Wilkinson Basin Research Articles

Differential piezometer probe for an in situ measurement of sea-floor

A telemetering differential piezometer was designed and constructed at the Norwegian Geotechnical Institute to measure a minimum differential pressure (pore-pressure referenced to hydrostatic pressure) of 34 kPa and a maximum pressure of 294 kPa in water depths of up to 500 m. An emplacement system was built at the University of Illinois, One successful in situ test in the Wilkinson Basin, in a water depth of 274 m, yielded a maximum excess pore-pressure of 59 kPa after the probe was driven about 3·2 m into the silty-clay bottom. An excess pore-pressure of 9·8 kPa was measured 5–10 h after emplacement of the probe. Implications of excess pore-pressures cyclically generated by storm and internal wave loading of sea-floor soils is discussed. It is concluded that a better understanding of submarine slope stability through the use of the effective stress principle should now be possible by measuring pore-pressure in situ. Un télémètre/piézomtre différentiel fut projeté et construit à l'institut Gèotechnique Norvégien, afn de mesurer une pression différentielle minimum (pression interstitielle se rapportant à la pression hydrostatique) de 34 kPa et une pression maximum de 294 kPa dans des profondeurs d'eau atteignants jusqu'à 500 m. Un système de mise en place fut construit à l'université d'Illinois. Le succés d'un test in situ dans le bassin de Wilkinson, dans une profondeur d'eau de 274 m, produisit une pression interstitielle avec un excés maximum de 59 kPa, après avoir enfoncé la sonde d'environ 3·2 m dans le fond de l'argile limoneuse. Un excès de pression interstitielle de 9·8 kPa fut mesuré cinq à dix heures après la mise en place de la sonde. On discute des conséquences d'exces de pression interstitielle engendrés d'une façon cyclique par la tempête et par le chargement interne de sols provenant du fond de la mer, produit par les vagues. On conclut qu'une meilleure compréhension de la stabilité sous-marine d'une pente par l'utilisation du principe de contrainte effective, devrait maintenant être possible, en mesurant la pression différentielle in situ.

Consolidation properties of Wilkinson basin soils : Dzwilewski, PT Univ. Calif. Berkeley, USA Richards, AF Lehigh Univ. Bethlehem, PA. USA Technical note. 2F, 13R J. Geotech. Engng Div. ASCE, V100, N. GT10, Oct. 1974, P1175–1179

Consolidation properties of Wilkinson basin soils : Dzwilewski, PT Univ. Calif. Berkeley, USA Richards, AF Lehigh Univ. Bethlehem, PA. USA Technical note. 2F, 13R J. Geotech. Engng Div. ASCE, V100, N. GT10, Oct. 1974, P1175–1179

Geotechnical and Chemical Property Relationships for Wilkinson Basin, Gulf of Maine, Sediments

ABSTRACT Nine 1.5 m sediment cores from the Wilkinson Basin Geotechnical Test Area were used in an investigation of the interrelationships among geotechnical properties and major ion concentrations in the pore water. The ionic composition of the pore water in the upper 10 cm does not differ significantly from the overlying sea water. Sodium, potassium and chloride ion concentrations do not change significantly with depth in the sediment. Anaerobic oxidation of organic matter is indicated by the decrease in pore water sulfate ion concentration with increasing depth. Pore water concentrations of magnesium and calcium also decrease with depth while the carbonate mineral fraction of the sediment increases, suggesting carbonate precipitation in consequence of oxidation. Simple quantitative relation hips among the measured chemical and geotechnical properties are not apparent. The normalized shear strength, n/z, decreases with decreasing sulfate ion concentration, indicating possible strengthening by labile organic matter.

Triassic Rift Structure in Gulf of Maine

Interpretation of seismic-reflection profiles across the Gulf of Maine and Georges Bank reveals that the region underwent major crustal rifting during the Triassic Period. Three rift systems are delineated: the Fundy fault system extending southwest from the Bay of Fundy to the central Gulf of Maine, the Wilkinson basin system striking north-south in the western Gulf, and the Georges basin/Georges Bank system which parallels the Fundy fault system and underlies the southern Gulf and outer continental shelf. A detailed comparison between these three systems and the Triassic Newark Group onshore suggests similar tectonic histories. A stress-strain analysis of the Triassic fault pattern in the Gulf of Maine shows that the overall stress field associated with continental sepa ation in this area can be defined by a left-lateral shear couple. An attempt then is made to relate the observed structures and related stress conditions to a sequence of six tectonic episodes beginning with initial graben formation in Late Triassic and ending with regional collapse and the onset of geosynclinal deposition in the Middle Jurassic to Early Cretaceous. In addition to Triassic rifting, the interpretation of the seismic-reflection profiles coupled with refraction data and the analysis of bedrock samples collected during the study by the submersible Alvin provides new insight into the distribution of coastal-plain sediments and the nature of Carboniferous-Permian tectonic activity. The restricted distribution of intermediate compressional wave velocities (4.6 to 5.1 km/sec) and that of late Paleozoic K-Ar radiometric ages to the northwestern Gulf of Maine, suggest the presence of a linear Carboniferous-Permian basin extending 250 km northeast from Boston into the gulf. This basin is thought to be part of a large right-lateral shear zone developed during the final phase of continental collision, when the proto-Atlantic Ocean was closed nd the continental landmasses slid past one another, creating a structural weakness that may have controlled subsequent continental separation in the Triassic.

Comparison of sedimentation-compression curves obtained by nuclear and gravimetric methods for Wilkinson Basin, Gulf of Maine, sediments

Accurate measurements of bulk density are important in the construction of sedimentation-compression curves of water-saturated, fine-grained cohesive sediments for consolidation studies. The bulk density of five cores was measured nondestructively on a laboratory nuclear transmission densitometer and the water content, expressed as void ratio, was derived from an empirical relationship between bulk density and water content. Bulk density and water content measured destructively was determined every 10 cm for a comparison with the nuclear data. Agreement between nuclear and gravimetric methods was within ± 0.02 g/cm 3 for bulk density ranging from 1.2 to 1.4 g/cm 3. Sedimentation-compression curves made using both methods were in generally good agreement. In the Wilkinson Basin, void ratio gradually decreases with increasing effective overburden pressure. This indicates that consolidation by dewatering the surficial sediments is normal and that the environmental conditions during deposition remained relatively constant.

Consolidation Properties of Wilkinson Basin Soils

The Geotechnical Test Area in the Wilkinson Basin located in the Gulf of Maine is a topographic depression laying within the 260-m (850-ft) depth contour and it encompasses an area of about 400 km² (4,300 ft²). This note reports on the consolidation properties of the Wilkinson Basin soils as determined from consolidometer tests. The study objectives were to determine the compression index and the coefficients of consolidation, to investigate the effects of using back pressure to assure 100% saturation on the soil being tested, and to develop predictor equations for the compression index in terms of other geotechnical properties.

Some chemical, mineralogical, and geotechnical analyses of four cores from the Wilkinson Basin

Studies have been made of the bulk chemistry and mineralogy of four clay cores from the Wilkinson Basin, Gulf of Maine, in an attempt to explain their geotechnical properties. Correlations between geochemistry and shear strength and sensitivity are low.

Variability of Geotechnical Properties of Lutite in Wilkinson Basin, Gulf of Maine, as Measured in Place from Submersible Alvin: ABSTRACT

End_Page 647------------------------------Dives in 240-250 m water depths were made by the submersible Alvin in the northwest and central parts of the Wilkinson basin in July 1971. The submersible was instrumented with a probe consisting of a nuclear transmission densitometer, which directly measured bulk density and, indirectly, water content at 0.8-cm intervals, and with a static cone penetrometer, which indirectly measured shear strength at 2.5-cm intervals. The probe was pushed at a constant rate to a maximum depth of 1.45 m by means of a rack and pinion drive actuated by the Alvin's mechanical arm. Three sites, located several hundred meters apart, were occupied on each of the 2 dives. Previously measured geotechnical properties in the Wilkinson basin, a postglacial sedimentary basin of lutite, indicated relatively uniform areal and vertical (to 3 m below the bottom) conditions. The Alvin studies showed a horizontal and vertical heterogeneity much greater than expected. The presence in limited areas of high shear strength strata, not directly correlatable with bulk density or water content, and other apparent small-scale anomalies are not easily reconciled with conventional depositional patterns generally believed present in marine sedimentary basins. End_of_Article - Last_Page 648------------

Foraminiferal Studies: Wilkinson Basin, Gulf of Maine: ABSTRACT

Sediment samples from a 280-m core taken in the Wilkinson basin, Gulf of Maine, have been studied for foraminiferal content. The sediments are dusky yellowish-brown silty clay and presumably represent particles carried by glacial meltwater into the Atlantic Ocean during the latest period of continental deglaciation. Several features set the foraminiferal fauna apart from the normal open-shelf fauna. There is a distinct lack of arenaceous species in these sediments. Eggerella advena, customarily abundant, is lacking, and only Hyperammina elongata and Trochammina inflata are present in trace quantities. Bolivina fragilis and B. pseudoplicata dominate the calcareous fauna, with fewer Glandulina laevigata, Guttulina glacialis, and Nonionella labradorica present. The species abundance and diversity are large and compare favorably with other northern areas. The faunal list from the basin does not compare with species found at comparable depths on normal shelves. The fauna appears mixed with shallow-water elements derived from relict sediments on the shelf and bordering the basin. By far the majority of species found are from depths of 60-75 m. A few ubiquitous species, Bulimina marginata and Cassidulina norcrossi are found at greater depths. Planktonic/benthonic ratios increase upward attesting to an influx of planktonic forms associated with rising sea level. End_of_Article - Last_Page 337------------