O. V. Levchenko

Contourite systems in the region of the southern São Paulo Plateau escarpment, South Atlantic

Seismoacoustic investigations with a high-resolution parametric echo-sounder “SES 2000 deep” carried out on cruises 33, 35, and 37 of the R/V Akademik Ioffe revealed several erosional-depositional contourite systems on the São Paulo Plateau escarpment and its toe in the South Atlantic. Two contourite terraces related to interfaces between different water masses are observable on the escarpment. These terraces presumably reflect the activity of internal waves and turbulent eddies. The São Paulo contourite channel and genetically related drift are traceable along the escarpment toe. Changes in planktonic foraminiferal assemblages in Core AI-2563 retrieved from the summit of the São Paulo contourite drift suggest a shallowing of the Weddell Sea Deep Water mass during glacial times. It is established that the contour current of the Weddell Sea Deep Water and Lower Circumpolar Water considerably affect the formation of contourite depositional systems on the escarpment and its toe.

New result of the seismic facies analysis of the Quaternary deposits in the western Atlantic

1256 The regular Transatlantic geological–geophysical expeditions on R/Vs Akademik Ioffe and Akademik Sergei Vavilov (Shirshov Institute of Oceanology, Rus sian Academy of Sciences) starting from 2009 have made the foundation for studying the inner structure and occurrence conditions of the main genetic types of deep water Quaternary bottom sediments of the Atlantic Ocean along the submeridional geotraverses [1–4]. The SES 2000 deep parametric narrow angle profiler used in these expeditions allows obtaining high resolution (tens of centimeters) seismoacoustic sections of bottom sediments with a depth extent of up to 100 m beneath the bottom at sea depths of up to 6000 m. For lithological–stratigraphic interpretation of the distinguished reference acoustic images (seis mofacies) at the key points of the seismoacoustic pro file, sediment cores are collected. A direct correlation between the sections from boreholes of deep sea drill ing, crossing the seismic profile, enables us to reveal sedimentation mechanisms leading to formation of seismofacies [1, 2, 4].

New data on composition and structure of the Pernambuco Seamounts, Brazil basin, south Atlantic region

330 Research vessel Akademik Ioffe performed investi gation of the Pernambuco Seamounts in the Brazil basin in the profile of cruise 33 in 2011. The chain of seamounts is oriented in the northwestern direction and consists of individual roughly NS , roughly EW , NE , and NW trending segments. The two highest seamounts in the northern part of the chain were investigated. They characterize roughly NS and EW trending segments conjugated at a right angle. The structure of the upper part of the sedimentary cover was studied with a SES 2000 deep seismoacoustic profilograph, while the bottom relief was investigated with an ELAC echosounder. Judging from the mor phology of slopes, the seamounts are characterized by a three level structure reflecting a few stages of their formation. Formation of swells was followed by devel opment of volcanic edifices emergening above sea level. Extinction of volcanic activity and marine abra sion of volcanic summits were followed by growth of carbonate platforms. On the slopes and at the feet of paleovolcanoes, there are abundant landslide and coarse grained debris deposits overlapped by pelagic stratified sediments. The latter are occasionally bro ken by neotectonic deformations.

Slump structures in quaternary slope sediments of the northern Derbent Basin (Caspian Sea)

During Cruise 20–3 of the R/V Rift (April, 2006), the area that includes the shelf and slope of the Derbent Basin in the northern Middle Caspian was studied using the continuous seismoacoustic profiling method. In accordance with the previous standpoint, two Pleistocene deltaic complexes formed in the Enotaevian and Mangyshlakian time are defined in this area. The seismoacoustic records obtained for the northern slope of the Derbent Basin demonstrate the development of specific rootless exogenic-gravitational fold structures in the upper (∼150–200 m) Quaternary part of the sedimentary sequence. The Quaternary section encloses angular unconformities indicating the pulsating mode of gravitational processes in the northern slope of the basin. South-dipping gravitational normal faults (and/or normal fault-related flexures) displacing the bottom surface and uppermost sedimentary layers (with vertical amplitudes up to 5–6 m) were defined in the southern part of the study area. Several impulses of the submarine slump structures predated and accompanied the deposition of the upper deltaic sequence (Mangyshlakian), although their most intense formation took place later during the Novocaspian (Holocene) time. Thus, the structural analysis of the seismoacoustic data revealed intense development of different-origin and different-age gravitational structures within the Quaternary sediments in the northern slope of the Derbent Basin. These results should be taken into consideration when designing, building, and operating submarine constructions in order to prevent potential natural hazards and reduce their consequences.

Tectonics of the Ninety-East Ridge

New multibeam bathymetry data and multichannel seismic profiles over 7 detailed survey sites collected during cruise no. KNOXRR06 of the R/V Rodger Revelle in 2007 fundamentally expanded the concepts about the structure of the sedimentary cover of the Ninety-East Ridge, which were based on the results of previous studies. They allow making a step forward in interpreting the nature of the unconformities and deformations. The deformation pattern of the sedimentary cover suggests three stages of tectonic activity over the ridge during the Paleocene, Eocene, and Late Miocene. As suggested by the high present-day seismicity, the last stage might continue to the present time. In addition, indirect criteria indicate young intraplate volcanism at the last stage. New data is reviewed in the context of two models of the ridge formation. The first model follows a well-known concept of the ridge representing a trace of the Kerguelen hotspot. The second one relates large outflows of basalts to the development of a giant fracture under conditions of global extension, which continued later during the sedimentary cover formation. Additional integrated geological and geophysical studies are required for understanding the nature of this unique feature.

Multidisciplinary Investigations along the Transatlantic Transect Ushuaia (Argentine)-La Manche Strait: Cruise 33 of the R/V Akademik Ioffe

Altogether, 16 specialists participated in the expe� dition: seven from the Institute of Oceanology of the Russian Academy of Sciences, three from the Institute of Geology of the Russian Academy of Sciences, three from the Institute of Atmosphere Physics of the Rus� sian Academy of Sciences, two from Pomeranian Uni� versity, and one from Innomar Technologie GmbH (Rostock, Germany). The chief scientist of the expe� dition O.V. Levchenko guided the seismoacoustic research and the deputy chiefs I.O. Murdmaa and S.G. Skolotnev were responsible for the sediment cor� ing and bedrock dredging, respectively. The work was fulfilled by three research teams: the seismoacoustic (leader A.D. Mutovkin), the geology (leader E.I. Ivanova), the atmospheric aerosol and biooptics (leader E.I. Grechko).

First data about the geochemistry and geological structure of underwater seamounts between Ascension and Bode Verde transform fracture zones in the Brazilian Basin (South Atlantic)

56 The origin of large linear structures in the oceanic crust is one of the pressing problems in marine geol ogy. During the 32nd cruise of RV Akademik Ioffe (2010), geological works were carried out at the Brazil Basin–Mid Atlantic Ridge (MAR) boundary between the passive parts of the Ascension and Bode Verde transform fracture zones in the zone of Middle–Late Eocene age of the oceanic crust (16–18 magnetic anomalies [1]). On the bathymetric structural scheme [2] (Fig. 1), a northwest trending linear chain of nine seamounts with a total length of ~460 km is shown. During field works, the two highest seamounts in the northern part of the chain were investigated. The study of underwater topography using the ELAC echo sounder showed that the top of the north ern seamount (the top coordinates are 10°20.01 S to 20°28.80 W) is located at a depth of 1127 m (Fig. 2); its elevation over the adjacent depression bottom is 3625 m, and that over the seamount base is about 3250 m. The mountain has a conical shape; however, on the western slope in its apical part, several ridges of 150 m in height over a wide terrace are observed (Fig. 2). It is probable that these ridges form the outer edge of the semi caldera. The top of the southern seamount is at a depth of 1501 m. Its elevation over the depression bottom is about 3370 m; above the seamount base it is 3270 m; and the top coordinates are 10°43.79 S and 19°51.14 W. The width of the depression enclosing the chain of seamounts is about 11 km, and its maximum depth is 4765 m. In the northern seamount (Fig. 1), according to the acoustic profile, recorded using the narrow beam parametric SES 2000 profilograph, the visible thickness of the sedimentary cover reaches 80 m (Fig. 3). It seems likely that the regular bedding observed indi cates the alternation of pelagic sediments and bioclas tic turbidites. This sequence on the slopes of the buried paleovolcanoes has a chaotic structure with landslide bodies and erosional channels. In the center of the depression, a diapir like penetration structure of about 2 km across is observed. It penetrates lower sed imentary layers, which ride up slightly along buried slopes of this structure. The upper layers of 20 m thick are not penetrated. However, they are bent upwards at the point of contact with this diapir like structure.

Estimation of parameters of the flows forming sediment waves on the western slope of the middle caspian sea

By means of mathematical modeling, the parameters of flows forming sedimentary waves on the western slope of the Derbent basin were estimated. The height of these flows depends on the slope steepness and varies from 25 to 170 m to reach its maximum values at gentler slope areas. However, the flow rate is independent of the slope steepness and depends only on the concentration of sediment matter supplied by the flow. At the upper part of the slope (the flow starting), the rate amounts to 0.4–1.4 m/s, being almost halved at the depths where the sedimentary waves are damped. The present rates of near-bottom currents show pronounced seasonal differences, and their values are close to flow rates obtained by numerical modeling.

Miocene volcanism of the Ninetyeast Ridge

The East Indian or Ninetyeast Ridge is one of the most unique and enigmatic structures of the oceanic bottom. It is characterized by a linear shape and extends in the meridional direction via the eastern part of the Indian Ocean for almost 5000 km and is approx� imately 200 km wide (Fig. 1). The ridge was discovered in the 1920s and first studied in detail in 1963 during the R/V Vityaz scientific expedition. In subsequent years, geological–geophysical studies of the ridge were conducted in cruises of research vessels from the Shir� shov Institute of Oceanology, RAN [1] and other Rus� sian and foreign scientific organizations. Most of the geological–geophysical data were obtained there in the 1970s–1980s.Since that time, the scientific equip� ment and study methods have become substantially better. Despite the sufficient knowledge of the ridge structure owing to numerous oceanological expedi�

On Gravitational Instability of Quaternary Sediments on the Western Slope of the Derbent Basin (Caspian Sea)

1179 The problem of gravitational instability of Quater� nary (primarily, Holocene) sediments on underwater slopes of the Caspian Sea is of both scientific and quite apparent applied significance in connection with the assessment of geological risks during development of oil and gas deposits [1–8]. The analysis of slope stabil� ity is also of primary importance for designing differ� ent engineering constructions such as offshore drilling platforms, underwater pipelines, communication cables, and others. The slopes of the Derbent Basin represent one of the most dangerous areas of the Caspian Sea with respect to the development of gravitational–slumping processes (Fig. 1). In our previous works on the Qua� ternary geology of the central Caspian Sea [4, 6–8], we noted exogenic–gravitational rootless folds in sed� iments of the eastern [7, 8] and northern [4, 6] slopes,