Seismic evidence of small-scale lacustrine drifts in Lake Baikal (Russia)

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<ul><li><p>Marine Geophysical Researches 22: 445464, 2001. 2002 Kluwer Academic Publishers. Printed in the Netherlands. 445</p><p>Seismic evidence of small-scale lacustrine drifts in Lake Baikal (Russia)</p><p>S. Ceramicola1,2,, M. Rebesco2, M. De Batist1 and O. Khlystov31Renard Centre of Marine Geology - RCMG, Gent, Belgium;2Istituto Nazionale di Oceanografia e di Geofisica Sperimentale OGS, Trieste, Italy;3Limnological Institute, Irkutsk, Russia;Author for correspondence (Tel: +39-040-2140341;Fax: +39-0404-327307; E-mail: sceramicola@ogs.trieste.it)Received 12 July 2001; accepted 20 December 2001</p><p>Key words: contourite, drift, fault, seismics, Lake Baikal, Russia</p><p>AbstractHigh resolution, single-channel seismic sparker profiles across the Akademichesky Ridge, an intra-basin structural high in Lake Baikal (Russia),reveal the presence of small sediment mounds and intervening moats in the upper part of the sedimentary cover. Such features interrupt thegenerally uniform and even acoustic facies and are not consistent with the hemipelagic sedimentation, which is expected on such an isolatedhigh and which would produce a uniform sediment drape over bottom irregularities. The influence of turbidity currents is excluded since theridge is an isolated high elevated more than 600-1000 m above adjacent basins. The mounded seismic facies, including migrating sedimentwaves and non-depositional/erosional incisions, strongly suggest that sediment accumulation was controlled by bottom-current activity. Weinterpret the mounds as small-scale (&lt; few tens of km2 in area) lacustrine drifts. Four basic types of geometry are identified: 1) slope-plasteredpatch sheets; 2) patch drifts; 3) confined drifts; 4) fault-controlled drifts. The general asymmetry in the sedimentary cover of the ridge, showingthicker deposits on the NW flank, and the common location of patch drifts on the northeast side of small basement knolls indicate that depositiontook preferentially place at the lee sides of obstacles in a current flowing northward or sub-parallel to the main contours. Deep-water circulationin the ridge area is not known in detail, but there are indications that relatively cold saline water masses are presently flowing out of the CentralBasin and plunging into the deep parts of the North Basin across the ridge, a process that appears to be driven mainly by small differencesin salinity. We infer that the process responsible for the observed bottom-current-controlled sedimentary features has to be sought in theselarge-scale water-mass movements and their past equivalents. The age of the onset of the bottom-current-controlled sedimentation, based on anaverage sedimentation rate of 4.0 cm/ky, is roughly estimated to be as least as old as 3.5 Ma, which is generally regarded as the age of the onsetof the last major tectonic pulse of rift basin development in the Baikal region.</p><p>Introduction</p><p>The term drift or contourite drift is commonly usedto define deep-sea accumulations of sediment that aredeposited or significantly affected by bottom currents,and to distinguish this type of deposits from those con-trolled by other deep-sea sedimentation processes (i.e.,turbidites, pelagites, debrites, etc.). It may encompassmany types of sediment accumulations of variablesizes and shapes (mounds, sediment waves, etc.) andof erosional features (channels, furrows. . . ), and itmay involve many different types of bottom currents:contour currents s.s., thermohaline or wind-driven cur-rents, and other types of flow in deep water (i.e.,&gt;300 m water depth) such as internal tides and down-or up-welling flows along slopes or within canyons.For a better understanding of all processes involved incurrent-controlled sedimentation, it is therefore essen-</p><p>tial to accurately describe and study all the varietiesof contourite drifts, including those occurring in lesstypical environments, such as on continental shelvesor in lakes.</p><p>Johnson et al. (1980) and Halfman and John-son (1984) already reported the presence of current-controlled sedimentary features in Lake Superior(USA), consisting of a.o. furrows, scours and a thincurrent-controlled sedimentary cover (see also Floodand Johnson, 1984). Other examples of current-controlled deposits, such as sediment waves and ero-sional moats, have been observed in some of the largerEast-African Rift lakes (Johnson, 1996, and referencestherein). In this paper, we report the existence of sedi-ment drifts in another large lake (Lake Baikal, Russia),with dimensions that are significantly larger than thosedescribed in Lake Superior and the African lakes. To</p></li><li><p>446</p><p>Figure 1. Location map. Track lines of the seismic survey are superimposed upon bathymetry (USSR Ministry of Defence, 1992). Contoursevery 100 m. Thick lines indicate the profiles shown in this article. Approximate location of the drifts is outlined by the grey shaded pattern.The grey arrows indicate the inferred direction of bottom-current flows: (1) sub-parallel to the contours in the sill between NWHB and SEHB;and (2) across the sill to the southwest of the NWHB.</p><p>our knowledge, this is the first report of lacustrinedrifts with dimensions comparable to those of theirdeep-sea counterparts. Another particular characteris-tic of the Lake Baikal drifts is that their occurrenceappears to be restricted to the crest of an intra-basinhigh, and that they do not occur on the deep basinfloors or along the basin margins.</p><p>Study area</p><p>Lake Baikal, the worlds largest and deepest fresh-water lake, occupies the central third of the Baikal RiftZone. It is roughly 600 km long, 30 to 87 km wide,and contains a total water volume of about 23,000 km3(Galazy, 1993). The lake surface lies at an elevation of</p><p>465 m. The lake is subdivided into three major bathy-metric basins: the North, Central, and South Basins,about 900, 1,600, and 1,400 m deep, respectively(Figure 1). The basins, each about 200 km long, areseparated by structural highs that rise to a depth ofabout 300 m: the Akademichesky Ridge between theNorth and Central Basins, and the Selenga Delta areabetween the Central and South Basins.</p><p>The investigated area is the Akademichesky Ridge(i.e., the underwater prolongation of Olkhon Island).It is 60 km long and 20 km wide, and is composedof two SWNE striking structural blocks (Ceramicolaet al., 1998; Ceramicola, 2001; Mats et al., 2000): theNWHB (NorthWestern Horst Block) and the SEHB(SouthEastern Horst Block) separated by an elongated</p></li><li><p>447</p><p>Figure 2. Dip seismic profile 53 across the NWHB (above), and schematic line drawing with reflector terminations indicated by short ar-rows (below). For location see Figure 1. Crossing with profile 24 (Figure 3) is shown. The filling facies of Unit A shows discontinuousreflectors onlapping the acoustic basement and erosional truncation at the upper boundary. The patch drift within Unit B is characterised byhigher-amplitude, laterally continuous reflectors showing bi-directional downlap.</p><p>sill (Figure 1). The depth of the NWHB ranges from300 m in the SW to 500 m in the NE; that of the SEHBfrom 300 m in the SW to 200 m in the NE.</p><p>The ridge is an isolated structural high elevatedmore than 6001,000 m above the adjacent basinfloors, which separate it from the lake borders andentirely shield it from turbidites and underflow de-posits. All sediments are delivered to the ridge through</p><p>hemipelagic settling. Sediments consist of alternationsbetween diatomaceous oozes and diatom-barren muds(Colman et al., 1995), deposited with an average sedi-mentation rate of 4 cm/ky (BDP Members, 1998), andshow no traces of turbidites (graded beds, consistentrepetition of facies. . . ) or any other mass-movementdeposits. For this reason, the area has been a primetarget for sediment coring and deep drilling for several</p></li><li><p>448</p><p>Figure 3. Strike seismic profile 24 across the NWHB (above), and schematic line drawing with reflector terminations indicated by short arrows(below). For location see Figure 1. Crossing with profile 53 (Figure 2) is shown. The patchy, mounded sedimentary cover within Unit B isshown to be asymmetrically distributed over the basement highs (a), to have no apparent connection with the basement morphology (b), or tomaintain the underlying topography (c).</p><p>years, because this type of sedimentary environmentis believed to hold a valuable record of paleoclimatechanges (BDP Members, 1998; Grachev et al., 1997,1998; Kuzmin et al., 1997, 2000).</p><p>In contrast, the sediments accumulating in the deepbasins of Lake Baikal are predominantly turbiditesand underflow deposits, which are controlled by thetectonic setting, the basin morphology and climate</p><p>changes (Nelson et al., 1995; Back et al., 1998, 1999).Basin-floor sedimentation rates vary from 16 cm/ky(Northern Baikal Basin) to 74 cm/ky (Southern BaikalBasin), with maximum values of 120 cm/ky near theSelenga Delta (Edgington et al., 1991).</p><p>The Baikal catchment area is approximately570,000 km2 (Galazy, 1993). The Selenga, Upper An-gara and Barguzin Rivers are the three main inflows</p></li><li><p>449</p><p>and deliver 72% of the total water supply. The AngaraRiver, at the southwestern end of the lake, is the onlyoutflow. The Barguzin River, the only tributary locatednear Akademichesky Ridge, has a catchment area ofabout 22,000 km2 and flows into the northern part ofthe Central Basin.</p><p>Lake Baikal behaves as a typical north-temperatedimictic lake with normal overturns in spring and au-tumn within the upper 200300 m of its depth. Deep-water renewal is relatively fast and does not exceed19 years (Hohmann et al., 1997). In the deeper partof the water column, localised deep mixing of densewater may occur due to a strong wind effect (Brad-bury et al., 1994). Moreover, Hohmann et al. (1997)suggested the presence of important inter-basin water-mass exchanges caused by salinity differences. Theyidentified two main processes: (1) spring inflow ofcold and relatively saline water from the Selenga Riverinto the Central Basin; and (2) deep-water formationtriggered by the small salinity difference between theCentral and North Basins. Thus, the AkademicheskyRidge is considered a threshold across which the coldsaline water flows from the Central Basin into the deeppart of the North Basin (Hohmann et al., 1997).</p><p>Methods</p><p>The geophysical dataset used for this study consistsof about 1,000 km of single-channel high-resolutionreflection seismic profiles that have been acquired bythe Renard Centre of Marine Geology (Belgium), incollaboration with the Limnological Institute (Rus-sia). The profiles are generally oriented perpendicularand parallel to the ridge axis (Figure 1). The distancebetween profiles is on average 5 km.</p><p>The seismics were shot using RCMGs Cen-tipede multi-electrode sparker as a seismic source,and a single-channel streamer as receiver. The seis-mic source was operated at 500 J. During the sur-vey, an analog bandpass-filter of 2002,000 Hz wasused to eliminate electric and ships noise. The signalwas recorded with the ELICS Delph2 digital data-acquisition system. Shot spacing was 25 m and thesampling interval was 0.25 ms.</p><p>Data show little ghost or bubble effects and atypical penetration of up to 400 ms TWT. The theo-retical vertical resolution (Rayleigh criterion) is about0.75 m, although in practice the length of the sparkerpulse may shift this to slightly higher values of around1.0 to 1.2 m, in particular for the lake-floor return and</p><p>to a lesser extent for deeper reflections. No deconvolu-tion was applied to the data. The good signal-to-noiseratio of the data required only minimal post-cruiseprocessing, which included automatic gain control(AGC) and water column muting, both essentially fordisplay purposes. Some selected profiles were alsomigrated.</p><p>Description of the results</p><p>The acoustic basement on our seismic data is markedon top by a few very high amplitude, laterally discon-tinuous reflections. Above it, two sedimentary unitsare identified in the seismic succession: Unit A andUnit B (the latter directly overlays the basement wherethe former is absent).</p><p>Unit A (Figure 2) is observed only on the NWHB(Ceramicola et al., 1998; Ceramicola, 2001; Matset al., 2000). The average thickness of Unit A is about30 ms TWT and it never exceeds 100 ms TWT. Inter-nal reflectors generally show low amplitude and lowlateral continuity. Internal geometry is complex andincludes internal unconformities. It shows both a fill-ing facies within depressions and a prograding facieson the flanks of basement blocks. In half-graben-like structural depressions, the former may have awedge shape thickening towards the bounding faultand shows sub-parallel, concave and undulated re-flectors. The latter has a clinoform shape formed bysub-parallel, oblique to sigmoidal reflectors. Both fa-cies show distinct onlap at the lower boundary andlocal erosional truncations at the upper boundary.</p><p>Unit B extends across both the NWHB and SEHB,and in the Maloye More basin (Ceramicola et al.,1998; Ceramicola, 2001; Mats et al., 2000). Thicknessof Unit B ranges from a few milliseconds above highs,to about 1 sec above depressions. Most of the unitshows a draping facies characterised by the presenceof continuous, sub-parallel, concordant reflectors. Theamplitudes of the seismic reflections are variable butgenerally much higher than in Unit A (Figure 2).</p><p>However, the sediments on the crest of the ridgehave a unique undulating and patchy morphology,punctuated by several sediment mounds (Figures 2and 3). Moreover, sediment thickness of Unit B isasymmetric on the two ridges flanks, with a largerthickness on the northern flank than on the south-ern (Figure 4). Conversely, no comparable thicknessThickness is given in this paper as milliseconds (ms) two-way-travel-time (TWT).</p></li><li><p>450</p><p>Figure 4. Dip seismic profile 48 across the top of the NWHB (above), and schematic line drawing with reflector terminations indicated byshort arrows (below). For location see Figure 1. The prograding facies of Unit A shows oblique to sigmoidal reflectors onlapping the acousticbasement. Erosional truncation at the upper boundary is more evident on the SE side of the ridge, whereas downlap terminations withinsigmoidal units are more common on the NW side. A small asymmetric drift controlled by active faulting is present within Unit B at a slopechange on the SE side of the NWHB. This patch drift is characterised by higher-amplitude, laterally continuous reflectors. Downlap of reflectorsis widespread on the SE side and in the lower part of the NW side, where a small moat is present between the drift and the ridge.</p><p>asymmetry is observed within Unit A. The patchymorphology of the ridge is produced where the gen-eral uniform configuration of Unit B is interrupted bythe presence of sedimentary mounds overlying lap-out (both onlap and downlap) terminations (Figure 5),sediment waves (Figures 5 and 6), and linear incisions</p><p>(Figure 7) which are all observed intermittently alongthe entire length of the ridge.</p><p>Sedimentary mounds are observed both on theslopes and on the crest of the NWHB, an...</p></li></ul>

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