Geotechnical Stratigraphy of Sediments of the Northwest Gulf of Mexico Continental Slope
Authors: Bryant William R. , Bean Daniel , Liu Jia Yuh , Dunlap Wayne , Silva Armand 
Published: October 2008

Geotechnical stratigraphy is the distribution and succession of the physical properties of layered sediment and their succession and interpretation in terms of mode of origin and their geological and geotechnical history. Geotechnical stratigraphy and high-resolution seismic stratigraphy form a method by which the geotechnical nature of the sediments that cover a large area can be efficiently assessed. In 1998 fifty jumbo piston cores up to 20 meters in length were collected on a traverse from the upper to the lower continental slope in the northwestern Gulf of Mexico. Detailed geotechnical properties, bulk density, water content, porosity were determined at 4cm intervals with a GEOTEK core logger. Shear strength and magnetic susceptibility were also determined at 4cm intervals. High-resolution seismic profiles (3.5 kHz) were obtained across the continental slope by the use of TAMU Deep Tow System. The long piston cores were taken along the seismic tracks. Bulk density profiles of the cores showed a remarkable and very detailed similarity between cores that cover a range over a distance of 160 km, from the upper continental slope to the lower continental slope. Markers such as an ash layer dated as being 29,800 years old was found in the majority of cores recovered as was increasing and decreasing amounts of forams that results in a distinct bulk density profile that is traceable across the slope. As expected, on basin walls and floors, slump and debris flow deposits break the common geotechnical property profile. The majority of sediments recovered were highly bioturbated hemipelagic silty clays with varying amounts of forams and highly laminated unbioturbated silt, clayey silt and silty clay that contained no forams. The results of this study suggests that geotechnical properties of surficial continental slope sediments can be traced between core sites with a high degree of reliability by the use of high-resolution seismic data coupled with strategically placed large-diameter piston cores.

Introduction

"Stratigraphy is the great unifying agency of geology that makes possible the synthesis of a unified geological science from its component part" (Weller, 1947).

There are many components of stratigraphy:Seismic stratigraphyLithostratigraphyMagnetomagnetoPedostratigraphyChronostratigraphyBiostratigraphyGeotechnical stratigraphy

Geotechnical stratigraphy is the distribution and succession of the physical properties of layered sediment and their succession and interoperation in terms of mode of origin and their geological and geotechnical history. It is our contention that knowledge of the shallow subbottom sediment lithostratigraphy and geotechnical stratigraphy is absolutely necessary in order to understand the significance of the interactions of sediment properties and their relationship to engineering characteristics. Geotechnical stratigraphy will enables one to determine the normal sequence of geotechnical properties and map any deviations resulting from such events as erosion, mass-wasting, creep, faulting and slumping and significant diagenetic changes.

A recent and ongoing research project funded by NSF and 7 industrial groups is the first application of high-resolution geotechnical stratigraphy and high-resolution seismic stratigraphy on a large scale for a continental slope environment.