with low density. Generally, the interface between water
and sediment forms a distinct boundary between high and low
impedance contrast and thus can be easily defined. However, in
some cases (e.g. Fig. 3) the boundary remains unclear, as the
measured water depth differs remarkably from the acoustic signal.
Towards the basin margins the thickness of this unit decreases to
less than 2 m and onlaps underlying sequences.
Layers of Unit II appear as densely-spaced high contrast reflections of about 1e2 m thickness. Parallel-running reflections indicate almost undisturbed layers, forming an overall onlap pattern.
However, differences occur along track C, where cross-bedded
signatures, sigmoidal clinoform and downlap structures dominate
(Fig. 5C), indicating different sediment composition. They are
assigned to sub-Units IIaed.
Unit III belongs to the downward parts of the entire sequence
and is partly masked by reflection multiples. Densely-spaced
reflections occur as chaotic and occasionally hummocky structure
of >10 m thickness, partly alternating with reflections of low
Y. Zhang et al. / Quaternary International 290-291 (2013) 57e67
Fig. 3. Detail 3 of seismic profile S110523-1, lake Kotokel, (see Fig. 2). A: Original; B: With marked layer boundaries.
Fig. 4. Seismic transect S110523-2 along the north-eastern side of lake Kotokel. A: Original, B: Marked units with location of close-up figure, C: Detail of marked area in B.
61
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Y. Zhang et al. / Quaternary International 290-291 (2013) 57e67
Fig. 5. Seismic transect S110523-4. A: Compressed original profile with boxes of detail Figs. 1 and 2a/2b; B: Detail 1 e original transect (S110523e4.2); C: Detail 1 with marked layer
boundaries.
contrast. The latter may be masked by degassing processes, as
described by Dietze et al. (2010). The unit forms a distinct boundary
towards the overlying sequence.
4.2.1. Transect S110523-1 (Track A)
This transect is about 11 km long and displays the sediment
structure crossing the central part of the lake and the core site KTK 2
(Fig. 2). The main part shows undisturbed and horizontally oriented
units with clear boundaries between them. A detailed close-up
figure (Detail 2 in Fig. 2B) displays the units around the drill site
KTK 2. According to the results of sediment analysis (Tarasov et al.,
2009; Shichi et al., 2009; Bezrukova et al., 2008, 2010, Fig. 2C); the
lithology comprises i) soft brownish-black gyttja (0e660 cm depth,
Unit I), ii) grayish-black slightly laminated clay (660e740 cm depth,
Unit II) and iii) grey silty clay followed by dark-grey silty clay (740e
1010 and 1010e1253 cm depth, respectively, Unit III). Compared
with the seismic stratigraphy, Units I and II can be assigned to the
upper two layers of the sediment record. Low impedance contrast of
Unit I results from the occurrence of soft gyttja on top of the section.
Any distortions of sediments do not exist or remain invisible.
According to the chronology, this sequence has been deposited
during the Late Pleistocene transition and the Holocene (e.g.
Bezrukova et al., 2010). Parallel-oriented reflections with high
impedance contrast in Unit II can be attributed to the appearance of
laminated dense clay, which seems to be almost undisturbed. They
represent a deposition during the Late Glacial period. Conversely,
the two silty clay layers in the KTK 2 profile differ only in color and
hence cannot be clearly distinguished by variations in reflection
contrast but probably by the orientation of the reflections. The
upper silty clay layer seems slightly stratified (Unit IIIa), whereas the
lower one appears as diffuse reflections without any orientation
(Unit IIIb). The boundary between both layers, however, seems to be
undulated and partly unclear. According to the seismic stratigraphy
it is likely that the Unit III with a comparable lithology continues
downward for several tens of meters, although masked by multiples
at ca 26 m depth.
Surprisingly, the layers of Units II and III at the beginning of track
A (Detail 1 in Fig.2A) are highly distorted with strongly undulated
boundaries along the units, whereas the toplap sediments of the
upper Unit I remain unaffected. The seismic reflections in Unit II
indicate sub-parallel structures with signs of pseudo clinoform
facies. A distinct updoming of sediments occurs in the middle part
and indicates locally vertical flow of sediments. Slightly uplifted
sediments nearby are of minor intensity but display a sharp vertical
offset of approximately 1 m in height. As both structures also occur
within the multiple reflection layers, they appear as significant
features within this part of the track. Undulated and distorted
sequences within generally diffracted signal contrasts are characteristic for Unit III. Furthermore, the boundary of multiple reflections do not follow the surface architecture of Unit II and thus may
Fig. 6. Details 2a and 2b within seismic transect S110523-4 (for location see Fig. 5). A: Original; B: With marked layer boundaries, C: Continuation of A; D: With marked layer
boundaries.
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Y. Zhang et al. / Quaternary International 290-291 (2013) 57e67
include further sedimentary structures of Unit III, tracing down to
>30 m depth.
As this part of the track is located in the center of the lake far
away (ca 1.5 km) from direct influence by near-shore or on-shore
processes, seismic events/tectonic impact has caused the distortion of the layers. Taking the current chronology and the almost
unaffected drape layer into consideration, this event happened
before the deposition of the upper gyttja sediments and thus dates
back to the Late Pleistocene (most likely younger than 15 ka BP).
Detail 3 of track A (Figs. 2A and 3) shows the southernmost part
of the track, passing the near-shore location there. The most
striking features here are the close contact to near-shore depositional conditions and a prominent vertical offset (normal fault) of
Units II and III by about 3 m. The upper layer (Unit I) close to the
shore comprises ca. 2 m thick sediments with alternations between
low and high contrast. Towards northern direction this unit
increases to about 6 m thickness. Unit II can be divided into two
sub-units (II a and II b, Fig. 3) of potentially different composition.
Whereas sediments in Unit IIa occur as densely-spaced high
contrast reflections (onlap feature), layers of Unit IIb show
sigmoidal and sigmoid-oblique clinoforms close to the shore line.
They indicate sediment transport from the land and sub-aqueous
near-shore deposition. They most likely differ from clay deposits
described at location KTK 2. Sediments of the underlying Unit III
cannot be differentiated in detail and generally show diffracted
reflections with few signs of undulation. Very low impedance
contrast beneath the normal fault may be due to gas appearance
that masks all reflections. The tectonically induced subsidence of
the lower layers in Units II and III falls in the same period assumed
for the distortions in Detail 1 of this track.
4.2.2. Transect S110523-2 (Track B)
This 5.8 km long transect covers the north-eastern part of the
lake from the near-shore region towards the beginning of track B
(Figs. 1 and 4). Water depth did not exceed 3 m. A cursory view
along the seismic profile indicates a very flat and homogenous lake
bottom (Fig. 4A). Some important differences to the previous
transect are worth to be noted. First, the densely-spaced high
contrast of reflections marking the water column does not match
the true water depth. Parts of this parallel structure represent Unit I.
Perhaps acoustic artifacts dominate this upper part of the profile.
However, the proximity to the inflowing region of the Istok River
may have influenced the sediment composition of the gyttja which
normally occurs as low impedance contrast as seen in the lower
part of Unit I. This assumption however, needs field confirmation. In
this transect, Unit I is only 2e4 m thick.
Unit II appears as reflections with high contrast, almost horizontally oriented with clear unconformity surfaces at the upper and
lower boundaries. However, sigmoidal clinoform structures are
developed close to the northern shoreline and indicate fluvial
deposition there. Two striking features occur close to the present
drainage area of the Istok River, and thus may be attributed to
former river channels that emerged as drainage pathways during
a former low stand of the lake. The difference between the channel
bottom and the present lake level is ca. 10 m. These channels were
successively filled by the sediments of Unit II. According to the
chronology (Fig. 2C) these features may have formed during the
final stage of the Last Glacial Maximum (LGM) between ca. 20 and
15 ka BP.
Unit III can be divided into two sub-units mainly based on slight
changes in reflection contrast. A sub-aqueous prograding delta in
chaotic clinoform seems to have developed close to the northern
shore, along with the deposition of sediments in Unit III. A different
orientation of reflections at 20e30 m depth in the north-western
(left) part of the track indicates the continuation of sediments in
Unit III down to >30 m depth, although this area is already masked
by multiples.
4.2.3. Transect S110523-4 (Track C)
Transect S110523-4 runs over a distance of 11.6 km along the
western part of the lake from south-west to north-east and passes
the north-eastern shore-regions of the island (Fig. 5). Similar to the
transect crossing the center of the lake (Fig. 2), the first 3 km passage
shows an almost flat lake bottom, and the same divisions of layers
(Units IeIII) without remarkable distortions. However, the seismic
stratigraphy close to the lake shore (middle part of the track, Fig. 5A:
Detail 1) changes towards a series of high contrasts with different
structure, indicating fluvial deposits with typical cross-bedded
features (Unit IIc) and downlap facies (Unit IIb) beneath a 4 m thick
sequence of low contrast reflections (Unit I). These units indicate fan
development covering bedrock. Unit IIa (onlap/sigmoidal clinoform)
is truncated here. Reflections of medium contrast in Unit IId ca.100 m
farther south do not show strong differences to the underlying Unit
III and thus may be composed of similar material, which can be
identified as a sand bar. It developed lakeward close to the former
coast and in close proximity to the former fluvial fan. However, its
relatively sharp boundaries at both sides may also indicate tectonically induced slight offsets. All features can be considered as old (Late
Glacial) and are not recently formed.
Approximately 300 m further north-east, Unit IIa (onlap facies)
continues and appears as horizontally-oriented densely-spaced
high contrast reflections as described before. However, the entire
sequences experienced a distinct vertical offset of about 2 m over
ca. 400 m distance (Fig. 5C) due to a rapid uplift (perhaps single
event). Slope material along the uplifted flanks is indicative of
a tectonic process prior to the deposition of Unit I.
Close to the island, a further 3 m high offset of the entire sediment package appears as a sudden uplift, probably contemporary
with the previously described sequence and confirms important
tectonic impact. The uplift also affected nearby sediments by
distinct displacements along the fault zones (Fig. 6, Detail 2b), as
recognized by deformed structures of low/medium contrast.
Further striking features in this section are hummocky mounds
(updoming features) which appear as high contrast reflections
within Unit I (Fig. 6 AeD) ca. 13 m below the lake surface. They are
associated with layers from Unit II and form single mounds of ca.10 m
width and 2e4 m height. It is uncertain how these mounds were
formed. A possible explanation could be that gas expulsions affected
the upper layer of Unit II and forced the sediments to inject into the
existing soft gyttja. Deformed structures within the mounds are
evidence of updoming from the bottom layer. As these mounds are
relatively large, ascending gas pressure must have been very strong
to induce updoming. Low impedance contrast in the vicinity of these
mounds indicates the presence of degassing processes which have
masked respective sediment layers (Unit III) and are potential causes
for post-sedimentary changes in sedimentary structure. The presence of abundant organic matter and its decomposition on the one
hand as well as ascending gases from the deeper underground along
fracture zones may be possible sources for gas emissions.
As these features only occur between the faults along the western
track C (Figs. 5 and 6), seismic shaking/tectonic movement in that
region not only caused the offset of sediment packages but also
updoming processes of layers from Unit II. This probably may have
also occurred before the upper layer (soft gyttja) was deposited. In
this case the deformation happened during pre-Holocene time.
A further explanation could be that during a lake low stand (less
than 1 m thick water body) or even complete desiccation periglacial
processes could reach the lake bottom in winter time and induced
the formation of frost mounds, which were preserved before the
upper sediments (Unit I) were deposited. However, as the
Y. Zhang et al. / Quaternary International 290-291 (2013) 57e67
occurrence of these features are restricted to the fault zones and not
widespread over the entire basin, the hypothesis of periglacial
processes seem to be unlikely, although a respective lake lowstand
seem to have existed during the LGM.
4.3. Spatio-temporal sediment distribution pattern and tectonic
implications
The seismic profiles indicate that the three depositional units
are distributed over the entire lake basin. The soft gyttja reaches
maximum thickness of 7e8 m in most parts of the lake, thinning
out towards the shorelines. The 6 m isopach roughly follows the
1.5 m isobath, except for the north-eastern part of the lake, where
the gyttja is considered to be markedly thinner (Fig. 7), probably
due to the influence of the Istok River. This gyttja (Unit I) was
deposited from the Late Glacial (ca. 15 ka BP) throughout the
Holocene (Bezrukova et al., 2010) and leveled the older undulated
lake bottom relief. Conversely, Unit II appears within the entire
lake basin as a 3e4 m thick layer of partly laminated clay and silty
clay or sandy material at near-shore locations. This layer follows
the pre-relief of the lake bottom and may have been formed as
lake sediments during the Late Pleistocene between 25 and
roughly 15 ka BP, according to the chronology as shown in Fig. 2.
Unit III sediments, composed of black silty clay, occur in the entire
lake basin and form the bottom lake sediments of unknown
thickness. The seismic profiles indicate that this sequence may
continue down to 20 m sediment depth or even deeper. They date
to older than 32 ka BP (Bezrukova et al., 2010). As this unit is
masked by reflection multiples, a reliable calculation of the total
thickness remains uncertain. However, they indicate a long term
deposition within an existing lake during MIS 3 or even older
periods. Channel formation at the north-eastern side of the lake
are evidence of higher fluvial activity of the Istok drainage that
eroded the sediments of Unit III during a time of approximately
10 m lower lake level than today. According to the chronology, this
65
phase of fluvial activity should have occurred >25 ka BP and thus
falls into the period of pre-LGM.
The most striking features of all seismic profiles are related to
tectonic activity that also affected the basin structure and its
sediments. This influence is not very surprising as the Baikal Rift
system and its attached small rift basins such as Kotokel basin are
dominated by extensional tectonics with strike-slip components
(Ten Brink and Taylor, 2002) which are still active (Déverchère et al.,
2001). Although mapping results of faults in the vicinity of Lake
Kotokel differ in location and strike direction (see Davidov, 1974;
Galaziy, 1993; Shchetnikov et al., 2012), they all show the general
framework of SW-NE trending large-scale tectonic structures that
influence the entire region (see also Fig. 1B). The finding of
tectonically induced sediment offsets in Lake Kotokel can be
regarded as continuations of faults that cross the western part of
the lake basin, documented by the sediment Units II and III along
the track A (No. 1 in Fig. 7A). They exactly follow the fault south of
the lake basin that was mapped by Davidov (1974) and Shchetnikov
et al. (2012). A second fault (No. 2 in Fig. 7A) seems to run at 90 to
fault no.1, following the elongated depression between the island
and land. This may indicate that the island was separated from the
land by tectonic movement. A third fault (No. 3 in Fig. 7A) in the
eastern part of the basin can be assumed from sediment disturbance and offsets in the northern part of track A. Its direction
remains unclear, but may continue southwestward towards the
fault line on land which was mapped by Galaziy (1993). However,
tectonic structures crossing the southern part of the lake and core
site KTK2 could not be traced. A further fault seems to occur in the
north-western part of the basin as indicated by small sediment
offsets in a short seismic track (Fig. 1C, not shown as figure).
Following the bathymetry of the lake, it seems plausible that this
fault may continue southwestward towards fault No. 2. However,
the direction of faults nos. 2 and 4 need to be confirmed by further
investigations. All documented faults affected Units II and III by
vertical offsets but not Unit I. The soft gyttja of Unit I muted
movements and prevented their preservation. Furthermore the
very low seismic reflectivity of Unit I may be also a reason that they
are not displayed. Taking the recent seismic activity in the Baikal
region into consideration, it is assumed that the faults around Lake
Kotokel are still active. However, the oldest tectonic movement,
preserved in the sediments must have started after the deposition
of Unit II layers ca. 15 ka BP ago, as they are the youngest visible
sequences affected by offsets.
5. Conclusion
Fig. 7. Lake Kotokel, bathymetry, faults and sediment distribution. A: Detected and
assumed fault lines (with numbers) crossing the lake basin; B: Distribution pattern of
gyttja > 6 m thickness within the lake basin.
Detailed studies on basin morphology and seismic stratigraphy
of Lake Kotokel along selected transects demonstrate that the
relatively flat lake bottom is the result of Holocene sedimentation
which leveled former undulated surfaces of Late Pleistocene age.
The seismic stratigraphy at the drill site KTK 2 could be confirmed
by lithological investigations (Bezrukova et al., 2010), which supported the extrapolation of similar layers at many sites within the
lake basin but with differences in individual thickness. However,
the finding of tectonic impact, revealed by sequential vertical
offsets of entire sediment packages (up to 3.0 m) indicate that
known faults along the western and eastern margins of the lake
(e.g. Davidov, 1974) continue within the lake. Comparable vertical
offsets were also found in the other seismic profiles not shown
here. They were most likely active since about 15 ka BP. However
deformations within the Holocene sediments could not be traced,
although the area is considered to be still tectonically very active
(Ufimtsev et al., 1998; Déverchère et al., 2001; Ten Brink and Taylor,
2002; Shchetnikov, 2007). As a result, only a few sites contain
undisturbed sediment sequences covering the Holocene and Late
66
Y. Zhang et al. / Quaternary International 290-291 (2013) 57e67
Pleistocene depositional units. The penetration depth of the
acoustic signals and masking effects by multiples indicate sediment
thickness of minimum 30 m at least. Fossil fan deposits could be
identified along some near-shoreline locations. There is also
evidence for fluvial impact from the Istok River towards the lake as
remnants of river beds extend ca 2 km into the lake basin. They
were most likely active during pre-LGM time under a much lower
lake level. According to seismic stratigraphy, promising sites for
further sediment drilling are restricted to the southern part of the
lake basin.
Acknowledgements
This work contributes to “Bridging Eurasia” research initiative
supported by the German Research Foundation (DFG TA 540/4-1,
TA 540/5-1), Center of International Cooperation, Freie Universität
Berlin and Russian Foundation for basic research (project N 12-0500476) and by Baikal-Hokkaido Archaeology Project (BAHP)-the
Major Collaborative Research Initiative financed by Social Sciences
and Humanities Research Council (Canada). We are thankful to Prof.
Pavel Tarasov who initiated the first Bridging Eurasia workshop at
the FU Berlin in May 2010 and helped to organize the cooperation
between the Chinese, German, Russian and Canadian partners and
supported us in discussing the obtained results. Furthermore we
thank Prof. Andrzej Weber and Andrea Hiob (University of Alberta,
Canada) and Prof. Mikhail I. Kuzmin (Institute of Geochemistry,
Russian Academy of Sciences, Irkutsk), for fruitful discussions and
additional financial support. Special thanks are also given to Marina
Khomutova, RAS Irkutsk who strongly supported us in administration and customs procedures.
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