Robert E. Houtz

Sediments and Structure of the Japan Sea

Seismic reflection (profiler) traverses of the Japan Basin, Yamato Basin, and intervening Yamato Ridge reveal horizontally stratified sediments over weakly stratified sediments. The basement surface is rough in some places and smooth in others and rises with the topography of Yamato Ridge and the lower continental slopes of Siberia and Japan. Compared to the Japan Basin, Yamato Basin has a shallower sea floor and thinner sediments. In each basin, the sediments decrease in thickness outward from a center. Wide-angle reflection and refraction data from 65 sonobuoy stations made en route give velocities in the sediments that range from 1.6 to 3.2 km/sec. Smooth oceanic basement (or layer 2) has two refracting layers, 3.5 and 5.8 km/sec; rough oceanic basement is typified by the 5.8 km/sec velocity alone. Layer 2 is thicker in the Yamato Basin than in the Japan Basin because of a greater amount of 3.5-km/sec capping material. Layer 3, of velocity about 6.8 km/sec, lies at nearly the same depth beneath the basins and Yamato Ridge. The results of profiler-sonobuoy measurements combined with the results of earlier two-ship seismic refraction measurements indicate that the Japan Basin and Yamato Basin are underlain by oceanic crust which in turn is covered by sediments (and volcanics?) that have built a shallower sea floor than that in the western North Pacific basin. Yamato Ridge appears to be mainly a pile of volcanics resting on an oceanic layer at normal depth. The crust of Yamato Basin may also have been modified to the extent that it has a thicker than normal layer 3 and a low-velocity mantle. Other profiler-sonobuoy data, gathered in the strait between Japan and Korea and across the Japan Sea margin of Southwest Japan and Northeast Japan, are presented. A number of profiler crossings of Toyama Channel indicate its formation by turbidity currents. Turbidity flows in this channel and in other channels transport sediment to the abyssal plain.

Kerguelen Plateau bathymetry, sediment distribution and crustal structure☆

Abstract The Kerguelen Plateau is a large topographic high situated in the south-central Indian Ocean, adjacent to Antarctica. The plateau rises some 2000 fathoms (∼ 3700 m) above the adjacent deep seafloor and, along its NW-SE-trending axis, exceeds 2000 km in length. The rugged northern part of the Kerguelen Plateau is dominated by NW-SE-trending normal faults and associated sediment-filled troughs and is believed to be young relative to the rest of the plateau. The central and southern parts lie in deeper water and are topographically subdued; fault trends in the central region are N-S, in the southern they are NW-SE, identical to the northern region. Sediments of Cenomanian age were sampled by a piston core taken on the eastern flank of the central region; we judge that the core penetrated the acoustic basement. Seismic profiles and piston cores from the central and southern parts of the plateau reveal a prominent sub-bottom reflector of Eocene age. It appears that significant areas of the plateau were peneplaned during the Eocene; this event was followed by general subsidence and normal faulting. Beneath the acoustic basement in the central plateau, sonobuoy measurements reveal a layer up to 1 km thick with a refraction velocity of 3.7 km/sec, underlain by material with an average velocity of 4.9 km/sec, presumably a crystalline basement. The 4.9 km/sec material must be considerably older than Cenomanian. No significant structural connection between Antarctica and the Kerguelen Plateau was observed. A sediment isopach map of the plateau and the deep ocean area to the east has been compiled. A sediment ridge paralleling the eastern margin of the plateau in abyssal depths appears to be the result of deposition by a westward-flowing bottom current that has been deflected northward by the plateau. Gravity data indicate a crust about 20–23 km thick beneath the plateau. We interpret this intermediate crustal thickness, the short-wavelength large-amplitude magnetic anomalies observed over the plateau, and other evidence discussed in the text, as evidence that the plateau is probably oceanic in origin. Anomalous depths to the top of the oceanic crust adjacent to the northeastern edge of the plateau and the plateau elevation itself are interpreted to result from a mantle hot spot, now centered beneath the active volcanoes in the northern part of the plateau. We suggest that the composite Kerguelen Plateau is an uplifted remnant of the Cretaceous ocean basin that existed to the west of Australia following the separation of India from Western Australia-Antarctica. Furthermore, it is speculated that the time transgressive eastern margin of the plateau has been created by a modest northerly drift of the locus of volcanic activity relative to the Antarctic plate.

Sediment Thickness and Depth to Basement in Western North Atlantic Ocean Basin

More than 200,000 km of single-channel and multi-channel seismic reflection data are used to construct an isopach map of sediment thickness and a map of depth to basement in the western North Atlantic Ocean basin, including the continental margins of eastern North America. Velocity regression equations and velocity estimates determined statistically from 17 provinces in the basin are used to convert reflection time in the sediments to thickness. The region between 15°N and 45°N lat. and between 40°W and 85°W long. is portrayed in Mercator projection at the scale of 1.0 in. per degree longitude (about 1:4,383,000 at equator). In this paper we outline the methods used for map preparation and discuss the most prominent features observed in basement struct re and in sediment thickness distribution.

Barents Sea continental margin sonobuoy data

Results from 68 closely spaced sonobuoys obtained aboard R/V Bartlett are combined with 30 published sonobuoy results obtained aboard R/V Vema to provide about 300 velocity solutions from the Barents Sea area and its western continental margin. Sections have been compiled from the closely spaced Bartlett stations, and statistical regressions of sediment sound velocity versus depth were made with all the available data. It was found that distinct geologic provinces from this region can be characterized with five separate velocity-depth curves, based on the regressions. The very low velocity gradient in the thick Tertiary wedge of sediments just west of the Barents shelf probably indicates rapid, deltaic deposition. More than 9 km of subbottom penetration was achieved in this Tertiary wedge.

Structure of the northern Brazilian continental margin

Results from two-ship seismic refraction profiles and supplementary data from several sonobuoys show that typical oceanic crust underlies both the landward and seaward sides of the North Brazilian Ridge. The Ceara Rise is also an oceanic structure, probably formed by local tectonic activity. Sediments of the upper Amazon Cone are more than 11 km thick, but they thin both landward and seaward. Owing to the great sedimentary thickness, refraction arrival times from oceanic basement could not be identified, but basement is assumed to extend under the cone.

Preliminary Study of Global Sediment Sound Velocities from Sonobuoy Data

Sonobuoy records from Lamont-Doherty’s collection are routinely reduced and analyzed for velocity information. Computed solutions are stored on disk where they are easily available for global studies. Velocity gradients (valid for 5 to 10 Hz waves) have been computed statistically from 16 geographical regions, based on sonobuoy interval velocity solutions from all over the world.

Sound-velocity characteristics of sediment from the eastern South American margin

The eastern margin of South America has been divided into 14 zones, each with a characteristic sound-velocity distribution relative to depth below the sea floor, as determined from sonobuoy data. Regressions of sound-velocity data determined from one-way travel time for each region can now be used to compute thickness from vertical reflection data. The velocity characteristics of the Pelotas Basin and Pelotas Rise are similar to those of the Amazon Cone and some other large deltaic deposits, which suggests that the discharge of the Rio de la Plata may have formerly been diverted along the shelf to the north. The velocity characteristics of the Rio Colorado Basin show that it is distinct from and cuts across the Argentine shelf and Argentine Rise. The Falkland Plateau velocity data are identical to the Argentine Rise contourites, suggesting a common provenance. Velocity characteristics in the adjacent Pelotas and Santos Basins are quite different, suggesting that the Torres Arch has been an important barrier for some time.

The Campbell Plateau and its relationship with the Ross Sea, Antarctica

Abstract Reconstructions of Gondwanaland place the Campbell Plateau adjacent to the eastern Ross Sea continental shelf and western Marie Byrd Land prior to approximately Late Cretaceous times. Seismic reflection data show that many basement structures on the Campbell Plateau trend in an approximately northeast—southwest direction, parallel to the axis of rifting of New Zealand from Antarctica. The only structures which may have been continuous in pre-rifting times are the Campbell Basin, underlying the southwest part of the Campbell Plateau, and a deep sedimentary trough underlying the eastern Ross Sea. A linear belt of large-amplitude magnetic anomalies trends east—west across the northern Campbell Plateau, apparently crossing the eastern shelf edge obliquely. They are probably caused by magnetic rocks of pre-Late Cretaceous age.

The Margin South of Australia — A Continental Margin Paleorift

Several important questions remain unanswered about the details of evolution and crustal structure of passive continental margins. These questions include the location of the boundary between oceanic crust and continental crust, the nature and present location of the continental rift which precedes the opening by sea-floor spreading and the origin of the Magnetic Ouiet Zone which often exists over continental margins.

Comparison of sediment sound-velocity functions from conjugate margins

A comparison of velocity functions from 15 regions within the conjugate margins of the Norwegian-Greenland Sea, Southeast Indian Ocean, North Atlantic, and South Atlantic Ocean shows that velocity functions are not significantly different across most conjugate margins. One major exception is the segment from the Blake Plateau to New England in the western North Atlantic, and its counterpart in northwest Africa. Here the velocity gradients are consistently steeper on the African side compared with those on the North American side. The reason for this unexpected difference was sought in the geologic literature of the two margins. Published studies of these two margins show that although total subsidence and sediment thickness of each of the margins are about equal, the older beds (pre-Cenozoic) are much thicker on the African side. This fundamental difference in depositional history seems to be the major cause of the difference in velocity profiles. Possible differences in the distribution of salt along the margins of Africa and North America are not considered a likely source for the trans-Atlantic differences in the velocity functions. The North Atlantic data therefore illustrate that velocity increases more rapidly with depth in sections biased toward greater proportions of older beds; that is, sedimentation rates decrease as the basin ages. These studies also reveal that sound-velocity profiles are sensitive to the early conditions of deposition on passive margins.