The porosity and yield stress of clay-rich marine sediments can be sensitive indicators of stress and stress history, if testing of these sediments is done carefully and the results are interpreted with an understanding of sediment mechanical behavior. Consolidation tests on a nannofossil clay from 619 mbsf in Hole 897D indicated a very low yield stress (about 1 MPa), but at a porosity that was also quite low with respect to that of the same sediment when it was disaggregated and consolidated from a slurry. Moreover, no evidence for cementation of the natural sediment was observed, despite its calcareous nature. The lower porosity of the natural sample was shown to result from the much slower geological rates of consolidation than the laboratory rates. The lower compressibility of the natural sample was attributed to "delayed consolidation" during the subsequent laboratory consolidation.
  Together with the downward increase in porosity and intense fracture/vein development observed at Site 897, the low yield stress indicates near-lithostatic pore pressures in the basal sediments over much of their history and probably at present. This condition would suggest that fluid is rising from below and probably moving laterally toward egress to the ocean at the ridge top. The source and hydrodynamic setting of this fluid are as yet inadequately explored.

Date of initial receipt: 30 November 1994
Date of acceptance: 27 March 1995

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