Geotechnical Properties of Sediments in a Carbonate Slope Environment: Ocean Drilling Site 630, Northern Little Bahama Bank

Dawn Lavoie

Site 630 was cored three times during Leg 101 of the Ocean Drilling Program, the second time so that whole round samples could be obtained for geotechnical analyses. Consolidation studies, performed so changes in the coefficients of consolidation, compressibility, and permeability could be related to the overburden pressure, revealed that sediments recovered to 79.4 mbsf (meters below sea floor) are not significantly consolidated. This may be a function of total carbonate content or of differences in mineralogy of carbonate constituents. The lower total porosity and higher permeability of 630B-2H5 relative to 630B-5H6 and 630B-9H6 is explained in terms of effective interparticle porosity and matrix composition.

Results from triaxial shear testing and direct shear testing indicated that 630B-2H5 has a higher degree of cohesion and higher angle of internal friction (^phgr) than sediment recovered deeper in the section, although the angles of internal friction from all three samples are lower than typical angles measured from deep foraminiferal oozes. Attempts to compare these results qualitatively with undrained shear strength measured on sediments from hole 630A suggest that vane shear measurements may be meaningless when made on sediments that depend on overburden pressure to maintain strength.

Calculations based on consolidated and drained shear strength suggest that if the sediment column is homogeneous, the slope will be stable up to 14° of inclination. If the inhomogeneities not tested in the section, such as the turbidite layers known to be present, are weaker than the sediments tested, the stable slope inclination may be less than calculated.

AAPG Search and Discovery Article #91038©1987 AAPG Annual Convention, Los Angeles, California, June 7-10, 1987.