Adjacent Deep-sea Floor Research Articles

Three-dimensional bottom diffraction in the North Pacific.

A significant aspect of bottom-interaction in deep water acoustic propagation, from point sources to point receivers, is the diffraction (or scattering) of energy from discrete seafloor locations along repeatable, deterministic paths in three-dimensions. These bottom-diffracted surface-reflected (BDSR) paths were first identified on the North Pacific acoustic laboratory experiment in 2004 (NPAL04) for a diffractor located on the side of a small seamount. On the adjacent deep seafloor, ambient noise and propagation in the ocean sound channel were sufficiently quiet that the BDSRs were the dominant arrival. The ocean bottom seismometer augmentation in the North Pacific (OBSANP) experiment in June-July 2013 studied BDSRs at the NPAL04 site in more detail. BDSRs are most readily identified by the arrival time of pulses as a function of range to the receiver for a line of transmissions. The diffraction points for BDSRs occur on the relatively featureless deep seafloor as well as on the sides of small seamounts. Although the NPAL04 and OBSANP experiments had very different geometries the same diffractor location is consistent with observed arrivals in both experiments within the resolution of the analysis. On OBSANP the same location excites BDSRs for 77.5, 155, and 310 Hz transmissions.

Echinoderid mud dragons (Cyclorhagida: Kinorhyncha) from Senghor Seamount (NE Atlantic Ocean) including general discussion of faunistic characters and distribution patterns of seamount kinorhynchs

Investigation of Kinorhyncha from Senghor Seamount (tropical northeastern Atlantic Ocean) and the adjacent deep-sea floor revealed seven species of Echinoderes. Of these, Echinoderes kaempfae sp. nov. and Echinoderes schwieringae sp. nov. are described as new species. Echinoderes multiporus originally found on Eratosthenes Seamount, Mediterranean Sea, and Echinoderes cf. adrianovi originally found off Florida, northwestern Atlantic Ocean, were also recovered. In addition, three additional species, which possibly represent undescribed species, occurred on Senghor Seamount. Because of the low number of available specimens or necessity of further observations of closely related congeners for these three undescribed species, we provide only a brief description of them without naming them as new species. The comparison of each species with morphologically similar congeners is also provided. Based on all records of kinorhynchs from seamounts and the adjacent deep-sea floor in the Northeastern Atlantic Ocean, Mediterranean Sea, and Arctic Sea, it is suggested that (1) the kinorhynch fauna on seamounts is partly unique, (2) seamount kinorhynchs originate both from shallow waters and the deep sea, and (3) a few kinorhynch species diversified on seamounts.

Methane-derived authigenic carbonates on accretionary ridges: Miocene case studies in the northern Apennines (Italy) compared with modern submarine counterparts

Abstract We present new field data from three outcrops of Miocene methane-derived authigenic carbonates in the foredeep of the northern Apennines that contain chemosynthetic fauna and record a long history (∼1 Ma) of shallow fluid seepage linked to seafloor anaerobic oxidation of methane. The studied outcrops show similar features in terms of carbonate morphology, facies, spatial distribution and lateral and vertical contacts with the enclosing sediments. Methane-derived carbonates occur in two structural positions: 1) on the slope of the accretionary wedge in hemipelagites draping buried thrust-related anticlines, and 2) at the leading edge of the deformation front in the inner foredeep, within fault-related anticlines standing above the adjacent deep seafloor as intrabasinal ridges. We compare fossil seeps with two extensively investigated modern analogues: the Hikurangi Margin, offshore New Zealand and Hydrate Ridge, on the Cascadia margin, offshore the U.S.A. These analogues share a similar compressive structural setting and are marked by the presence of variably extensive and voluminous methane-derived carbonate bodies and chemosynthetic fauna on the present-day seafloor. The comparison allows us to propose a model for the evolution of fluid seeps on thrust-related ridges. At the deformation front, uplift and geometry of the anticlinal ridges are controlled by the growth of splay faults, mostly blind, connected to the basal detachment, favoring the migration of fluids toward the incipient anticline. Fold development generates extensional stresses in the hinge zone of the anticline, promoting the development of normal faults; fluid migration pathways and seafloor seeps shift from the forelimb toward the crest of the ridge as the structures evolve. In the slope setting, far from the deformation front, thrust faults and extensional faults in buried anticlines remain the main fluid migration pathways to sustain seepage at the seafloor. After reaching a mature stage within the wedge, the structure is less active and buried in the slope environment of the evolved prism.

Morphology and shallow structure of seafloor mounds in the Canary Basin (Eastern Central Atlantic Ocean)

The increasing volume of high-resolution multibeam bathymetry data collected along continental margins and adjacent deep seafloor regions is providing further opportunities to study new morphological seafloor features in deep water environments. In this paper, seafloor mounds have been imaged in detail with multibeam echosounders and parametric sub-bottom profilers in the deep central area of the Canary Basin (~350–550 km west off El Hierro Island) between 4800 and 5200 mbsl. These features have circular to elongated shapes with heights of 10 to 250 m, diameters of 2–24 km and with flank slopes of 2–50°. Based on their morphological features and the subsurface structures these mounds have been classified into five different types of mounds that follow a linear correlation between height and slope but not between height and size. The first, second (Subgroup A), and third mound-types show heights lower than 80 m and maximum slopes of 35° with extension ranging from 2 to 400 km2 and correspond to domes formed at the surface created by intrusions located at depth that have not outcropped yet. The second (Subgroup B), fourth, and fifth mound-types show higher heights up to 250 m high, maximum slopes of 47° and sizes between 10 and 20 km2 and are related to the expulsion of hot and hydrothermal fluids and/or volcanics from extrusive deep-seated systems. Based on the constraints on their morphological and structural analyses, we suggest that morphostructural types of mounds are intimately linked to a specific origin that leaves its footprint in the morphology of the mounds. We propose a growth model for the five morphostructural types of mounds where different intrusive and extrusive phenomena represent the dominant mechanisms for mound growth evolution. These structures are also affected by tectonics (bulge-like structures clearly deformed by faulting) and mass movements (slide scars and mass transport deposits). In this work, we report how intrusive and extrusive processes may affect the seafloor morphology, identifying a new type of geomorphological feature as ‘intrusive’ domes that have, to date, only been reported in fossil environments but might extend to other oceanic areas.

Three new species of Echinoderidae (Kinorhyncha: Cyclorhagida) from two seamounts and the adjacent deep-sea floor in the Northeast Atlantic Ocean

Three new species of Echinoderidae (Kinorhyncha: Cyclorhagida) from two seamounts and the adjacent deep-sea floor in the Northeast Atlantic Ocean

Erosional offshore sand ridges and lowstand shorefaces; examples from tide- and wave-dominated environments of France

ABSTRACT Offshore sand are described from many continental shelves in the world, as well as in the stratigraphic record, where they commonly are productive reservoirs. Some of these ancient sand bodies, initially interpreted as sand ridges, are now reinterpreted as lowstand shoreface deposits. Modern sand bodies, in contrast, have received relatively little attention with regard to reinterpretation of their origin, largely because of the lack of information about their internal structure. Improved techniques in acquisition and processing of very high-resolution seismic profiles, along with some shallow cores, allow us to reconstruct the architecture of sand bodies from the Celtic Sea (tide-dominated) and the Gulf of Lions (wave-dominated) shelves of France, in water depths of 100-170 m. In both areas, our investigations demonstrate that these particular sand consist mainly of lowstand deposits (estuarine/deltaic systems, sharp-based shorefaces), reworked during transgressions. In the Celtic Sea, intense erosion by combined waves and tidal currents resulted in the shaping of shore-oblique ridges by cannibalization of older lowstand deposits. In the Gulf of Lions, the shore-parallel orientation of the lowstand shorefaces has been preserved, leaving an ancestral sand body with reworked (transgressive) surface deposits. Understanding the architecture and distribution of offshore sand requires taking into consideration not only the effects of relative sea-level changes and sediment supply, but also the role of hydrodynamical processes. The erosional sand we describe represent a new category of outer-shelf sand bodies, a combination of the autocyclic examples described by Houbolt (1968) in the southern North Sea and the allocyclic lowstand shorefaces mainly described in the stratigraphic record of the Western Interior Seaway of North America. Our findings have applications for predicting the geometry of ancestral sand and their orientation and position with respect to paleo-shorelines. The magnitude of erosional processes also implies that a large amount of shelf sediment (mainly sand) was transferred to the adjacent deep-sea floor during the early transgression.

Insular Erosion, Isostasy, and Subsidence

Organic reefs and shore erosion record the intersection of sea level with islands. From this record it is possible to reconstruct the history of vertical movement of the islands and the adjacent deep sea floor, including midplate swells. As judged by coral thickness, islands with barrier reefs sink as though they were on thermally youthful crust regardless of the actual age. Reefless islands do not sink until truncated by erosion. Apparently, thermal subsidence is balanced by isostatic uplift in response to erosion. Barrier reefs prevent wave erosion of encircled volcanoes and capture products of stream erosion so that isostatic uplift is eliminated. Insular shelves widen initially at rates of 0.6 to 1.7 kilometers per million years; the rates decrease with time. Thus the subsidence of islands depends on the size of the is land and the presence of reefs, and it may not always be the same as that of the surrounding oceanic crust.

Kerguelen Plateau bathymetry, sediment distribution and crustal structure

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.

Structural Framework of Continental Margin in South China Sea

A 16,000-km series of seismic and geomagnetic traverses in the Gulf of Thailand, northern Sunda Shelf, and the adjacent deep-sea floor was completed during 1969. The results show the presence of three large sediment-filled basins separated by swells or ridges. Westernmost is the Gulf of Thailand basin that extends almost from Bangkok to Singapore. A buried ridge separates this basin from two other basins on the northeast, the Mekong and the Brunei-Saigon. Both of the latter basins reach the adjacent land areas, and the Brunei-Saigon basin appears to be the main part of the Northwest Borneo geosyncline. The Mekong and Brunei-Saigon basins are limited on their northeastern sides by a narrow ridge that lies beneath the outer continental shelf and the upper continental slope, a feature termed the Peripheral Ridge. Sediments in all three basins are thicker than 2 km and they may be thicker than 4 km. They contain tectonic folds, unconformities, faults, and at least a dozen structures believed to be diapiric intrusions. The deep-sea floor in the northeast beyond the continental slope contains an abyssal plain and also a broad comparatively level plateau that is dotted with seamounts, some of which are capped with coral atolls. Sediments in this deep region, the China basin, are of two distinct ages separated by a widespread unconformity. Inferences from geologic studies on land suggest that the basin in the Gulf of Thailand was formed during Late Cretaceous time, and the present form of the ones beneath the Sunda Shelf was acquired subsequently. Sediments have accumulated in them ever since, with interruption by tectonic activity and erosion. Prospects for oil and gas appear to be favorable, as also is indicated by the intense exploration and test drilling off Borneo and in the southern part of the Gulf of Thailand by international companies. Other parts of the sediment-filled basins deserve attention as well.

Peru-Chile Trench Sediments and Sea-Floor Spreading

The hypotheses of sea-floor spreading and plate tectonics require the removal of sediment from oceanic trenches either by crustal underthrusting or by folding against the base of a continental or insular margin. Accordingly, over a period of time the volume of sediment removed by way of spreading must be equal to the difference between the observable volume of undeformed terrigenous deposits in a trench and the volume contributed to it by continental erosion. To assess possible sediment loss from the central Chilean segment (23°–44° S.) of the Peru-Chile Trench, we have compared the volume of terrigenous deposits overlying the land, the continental margin, and filling the trench with that expected from continental denudation. Our data indicate that an episode of sediment removal occurred at the base of the margin and adjacent deep-sea floor in Late Cretaceous and perhaps earlymost Tertiary time and may imply spreading. Nearly 100 × 10 3 km 3 of deposits of Tertiary age, chiefly Eocene to Pliocene, have accumulated on the margin, and perhaps an additional 5 × 10 3 km 3 in the trench. This amount of offshore sediment could be supplied by fairly low rates (3 cm/10 3 yrs) of Tertiary erosion. However, many uncertainties in our denudation-sedimentation budget make it impossible to determine whether or not sediment reaching the base of the margin was removed tectonically in Tertiary time. Between 27° and 44° S., the trench contains nearly 70 × 10 s km 3 of turbidite deposits that we believe accumulated during late Cenozoic periods of glacially lowered sea level. The volume of turbidites in the trench is virtually equal to that expected from continental erosion, which is estimated to have probably been no greater than 5 cm/10 3 yr for the arid region between 27° and 31°, and 50 cm/10 3 yr for the humid and partially glaciated region from 36° to 42°. During this time of rapid erosion and trench filling, magnetic data indicate that convergence of lithospheric plates was taking place below the trench at a rate between 5 and 10 cm/yr. If turbidite deposits were swept from the trench at these rates, then continental denudation must have been exceedingly rapid: 20–40 cm/10 3 yr for the arid zone, and 110–165 cm/10 3 yr for the partially glaciated region. If more conventional estimates of erosion are valid, then either (1) late Cenozoic underthrusting has not taken place (or at a rate much slower than that implied by geophysical data), or (2) underthrusting at the prescribed rates has not involved the removal of a significant volume of sediment from the trench.