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

High 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 the generally uniform and even acoustic facies and are not consistent with the hemipelagic sedimentation expected on such an isolated high, which would produce a uniform sediment drape over bottom irregularities. The influence of turbidity currents is excluded since the ridge is an isolated high, elevated more than 600-1000 m above adjacent basins. The mounded seismic facies includes migrating sediment waves and non-depositional/erosional incisions that strongly suggest sediment accumulation was controlled by bottom-current activity. We interpret the mounds as small-scale (few tens of km2 in area) lacustrine drifts. Four basic types of geometry are identified: 1) slope-plastered patch sheets; 2) patch drifts; 3) confined drifts; 4) fault-controlled drifts. The general asymmetry in the sedimentary cover of the ridge, showing thicker deposits on the NW flank, and the common location of patch drifts on the northeast side of small basement knolls, indicate that deposition preferentially took place on the lee sides of obstacles to a current flowing northward or sub-parallel to the main contours. Deep-water circulation in the ridge area is not known in detail, but there are indications that relatively cold saline water masses are presently flowing out of the Central Basin and plunging into the deep parts of the North Basin across the ridge, a process that appears to be driven mainly by small differences in salinity. We infer that the process responsible for the observed bottom-current-controlled sedimentary features has to be sought in these large-scale water-mass movements and their past equivalents. The age of the onset of the bottom-current-controlled sedimentation, based on an average sedimentation rate of 4.0 cm/ky, is roughly estimated to be at least 3.5 Ma, which is generally regarded as the age of the onset of the last major tectonic pulse of rift basin development in the Baikal region

Determining the structure of a large tilted block between two major boundary faults in a continental rift (central Lake Baikal): a reflection seismic study

Between the major boundary faults of the central part of Lake Baikal (ie. the Ol’khon fault and the Primorsky fault), a structurally complex tilted area exists that is strongly influenced by the interaction between these two faults. This area, that is about 30 kilometer wide and a 100 kilometers long, consists of three main parts: Pri-Ol’khon, Ol’khon-island and the submerged Maloe More depression. It is believed that the area formed by the gradual propagation of the Primorsky fault in a southeast direction towards the Ol’khon fault.During the summer of 2001 a large amount of high resolution reflection seismic profiles were shot in Maloe More (>600 km), that could be used to get a better insight in the structural development of the area, and in the geometry of its different sub-blocks and basins. In a first stage we have investigated the morphology of the basement underneath the sedimentary cover, and we determined which structures were fault related and which not. Age constraints on the subsequent evolution came from the correlation of the sedimentary units in Maloe More with deposits on Ol’khon-island, and with data from the long BDP-cores in a nearby area (Academician Ridge).The depth of the basement gradually increases from the southwest towards the northeast, and its morphology is characterised by several ridge structures and faults that strike at high-angle to the main faults. Several of these ridges border basins that contain relatively old sediments (Miocene age; Unit A) later overlain by younger units. Therefore the main basement structures of the Maloe More area should be older than the general believed age for the southward propagation of the Primorsky fault (1 Ma according to earlier models). Moreover the occurrence of relatively thick deposits of unit A in the southwestern extremity of Maloe More and in Ol’khon-gate contradicts the idea that these parts of the area are the youngest, being submerged only recently.Instead, older (isolated) sedimentary traps and lacustrine environments must have existed in this area. Faulting in the younger sediments however shows that the presentday activity of the major boundary faults, still has a pronounced effect on the local structure between them. Some of the formed basins are still determined by displacements on the older structures.For this study we have tried to determine the evolution of the Maloe More area, based on its interpreted structure and the relation with overlying sedimentary deposits, and we have tried to link our observations with existing models for the development of the Primorsky and Ol’khon faults

Propagation of the Primorsky Fault in the central part of Lake Baikal and the evolution of Maloe More

The Primorsky Fault is one of the two major western boundary faults in the central part of Lake Baikal. According to the existing fault growth model (e.g. Agar and Klitgord, 1995), this fault has propagated gradually in a southward direction. During this propagation, the Primorsky Fault has cut through the footwall of the Ol’khon Fault, which is the other major boundary fault 35–40km to the south-east. This propagation has controlled the submergence of the Ol’khon Region which forms a large tilted block between both faults.Based on the interpretation of high-resolution reflection seismic profiles of the submerged part of the Ol’khon Region (ie. Maloe More), different depocentres have been identified in the hanging-wall region of the Primorsky Fault. These depocentres correspond to small basins that are separated from each other by distinct basement ridges, with an orientation that strikes almost perpendicularly to the Primorsky Fault. The occurrence of the oldest sedimentary deposits (Unit A, Miocene age) in depocentres in the southern part of Maloe More, indicates that old sedimentary traps and lacustrine environments must have existed in the area. This finding contradicts the existing growth model for the Primorsky Fault, which assumes that only a recent (ca. 1Ma) and gradual propagation of the fault is responsible for the increasing subsidence in Maloe More. In the different sub-basins, younger sediments (Unit B, Upper Pliocene) overlie the deposits of Unit A. Nevertheless, the upper parts of Unit B are also present on the different basement ridges. The thickness of Unit B is on the northeastern ridges in Maloe More considerably greater than on those more to the south-west, indicating that they have been submerged for a longer time. Careful investigation of a RESURS satellite image of the area has revealed a possible segmentation of the Primorsky Fault, with segment boundaries occurring at the location of the different basement ridges in Maloe More.We believe that the growth of the Primorsky Fault can therefore be described in two different stages. A first stage, during the deposition of Unit A, was characterised by the evolution of 5 different (isolated) segments that defined small basins in Maloe More. The observed basement ridges corresponded at that time to intrabasin highs that resulted from the displacement deficit between the different fault segments. Increasing extension lead to the further growth of the segments, causing a final linkage between them. This linkage marks the onset of a second stage, which was achieved during the deposition of Unit B. Linkage between fault segments caused a displacement increase (mainly at the former location of the segment boundaries), resulting in the submergence of the basement ridge. Seen the thicker deposits of Unit B on the northeastern ridges in Maloe More, we believe that the segment linkage was first established between the northernmost fault segments of the Primorsky Fault. Subsequent linkages between other segments more to the south, and the associated post-linkage displacement increases, caused the further submergence of Maloe More towards the southwest in later stages

Stratigraphic and structural controls on the location of active methane seeps on Posolsky Bank, Lake Baikal

The distribution and origin of shallow gas seeps occurring at the crest of the Posolsky Bank in Lake Baikal have been studied based on the integration of detailed seismic, multibeam and hydro-acoustic water-column investigations. In total 65 acoustic flares, indicating gas-bubble release at the lake floor (seepage), have been detected within the 630 km² area of the Posolsky Bank. All seeps are located on the Posolsky Fault scarp near the crest of the Posolsky Bank or on similar locations in water depths of -43 m to -332 m. Lake Baikal is the only fresh-water basin in the world where gas hydrates have been inferred from BSRs on seismic data and have been sampled. Our seismic data also portray BSRs occurring up to water depths of -300 m, which is much shallower than the previously reported -500 m water depth. Calculations for hydrate stability, heat flow and topographic effect based on the BSR occurrence and multibeam bathymetry allowed the determination of a methane-ethane gas mixture and heat-flow values wherefore gas hydrates could be stable in the lake sediments at the given ambient conditions. None of the seeps associated with the Posolsky Bank have been detected within this newly established gas-hydrate stability zone. Our observations and data integration suggest that the seeps at the crest of Posolsky Bank occur where gas-bearing strata are cut off by the Posolsky Fault. These gas-bearing layers could be traced down the Posolsky Bank to below the base of the gas-hydrate stability zone (BGHSZ), suggesting that the detected seeps on the crest of the Posolsky Bank are mainly fed by gas coming from below the BGHSZ

Baikal mud volcanoes: thermal features of dynamic gas hydrate systems

In Lake Baikal shallow gas hydrates have already been identified in five mud volcano/seep structures through joint Russian, Japanese and Belgian research. These mud volcano/seep structures are found at different water depths (from 1380 m to as shallow as 440 m) and contain shallow hydrates of both structure I and II. Bottom Seismic Reflections (BSRs), indicative for the presence of deep-seated hydrates, has been observed on nearby seismic profiles. We will report on detailed thermal investigations in association with gravity coring performed over the last three years in the following gas hydrate containing mud volcanoes: “K-2”, “Malenkiy” and “Bolshoy”.The “K-2” mud volcano is located on the flanks of the Kukuy Canyon at a water depth of 900 m water depth. This oval structure of 60 m in height and 800 m in diameter consists of two separate mud volcanoes corresponding to two culminations. Sediment cores have been retrieved in more than 75 sites (15 contained hydrates), with temperature sensors attached to the corer in 22 occasions. Shallow hydrates were only found in two zones of not more 50-100 m diameter: on the top and between the two culminations. These zones also stand out by anomalous low (30-43 mK/m) and high (90-113 mK/m) thermal gradients in comparison to what is measured outside the mud volcano (60-70 mK/m). Cores with hydrates were directly correlated to low thermal gradient and large non-linearity in the temperature-depth profiles. This can be explained in three ways: (1) heat absorption by hydrate dissociation; (2) topographic effect combined with a dynamic hydrate system; and (3) infiltration of cold lake water, possibly induced by local convection and/or water segregation. The localized occurrence of hydrates within the mud volcanoes and a close relation to thermal anomalies was also observed in the mud volcanoes “Malenkiy” and “Bolshoy”, located at a water depth of about 1380m. More than 30 gravity cores in both structures indicate zones with shallow hydrates in local depressions and on culminations. Thermal stations show the presence of anomalous thermal gradients, up to 180 mK/m, at short distances of background values.The mud volcanoes in Lake Baikal do not display a strong activity in terms of acoustic flaring in the water column (almost absent) and large-scale temperature anomalies (< 1 °C). However, they comprise local shallow hydrate systems in close association with anomalous low and high thermal gradients. A dynamic nature of the hydrate system in “K-2” mud volcano has been supported by small shifts of the hydrate occurrence zone within the three year period of investigation

Active destabilisation of gas hydrate accumulations in Lake Baikal by tectonically induced fluid-flow

Multi-channel seismic profiling and deep drilling have evidenced the presence of gas hydrates in Lake Baikal, Siberia. They occur in the deep basins around the large Selenga River Delta. The presence of the hydrates is evident on seismic records by virtue of a distinct high-amplitude, reversed-polarity, cross-cutting BSR. Locally, however, the BSR shows a very anomalous behaviour. In the vicinity of some of the main, active, intra-basin faults, its depth strongly fluctuates, with undulations (positive as well as negative compared to background positions) and vertical displacements of several hundreds of ms TWT. Locally, the BSR is even entirely disrupted by vertical ‘chimneys’ that reach up to the lake bottom.High-resolution deep-tow side-scan sonar mosaics over one of such areas of deformed and disrupted BSR show a cluster of morphological irregularities on the lake floor, in contrast to areas above a regular BSR where the lake floor is absolutely regular and flat. Four large irregularities - aligned parallel to the fault – were discovered, one of them coinciding with one of the ‘chimneys’. They were mapped in detail by bathymetric sounding and proved to be either elevations (mud volcanoes ?) or depressions (craters) at the lake floor. Echosounding has also shown venting associated with these features, which is evidenced by an acoustically non-transparent plume, reaching 10-25 m above the bottom (in other places in a similar context plumes were observed of > 200 m of height). CTD-profiling, which shows very little change in bottom-water temperature at the venting sites, suggests that the plumes represent cold seeps.Heat-flow values measured over the area show a good correlation with changes in BSR depth: values vary between 50-60 mW/m² to 80-90 mW/m². In the craters, heat-flow values are highest, but they do not exceed 165 mW/m². Our observations suggest that the Baikal hydrates are locally - along particular segments (about 15 km long) of active faults - destabilizing by tectonically controlled upward flow of fluid and heat, and that this results in active venting of gasses and/or fluids at the lake floor

ВОЗРАСТ ГРЯЗЕВОЙ БРЕКЧИИ ГРЯЗЕВЫХ ВУЛКАНОВ АКАДЕМИЧЕСКОГО ХРЕБТА ОЗЕРА БАЙКАЛ

LakeBaikalis the only freshwater reservoir on Earth with gas-hydrate accumulations in its bottom sediments, partly due to the activity of mud volcanoes. This paper describes a group of mud volcanoes recently discovered on the slope of the Academician Ridge between the northern and central LakeBaikalbasins. Our analysis of diatom skeletons in the mud breccia sampled from the study area shows a high abundance of Cyclotella iris et var. These extinct species were also discovered in a core sample from BDP-98 borehole. Based on the biostratigraphic and seismostratigraphic correlations, the age of the mud breccia in the studied mud volcanoes ranges from the Late Miocene to the Early Pliocene (4.6 to 5.6 Ma). The correlations suggest that the material originated from a depth of less than310 m below the lake bottom.Озеро Байкал является единственным пресноводным водоемом, в донных отложениях которого обнаружены скопления газовых гидратов, часть из них связана с деятельностью грязевых вулканов. В настоящей работе представлена группа грязевых вулканов, обнаруженных с помощью съемки многолучевым эхолотом на склоне подводной возвышенности Академический хребет между средней и северной котловинами озера Байкал. Анализ скелетов диатомей в грязевой брекчии выявил в массе вымерший вид Cyclotella iris et var., который ранее был обнаружен в керне скважины BDP-98. При помощи биостратиграфической и сейсмостратиграфической корреляции было установлено, что материал, входящий в грязевую брекчию исследуемых грязевых вулканов, имеет возрастной интервал от позднего миоцена до раннего плиоцена (от 5.6 до 4.6 млн лет) и мог быть поднят с глубины не более310 м ниже дна.