Floor Off Research Articles

Acoustic, magnetic and seismic investigation of the sea floor off the uraniferous district of Piriac, France

A geophysical survey was carried out off the Piriac coast in France. A sidescan sonar, a high-accuracy magnetometer and a high-resolution seismic system in conjunction with a very precise positioning system were used together. These geophysical methods were employed in order to follow the offshore extension of a metamorphic synform including pitchblende deposits and to identify the nature of the surrounding bedrock. Following the results of the seismic and the magnetic survey, complemented by onshore core-sample magnetic studies, it was possible to outline the termination of the offshore synform extension and to locate a new marine metamorphic unit to the west of the synform. This unit includes some strong magnetic anomalies which may be associated with mineralized districts. Moreover, the most eastern anomaly could represent the prolongation of the synform, displaced by a fault towards the northwest. A mosaic of the extremity of the western synform was compiled with the sidescan sonar images. The different rock types and sediments, extensive fracturing of the granitic basement and metre-scale ripples in the sediments are clearly visible, allowing a geological interpretation of the end of the synform to be made at a scale of 1 2500 . The synthesis of all data including sidescan sonar mosaic, seismic and magnetic maps and the magnetic study of onshore core samples contributes considerably to the knowledge of the sea floor off Piriac and should be valuable for uraniferous mineral prospecting of the western part of the synform.

Hirsutia bathyalis, a new unusual deep-sea benthic peracaridan crustacean from the tropical Atlantic

ABSTRACT A new genus and species, Hlrsutia bathyalis, created for a single, small peracaridan crustacean collected at a depth of 1,000 m from the sea floor off northeastern South America, provides the basis for one of two new families, the Hirsutiidae, of the new Order Mictacea erected by Bowman et al. (1985). Hirsutia uniquely possesses a number of morphological features found neither in other peracaridan orders nor in the second species, Mictocaris halope, and family, the Mictocarididae, of the Mictacea (Bowman and Iliffe, 1985). These include paragnaths that have elongate, slender anterior extensions that are probably sensory; an absence of grooming armature on the medial surface of the mandibular palp; unusually long setal armature on the pereiopodal basal-endopodal shaft relative to other peracaridan Crustacea; that arise from the posterior rather than the medial surface of the coxa as in all other known Peracarida, supporting Claus's (1885) century-old contention that oostegites are modified epipodites. Certain morphological features of Hirsutia suggest that it possesses the potential of being a facultative carnivore.

Geomorphic Features off Southern California as Seen by GLORIA Side-Scan Sonar System: ABSTRACT

Approximately 165,000 km2 of the sea floor off southern California was mapped during May 1984, as part of a USGS/IOS cooperative program to study the newly proclaimed Exclusive Economic Zone (EEZ) of the United States Pacific margin. The area was insonified using the Geological Long-Range Inclined Asdic (GLORIA), a long-range side-scan sonar system. Images were corrected for water-column velocity anomalies, for along-track distortions caused by variations in ship speed, and for slant-range distortions caused by acoustic ray travel paths. A photo-mosaic of the overlapping sonographs has been compiled at a scale of 1:375,000. The basins of the inner California continental borderland are characterized by both sinuous channel and fan complexes and by feathery acoustic patterns indicating active sediment transport. In contrast, outer borderland basins appear to be more sediment starved, exhibit large areas of sediment failure, and show significant structural influence. West of Patton Escarpment, the sonographs are dominated by acoustic patterns showing volcanic ridges and seamounts and by deposits of the Monterey and Arguello Fans. Arguello Fan, for example, exhibits multiple sinuous channels that have transported sediment 60 km south from the canyon mouth. These channels coalesce into a single 100-km long, westward-meandering channel that terminates in a 600-m deep box canyon. A zone of sediment failure is identifiable on the north levee of an upper fan channel. Tectonic trends associated with oceanic basement are highlighted by the terminus of the west-trending Murray Fracture Zone and by the prevailing northeast trend of volcanic ridge and seamount chains. End_of_Article - Last_Page 251------------

Pockmark-like structures on the Poverty Bay sea bed — possible evidence for submarine mud volcanism

Abstract Side-scan sonographs of the shallow (2–22 m) sea floor off Gisborne Harbour, northern Poverty Bay, New Zealand, include areas of scattered to locally prominent, subcircular, flat-floored depressions, resembling pockmarks, typically 5–20 m across and 0.3–0.5 m deep, in muddy very fine sands. Because of their geologic setting, it is speculated that the structures are exhalative pits for ascending streamers of mud, water, and/or natural gas from underlying diapiric intrusions of predominantly Paleogene shale, and that they may be submarine equivalents of the subaerial quiescent mud volcanoes occurring in the Gisborne district. Because of their possible significance in hydrocarbon prospecting, tsunami generation, and siting of subsea cables, pipelines, and offshore structures, they require further research.

Heavy Mineral Assemblages of Sandstones of the Miocene Yaeyama Group Obtained from a Borehole of MITI Miyakojima-Oki

Heavy mineral assemblages of sandstones of the Miocene Yaeyama Group were examined using core samples from a borehole of MITI Miyakojima-Oki drilled at the sea floor off Miyako-jima, Ryukyu Islands.Identified heavy mineral species are garnet, zircon, tourmaline, amphibole, epidote, rutile, pyroxene, glauconite, chlorite, monazite, staurolite and anatase, and among them, garnet appears in highest frequency in all specimens. Characteristic mineral as staurolite is found in many samples inspite of its low frequency. These minerals are presumed to have been supplied from the source area consisting of granitic rocks and metamorphic ones. Such assemblages resemble to those of the same group and its equivalents distributed in small islands such as Iriomote-jima, Yonaguni-jima and also in Formosa. This fact may suggest a paleogeographical relevancy between a source land near Miyako-jima and that adjacent eastward to Formosa which were named as the East China Old Land (LIEW and LIN, 1974).

Submarine Canyons: Multiple Causes and Long-Time Persistence

The actual investigation of submarine canyons as field work was begun about 50 years ago. A large amount of factual information has accumulated as result of operations of deep diving vehicles, first in the Pacific Coast canyons and more recently in the remarkable dives of the Woods Hole minisubmarine Alvin into East Coast canyons. Taking the results of these recent dives and combining them with earlier investigations, including much work done by the French in the Mediterranean as well as our extensive studies off California and Baja California, we can now say with some confidence that these amazing deep excavations into the sea floor off so many coastal areas can be explained. New methods such as side scanning have also given us a greater understanding of the exact character of submarine canyons, particularly in the Bay of Biscay. The development of multichannel sonar has greatly increased our knowledge of the nature of continental margins and hence their history. This has given us more insight into the history of canyon development, particularly off the east Coast where drilling for oil and gas has become so important. In the past we have seen a great variety of hypotheses for explaining submarine canyons. Unfortunately almost all of these have been based on information from a small selection of the canyons, usually from one area. From the new information, it is evident that canyons are of composite origin and that many of the hypotheses suggested in the past were partly correct but did not appreciate that coordination of other processes was required. Thus there is growing evidence that, in the history of many canyons, there was a period in which subaerial erosion was an important precursor, but that present features are predominantly the result of marine erosion. Those advocating turbidity currents as the unique cause of canyons failed to appreciate that debris flows down the incipient valleys, as ell as other types of landslides, could be an almost equally important factor in marine erosion. The great effect of biologic activity on the rock walls of incipient canyons has been almost completely neglected in explanations, and various types of currents such as those of the tides have been left largely out of the picture. Perhaps the most important feature absent in these various hypotheses has been the realization that canyons may well be the result of a long period of formation, much longer than the short episodes of Pleistocene glacial sea-level lowering features which commonly cut into hard crystalline rock. New information is showing that the canyons may date back to at least the Cretaceous.

The Dellwood knolls and their role in triple junction tectonics off northern Vancouver Island

The Dellwood knolls are two small topographic features on the ocean floor off northern Vancouver Island. They have been proposed as a spreading centre connecting the Explorer ridge to the Queen Charlotte fault and the location of a triple junction between the Pacific, American, and Juan de Fuca plate systems.Detailed geophysical surveys and ocean-bottom seismometer deployments confirm that they are the site of active seismicity and recent volcanism. Modelling of the magnetic anomaly field shows that it is almost entirely produced by normally magnetized material, supporting geological evidence that the knolls are probably less than 1 Ma old. Although the two knolls are separated by a narrow valley with some downfaulting, they do not form a clearly linear spreading rift.Assessment of their role in the plate tectonics of the region suggests that spreading at the knolls was initiated around 1 Ma ago in crust now 4.5 Ma old as part of a complex, northwesterly ridge migration process at the northern end of the Explorer ridge. Reconstruction of this process, which involves asymmetric spreading and ridge jumping, provides an explanation for the creation of the associated Paul Revere and Winona ridges.

Acoustic stratigraphy and current-generated bedforms in deep ocean basins off southeastern Africa

A bathymetric map of the deep-sea floor off southeastern Africa shows the Agulhas Plateau to be separated from the continental margin of southeastern Africa by a narrow (50 km) elongate depression, the Agulhas Passage, which acts as a deep-water connection between the Agulhas and Transkei basins. Three regionally developed sediment layers occur in the deep (> 4500 m) Transkei Basin/Agulhas Passage area. With the aid of a simple ocean crust sinking/carbonate compensation level (CCL) model these layers are related to the sedimentation history of the area. The model suggests that acoustic basement (Horizon X) represents Lower Cretaceous limestones draped over oceanic basement, and that this is overlain by an acoustically transparent sequence of pelagic/terrigenous material (Horizon A) that was deposited during a lengthy period (95 m.y.) beneath the carbonate compensation level. Post-Late Miocene sedimentation (Horizons B and C) has probably taken place above the CCL. Local sedimentation has always been influenced by strong sea-floor currents, but since middle Palaeogene times these currents have operated on a regional scale and have generated numerous large ridge and billow-like bed forms.

Uranium-series disequilibrium studies in phosphorite nodules from the west coast of South America

Sedimentary phosphorites occurring on the sea floor off Peru and Chile have been analyzed for U and Th isotopes, to establish their ages and hence obtain clues for their mode of formation. Fission-track distribution studies indicate that the U is primarily associated with the apatite fraction. Uranium-series disequilibrium methods, therefore, should be applicable, if the U incorporation is syngenetic with the apatite. The fractionation of U isotopes between oxidation states in the relatively young phosphorites from South America is low compared to that in older deposits. This supports the contention of Kolodny and Kaplan (1970) that the major mechanism of 234U 238U fractionation is displacement of 234U atoms into sites where they are more ‘oxidizable’ than the 238U parent. Age estimates based on 234U(IV) and 230Th contents are internally consistent and range from late Pleistocene to Recent. The results indicate that marine phosphorites are currently forming in this area of intense oceanic upwelling. The age pattern during the last 150,000 yr suggests a correlation with eustatic high sea level stands and implies that conditions were more favorable for apatite genesis in this area during interglacials rather than during glacial times.

Sedimentation by non-tidal currents around northern Denmark

Side-scan sonar surveys of the shallow sea floor off northern Denmark have shown the presence of a number of bedforms of mobile sand which can be associated with nearby sand deposits. Both are of interest as they appear to have been formed largely by the strong, wind-induced, intermittent, coastal current, which converges from between west and south, and flows eastward past the north Danish coast.

COMPRESSIONAL‐WAVE ATTENUATION IN MARINE SEDIMENTS

In‐situ measurements of compressional (sound) velocity and attenuation were made in the sea floor off San Diego in water depths between 4 and 1100 m; frequencies were between 3.5 and 100 khz. Sediment types ranged from coarse sand to clayey silt. These measurements, and others from the literature, allowed analyses of the relationships between attenuation and frequency and other physical properties. This permitted the study of appropriate viscoelastic models which can be applied to saturated sediments. Some conclusions are: (1) attenuation in db/unit length is approximately dependent on the first power of frequency, (2) velocity dispersion is negligible, or absent, in water‐saturated sediments, (3) intergrain friction appears to be, by far, the dominant cause of wave‐energy damping in marine sediments; viscous losses due to relative movement of pore water and mineral structure are probably negligible, (4) a particular viscoelastic model (and concomitant equations) is recommended; the model appears to apply to both water‐saturated rocks and sediments, and (5) a method is derived which allows prediction of compressional‐wave attenuation, given sediment‐mean‐grain size or porosity.

Velocities of compressional and shear waves in marine sediments determined in situ from a research submersible

In situ measurements of the velocity and attenuation of compressional waves, and velocities of Stoneley waves from which shear-wave velocities were computed, were made at three stations in the sea floor off San Diego, California, from a research submersible; a fourth station was occupied by divers. Sediment types and water depths ranged from medium sand at 20 meters to clayey silt at 986 meters. Sediment densities, porosities, and grain sizes were measured in sediment samples taken at each station. These unique data allowed tentative evaluations of models and equations, and computations of constants, for elastic and viscoelastic saturated, porous media. For the range in sediment types, results included:Property Sediment type Sand Clayey siltRatio, Vp sediment/Vp bottom water 1.20 0.98Attenuation, db/m at 14 kHz 7 1Sediment density, g/cm3 2.01 1.33Shear-wave velocity, m/sec 197 88Rigidity modulus μ × 1010 dynes/cm2 0.08 0.01Lame's constant λ × 1010 dynes/cm2 6.34 2.80 It is concluded that either Hookean-elastic or viscoelastic equations can be used to compute compressional-wave and shear-wave velocities and other elastic constants. To account for absorption in sediments, a ‘viscoelastic’ solid model is used in which frequency-independent, complex Lame constants, (μ + iμ′) and (λ + iλ′) replace μ and λ in the equations of elasticity; with this model and equations, and data taken on the four stations, μ, μ′, λ and λ′ were computed. In this viscoelastic model, the mechanics of absorption are not specified, and the assumption of frequency-independent, complex Lame constants leads to a linear dependence of attenuation on frequency and no dispersion of velocity with frequency in the range of frequencies of interest in underwater acoustics and marine geophysics. The evidence of this study and others indicates that almost all open-ocean sediments have finite regidities and transmit shear waves.

Present day ecology of benthonic foraminifera from an Upper Miocene coral thicket from Wairarapa, New Zealand

Comparison of Foraminifera from an Upper Miocene coral thicket at Hinakura, Wairarapa and from the sea floor off northeastern New Zealand indicate a depth of formation for the thicket of about 2,000 m at temperatures 2.5–3°C. Previously cited evidence for a depth of formation of less than 1,000 m from corals and O 18/O 16 ratios is shown to be inconclusive.

Clues to Gold Detected on Sea Floor off Oregon

Clues to Gold Detected on Sea Floor off Oregon

Probing Deeper, Higher, Farther

Probing Deeper, Higher, Farther

Distribution of Interstitial Salts in Drill Cores from Atlantic Ocean Floor off Florida: ABSTRACT

Interstitial water from five Paleocene to Recent core series, taken on the J.O.I.D.E.S. offshore drilling project, has been analyzed for chloride and major cations. The cores were obtained at depths to 300 meters below the sea bottom on the continental shelf, the Florida-Hatteras slope, and the Blake plateau. Samples from several holes show a marked downward increase in chloride concentrations, with maximum Cl- greater than 26 ^pmil, equivalent to a salinity greater than 47 ^pmil. It appears that forces End_Page 625------------------------------ tending to concentrate brines in deeper parts of many older sedimentary basins may operate at depths of only a few hundred feet in young sediments. The downward increases in salinity can not be accounted for easily by such mechanisms as molecular filtration. However, a combination of pressure-induced diffusion and migration induced by the geothermal gradient (Soret effect) tends to pump salts downward and appears to be a promising explanation for the increase in salt content with depth. Total water content in the cores is uneven, partly because of irregular carbonate cementation. The cementation may be related partly to changes in ionic composition noted in the interstitial waters. Fresh waters have been detected in marine strata under the Atlantic Ocean as far as 60 miles from shore. The water-bearing zones are believed to be extensions of land aquifers and may discharge in the slope regions. End_of_Article - Last_Page 626------------

Eolian sedimentation in the pacific off Northern Mexico

A sound-reflecting underbottom layer extends over a wide area of the Pacific Ocean floor off Baja California, Mexico. The top of this reflecting layer consists of a silty clay with a silt fraction of clastic minerals, dominantly quartz, alien to the local province of vulcanism. Investigation of the possible sources and transport modes for this silt indicates that it was windborne from arid continental areas during a period in the past when easterly dust storms had a higher transport efficiency than at present. Radiocarbon dating of the immediately overlying sediment indicates the age to be more than 40,000 years. The surface of the quartz grains is overgrown by chlorite. A possible origin for this coating by diagenetic transformation of desert varnish is suggested. The composite sediment resulting from deposition of eolian clastic silt together with pyroclastics and argillaceous components, closely resembles greywacke in texture and composition.

Origin and Distribution of Glauconites and Related Clay Aggregates on Sea Floor off Southern California: ABSTRACT

Studies have been made of about 1,300 glauconitic samples from the sea floor off southern California, in Monterey Bay, California, and off Cedros Island, Baja California. The principal depositional environments are continental terraces, banks, ridges, and basin slopes. Clay aggregates that probably would be designated as glauconite, according to contemporary field usage of the term by geologists, grade from relatively soft, pale yellow-green, highly expandable montmorillonitic types to dark green, illitic types. As lattice thickness [d(001)] of these different types increases, the refractive index and potassium content decreases. The organic nitrogen content is greatest in highly expandable varieties. Glauconite may compose from less than 1 per cent to about 80 per cent of the upper few centimeters of sediment. Maximum concentrations occur in the sediments from the outer shelf and upper slope areas, but the distribution is patchy, both areally and vertically, within these sediments. Glauconite is rare in water depths of less than 100 feet. Off southern California, glauconites that probably have been deposited from turbidity currents occur in near-surface sediments from the continental rise and on the abyssal floor. Living benthonic Foraminifera of the same species as those that are filled with expandable glauconite occur in water depths that range from approximately 100 feet to 6,000 feet. Specimens of the genus Cassidulina are common in Foraminiferal faunas that contain abundant glauconite-filled tests. Glauconite replaces argillaceous sedimentary rock, mineral grains, organic carbonate, and probable fecal pellets. Some spheroidal-ellipsoidal and irregular-papillate forms appear to be of accretionary origin. The mineralogy and morphology of fillings in Foraminifera indicate that they are probably deposited by direct precipitation. Distinctive morphological varieties of glauconite are not everywhere randomly distributed, but are commonly concentrated in localized areas, which indicates that variations in morphology may be caused by local conditions of origin. Glauconites have been recognized that are reworked from Pliocene, and perhaps Miocene, submarine outcrops. Foraminiferal tests that are filled with glauconite range in age from Pliocene to Recent and from Pleistocene to Recent. No direct evidence of present-day formation of glauconite has been found. End_of_Article - Last_Page 275------------

Terrestrial to Marine Transition of Heavy Minerals: ABSTRACT

Heavy-mineral analyses show that the main supply of sand for the beaches and sea floor off Southern California is provided directly by the adjacent land areas. The longshore movement of sand is only of local importance because obstacles along the coast, such as submarine canyons and points of land, limit the longshore movement of sand to small segments along the coast. A close examination of a coastal segment shows marked differences between Recent marine and nonmarine sediments, and suggests that longshore drift of sand within a segment is prominent. The concept of time, as used in correlation of heavy-mineral samples, should include consideration of source rocks and drainage patterns, and should not be based solely on depositional features. End_of_Article - Last_Page 260------------

Experimental Turbidity Currents on the Sea Floor

Marine turbidity currents provide an explanation, frequently the best or only explanation, for a wide variety of phenomena. The circumstantial evidence for their efficacy is very convincing, but there is scant empirical evidence bearing on their mode of origin and none on the presence of high-density, high-velocity currents in the ocean. Experiments intended to simulate the conditions of nature have been conducted on the sea floor off San Diego, California, in an attempt to produce high-density currents. The experiments were conducted by sliding a mass of pre-mixed sediment from an elevated, tilted container 40 feet below sea-level on a naturally sloping sea floor and observing the resulting flow directly by utilizing SCUBA. The initiation of a flow appears sensitive to the proportions of the sediment grain sizes and therefore cohesiveness of the sediment mass, the method of mixing, and total volume. Initiation on an open slope appears much less likely than on the walls of a confined feature such as a gully or canyon. Natural analogies do not tend to support the existence of high-density, high-velocity, marine turbidity currents.