Geotechnical Characteristics of High-Latitude Biosiliceous Sediments

Alan Pittenger, Frank R. Rack, Elliot Taylor, William Bryant

Results from Ocean Drilling Program Legs 104 and 113 indicate that high-latitude pelagic sediments having a significant component of biogenic silica are characterized by distinctive physical properties, depth profiles, and geotechnical test results. Depth profiles of bulk density, water content, and porosity in siliceous sediments often show no change with depth over the first few hundred meters of burial. Occasionally, these sediments show reversed depth trends, i.e., density decreases while porosity and water content increase with depth in some silica-rich intervals. In siliceous sediments, the relative abundance of silica appears to be more important than overburden pressure in determining the trend of physical properties. These trends are anomalous, resulting in stron physical contrasts with other sediment types. Measurements of undrained shear strength in the siliceous sediments also show anomalous depth trends. Within these sediments, strength generally increases with depth, as expected, but this strengthening occurs without an associated decrease in water content.

The results of back pressure consolidation and falling-head permeability tests indicate that siliceous sediments typically have relatively high initial void ratios and in-situ coefficients of permeability, are underconsolidated, and exhibit considerable consolidation during testing. Laboratory response to loading is therefore different than the response exhibited by these sediments to naturally occurring overburden loads.

The unusual geotechnical character of siliceous sediments has important implications for slope stability, structural control in sediment deformation, and seismic stratigraphy. These intervals are often relatively less dense, have higher water content, and may therefore serve as planes of weakness along which slope failures can develop. Siliceous intervals tend to be more unconsolidated and permeable and may serve to structurally localize zones of decollement development within subduction complexes. The low density of these intervals relative to surrounding sediments may also result in characteristic responses of seismic signals, thus enabling siliceous intervals to be mapped on the basis of their seismic character.

AAPG Search and Discovery Article #91022©1989 AAPG Annual Convention, April 23-26, 1989, San Antonio, Texas.