ABSTRACT
Shelf deposits of the active Balize Lobe of the Mississippi River delta, are constantly displaced from their original depositional environments by a variety of deformational and mass-movement processes. Consequently, hydraulically controlled sedimentation patterns are altered in favor of sediment displacement to deeper water settings. High-resolution seismic and side-scan sonar surveys have shown that complex mudflow systems are the most important means of sediment transport from the upper and intermediate delta front to deeper shelf and upper-slope environments. With expanding exploration and production of hydrocarbons from shelf depths and deeper, it has become important to identify and understand both the surficial and subsurface characteristics of sediments associated with sea floor instabilities.
The sedimentology of mudflow deposits has been determined from detailed x-ray radiography (for sediment structure) and x-ray diffraction (for clay mineralogy) analyses evaluated in conjunction with their geotechnical properties (shear strength, Atterberg limits, etc). Sediments from each major part of the mudflow system (gully heads, chutes, and mudflow lobes) share a set of common sedimentary structures. The most complicated deposits are the wide-spread and rapidly deposited mudflow lobes. They are composed of overlapping wedges of highly remolded, low-shear-strength deposits, separated by thin, interlobe units of acoustically reflective and slowly accumulated hemipelagic sediment. Gas-related features, convolute structures, inclined bedding, evidence of flowage, and indications of thorough mixing are found in mudflow lobe deposits. Clay mineral assemblages are typical of rapidly deposited pro-delta clays (smectite:kaolinite:illite ≈4:4:1). In contrast, thin interlobe units and distal shelf sediments contain evidence of biogenic activity (micro-organism tests, burrows, and shell fragments) and diagenetic products. Interlobe and distal shelf deposits have clay mineral suites characterized by an increase in kaolinite and illite at the expense of smectite, which allows for distinction of individual flows and the general mudflow base.
