Stability of Mississippi Delta Sediments as Evaluated by Analysis of structural Features in Sediment Borings

Abstract

ABSTRACT Numerous failures of manmade structures in the active Mississippi River Delta have initiated various attempts to distinguish stable from unstable areas as well as to determine the effective depth-of sediment movement or potential movement. To investigate the sedimentological and geochemical properties of Recent deltaic sediments, four borings were taken from areas ranging from one considered stable (wellstratified and acoustically transparent sediments) to one where a suspected mudflow feature (acoustically amorphous) occurred. By means of X-ray radiography the internal sediment characteristics were analyzed. Results of this analysis were compared with geochemical and-geotechnical data to better define the type and depth of sediment deformation. Disturbed sediment is easily identified using the X-ray radiography method. Several types of sediment deformation are identifiable from X-ray radiographs: flowage, fracturing, microfaulting, mechanical disturbances owing to the coring procedure, and disruptive features associated with gas expansion and migration. Significant differences exist between the assemblages of sedimentological, geochemical, and geotechnical properties of deltaic sediments which have been deformed as opposed to those which have not. Boring 2, taken in an acoustically amorphous region, is the most extensively deformed. Flow structures are common throughout the boring to near the base (~ -49 m), at which point fracturing and microfaulting occur. Sediment structure, as well as anomalous profiles of geochemical and geotechnical data, strongly indicate deformation owing to mass movement (mudflow), possibly of a convective nature. Boring 5, -also taken in an acoustically amorphous area, shows intricately laminated sediments but no convincing evidence of deformation. Borings lA and 3A, from acoustically transparent sediments, exhibit classical geochemical and geotechnical profiles and are intermediate cases between Borings 2 and 5 with regard to internal sediment characteristics. Sediment deformation in these borings is restricted to approximately the upper 20 m. If mass movement has occurred, only the sediments of this near-surface zone have been involved. Anomalously high shear strength profiles in Borings 3A (--8 m) and 5 (-21 m) define "crust Zones" which appear to be related to an increase in silt content as interpreted from the X-ray radiographs. A transition in depositional environment from prodelta to interdistributary shelf accounts for this feature in Boring 5. However, the process of silt accumulation in the crust zone of Boring 3A is still somewhat problematical. INTRODUCTION Sediment deformation within the Mississippi River delta complex has resulted in numerous breaks and relocations of pipelines, as well as the loss of oil platforms and conductors. Recent studies of damage sustained by manmade structures in South Pass Block 70 1,2 proved rather convincingly that loss of support and resulting damage were directly attributable to sediment movements initiated by wave perturbations during the passage of a violent tropical storm. Although there is little doubt that throughout the subaqueous delta and shelf region sediment deformation occurs on many scales and is triggered by several possible mechanisms, data are insufficient to predict how, when, and to what depth failures may occur.