Characteristics and mechanics of subaqueous debris flows

Mahgoub, Abdelmagid (1998) Characteristics and mechanics of subaqueous debris flows. Masters thesis, Memorial University of Newfoundland.

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Abstract

Debris flows are gravity-driven mixtures of sediment and water that have considerable yield strength. Deposits of debris flows are common features of many modern and ancient continental margins. In this thesis, an integrated approach of marine, outcrop, and laboratory investigation is used to study the character and behaviour of the subaqueous debris flows. -- Marine seismic data, obtained from the Northeast Newfoundland Slope, Baffin Bay, Delaware Slope, and South China Sea, were used to unravel certain geometrical aspects such as the overall shape of the deposits, slope angles, basal erosion, and relation to surrounding sea floor. Generally the deposits appear as well defined, seismically transparent lenses aligned downslope for a distance of 70-1700 km from the shelf edge. These lenses have central thickness ranging from few meters to few tens of meters. They are 0.5-75 km long and 0.5-25 km wide. Flows appear to have travelled on very gentle slopes (1.5° to about 0.1°) without significantly eroding the underlying sediments. Piston cores from debris-flow deposits reveal structureless, poorly sorted pebbly mud with numerous lithic fragments and few soft mud clasts. -- More detailed information pertaining to structures, fabric, grain-size distribution and the mineralogy of the fine fraction is obtained from outcrops of the Cow Head Group, Western Newfoundland and Fraser River Valley, Central Interior of British Columbia. Deposits typically exhibit extremely poorly sorted massive diamict beds. These are mostly matrix supported with the matrix being mostly sand. A few beds are clast-supported and show crude inverse grading at their bases. Most beds show no preferred clast orientation and exhibit weak positive correlation between bed thickness and maximum particle size. -- Variables such as shear strength and pore-fluid pressure, necessary to an understanding of flow support mechanisms, are reported from laboratory experiments using reconstituted slurries of seawater and debris from marine cores. Reconstituted debris-flow slurries develop high values of excess pore-fluid pressure (0.7-2.5 times hydrostatic pressure, depending on depth and sediment concentration). These high values, which are maintained for several hours, are mainly due to poor sorting and high amount of fines. High pore-fluid pressure effectively mobilised debris flows by reducing the total normal stress and consequently the shear strength of the debris material.

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