Armand J. Silva

Analysis of a longitudinal ripple from the Nova Scotian continental rise

Abstract A longitudinal ripple was obtained in a box core taken from the Nova Scotian continental rise in July 1982. Soft brown mud comprising 1.5—10% sand, ∼60% silt and ∼35% of μ m clay forms the 5 cm high and ∼40 cm wide ripple. A maximum thickness of 8 cm of this mud under the crest, thinning to 2 cm on the ripples flank, overlies stiff muddy foram ooze. The vane shear strength of the brown mud is ∼0.4 kPa whereas that of the ooze is ∼4 kPa. X-radiographs show the mud to contain many fine burrows and a few larger ones as well as winnowed horizons rich in foraminifera of both benthonic and planktonic origin. Gentle wet sieving of the sand fraction on board ship showed the sand fraction to contain very few faecal pellets. Thus it appears unlikely that bedload movement played a large part in formation of the ripple. Rapid initial deposition from a concentrated suspension is suggested to have formed the structure, but this was followed by periods of erosion to yield winnowed horizons and further rebuilding with material deposited from suspension. Radiochemical data ( 234 Th and 210 Pb) and X-radiographs suggest intense particle mixing ( D B ∼90 cm 2 yr −1 ) and rapid sediment deposition (∼1.5 cm month −1 ). Thus it is possible that the bedform (and associated structures) are destroyed and recreated on a time-scale of a few months.

Consolidation Properties and Stress History of Some Deep Sea Sediments

This paper focuses on the consolidation properties and analysis of stress conditions of deep sea sediments in two regions: the North Western Atlantic in the vicinities of the Bermuda Rise and Blake Bahama Outer Ridge, and the North Central Pacific between the Murray and Mendocino Fracture Zones. One-dimensional consolidation tests have been conducted on over 100 samples obtained from a variety of different types of core samplers, including box, standard piston, giant piston (i.e. large-diameter), large-diameter gravity and Kasten, taken to depths of over 30 m. The sediments from the Atlantic sites are primarily illitic clays with varying amounts of calcium carbonate, smectite and silt content. In the Pacific sites the sediments range from illite-rich to smectite-rich pelagic and authigenic clays.

Geotechnical properties of ocean sediments recovered with the Giant Piston Corer: Blake—Bahama Outer Ridge☆

Abstract Consolidation tests were conducted on sediment samples from four cores (17–31 m lengths) obtained with the Giant Piston Corer on the Blake—Bahama Outer Ridge (water depths 4758–4962 m). Three of the cores (GPC-7, 8, 9) had similar composition (calcareous lutites) and texture (60% clay) and similar vertical variability in water content, shear strength and carbonate profiles with very large decreases in water content (average decrease of 52%) occurring at 20 m depth. The fourth core (GPC-11) has considerable sand- and silt-sized material (calcareous tests) and much higher shear strength. The consolidation results of core GPC-11 showed a significant amount of overconsolidation suggesting that more than 10 m of sediment has been eroded and that the core was taken in the bottom of a large erosional furrow. Correlations between consolidation parameters and composition are proposed through use of a mineralogy factor ( E ) such that the mineralogy of the clay fraction is combined with effects of stress history and clay fraction influence as follows: E=[(M ∗ + I) − (%CaCo 3 )] × [(w 0 − w p ) % where: • M∗ = percentage of montmorillonite plus mixed-layer montmorillonite—illite, • I = percentage of illite, • w 0 = in-situ water content, and • w P = plastic limit. The following regression-line equations were obtained based on consolidation test results from Blake—Bahama Outer Ridge: Compression Index = C c = 0.0162 E − 0.0333 Coef. of Secondary Compression = C ∝ = (0.461 E − 0.126) · 10 −3 Although clay mineralogy does have an effect on consolidation behavior, the effects of consistency coupled with percentage of calcium carbonate exert the greatest influence on the consolidation characteristics of these sediments.

Investigation of mass wasting on the continental slope and rise

Abstract A methodology is presented for the investigation of submarine slope instability. This approach integrates the results of a variety of tools and methods, including multibeam, side scan and subbottom acoustic data, geotechnical sampling and testing, and advanced dating techniques. For slides/slumps that have the potential for causing tsunamis, the use of long, large-diameter piston corers is a cost-effective way of obtaining good quality sediment samples. Using a Multi-Sensor Core Logger and geotechnical analyses, a baseline of sediment properties is developed from nearby stable, unfailed regions, which is then compared to more active areas where faulting and instability is suspected. Clear evidence of past and recent slope failures can often be obtained from the stress history of the sediment derived from consolidation test results and from careful identification of naturally deposited sediments overlying relic failures. This methodology was applied for the investigation of slope failures along a study corridor in deepwater Gulf of Mexico. Large failures were identified from multibeam data, and coring locations were chosen based on subbottom and side scan data obtained with a deep-tow system. Buried failures were identified from detailed bulk density measurements, geotechnical test results, and paleomagnetic and 14 C dating techniques. Dating of these failures can provide information regarding the triggering mechanisms of these events, which can be used to assess the current risk of new instabilities.

Permeability of deep‐sea clays: Northwestern Atlantic

Abstract Permeability characteristics of deep‐sea sediments from the East Bermuda Rise and Blake Bahama Outer Ridge areas of the northwestern Atlantic were investigated. The samples were predominantly clay sized, although there were significant percentages of coarser‐grained material. The coefficients of permeability were directly measured by imposing a hydraulic head across the sample and were calculated according to the theory of one‐dimensional consolidation. Darcys measured coefficients of permeability ranged from 7.93 ts 10‐7 cm/sec at a void ratio of 3.66 to 1.10 ts 10‐8 cm/sec at a void ratio of 1.10. Of the two methods (log of time and square root of time) the square root of time method agreed more with the measured permeability values. This is related to the square root of times reliance on the earlier stages of consolidation, when the excess pore pressures across the sample are greater and secondary effects are reduced. Existing theory based on the physiochemical nature of the clays was inadeq...

Consolidation and permeability characteristics of high-porosity surficial sediments in Eckernförde Bay

Back-pressured, constant-rate-of-deformation consolidation, and permeability tests were conducted on 21 “undisturbed” samples from Eckernförde Bay in the Baltic Sea. The soft fine-grained sediments have very high in-situ void ratios and are highly compressible. The compression index decreases slightly in the upper 40 cm but remains essentially unchanged below 40 cm at an average value of 3.5 to a depth of 260 cm. Recompression indices range from 5 to 19% of the virgin indices. The preconsolidation stress is consistently higher than the overburden stress, particularly near the surface. Permeabilities at in situ void ratios vary between 3 × 10−4 and 10−6 cm s−1, with the relationship between void ratio and the logarithm of permeability being linear.

Geotechnical characterization of surficial high-porosity sediments in Eckernförde Bay

Geotechnical studies of high-porosity, finegrained sediments from Eckernförde Bay, part of the Coastal Benthic Boundary Layer program, included coring, subsampling, and on-board testing during three field experiments, and a laboratory testing program to determine the mechanical properties of the seabed. The sediments consist of high-porosity (86–94%), organic-rich (10–20%) silty clays with varying amounts of methane gas. The surface 5–10 cm are characterized by unusually high water contents (400–600%), very low shear strengths (<1 kPa), and pronounced shear thinning behavior. Below that depth sediments are somewhat more competent, exhibiting water contents of 250–300% and higher strengths (> 1 kPa).

Marine sediment acoustic measurement system

Abstract This paper describes a system that has been developed to measure compressional wave speed in cored marine sediments onboard ship. The structure enables one to secure an extruded core sample to its base and to move acoustic probes to a desired location, implant them to a specified depth into the sample and perform the measurement. The acoustic measurement system is a pulse-time delay system measuring time difference over a fixed path length and the temperature of the sediment. The time difference and temperature measurement systems are comprised of task oriented components and are housed in a single portable box. The system is adaptable to the various sample sizes obtained with the coring apparatus presently in use. Initial field tests indicate that ship motion has no effect on the system. Data collected from cores has been classified according to sediment type and displays good agreement with data presented by Hamilton (1970). The difference in compressional wave velocity, based on sediment type, for the two studies is ⩽ 5 m per sec.

Geo-acoustic characterization of calcareous seabed in the Florida Keys

Abstract Geotechnical and acoustic measurements on a set of 35 gravity cores and 11 box cores from two calcareous seabed locations in the lower Florida Keys that are characterized by contrasting environmental settings show significant differences in terms of vertical profiles of physical, acoustic, and geotechnical properties. The lower energy study site of the two is sheltered by the adjacent Dry Tortugas platform complex and reveals a higher porosity surface interval with significant changes in water content, density, and compressional wave velocity within the upper 25 cm. Sediment cores from open-water locations, such as those collected in the study area north of the Marquesas Keys, exhibit higher, less variable densities and lower velocities within the top 25 cm. This is attributed to consolidation associated with cyclic pressure variations from surface swells and strong tidal currents. Acoustic subbottom profiles display good correlation with shell-lag deposits observed in the gravity cores, although acoustic records lack the vertical resolution to detect variations in physical and acoustic properties on the order of those measured in this study. Calculated impedances at depths below 25 cm are significantly higher in the Dry Tortugas area and hence penetration of 4- and 15-kHz acoustic signals is less than at the Marquesas study site. From a geotechnical point of view, the sediments at both sites can be considered to behave like granular materials with little or no plasticity, no significant cementation, low compressibility, permeability highly dependent on void ratio, and moderate to high friction angles. A comparison with deep-sea sediments of mixed mineralogy shows that the effect of increasing calcium carbonate with decreasing clay content is to decrease plasticity and compressibility, and to increase friction angles. In other words, sediments shift from a cohesive to a granular nature as the carbonate content increases.

Physical and acoustic measurements on cohesionless sediments from the northwest Florida Sand Sheet

The effects of grain size and density on compressional wave speed and attenuation are investigated for a clastic silica sand from a seabed study site south of Panama City, Florida, using an automated core logging device that allows for highly accurate, non-destructive, fine-scale measurements to be taken on unopened core sections. Measurements were conducted on relatively undisturbed cores obtained using a large-diameter gravity corer, as well as on reconstituted sections containing sediment segregated into narrow grain size ranges. Findings indicate that whereas density is the primary physical sediment attribute controlling speed, attenuation at 500 kHz is primarily a function of grain size and grain structure. Sandy sediments, particularly those with narrow sorting, are susceptible to liquefaction, which can reduce attenuation dramatically.