Mechanical tests on claystone from Ocean Drilling Program Hole 1070A (Leg 173) - implications for elevated pore-fluid pressure in sediments within the ocean - continent transition zone, West Iberia

摘要

Laboratory geotechnical deformation tests on a claystone test specimen from 619 m depth in Ocean Drilling Program Role 1070A provide evidence of low effective yield stresses and capillary pressures (1.2-1.5 MPa) in the sediment section near the acoustic basement. Effects from negative pore-fluid pressure, and the presence of expansive clay minerals probably account for the drastic change in the mechanical behavior of the sample below and above the yield stress. The results from the geotechnical tests, together with high porosity values, suggest that the pore-fluid pressure is high in these basal sediments. The intense fracture/vein development in the basement suggests that the pore-fluids originate from deeper sections in the basement. Porosity anomalies are found near the basement in three boreholes drilled in the western part of the Iberia Margin. The anomalies are interpreted to signal elevated pore-fluid pressure. Consequently, a zone of overpressure possibly exists in the western part of the ocean-transition zone of the West Iberia margin, which thickens towards the west.