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Results for “Geo-Marine Letters”

Search indexed USGS publications on groundwater, aquifers, geologic maps, mineral resources and earthquakes. Explore source records by subject and place.

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At least 37 records · Page 2Linked to original sources
USGS1989

Pockmarks in the floor of Penobscot Bay, Maine

Hundreds of depressions (pockmarks) were found within a 40 square kilometer area of the sea floor near the head of Penobscot Bay, Maine. These roughly circular depressions range in diameter from 10 to 300 meters and extend as much as 30 meters below the surrounding sea floor. The pockmarks have formed in marine mud of Holocene age, which unconformably overlies glaciomarine deposits. The presence of shallow interstitial gas in the mud suggests that the pockmarks are related to the excipe of gas from the sediments, although other factors must be involved. ?? 1989 Springer-Verlag New York Inc.

Geo-Marine Letters↗
USGS1985

Morphology of the Ebro fan valleys from SeaMARC and sea beam profiles

The northern continental slope off the Ebro Delta has a badland topography indicating major slope erosion and mass movement of material that deposits sediment into a ponded lobe. The southern slope has a low degree of mass movement activity and slope valleys feed channel levee-complexes on a steep continental rise. The last active fan valley is V-shaped with little meandering and its thalweg merges downstream with the Valencia Valley. The older and larger inactive channel-levee complex is smoother, U-shaped, and meanders more than the active fan valley. ?? 1985 Springer-Verlag New York Inc.

Geo-Marine Letters↗
USGS1985

Physiography and deposition on a distal deep-sea system: The Valencia Fan (Northwestern Mediterranean)

The Valencia Fan developed as the distal fill of a deep-sea valley, detached from the continental slope and the main sedimentary source. A survey of side-scan sonar, Sea Beam and reflection seismics shows that the sediment is largely fed through the Valencia Valley. The upper fan comprises large channels with low-relief levees, and the middle fan has sinuous distributary channels. Depositional bedforms predominate on the valley floor and levees, and erosional bedforms are common in the valley walls. A change to slope on the fan apex and the presence of volcanoes on the upper fan are the main factors influencing fan-growth pattern. ?? 1985 Springer-Verlag New York Inc.

Geo-Marine Letters↗
USGS1984

Transport of clays in the eastern part of santa barbara channel, California

A record discharge of about 54 ?? 106 metric tons of predominantly fine-grained detrital sediment was introduced during 1969 into the eastern part of Santa Barbara Channel from the Santa Clara and Ventura Rivers. The clay-size fraction from bottom samples collected during a time-series of about 18 months revealed movement across the shelf and into the adjacent basin in a repetitive sequential pattern. Light transmission profiles show resuspension and transport of sediments at depths of up to 225 m. This transport and distribution history is attributed to seasonal variations in the vertical and lateral position of a poleward-flowing current. ?? 1985 Springer-Verlag New York Inc.

Geo-Marine Letters↗
USGS1984

Natural radioisotope210Pb as an indicator of origin of fine-grained sediment

Distribution of210Pb in sediments on the South Texas Continental Shelf is related to dynamics of the sedimentary transport processes. This radioisotope, whose concentration is time-dependent, defines three depocenters on the shelf. In addition, the variation of210Pb activity at the sediment/water interface delineates areas of terrigenous sedimentation from hemipelagic sedimentation. ?? 1985 Springer-Verlag New York Inc.

Geo-Marine Letters↗
USGS1984

Creep behavior of submarine sediments

A series of experiments on drained creep of marine sediment indicates that strength degradation results from the creep process, which implies an associated reduction in slope stability. Furthermore, the highest creep potential of a sediment may be at its preconsolidation stress. Results from the experiments on samples from Georges Bank continental slope were also used in conjunction with a preliminary theoretical model to predict creep displacements. For the case illustrated in this report, steep slopes (>20??) and thick sections (>30 m) give rise to substantial creep and probable creep rupture; as angles or thicknesses decrease, displacements rapidly become negligible. Creep may be a significant geologic process on many marine slopes. Not only can it cause major displacements of surface sediment, but it may also be the precursor to numerous slope failures. ?? 1985 Springer-Verlag New York Inc.

Geo-Marine Letters↗
USGS1988

Acoustic stratigraphy and hydrothermal activity within Epi Submarine Caldera, Vanuatu, New Hebrides Arc

Geological and geophysical surveys of active submarine volcanoes offshore and southeast of Epi Island, Vanuatu, New Hebrides Arc, have delineated details of the structure and acoustic stratigraphy of three volcanic cones. These submarine cones, named Epia, Epib, and Epic, are aligned east-west and spaced 3.5 km apart on the rim of a submerged caldera. At least three acoustic sequences, of presumed Quaternary age, can be identified on single-channel seismic-reflection profiles. Rocks dredged from these cones include basalt, dacite, and cognate gabbroic inclusions with magmatic affinities similar to those of the Karua (an active submarine volcano off the southeastern tip of Epi) lavas. ?? 1988 Springer-Verlag New York Inc.

Geo-Marine Letters↗
USGS1988

An Angus/Argo study of the neovolcanic zone along the East Pacific rise from the Clipperton fracture zone to 12°N

Still photographs and video images collected along the Neovolcanic Zone of the East Pacific Rise from 10°15′N to 11°53′N show that recent volcanic sheet flows, possibly less than 100 years old, are superimposed on an older sediment-laden pillow terrane. This recent activity is restricted to a narrow zone that crosses two topographic highs at 10°55′N and 11°26′N and diminishes along-axis away from these highs. The association of recent sheet flows with older flows and collapse structures on the overlapping spreading centers at 11°45′N supports the evolutionary model for the occurrence and evolution of overlapping spreading centers by MacDonald and others (1986, 1988).

Geo-Marine Letters↗
USGS1988

Sediment waves on the tiber prodelta slope: Interaction of deltaic sedimentation and currents along the shelf

A regressive depositional sequence has been prograding on the northeastern Tyrrhenian Shelf since the establishment of the present high stand of sea level. Thickness and distribution of this prograding sequence are chiefly controlled by the Tiber Delta sediment source and the oceanographic conditions on the shelf. Wavy bedforms characterize the Tiber prodelta slope between 35 and 100 m water depth. On 3.5 kHz subbottom profiles, these bedforms show the same morphology and internal depositional geometry as most of the deep-water examples of sediment waves. ?? 1988 Springer-Verlag New York Inc.

Geo-Marine Letters↗
USGS1988

Hydrocarbon gas in sediment of the Southern Pacific Ocean

Methane, ethane, ethene, propane, and propene are common hydrocarbon gases in near-surface sediment from offshore areas in the southern Pacific Ocean near Papua New Guinea, the Solomon Islands, Vanuatu, Tonga, New Zealand, and Antarctica. Sea floor sites for sampling of sediment were selected on the basis of anomalies in marine seismic records, and the samples were intentionally biased toward finding possible thermogenic hydrocarbon gases. In none of the areas, however, were thermogenic hydrocarbons clearly identified. The hydrocarbon gases that were found appear to be mainly the products of in situ microbial processes. ?? 1988 Springer-Verlag New York Inc.

Geo-Marine Letters↗
USGS1989

Depositional history of the Lagniappe Delta, northern Gulf of Mexico

The northern Gulf of Mexico continental shelf is characterized by superimposing deltas. One such delta, informally named Lagniappe, extends east of the Mississippi Delta from mid-shelf to the continental slope. This late Wisconsinan delta is adjacent to, but not associated with the Mississippi Delta complex: the fluvial source was probably the ancient Pearl and/or Mobile Rivers. The fluvially dominated Lagniappe Delta is characterized by complex sigmoid-oblique seismic-reflection patterns, indicating delta switching of high-energy sand-prone facies to low-energy facies. The areal distribution and sediment thickness of the delta were partially controlled by two diapirs. ?? 1989 Springer-Verlag New York Inc.

Geo-Marine Letters↗
USGS1991

Consolidation and erosion of deposited cohesive sediments in Northern Chesapeake Bay, USA

Deposits of dredged cohesive sediments were monitored for changes in volume, bulk characteristics, and susceptibility to resuspension and erosion at disposal sites in Chesapeake Bay. There is a 23-48% volume reduction during the first six months, with correspondingly greater changes over longer time periods. A bulk density increase from 1.15 to 1.3 g/cm3 due to dewatering and compaction accounts for the majority of the volume change. Tidal current induced resuspension is a minor process. The observed suspended sediment load can be accounted for by erosion of only a fraction of a millimeter of sediment on each tidal cycle. ?? 1991 Springer-Verlag New York Inc.

Geo-Marine Letters↗
USGS1989

Sea-floor observations in the tongue of the ocean, Bahamas: An Argo/SeaMARC survey

SeaMARC side-scan sonographs and Argo video and photographic data suggest that the recent sedimentary environment of the floor of the Tongue of the Ocean is controlled by an interplay of turbidity current flow from the south, sediment spill-over from the carbonate platform to the east (windward side), and rock falls from the west carbonate escarpment (lee side). The spill-over forms a sandy sedimentary deposit that acts as a topographic obstruction to the turbidity current flow from the south. This obstruction is expressed by the westward migration of a northwest-southeast oriented turbidity-current-cut channel. ?? 1989 Springer-Verlag New York Inc.

Geo-Marine Letters↗
USGS1981

210 Pb method for estimating the rate of carbonate sand sedimentation

The plot of 210 Pb activity against depth in carbonate sands on the Virgin Island Bank is a negative asymmetric hyperbolic curve. As depth increases, an initial rapid decrease in 210 Pb activity caused by the decay of unsupported 210 Pb and 226 Ra is followed by increasing activity as a result of 210 Pb achieving equilibrium with in growing 230 Th. As this curve is time dependent, an estimate of the relative ages in carbonate sequences and the rates of net carbonate accumulation can be made. The ease of 210 Pb activity determinations makes this procedure an attractive method in obtaining carbonate sand accumulation rates.

Geo-Marine Letters↗
USGS1981

Response of the suspended sediment transport system to continental shelf dynamics

Surface currents influenced by a wind-driven upwelling event in San Pedro Bay moved total suspended matter (TSM) confined to the inner shelf on 19 April 1978 seaward, so that by 27 April surface TSM had increased over the outer shelf. Near-bottom concentrations of TSM also increased across the shelf during this time. This is explained by sediment resuspended by large surface waves being advected from the inner shelf seaward by the mean flow after this flow had turned from southeasterly to southerly when upwelling ceased on 26 April. These complex shelf dynamics contribute to the off-shelf transport of mud to the slope and deep basins. ?? 1981 A. M. Dowden, Inc.

Geo-Marine Letters↗
USGS1984

Problems in turbidite research: A need for COMFAN

Comparison of modern submarine fans and ancient turbidite sequences is still in its infancy, mainly because of the incompatibility of study approaches. Research on modern fan systems mainly deals with morphologic aspects and surficial sediments, while observations on ancient turbidite formations are mostly directed to vertical sequences. The lack of a common data set also results from different scales of observation. To review the current status of modern and ancient turbidite research, an international group of specialists formed COMFAN (Committee on Fans) and met in September 1982 at the Gulf Research and Development Company research facilities in Pennsylvania. ?? 1984 Springer-Verlag New York Inc.

Geo-Marine Letters↗
USGS1984

Sedimentary, tectonic, and sea-level controls on submarine fan and slope-apron turbidite systems

To help understand factors that influence submarine fan deposition, we outline some of the principal sedimentary, tectonic, and sea-level controls involved in deep-water sedimentation, give some data on the rates at which they operate, and evaluate their probable effects. Three depositional end-member systems, two submarine fan types (elongate and radial), and a third nonfan, slope-apron system result primarily from variations in sediment type and supply. Tectonic setting and local and global sea-level changes further modify the nature of fan growth, the distribution of facies, and the resulting vertical stratigraphic sequences. ?? 1984 Springer-Verlag New York Inc.

Geo-Marine Letters↗
USGS1984

The Astoria Fan: An elongate type fan

The Astoria Fan, a modern system, is located on a subducting oceanic crust and fills a north-south-trending trench along the Oregon continental margin. Well-developed channels cross the entire fan length; they display classic inner-fan leveed profiles but evolve into distributaries in the midfan area where the gradient decreases sharply. During periods of low sea level, inner- and middle-fan channels funnel sand to distal depositional sites in the outer-fan area where the sand/shale ratios are highest. This pattern of sand displacement and efficiency of transport appears to be characteristic of elongate fans fed by a major river and submarine canyon. ?? 1984 Springer-Verlag New York Inc.

Geo-Marine Letters↗