Pore fluid overpressures as high as 2 MPa (full lithostatic) are observed at ODP Site 975 starting at depths of 145 mbsf (meters below the sea floor). Sediment porosity increases to near surface values below the top of overpressure, while compaction and pressures above the seal are normal. Spikes in the density log and degassing of core from the overpressured zone indicate the presence of gas. The high porosity sediments are far too permeable to allow the generation and maintenance of fluid overpressures by disequilibrium compaction alone. The presence of two fluid phases in a layered sequence of fine and coarse sediment suggests capillary sealing, and this is shown to be quantitatively feasible. ¿ 1998 American Geophysical Union