Abstract:
The mechanical stability of channel levees may be an important control on channel avulsion and sediment distribution patterns in submarine fans. Levee stability depends on the shear strength of the levee sediments, which in turn depends on how the sediments are deposited and compacted. Long, continuous sections of levee sediments on the Amazon Fan were recently cored during Leg 155 of the Ocean Drilling Program. The porosity-depth profiles from Sites 939 and 944, situated on the levee of the Amazon Channel, were analysed using a theoretical compaction curve derived from soil mechanics theory. By fitting the theoretical compaction curve to porosity-depth data, estimates of the compression index and the void ratio at 100 kPa effective stress were determined. The compression index of the levee sediments was estimated to be 0.58-0.68. The mechanical parameters of the levee sediments are consistent with published geotechnical data for remoulded clays. Undrained shear strength data from Leg 155 were analysed using an infinite slope analysis to determine the slope angle for failure as a function of burial depth. The variations in the shear strength measurements were bounded by critical slope angles of 1-3 degrees . The higher bound of 3 degrees tends to be consistent with maximum slopes measured near the levee crest near Site 938 on the Blue channel. The lower bound of 1 degrees appears to be consistent with the slope of the sediment deposit blanketing the slope away from the levee crest. The top 55 m of Site 941, sited in the Western Debris Flow, show a relatively undisturbed, normally compacted porosity profile, even though the sediments show soft-deformation features. In contrast, the core samples between 55 and 68 m show lower porosity values compared with the porosity profile from Site 939. This low-porosity zone was interpreted as a zone of reworked or sheared sediment that could be the failure surface of a translational slide.