1982 Theses Doctoral

Two processes affecting sediment stability and physical properties: hydrothermal circulation and rapid deposition

Abbott, Dallas Helen

We develop and test a method of in situ measurement of excess pore pressure, permeability and shear strength in deep sea sediments. Measurements from the Guatamala Basin confirm that thermally significant water advection can persist in a 200 to 300 meter thick sediment column. Guatamala Basin data and mathematical models suggest that independent water advection rates calculated from perturbation of chemical profiles may be too slow by an order of magnitude if nonlinear kinetics and sedimentary reactions are ignored. The water advection rates needed to perturb chemical and thermal profiles may be much closer than was previously assumed.

In situ measurements confirm that hydrothermally driven water movement is rapid enough and sediment permeability is low enough to lower apparent or actual sedimentation rates in hydrothermal areas by affecting 1) slope stability, 2) sediment shear strengths (erodability), 3) sediment compaction rates and 4) fine particle deposition.

Hydrothermally driven water movement appears to promote sediment slides and high heat flow on the tops of narrow cone shaped abyssal bills on the eastern Nares Abyssal Plain. The sedimentation patterns on top of the steep sided, cone shaped hills are significantly different from those on top of intermingled elongated, shallow sided hills. Sediment slides appear to be enhanced and turbidite deposition diminished on the tops of conical hills, despite similar heights above the plain as surrounding elongated shallow hills.

In situ measurements of shear strength and permeability from the Mississippi Delta give consistent results in different physiographic provinces. The most deformed sediments have the lowest shear strengths and permeabilities, as do most recently remolded sediments. Since the most deformed sediment differs little in composition from the least deformed, these results suggest that sediment slides may be much easier to continue than to initiate.