Sediment Strength and Hydrates Along the Northern Gulf of Mexico
Authors: Lowrie A. , McGee T. 
Published: October 2008

Ever-deeper petroleum exploration has revealed that the continental margin of the northern Gulf of Mexico is surprisingly complex, dynamic and structurally weak. Gas hydrates forming and dissociating, fluids - gases and liquids - migrating and being vented into the water column, buried organic material maintaining near-surface unconsolidation, sediments dewatering, smectite converting to illite, salt and/or shale moving in various directions and at varying rates are only some of the processes affecting the overall condition of the continental margin. In water depths of two to five kilometers, the hydrate stability zone may be a kilometer or more thick. Gases converting to hydrates at such depths create expansive forces that can shatter sediment. Such shattering means that conditions for Mohr-Coulomb deformation (homogeneous materials, uniform stresses and no zones of prior deformation or weakness) coexist with non-Mohr-Coulomb conditions at scales from centimeters to meters to regional.

Structural weakness varies with depth: in sediments less than 500-1000 meters deep and not yet much consolidated, clays are structurally weakest while deeper than a kilometer fluid-saturated sands become relatively weaker. Coupled to these structural strength variations due to sediment type are migrations of depositional loci due to changes in sea level. In this way sealevel position determines facility and intensity of mass-wasting. Complex patterns of sedimentation and structural strength result.

Understanding sediment strength is essential to developing detailed models of how contemporary continental margins operate. Applying sequence stratigraphy and an evolving tectonic paradigm permits a back-stripping of individual sequences to decipher paleo-conditions.

Introduction

The last few years have seen much progress in understanding the more subtle aspects of the structure of passive continental margins. Individual discoveries are largely descriptive at this time and there is no clear formulation of how geologic processes operate over time. When the phrase "geologic process" is employed what is meant is that a single process has been extracted from the totality of what occurs, geologically, in any given spot or region. For example, along the Louisiana upperslope, there are many processes going on simultaneously; a single process may be "deposition" which is singled out/extracted so that human minds, constructed to best handle only one topic at a time, may concentrate on this one topic, "deposition". The purpose of segregating diverse processes is to facilitate analysis of each one, with the results then suitably organized so that study of processes in the general, interrelating, can proceed. This segregation of processes is an artifice done for ease of study.

Naturally there is the understanding that many geologic aspects (processes within the region) are proceeding simultaneously as deposition is occurring. For example, within the context of deposition there may be variations in amount and type of deposition and even temporary cessations of deposition.