A Geologic And Geotechnical Analysis Of The Upper Continental Slope Adjacent To The Mississippi Delta
Authors: Booth James S. , Garrison Louis E. 
Published: October 2008

The upper Continental Slope immediately adjacent to the modern (Ba1ize) delta is well within the depositional influence of the Mississippi River, and this influence is increasing as the delta progrades. Thus; the likelihood of incipient large-scale deltaic processes, such as mass sediment movement, is steadily increasing as well. Because these processes may be a potential hazard to offshore petroleum development, and because of the opportunity to study such processes, this area is of prime interest.

The U.S. Geological Survey, therefore, conducted an intensive investigation of this upper slope environment, with the following objectives:establish the basic geologic and geotechnical character of the surficial sediments,gain an understanding of the recent geologic history, anddefine potential geologic hazards in the area.

Acoustic profiling and piston coring were the main elements in the program, with the latter being confined to three physiographica11y distinct provinces which included a major valley, a ridge adjacent to that valley, and a mudf1ow. A wide range of geologic and geotechnical analyses were performed on the samples.

Interpretation of seismic records suggests that the main effect of rapid deposition associated with the delta is yet to be felt by the upper Continental Slope. The area is geologically active, having experienced a major episode of slumping and being continuously affected by faulting and diapirism, but these events are basically unrelated to the Holocene Mississippi delta complex or to the growth of the Ba1ize lobe. The one major geologic feature that is related to current conditions is a&gt;30-m thick mudflow that is just beginning to encroach upon the upper slope. The rate of movement of the mudfl0w is not known.

Geologic analysis (chiefly core descriptions and CaCO3 data) of the sediments reveals a dramatic change from an environment dominated by biological activity in the past to one having more azoic characteristic at present. We believe that this change signaled the appearance of the Plaquemines or possibly the Ba1ize Delta. Small-scale slumping has been taking place in the study area continuously throughout the time represented by the cores. The frequency of this slumping may be increasing. Only a few meters of surficial sediment are affected, however. Characteristics of the three provinces could not be distinguished by the geologic data.

Geotechnical analysis, however, did permit recognition of the characteristics. The valley is typified by a more underconso1idated sediment than is either of the other two provinces. A higher clay content, perhaps in conjunction with a higher sedimentation rate, is the probable cause of this difference. The ridge sediments are more normally consolidated, are deposited at slightly lower rates, and contain less clay. Data from the mudf10w sediments do indeed indicate mechanical disturbance. The geotechnical data failed to show the environmental change revealed by the core descriptions and CaC03 data. This failure probably indicates a smooth transition from the environment dominated by biological activity to the more azoic one.