Richard H. Bennett

Clay Fabric of Selected Submarine Sediments: Fundamental Properties and Models

ABSTRACT Clay fabric of selected Mississippi Delta sediment and a red clay from the eastern equatorial Pacific is related to void ratio and depth of burial. Relatively high-void-ratio, naturally consolidated delta sediment is characterized by a fabric of randomly oriented domains and short linking chains with particles in edge-to-face and face-to-face contacts. Lower intermediate-void-ratio sediment showed greater particle-to-particle packing but a consistent predominance of randomly oriented domains with a few chains and a slight development of larger domains than observed in the high-void-ratio sediment. The deeply buried, very low-void-ratio sediment has highly oriented domains and thin, long voids. The red clay sample is characterized by flocs connected by long linking chains and a very hi h void ratio. Domains and turbostratic-type clay fabric are characteristic of high-porosity (high-void-ratio) Mississippi Delta sediments. Highly oriented domains and long chains are diagnostic of the deeply buried, very low-porosity delta muds. The clay fabric of natural (undisturbed) samples was compared with laboratory remolded samples for comparison with models suggested by earlier researchers. Observations using the transmission electron microscope (TEM) support the proposed fabric models for remolded sediment. Selected samples were studied using the scanning electron microscope (SEM). Tentative clay fabric models for undisturbed submarine sediment are proposed and related to void ratios.

Ambient and dynamic pore pressures in fine-grained submarine sediments: Mississippi Delta

A multisensor piezometer probe has measured pore pressures in fine-grained submarine sediments of the Mississippi Delta over a period of approximately eight months. Data presented here cover the initial 2650 hours of the experiment. Dynamic and ambient pore pressures were recorded. Analogue data collected from the time of probe insertion include decay characteristics, steady-state (ambient) excess pore pressures, and the response of pore pressures to surface wave activity. The probe was installed in 43–44 ft of water near an offshore platform in the East Bay area of the Delta. Sensors were located at 21, 41 and 51 ft below the mudline. Ambient excess pore pressures were determined to be 0.7, 2.8 and 6.6 psi (lb/in2) at the respective depths. The relatively high excess pressures and the measured laboratory wet unit weights of the soil result in a significantly low effective stress. Pressure fluctuations due to tidal and surface wave activity were observed to produce significant pore pressure response in these soils. Preliminary data obtained using high-airentry and corundum stones indicate that considerably more research is necessary in order to fully understand the peculiarities observed in the data and to assess the role of dissolved and free gas on the pore pressures in submarine sediments.

Mass movement of sediment on the continental slope and rise seaward of the Baltimore Canyon Trough

Abstract Sediment instability has produced large submarine slides on the continental slope and rise seaward of the coast of New Jersey and the Baltimore Canyon Trough. Sediment failure is confined to Quaternary material over a continuous mid-Pliocene unconformity. The thickness of Pleistocene sediment is greatest in proximity to Wilmington and Spencer Canyons, indicating that the canyons influence the amount of sediment on the lower slope and rise. Differences in thickness of sediment between both canyons suggest variations in the amount of sediment each transported. Sediment failure is ubiquitous in this area. Instability appears to be related to sediment thickness rather than to other trigger mechanisms. Rapid sediment accumulation and the probable low shear strength-under-consolidated condition of Pleistocene deposits on the mid-Pliocene unconformity are believed to be major factors controlling sediment instability in this area.

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.

Continental Slope Sediment Instability Northeast of Wilmington Canyon

A region on the U.S. East Coast exhibiting sediment instability is present on the continental margin seaward of the coast of Delaware and the Baltimore Canyon Trough area. A detailed bathymetric and seismic-reflection survey of a 7.5 by 13.0-km area, together with box, hydroplastic, and piston cores, affords an integrated study of a large submarine slide with a volume of 11 cu km on the continental slope northeast of Wilmington Canyon. The geotechnical properties and sedimentologic characteristics of the seafloor show variations that are associated with the slide. The slump block is believed to be composed of Pleistocene sediment with sediment failure occurring on the slope along a late Tertiary erosion surface which formed the gross morphology of the continental margin. he suggested mechanism for triggering the sediment failure is increased wave energy associated with a lower stand of sea level during the late Pleistocene. Mass wasting of material downslope by creep is a process which apparently still is occurring.

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....

Mississippi prodelta crusts: A clay fabric and geotechnical analysis

Abstract Crust”; zones, as defined by shear strength profiles, have been delineated in Mississippi prodelta sediments since the advent of the remote wire‐line probe. Attempts to map the distribution of the “crust”; on the prodelta have been hampered by sparsity of in situ measurements and its apparent lateral discontinuity. Evidence based on clay fabric, geotechnical properties, and the location of selected cores within certain geomorphic zones, suggests the “crusts”; are associated with zones of shearing during an episode of submarine sediment movement on the prodelta. The clay fabric within the “crust”; zone differs from that found above and below in the high‐porosity sediments. A noticeable degree of clay platelet preferential alignment is characteristic of sediments within the “crust.”; This preferred orientation occurs as localizations rather than as an overall alignment of all the particles. The clay fabric of the higher‐porosity sediments above and below the “crust”; is typified by randomly arrang...

Pore‐water pressure measurements: Mississippi delta submarine sediments

Abstract A pore‐water pressure probe (piezometer) was implanted in Mississippi delta sediments at a preselected site (Block 28, South Pass area, 29°00´N, 89°15´W) 145 m from an offshore production platform (water depth approx. 19 m) in September 1975. Total pore‐water pressures (uw ) were monitored for extended periods of time at depths of approximately 15 and 8 m below the mudline concurrently with hydrostatic pressures (u8 ) measured at depths of 15 m and approximately 1 m below the mudline. Relatively high excess pore‐water pressures, ue = (uw ‐u8 ), were recorded at the time of probe insertion measuring 99 kPa (14.4 psi) at 15 m and 50 kPa (7.3 psi) at 8 m. Six hours after the probe was implanted, excess pore pressures were still high at 81 kPa (11.8 psi, 15 m) and 37 kPa (5.4 psi, 8 m). Pore pressures appeared to become relatively constant at the 8‐m depth after 7 h had elapsed, and at the 15 m depth after 10–12 h. Excess pore‐water pressures averaged 72 kPa (10.4 psi, 15 m) and 32 kPa (4.6 psi, 8 m)...

Piezometer Probes for Assessing Effective Stress and Stability in Submarine Sediments

Multisensor piezometer probes were deployed at four different sites in the Mississippi Delta in water depths ranging from 13.5 to 43.6 m with sensor penetration depths of up to 15.6 meters. Absolute and differential pressure sensors were used to measure pore water pressure and excess pressures, respectively. The free water column pressure was measured with absolute pressure sensors. Pore pressures induced by probe insertion were determined as well as ambient excess pore pressures following the time-dependent decay of induced pressures. Significant differences in the pore pressures and related geotechnical properties were found between East Bay and Main Pass sediments. Generally higher probe insertion pressures and lower ambient excess pore pressures were characteristic of Main Pass compared to East Bay. Probe insertion pressures (Ui) were found to correlate well with the undrained shear strength (Su) of the sediments, indicating reasonably good agreement with the predicted relation: Ui = 6Su as suggested by an earlier study42. Using this relationship undrained shear strengths were calculated and compared with measured values.