Douglas N. Lambert

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

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.

Bottom processes, morphology, and geotechnical properties of the continental slope south of Baltimore Canyon

Abstract The continental slope south of Baltimore Canyon seaward of the coasts of Delaware and Maryland has a different morphology and sedimentary structure than adjacent portions of the continental margin. Ridges of sediment 600 m thick and transverse to the slope contain many unconformities that can be traced from ridge to ridge. The age of the sediment is inferred to be late tertiary to recent with the morphology related to a major drainage system. Physical properties of a suite of sediment cores display a pattern that varies in relationship to the morphology and depositional environment. Sedimentary structures and low shear strengths indicate instability of surficial sediments present on the upper slope and can be correlated with regions where the seismic reflection profiles show slumping has occurred. A veneer of sand overlying the general silty clay of the area is present on the upper slope and on the ridges indicating sand spillover from the shelf with a recent change in deposition pattern.

Rapid and reliable technique for determining unit weight and porosity of deep-sea sediments

Abstract A rapid and reliable technique is explained for determining unit weight and porosity of deep-sea sediment from water content and average grain density measurements. Comparisons are made between this method and the standard tube method (volumetric), with 77 samples. The correlation coefficient is found to be 0.994 with a standard deviation between the methods of ± 0.01 g/cm 3 . The two techniques used to determine porosity are found to have a correlation coefficient of 0.998 with a standard deviation of 0.42%. A nomographic chart is shown which permits rapid determination of unit weight using water content and average grain density.

Geotechnical properties of a submarine slide area on the U.S. continental slope northeast of Wilmington canyon

Abstract A relatively large submarine slide (slump block) and apparent unstable surficial sediments undergoing creep have been delineated in bathymetric and seismic reflection profiles along the U.S. Atlantic continental margin northeast of Wilmington Canyon. A downslope core transect was made over selected areas to assess the geotechnical properties of the sediments associated with the slide. Sediments are predominantly silty clays and clayey silts rich in illite, with lesser quantities of feldspar, kaolinite, chlorite, quartz, and smectite minerals. Surficial sediments (cored up to 12 m) upslope from the slump block reveal typical variations in the mass physical properties with core depth. Shear strength and wet unit weight show a steady increase with depth below the mudline commensurate with a decrease in water content. In contrast, surficial sediments downslope overlying the slump block generally have low shear strength and relatively high variability in other mass physical properties with core depth....

Reconnaissance in DSRV Alvin of a “fluvial-like” meander system in Wilmington Canyon and slump features in South Wilmington Canyon

Three dives in DSRV Alvin on the Atlantic Continental Rise in Wilmington and South Wilmington Canyons, off the east coast of the United States, allowed examination and sampling of morphological features, in water depths of 2,300 to 2,400 m, that were observed in midrange sidescan sonar data. In Wilmington Canyon, a “fluvial-like” meandering system was confirmed. The meandering channel had steep undercut outer banks and gently sloping inner banks. Localized slumping is inferred from many steplike depressions on the steep outer banks. Although the meander system is as much as 500 m wide and has many characteristics typical of a fluvial system, extreme depth and evidence of episodic ongoing sedimentary processes preclude a true fluvial origin. Currents of unknown origin—that is, downcanyon turbidity flows—appear to be the only agent capable of sculpturing the observed features. A meander system was not observed in South Wilmington Canyon. Channel-floor features, including deformed and displaced sediment, support a previous suggestion of large-scale slumping in the area. These sediments include upturned clay beds, disaggregated gravels, loosely bound gravel conglomerates in a reddish-brown matrix, and a tubular structure, resembling a tree-root cast, within a thinly bedded, reddish-brown sandstone.

Geology of Great Abaco Submarine Canyon (Blake Plateau): Observations from the research submersible “Alvin”

Abstract Nine dives in the research submersible “Alvin” were made into Great Abaco Submarine Canyon to depths ranging from 1850 to 3666 m. Our observations indicate that the walls of this canyon are distinctly terraced, consisting of nearly vertical to overhanging rock cliffs and intervening, less steep sediment-covered slopes. The wall rock consists mostly of massive, shallow-water limestones and dolostones of Cretaceous age, coated on exposed surfaces with manganese oxides. These rocks are heavily jointed/fractured and thus very blocky to angular in appearance, with sponges and other sessile organisms commonly attached. Talus slopes and sedimentary breccia deposits containing angular boulders are present at the base of these steep escarpments. Short-term bottom current measurements in the axis of the eastern part of the canyon indicate that currents are relatively weak, reaching velocities of only 10 cm/sec. This relatively placid setting is further corroborated by the abundance of turtle grass ( Thalassia ) found along the canyon axis. However, abundant subdued, symmetrical ripple marks and large scour depressions at the base of boulders, indicate that high-energy events sporadically impact the canyon axis. Contemporary erosional activity along the axis of the western (headward) part of the canyon appears to be more significant, as evidenced by asymmetrical ripple marks, sand waves and bioerosion. Great Abaco Canyon has evolved with time via a variety of processes, including: (1) faulting: (2) subsidence; (3) defacement; and (4) erosional down-cutting. The location, orientation and initiation of this canyon appear to be structurally controlled by the Great Abaco Fracture Zone during pre-Santonian time. Regional subsidence during the Mesozoic allowed the walls of Great Abaco Canyon to build vertically by accretion of shallow-water limestones, whereas joint-controlled defacement has widened the canyon while maintaining steep walls. Erosional down-cutting in the canyon axis by carbonate sediment gravity flows also appears to have been important episodically, particularly during the Miocene and Pleistocene.

Geotechnical properties of continental upper rise sediments—Veatch Canyon to Cape Hatteras

Abstract Geotechnical properties and brief core descriptions are given for 17 high‐quality gravity cores collected on the continental upper rise between Veatch Canyon and Cape Hatteras. Eleven of the cores were taken on four transects across the upper rise perpendicular to the continental slope. Six other cores were recovered from the axis and periphery of the outer Hudson Canyon. The upper 1m of sediment from the upper rise (not associated with the canyon) is generally bioturbated, olive‐gray silty clays containing few, if any, primary sedimentary structures. The cores peripheral to the outer Hudson Canyon contain numerous thin‐bedded mud turbidite sequences, indicating that the canyons levees were built up by over‐bank deposition of turbidity currents, particularly on the south side of the canyon. Geotechnical properties of the sediments from the upper rise display a reasonably high degree of uniformity over the expanse of the study area. Within‐core variation is often high and can be attributed to sli...

The Hudson Shelf Valley: Its role in shelf sediment transport

Abstract Freeland, G.L., Stanley, D.J., Swift, D.J.P. and Lambert, D.N., 1981. The Hudson Shelf Valley: its role in shelf sediment transport. Mar. Geol., 42: 399-427. The Hudson Shelf Valley was deeply incised by Pleistocene lowstands of the sea. During the Holocene transgression, fluvial, estuarine, and estuary-mouth depositional environments were displaced landward, and sedimentation in these environments partially filled the valley. Holocene sediment thickness in the seaward half of the valley is about the same as on the shelf to either side, but in the landward half of the valley, 22 m of sediment is present versus 5-10 m on the adjacent shelf. Sediment volume calculations in the upper shelf valley show that there is about three times the infill volume east of the thalweg as compared to the infill to the west. This configuration indicates that the valley has served as a sediment trap and records the westward migration of the thalweg during the Holocene. Most of the infill was the result of littoral drift which transported sand westward along the south shore of Long Island during the Holocene transgression. Modern sediment transport, as calculated from current-meter observations and inferred from bedform observations, is mainly south westward, along the regional trend of the isobaths and across the shelf valley. Therefore, infill on the eastern side of the shelf valley is continuing at present. Observations of the regional distribution of sediment types indicate that the asymmetrical valley fill is part of a pattern of bottom response to flow. The former drainage divides on both sides of the shelf valley are surfaced by coarse lag deposits on slopes facing northeast, into the direction from which the major storm flows come, while the down-current sides are mantled with fine sand.

Geotechnical properties of surficial sediments in a mega-corridor: U.S. Atlantic continental slope, rise, and deep-sea basin

Abstract Marine geotechnical studies at The National Oceanic and Atmospheric Administrations (NOAA) Atlantic Oceanographic and Meteorological Laboratories (AOML) for the past fourteen years have resulted in a considerable amount of sediment mass physical properties data, which have provided the basis for a comparative analysis of the surficial sediments of the U.S. Atlantic continental margin and deep-sea basin in a mega-corridor. A synthesis of available mass physical properties data for the Atlantic and Pacific basins was made by Keller and Bennett (1968, 1970). An extensive coring program initiated in 1974 by NOAA along the U.S. Atlantic outer continental margin (outer shelf, slope, and upper rise) between Hydrographer and Hatteras Canyons has provided data for an initial synthesis of the sediment geotechnical properties of the continental slope (Keller et al., 1979). Recent completion of laboratory analyses of the continental rise sediment cores (Lambert et al., in press) offer an opportunity to compare these suites of data. Although the data for the Atlantic deep-sea basin cover a considerably more extensive area than the data for the Atlantic continental margin, there is now adequate geotechnical properties information to show a sharp contrast between predominantly pelagic and hemipelagic deep-sea sediments and continental margin deposits which are primarily terrigenous in origin. In concert with the fining of sediments seaward off the continental margin (Bennett et al., 1977a; Keller et al., 1979), the mean water contents are higher on the rise (95%) than on the slope (88%) and deep basin (86%). In general, average water contents are significantly higher than the liquid limits for surficial slope, rise and basin sediments. Undrained shear strengths average 5–8 kPa for slope, rise and North Atlantic basin deposits, with values for the slope and rise sediments generally higher than the Atlantic basin deposits. Basin deposits have the greatest range in shear strength, as would be expected for such an extensive and sedimentologically diverse area. Sediments associated with the outer Hudson Canyon display the lowest average water content (73%), highest wet unit weight (1.60 Mg/m 3 , wet bulk density) and grain specific gravity (2.76, average grain density) as compared with continental slope, rise and deep-sea Atlantic basin deposits.