Assessment of the Stratigraphic Controls on Deltaic Subsidence in the Mississippi River Delta

Abstract

Louisiana’s coastlines are being lost due to a rise in sea level and land subsidence. This study isolates one aspect of land subsidence, called autocompaction, to access its contributions to overall subsidence. Autocompaction is the process where a growing sequence of sediments collapses due to an increasing overburden load. A total of 36 sediment cores from the Sale-Cypremort deltaic lobe were analyzed. Each core was divided into facies units of natural levee, marsh, poorly drained backswamp, and bay mud. A soil analysis was conducted along with the sediment cores. Each soil was identified as a facies type. By identifying facies, geotechnical parameters based on facies type were applied in an equation that solved for consolidation settlement, also called autocompaction (Sm). Autocompaction measures the decrease in layer thickness by vertical compression. The autocompaction values were compared to depth of facies, thickness of facies layers, as well as depth to Pleistocene. Results show that as thickness of facies layers increases, compaction increased. As depth to Pleistocene increased, compaction had a slight increasing trend. Natural levee facies can be considered firm and nearly incompressible, while marsh, poorly drained backswamp, and bay mud facies are soft and compressible.