George H. Keller

Geotechnical Properties of Continental Slope Deposits Cape Hatteras to Hydrographer Canyon

The continental slope off the northeastern United States commonly displays gradients ranging from 3 to 10 is heavily dissected by submarine canyons and valleys and is an area of considerable slumping activity A study of the geotechinal properties sediment texture shear strength water content wet bulk density porosity and Atterberg Limits of 73 sediment cores from 21 transects across the continental slope from Cape Hatteras to Hydrographer Canyon provides insight into the general distribution and variation of these properties within the near surface deposits of this province of the seafloor Although a general gradation in sediment texture from coarse to fine prevails in a down slope direction all along the continental slope fine grained sediments silty clay appear to comprise the predominant sediment type along the slope particularly within the central portion of the Middle Atlantic Bight This depositional pattern appears to account for the occurrence of generally higher water contents and porosities as well as the lower wet bulk densities found in the slope deposits of the Middle Atlantic Bight Relatively coarse grained sediments of low water content and porosity and high bulk density make up the slope deposits to the north of Block Canyon as well as in the general area of Cape Hatteras Higher values of shear strength 7 to 14 kPa 1 2 psi are commonly found in the lower mid to lower slope deposits except in the vicinity of submarine canyons where lower values 2 to 4 kPa 0 3 0 6 psi appear to be related to a combination of increaspd concentrations of organic matter and fine grained sediments Sediment sensitivities range from I to 12 with a mean of 3 giving the indication that these deposits may be slightly quick in places but they are predominantly classed as medium sensitive Porosities vary from 4482 with the higher values occurring along the lower slope The mean value of 71 for these sediments is slightly higher than that reported for the hemipelagic sediments of the North Atlantic An analysis of the plasticity characteristics of the mid and lower slope sediments indicates that they vary little from those of abyssal plain deposits which are classed as inorganic clays of low medium and high plasticity An exception is found in the central part of the Middle Atlantic Bight where a large proportion of the sediments are classified as organic clays of medium to high plasticity and micaceous Distribution of the mean values of the various geotechnical properties in the near surface 1 290 cm deposits of the slope indicates that there is a general increase in water content liquid limit plastic limit clay content and porosity from off New England towards Cape Hatteras Analyses of slope deposits using the infinite slope analysis method to determine the effects of overburden pressure on their stability revealed no indication of instability Factors other than overburden however undoubtedly playa role in the slumping of slope deposits

The Physical Properties of Peru-Chile Continental Margin Sediments--The Influence of Coastal Upwelling on Sediment Properties

Physical properties (sediment texture, water content, wet bulk density, grain specific gravity, porosity, shear strength, and Atterberg limits) were determined for near-surface sediments (0 to 4 m) from the Peru-Chile continental slope and eastern Nazca Plate. These properties are strongly affected by organic matter which is concentrated in the slope deposits as a result of the high productivity in coastal upwelling areas off Peru. Sediments with unique properties occur in a lens of organic-rich mud on a portion of the upper slope (10.5 degrees to 13.6 degrees S) which coincides with an area of intense upwelling. The mud lens sediments are anomalously fine-grained and have the highest water content and plasticity and lowest wet bulk density and grain specific gravity found along the margin. Variation of these properties on the mud lens, and on the remainder of the slope and Nazca Plate, correlates well with changes in organic content. The undrained shear strength of the mud lens sediments is higher than might be expected for their high water content, possibly resulting from the development of clay-organic aggregates. A distinctive feature of the upper slope mud lens is a surface layer (0 to 15 cm) characterized by organic carbon concentrations of up to 20 percent dry weight, extremely high water content (maximum 853% dry weight), and very low wet bulk density (minimum 1.09 Mg/m 3 ). Outside the mud lens organic matter is less abundant and the amount of clay-size material in the sediment exerts a greater influence on the physical properties. Progressive fining of the sediments from south to north along the margin and with increasing distance from shore is accompanied by an increase in water content and plasticity and a decrease in wet bulk density and shear strength. These sediments typically show less change in physical properties with depth of burial than those of the upper slope mud lens. Differences in sediment mineralogy and sedimentation rate also affect the physical properties, but to a lesser extent than the variation of organic content or amount of clay.

Bottom Currents in the Hudson Canyon

In-place measurements of the bottom currents in the Hudson Canyon reveal that the current regime is characterized by a pronounced reversal of flow up and down the canyon. Velocities are commonly of the order of 8 to 15 centimeters per second, reaching 27 centimeters per second on occasion in the upper and central portion of the canyon. Although α 2.5-day recording of currents showed a net transport upcanyon, a combination of 66 current measurements from the submersible Alvin, the analysis of sediment texture and organic carbon, and the determination of the benthic fauna-nutrient relationship indicate that over the long term there is a net transport of fine material through the canyon to the outer continenital rise.

Variations in the mass physical properties of selected submarine sediments

Abstract In recent years there has been an increasing interest in the mass physical properties of submarine sediments. This interest has been developed by the sedimentologist who, borrowing techniques from the field of soil mechanics, has adapted a new approach to some of his problems. Although only about one thousand sediment cores have been collected from the ocean basins and examined for their mass physical properties (water content, cohesion, density, porosity, Atterberg limits and sensitivity) they afford some insight into the range of variations that can be expected in the upper few meters of sea-floor deposits. Since extreme values are frequently more significant than the averages, the maximum and minimum values of selected mass properties, as well as average values, have been compiled for the North Atlantic and North Pacific basins. When these values were compared with similar data from Chesapeake Bay, Tokyo Bay, and Lake Pend Oreille in Idaho, it was found that although the ocean basin deposits exhibit greater extremes, these particular bay and lake sediments possess higher overall values. Comparison of the ocean basin data with their associated sedimentary deposits revealed some characteristic parameters for each of the three types of deposits considered (carbonates, terrigenous, and “red clay”). Some correlations are possible between the various mass properties and the sediment type or depositional area. However, it is not yet possible to classify with confidence, either an area or a submarine deposit by any of these parameters.

Time-lapse photography of the biological reworking of sediments in Hudson submarine canyon

ABSTRACT Time-lapse photography indicates that biological activity and reworking acted as a catalyst for sediment erosion by bottom currents during a two-day period at a depth of 360 meters in the Hudson submarine canyon. Decapod Crustacea were responsible for the major tracks and burrows. Deep, sharp tracks made by the crab Cancer borealis were smoothed by currents within three hours, but a large burrow was maintained with intermittent excavation by a smaller burrowing decapod crustacean. The asteroid (seastar) Henricia did not track but smoothed the bottom. A polychaete worm appeared to forage in the Cancer tracks. Bottom water turbidity varied abruptly at intervals of less than one hour.

Morphology and sedimentary processes in and around Tortugas and Agassiz Sea Valleys, southern Straits of Florida

Abstract Continuous seismic reflection profiling and new bathymetry data in the southern Straits of Florida over an area dominated by the Tortugas and Agassiz Valley systems have allowed a more detailed analysis of the morphology and sedimentary processes active in this region. Four dives in the submersible DSV “Alvin” supplement the seismic and bathymetric data. The continental slope in the study area can be divided into two physiographic provinces: (I) an irregular topography controlled by the Florida Escarpment west of Tortugas Valley; and (II) the remainder of the continental slope which contains the majority of features under investigation. Seismic data indicate that the valleys are being filled shoreward of 290 fathoms (530 m) by a wedge of prograding sediments derived from the Florida shelf. The morphology of the two valley systems reflects probable differences of origin. Tortugas Valley appears to have originated coincident with the eastern terminus of the Florida Escarpment and province-I-type topography. The Agassiz valleys may have an origin associated with jointing patterns observed by divers aboard DSV “Alvin”. Current meter readings and bottom photographs from “Alvin” indicate that currents are relatively sluggish and not very effective in the transport of sediment within the valleys. An area of undulations west of Pourtales Terrace was investigated and concluded to be erosional in origin. Slumping appears to have played a large part in shaping many features in the study area. The bottom morphology and sediment distribution on the continental slope and in the axis of the Straits of Florida suggest that bottom currents are active in shaping the entire area.

Geotechnical Properties and Stability Characteristics of Continental Slope Deposits Influenced by Coastal Upwelling: ABSTRACT

Studies to date on continental slope deposits of Peru and Oregon indicate that coastal upwelling indirectly contributes to the alteration of sediment mass physical properties and stability characteristics by concentrating organic matter in the underlying and nearby sediments. Those sediments in close proximity to areas of intense upwelling display distinctly different geotechnical properties than do those of comparable sediment type some distance away. The ability of organic matter to adsorb water and to aggregate clay-size particles to form an open fabric appears to result in exceptionally high water End_Page 944------------------------------ content (853% by dry weight), porosity (89%), and plasticity as well as very low bulk density (1.09 Mg/m3). The undrained shear strength (cohesion) of these sediments is also unexpectedly high, resulting apparently from some form of bonding of the sediment particles by organic matter. Sensitivity (ratio of natural to remolded or disturbed shear strength) is also unusually high (21), indicating a high susceptibility to failure if the sediments should become severely disturbed. All sediments along the margins behave as if they are overconsolidated. The greater the organic content the greater the degree of overconsolidation. In some areas this degree is on the order of six to seven times that of similar slope deposits but with relatively low organic contents. This degree of ov rconsolidation suggests that organic-matter related, interparticle bonding may be responsible for the apparent overconsolidation. End_of_Article - Last_Page 945------------

Cariaco Trench-Sediment Geotechnical Properties

ABSTRACT A study of the mass physical properties of a sediment core from the anaerobic Cariaco Trench shows there to be a noticeable difference in these properties relative to those of the eastern Caribbean and Atlantic basins, and an even greater difference from those of the anoxic Black Sea. The variations in physical properties appear to reflect the respective concentrations of organic carbon in the four areas compared.