Abstract: Oslofjord, Norway: Case Study in Applications of Marine Geotechnology to Sedimentology

Adrian F. Richards

Geophysical-geotechnical field work was done in 1962-1963, laboratory geotechnical and geochemical testing of high-quality cores was performed in 1963-1964, and the data were evaluated and synthesized in 1964-1974 for the Norwegian Geotechnical Institute's Oslofjord Project. This investigation will be used as a case study to demonstrate the applications of marine geotechnology to sedimentology. Sediment thickness in the four basins along the axis of the fjord ranged from 29 to 130 m; sediments at depths greater than about 9 to 14 m are Pleistocene age or older, on the basis of carbon-14 dates determined at shallower depths. Sedimentation rates, corrected for consolidation, ranged from 0.9 to 2.5 m/1,000-years. An acoustic reflector in the basins was correlated with the 10 000-11,000 B. P. recession of the glacial ice cap. The overconsolidation ratio (p_c/^sgr_v) is unity at burial depths greater than 4 to 5 m, indicating geotechnically normal consolidation. Quick (highly sensitive) clays had pore-water salinities of 29 to 38 parts per thousand. Typical nonquick silty clays had the following properties: clay-size 55% < 2 µm; bulk density 1.55 Mg/cu m; water content 86% dry weight; liquid limit 80%; plastic limit 31%; natural shear strength 8.8 kPa; sensitivity 7.2; organic carbon 0.7%; CaCO₃ 2.9%; and pore-water salinity 33.5 parts per thousand. The typical submarine clay has higher Atterberg limits compared to the uplifted marine clays adjacent to the fjord. At three selected burial depths, the change of properties a ong the fjord axis from Oslo south toward the Skagerrak was investigated: at the surface (0.1 m) the pore-water salinity and CaCO₃ consistently increased; at 2.8 m only CaCO₃ increased; and at 5.6 m both bulk density and CaCO₃ increased. Other properties were variable along the fjord axis. The magnitude of bioturbation was assessed using the concept of greatest difference between the minimum and maximum natural-shear strength; the amount of bioturbation was significant at the 2.8 and 5.6-m depth intervals in all of the basins.

AAPG Search and Discovery Article #90972©1976 AAPG-SEPM Annual Convention and Exhibition, New Orleans, LA