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At least 55 records · Page 3Linked to original sources
USGS2021

Influence of antecedent geology on the Holocene formation and evolution of Horn Island, Mississippi, USA

Horn Island, one of the two most stable barriers along the Mississippi-Alabama chain (Cat, East and West Ship, Horn, West Petit Bois, Petit Bois, and Dauphin), provides critical habitat, helps regulate estuarine conditions in the Mississippi Sound, and reduces wave energy and storm surge before they reach the mainland shore. However, important details of the formation and evolution of the island in response to sea-level rise, storms, and antecedent geology remain unclear. This study integrates 2200 km of high-resolution geophysical data, 35 sediment cores, and 18 radiocarbon ages to better understand the geologic history of the island. Incised valleys of the Biloxi and Pascagoula Rivers underlie Horn Island and played a profound role in the evolution of the system. Within the incised valleys, sandy paleochannel deposits represent potential sediment sources during island development. Scour associated with wave and tidal ravinement processes liberated sand from the paleochannels and along with numerous other sizable sand sources on the shelf contributed to the formation and continued maintenance of Horn Island. Based on radiocarbon ages, transgressive ephemeral islands/shoals with no preserved shoreface existed at least 8000 cal yr BP and were frequently overwashed when sea-level rise rates were ~ 4–5 mm/yr. Approximately 5000 cal yr BP, coinciding with a deceleration in sea-level rise to about 1.4 mm/yr and attendant increased sand supply, radiocarbon ages associated with Horn Island's barrier complex and lower shoreface indicate a period of island stabilization. Seismic and sediment core data show a long history of westward lateral migration by longshore currents through tidal ravinement and inlet fill. Subsurface sand packages associated with tidal inlet fill and paleochannels are available for ravinement and may be important sand sources for Horn Island to maintain subaerial exposure with the expected accelerated future sea-level rise.

Mississippi↗
USGS2012

Laboratory studies of volcanic jets

The study of the fluid dynamics of violent volcanic eruptions by laboratory experiment is described, and the important fluid-dynamic processes that can be examined in laboratory models are discussed in detail. In preliminary experiments, pure gases are erupted from small reservoirs. The gases used are Freon 12 and Freon 22, two gases of high molecular weight and high density that are good analogs of heavy and particulate-laden volcanic gases; nitrogen, a moderate molecular weight, moderate density gas for which the thermodynamic properties are well known; and helium, a low molecular weight, lowdensity gas that is used as a basis for comparison with the behavior of the heavier gases and as an analog of steam, the gas that dominates many volcanic eruptions. Transient jets erupt from the reservoir into the laboratory upon rupture of a thin diaphragm at the exit of a convergent nozzle. The gas accelerates from rest in the reservoir to high velocity in the jet. Reservoir pressures and geometries are such that the fluid velocity in the jets is initially supersonic and later decays to subsonic. The measured reservoir pressure decreases as the fluid expands through repetitively reflecting rarefaction waves, but for the conditions of these experiments, a simple steady-discharge model is sufficient to explain the pressure decay and to predict the duration of the flow. Density variations in the flow field have been visualized with schlieren and shadowgraph photography. The observed structure of the jet is correlated with the measured pressure history. The starting vortex generated when the diaphragm ruptures becomes the head of the jet. Though the exit velocity is sonic, the flow head in the helium jet decelerates to about one-third of sonic velocity in the first few nozzle diameters, the nitrogen head decelerates to about three-fourths of sonic velocity, while Freon maintains nearly sonic velocity. The impulsive acceleration of reservoir fluid into the surrounding atmosphere produces a compression wave. The strength of this wave depends primarily on the sound speed of the fluid in the reservoir but also, secondarily with opposite effect, on the density: helium produces a relatively strong atmospheric shock while the Freons do not produce any optically observable wave front. Well-formed N waves are detected with a microphone far from the reservoir. Barrel shocks, Mach disks, and other familiar features of steady underexpanded supersonic jets form inside the jet almost immediately after passage of the flow head. These features are maintained until the pressure in the reservoir decays to sonic conditions. At low pressures the jets are relatively structureless. Gas-particle jets from volcanic eruptions may behave as pseudogases if particle concentrations and mass and momentum exchange between the components are sufficiently small. The sound speed of volcanic pseudogases can be as large as 1000 m s −1 or as small as a few tens of meters per second depending on the mass loading and initial temperature. Fluids of high sound speed produce stronger atmospheric shock waves than do those of low sound speed. Therefore eruption of a hot gas lightly laden with particulates should produce a stronger shock than eruption of a cooler or heavily laden fluid. An empirical expression suggests that the initial velocity of the head of supersonic volcanic jets is controlled by the sound speed and the ratio of the density of the erupting fluid to that of the atmosphere. The duration of gas or pseudogas eruptions is controlled by the sound speed of the fluid and the ratio of reservoir volume to vent area.

Journal of Geophysical Research Solid Earth↗
USGS1997

Physical processes affecting the sedimentary environments of Long Island Sound

A modeling study was undertaken to simulate the bottom tidal-, wave-, and wind-driven currents in Long Island Sound in order to provide a general physical oceanographic framework for understanding the characteristics and distribution of seafloor sedimentary environments. Tidal currents are important in the funnel-shaped eastern part of the Sound, where a strong gradient of tidal-current speed was found. This current gradient parallels the general westward progression of sedimentary environments from erosion or non-deposition, through bedload transport and sediment sorting, to fine-grained deposition. Wave-driven currents, meanwhile, appear to be important along the shallow margins of the basin, explaining the occurrence of relatively coarse sediments in regions where tidal currents alone are not strong enough to move sediment. Finally, westerly wind events are shown to locally enhance bottom currents along the axial depression of the sound, providing a possible explanation for the relatively coarse sediments found in the depression despite tide- and wave-induced currents below the threshold of sediment movement. The strong correlation between the near-bottom current intensity based on the model results and the sediment response as indicated by the distribution of sedimentary environments provides a framework for predicting the long-term effects of anthropogenic activities.

Conference Paper↗
USGS2021

Preliminary assessment of the wave generating potential from landslides at Barry Arm, Prince William Sound, Alaska

We simulated the concurrent rapid motion of landslides on an unstable slope at Barry Arm, Alaska. Movement of landslides into the adjacent fjord displaced fjord water and generated a tsunami, which propagated out of Barry Arm. Rather than assuming an initial sea surface height, velocity, and location for the tsunami, we generated the tsunami directly using a model capable of simulating the dynamics of both water and landslide material. The fjord below most of the landslide source area was occupied by the Barry Glacier until about 2012; therefore, our direct simulation of tsunami generation by landslide motion required new topographic and bathymetric data, which was collected in 2020. The topographic data also constrained landslide geometries and volumes. We considered four scenarios based on two landslide volumes and two landslide mobilities—a more mobile, contractive landslide and a less mobile, noncontractive landslide. The larger of the two volumes is 689 × 10 6 cubic meters (m 3 )—larger than the volume estimate in a previous study—and reflects the largest plausible volume given current observational data. The considered scenario that generated the largest wave heights resulted in forecast wave heights of over 200 meters (m) in the northern part of Barry Arm, adjacent to the landslide source area and runup on the opposite fjord wall in excess of 500 m. Simulated wave heights in excess of 5 m in southern Barry Arm and in Harriman Fjord occurred within 10–15 minutes (min) of landslide motion. The simulated tsunami reached Whittier, Alaska, approximately 20 min after initial rapid landslide motion, with peak heights of just over 2 m in Passage Fjord, 500 m offshore Whittier, occurring 26 min after initial rapid motion. Time of peak wave heights was consistent with previous modeling. Although results are preliminary and can be refined with additional observations and analyses, they provide a refined assessment of the upper bound of the hazard presented by the Barry Arm landslides. The results herein support the National Oceanic and Atmospheric Administration’s National Tsunami Warning Center mission to detect, forecast, and warn for tsunamis in Alaska.

Alaska↗
USGS2006

Bedform distribution and inferred sand transport on Georges Bank, United States Atlantic continental shelf.

Four bedform provinces have been identified using sidescan-sonar and echo-sounding techniques: large sand waves superimposed on sand ridges, small sand waves, megaripples, and featureless seafloor. Sand-wave asymmetry and surface-sediment texture have been used to infer bedload transport paths and although the asymmetry of megaripples could not be determined, the occurrence of megaripples between the small sand-wave province and areas of featureless seafloor suggests a decreasing effectiveness of sand transport away from the bank crest. This sand dispersal pattern is further supported by the surface sediments which become progressively finer to the N and SW away from the crest of Georges Bank.-from Author

Massachusetts↗
USGS1997

Spatial earthquake hazard assessment of Evansville, Indiana

The earthquake hazard has been evaluated for a 150-square-kilometer area around Evansville, Indiana. GIS-QUAKE, a system that combines liquefaction and ground motion analysis routines with site-specific geological, geotechnical, and seismological information, was used for the analysis. The hazard potential was determined by using 586 SPT borings, 27 CPT sounding, 39 shear-wave velocity profiles and synthesized acceleration records for body-wave magnitude 6.5 and 7.3 mid-continental earthquakes, occurring at distances of 50 km and 250 km, respectively. The results of the GIS-QUAKE hazard analyses for Evansville identify areas with a high hazard potential that had not previously been identified in earthquake zonation studies. The Pigeon Creek area specifically is identified as having significant potential for liquefaction-induced damage. Damage as a result of ground motion amplification is determined to be a moderate concern throughout the area. Differences in the findings of this zonation study and previous work are attributed to the size and range of the database, the hazard evaluation methodologies, and the geostatistical interpolation techniques used to estimate the hazard potential. Further, assumptions regarding the groundwater elevations made in previous studies are also considered to have had a significant effect on the results.

Indiana↗
USGS1981

Bed form distribution and inferred sand transport on Georges Bank

Four bed-form provinces have been identified on Georges Bank using sidescan sonar and echo-sounding techniques: large sand waves, small sand waves, megaripples, and featureless sea floor. The large sand waves are found on the bank crest where the surface tidal currents are strongest, and are bordered, north and south, by areas of small sand waves and/or megaripples, formed where tidal currents are moderate in strength. Featureless sea floor is found farthest from the bank crest where surface tidal currents are weakest. Bed-form asymmetry and surface-sediment texture have been used to infer bedload transport paths on the bank. In the large sand-wave area, bed forms converge on linear northwest-striking ridges from both sides implying erosion from the troughs and growth of the ridges. The asymmetry of the small sand waves along the south side of the bank indicates that sand is also transported southward away from the bank. Though the bed-form asymmetry of megaripples could not be determined in this study, the occurrence of megaripples between the sand-wave provinces and areas of featureless sea floor suggests a decreasing effectiveness of sand transport away from the bank crest. This sand transport pattern is further supported by surface-sediment texture which becomes progressively finer both to the north and southwest away from the crest of Georges Bank.

Connecticut, Maine, Massachusetts, New Hampshire, ↗
USGS2014

Hydraulic criteria for sand‐waves

Sand‐waves on rivers are rhythmic successions of waves which occur at flood‐stages of streams heavily loaded with sediments. They take their name from the fact that sand and associated silts and gravels form a large part of the load transported by a river at such times. They seem to be peculiar to the Southwest and many vivid descriptions of them can be found in the literature of that region. R. C. PIERCE [see 1 of “References” at end of paper[, who observed many sand‐waves on the San Juan River in Utah, has described them as resembling in appearance “the waves thrown up by a stern‐wheel river steamboat.” He further describes their appearances as follows: “The sand‐waves are not continuous, but follow a rhythmic movement. At one moment the stream is running smoothly for a distance of perhaps several hundred yards. Then suddenly a number of waves, usually from six to ten, appear. They reach their full size in a few seconds, flow for perhaps two or three minutes, then suddenly disappear. Often, for perhaps half a minute before disappearing, the crests of the waves go through a combing movement, accompanied by a roaring sound. On first appearance it seems that the wave‐forms occupy fixed positions, but by watching them closely it is seen that they move slowly upstream. In the narrow parts of the stream the waves may reach nearly the width of the river, but in the wider parts they occupy smaller proportional widths. Usually they are at right‐angles to the axis of the stream, but at some places, particularly in the wider parts of the river, they may suddenly assume a diagonal position, moving rather rapidly across the stream in the direction toward which the upstream side of the wave has turned.” Many such descriptions may be found which in the main bear out PIERCE'S account, varying, however, as to size of wave, rate, and sometimes as to direction of movement.

Eos, Transactions, American Geophysical Union↗
USGS2025

Shallow geologic framework of the Mississippi Sound and the potential for sediment resources

The Mississippi Sound, an estuarine environment located between the mainland and barrier islands bordering the northern Gulf of America (formerly the Gulf of Mexico), serves as a vital ecosystem for the States of Mississippi and Alabama. Spanning approximately 100 kilometers from east to west and covering 1,400 square kilometers, the sound is home to marine industry and ports, and its shallow and brackish waters sustain a diverse array of marine life. Barrier islands along the southern edge of the sound separate the microtidal estuary from the Gulf of America. This protection from gulf wave action mediates current flow within the sound, resulting in predominantly fine-grained sediment deposition along the seafloor. This study, conducted by the U.S. Geological Survey in cooperation with the U.S. Army Corps of Engineers, provides insight on fluvial and tidal processes spanning the past 5,000 years. The report synthesizes existing research to provide a comprehensive overview of the sound geology, from Pleistocene origins to present-day morphology, and utilizes high-resolution single channel seismic profiles and sediment data to identify and map sedimentary deposits and morphologic features at and below the seafloor. Despite its ecological significance, the Mississippi Sound faces environmental challenges, including water-quality issues, habitat degradation, storm-induced erosion, and the ongoing threats of sea-level rise and environmental changes. This study uses the present-day understanding of the sound's geology to inform coastal management decisions, hazard assessment, and potential mineral resources.

Louisiana, Mississippi↗
USGS1980

Graded storm sand layers offshore from the Yukon Delta, Alaska

The northern Bering Sea has a history of severe storm surges. The most recent, and perhaps the worst in historical times, occurred in November, 1974 (Fathauer, 1975). Evidence of storm surge events is exhibited in sea-floor stratigraphy as well as shoreline flooding and indicates that significant widespread changes in sea-floor sedimentation take place (Nelson and Creager, 1977). These changes have implications for installations on the sea floor and for mass transport of pollutants. This paper describes the interbedded sand layers found in southern Norton Sound off the modern Yukon Delta that are deposited by the storm surge events. Such deposits are evident in both modern and ancient deposits of epicontinental shelves (Hays, 1967; Howard and Reineck, in press; Anderton, 1976.). These graded sand layers in very shallow water mimic many of the features of thin-bedded turbidite sands, although the shallow water deposits are thought to have a very different mechanism of deposition related to storm surge processes. Two factors in the oceanographic setting of northern Bering Sea magnify the effects of storm surge. The sea floor is very shallow (less than 20 m deep over wide areas) particularly in Norton Sound. Consequently there is intensive wave reworking which causes extensive sea-floor erosion, mass movement, displacement, and offshore progradation of significant amounts of sediment during storm surges; The second factor is a system of strong dynamic_ bottom currents that can move large amounts of sediment northward to Chukchi Sea during normal weather. Much more sediment is moved when the current is reinforced by relaxing of the sea surface set-up caused by storm surge (Fig. 1) (Flemming and Heggarty, 1966; Coachman et al., 1976; Cacchione and Drake, 1979; Schumacher and Tripp, 1979). Another important influence on the sedimentation in southern Norton Sound is the effect of seasonal processes on the Yukon River delta, During the winter months from November to May the Yukon River averages 40,000 cfs when the ice-covered river is fed mainly by base flow (Dupre, 1976). Within less than a week of river breakup, peak discharges may reach 1,000,000 cfs or more and then decline throughout the summer.

Alaska↗
USGS1976

Physical constraints on sounds generated by very small earthquakes

Vertical vibrations of the ground surface due to elastic waves from an earthquake will generate acoustic pressure waves in the overlying atmosphere. Sufficiently intense vibrations of the ground at frequencies greater than about 20 HZ can produce pressure waves that are within the audible range of many animals, including humans. People in the epicentral region of moderate to large earthquakes, for instance, commonly relate hearing sounds accompanying individual earthquakes (here we exclude sounds associated with mechanical resonances in structures such as houses). Descriptions of these sounds range from the report of distant guns to the rumble of thunder or the rushing of a wind (Davison, 1938; Lanchow Seismological Brigade, Appendix II). In many instances, the sounds are described as starting several seconds before the earthquake is felt. Experimental and theoretical verification that such earthquake sounds are generated by high-frequency ground motion from local earthquakes is based on simultaneous acoustic and seismic recordings recently obtained during a swarm of earthquakes in the Imperial Valley, California (Hill and others, 1976). These results show that for local earthquakes in the magnitude range 2 to-3, audible sounds are generated by the first arriving compressional (P) wave while perceptible shaking begins with the larger but slower shear (S) wave, explaining the reports that "earthquake sounds" are frequently heard several seconds before the earthquake is felt. A commonly proposed explanation for alleged anomalous animal behavior hours to days prior to large earthquakes suggests that animals with acute, high-frequency hearing respond to weak '''ultrasonic" sounds generated by small earthquakes occurring in the epicentral region of the impending event. Such earthquakes are presumably too small to be either felt or heard by humans in the region and too small to be routinely recognized as earthquakes on seismograms recorded on a local seismograph network. Evidence that such "popping and cracking" may occur in the region of an impending earthquake comes from laboratory experiments in which the rate of microfracturing in a rock sample subjected to large stress differences increases dramatically before catastrophic failure of the sample (Scholz, 1968; Stesky, 1975). In this paper we investigate the following question: What are the constraints on earthquake size (fault dimension and displacement) and hypocentral distance (distance from the earthquake focus to a point on the Earth's surface) such that animals noted for their acute hearing might react to the sound generated by the event while the same event would go undetected by humans (either by feeling, hearing, or routine identification on a local seismograph network)? Factors to be considered include: 1) the nature of the displacement spectral amplitudes of elastic waves radiated from an earthquake, 2) the effects of attenuation on the spectral components of elastic (seismic) waves over propagation paths from the focus to the Earth's surface, 3) the appropriate scaling of spectral amplitudes from moderate to very small earthquakes, 4) the coupling of elastic waves in the Earth to acoustic waves in the atmosphere, and 5) the threshold-of-hearing characteristics for various animals, including man. In considering these factors, we will find that, except for extremely small, shallow "earthquakes" (fracture dimension on the order of 10 cm and focal depths on the order of 10 m), acoustic emissions from premonitory fracturing ought to be equally audible to both people and animals in the immediate area of the fracturing.

Open-File Report↗
USGS2012

Shipboard magnetic field "noise" reveals shallow heavy mineral sediment concentrations in Chesapeake Bay

Shipboard magnetic field data collected over Chesapeake Bay exhibit low-amplitude, short-wavelength anomalies that most likely indicate shallow concentrations of heavy mineral sediments. Piston core layers and black sand beach samples exhibit enhanced magnetic susceptibilities and carry remanent magnetization, with mineralogical analyses indicating ilmenite and trace magnetite and/or maghemite and hematite. The anomalies are subtle and would be filtered as noise using traditional approaches, but can instead be highlighted using spectral methods, thus providing nearly continuous coverage along survey tracks. The distribution of the anomalies provides constraints on relevant sorting mechanisms. Comparisons to sonar data and previous grab samples show that two of three areas surveyed exhibit short-wavelength anomalies that are clustered over sand-covered areas, suggesting initial sorting through settling mechanisms. This is supported by a correlation between core magnetic susceptibility and grain size. Near the Choptank River, where sediment resuspension is wave-dominated, anomalies show a sharp decrease with seafloor depth that cannot be explained by signal attenuation alone. In Pocomoke Sound, where both tidal currents and wave-action impact sediment resuspension, anomalies show a more gradual decrease with depth. Near the mouth of the bay, where there is a higher influx of sediments from the continental shelf, short-wavelength anomalies are isolated and do not appear to represent heavy mineral sand concentrations. These combined observations suggest the importance of further sorting by erosional processes in certain parts of the bay. Additionally, comparisons of these data to cores sampling pre-Holocene sediments suggest that the sorting of heavy minerals in higher energy, shallow water environments provides a mechanism for correlations between core magnetic susceptibility and sea-level changes.

Maryl;Virginia↗
USGS2007

Field Demonstrations of Five Geophysical Methods that Could Be Used to Characterize Deposits of Alluvial Aggregate

Personnel from the U.S. Geological Survey and Martin Marietta Aggregates, Inc., conducted field demonstrations of five different geophysical methods to show how these methods could be used to characterize deposits of alluvial aggregate. The methods were time-domain electromagnetic sounding, electrical resistivity profiling, S-wave reflection profiling, S-wave refraction profiling, and P-wave refraction profiling. All demonstrations were conducted at one site within a river valley in central Indiana, where the stratigraphy consisted of 1 to 2 meters of clay-rich soil, 20 to 35 meters of alluvial sand and gravel, 1 to 6 meters of clay, and multiple layers of limestone and dolomite bedrock. All geophysical methods, except time-domain electromagnetic sounding, provided information about the alluvial aggregate that was consistent with the known geology. Although time-domain electromagnetic sounding did not work well at this site, it has worked well at other sites with different geology. All of these geophysical methods complement traditional methods of geologic characterization such as drilling.

Scientific Investigations Report↗
USGS2010

Maps and documentation of seismic CPT soundings in the central, eastern, and western United States

Nine hundred twenty seven seismic cone penetration tests (CPT) in a variety of geologic deposits and geographic locations were conducted by the U.S. Geological Survey (USGS) primarily between 1998 and 2008 for the purpose of collecting penetration test data to evaluate the liquefaction potential of different types of surficial geologic deposits (table 1). The evaluation is described in Holzer and others (in press). This open-file report summarizes the seismic CPT and geotechnical data that were collected for the evaluation, outlines the general conditions under which the data were acquired, and briefly describes the geographic location of each study area and local geologic conditions. This report also describes the field methods used to obtain the seismic CPT data and summarizes the results of shear-wave velocities measurements at 2-m intervals in each sounding. Although the average depth of the 927 soundings was 18.5 m, we estimated a time-averaged shear-wave velocity to depths of 20 m and 30 m, VS20 and VS30, respectively, for soundings deeper than 10 m and 20 m. Soil sampling also was selectively conducted in many of the study areas at representative seismic CPT soundings. These data are described and laboratory analyses of geotechnical properties of these samples are summarized in table 2.

Open-File Report↗
USGS2008

Sea-floor character and sedimentary processes in the vicinity of Woods Hole, Massachusetts

Continuous-coverage multibeam bathymetric models and sidescan-sonar imagery have been verified with high-resolution seismic-reflection profiles, sediment sampling, and bottom photography. Together these data layers provide detailed base maps that yield topographic, compositional, and environmental perspectives of the sea floor in the vicinity of Woods Hole, an important harbor and major passage between the Elizabeth Islands and Cape Cod, Massachusetts. Tidally dominated high-energy environments within Woods Hole have prevented deposition of Holocene marine sediments, exposed underlying glacial drift of the Buzzards Bay moraine, and winnowed finer grained sediments, leaving lag deposits of boulders and gravel. These conditions have also enlarged and preserved depressions in the moraine surface that were originally kettle holes and formed ebb-tidal deltas at the entrances to passages. Fields of transverse and barchanoid sand waves dominate across the southern part of the study area in Vineyard Sound, where benthic environments are characterized by processes associated with coarse-bedload transport. Transverse sand waves dominate near shoals where sediment supply is greater and have asymmetries that indicate that the shoals are shaped and maintained by clockwise gyres of net sediment transport. Barchanoid sand waves, which are most common where Holocene sediments are thinner, commonly align into elongate fields that have smaller isolated waves concentrated at the eastern ends and that progressively widen and have waveforms that increase in amplitude, wavelength, and complexity westward. The northern, protected parts of the Little and Inner Harbors are characterized by muddy sediment and processes associated with deposition. A pockmark field in Little Harbor and the muddy, organic-rich sediments that form a scarp along the edge of Parker Flat are evidence for the presence of submerged marsh deposits formed during the Holocene rise in sea level.

Massachusetts↗
USGS2022

New model of the Barry Arm landslide in Alaska reveals potential tsunami wave heights of 2 meters, values much lower than previously estimated

The retreat of Barry Glacier has contributed to the destabilization of slopes in Barry Arm, creating the possibility that a landslide could rapidly enter the fjord and trigger a tsunami. The U.S. Geological Survey (USGS) recently released a report documenting potential tsunami wave heights in the event of a large, fast-moving landslide at the Barry Arm fiord near Prince William Sound, Alaska (Barnhart and others, 2021). This new work shows that the largest plausible wave height is smaller than initial estimates published in Dai and others (2020), but waves still represent a substantial hazard to the people who live, work, and recreate in Prince William Sound. Thus, it is important that residents and visitors remain informed about this hazard and prepare accordingly.

Alaska↗
USGS1983

Sand-wave movement on Little Georges Bank

A 1-x-1.5-km area on Little Georges Bank (centered at 41?08?N., 68?04?W.) was mapped three times during a ten-month period by sidescan sonar and echo-sounding techniques to assess the morphology and mobility of sand waves on Georges Bank. Sand-wave amplitudes in the survey area ranged from 1-11 m although most were 5-7 m. Wavelengths were not constant as the crests were sinuous and in places, even bifurcated. The sand waves are asymmetrical with their steepest sides facing northwest; however, gradients of their steep sides mostly are 4?-10? which is well below the angle of repose for sand in water. Sand waves tended to have greater relief and a sharper asymmetry during the survey in September than during those in June or April. During the survey period the sand waves moved but the direction and rate of motion was variable. Even along an individual sand wave some parts moved as much as 60 m between surveys while other parts apparently remained stationary. The sand waves were asymmetrical, but movement was not consistently in the direction that the steep sides faced. Along the same sand wave, parts moved to the northwest while other parts moved to the southeast. Despite the complex pattern of sand motion, the mean displacement of the sand waves was below the resolution of the survey technique; to resolve it, a longer survey is needed.

Open-File Report↗
USGS1982

Geological interpretation of cone penetrometer tests in Norton Sound, Alaska

In situ cone-penetrometer tests at 11 stations in Norton Sound, Alaska, complement previous studies of geologic processes and provide geotechnical data for an analysis of sediment response to loading. Assessment of the penetrometer records shows that various geologic factors influence penetration resistance. On the Yukon prodelta, penetration resistance increases with the level of storm wave or ice loading. In central and eastern Norton Sound, thermogenic and biogenic gas, as well as variations in sediment texture and composition, effect a wide range of resistance to penetration. ?? 1982 A. M. Dowden, Inc.

Geo-Marine Letters↗