Sedimentary Controls on Subdelta Formation and Mass Transport in an Interdistributary Bay of the Mississippi River Delta: East Bay, Louisiana

Author

Giancarlo J. Portocarrero Cortez, Louisiana State University and Agricultural and Mechanical CollegeFollow

Semester of Graduation

Spring 2026

Degree

Master of Science (MS)

Department

Geology & Geophysics

Document Type

Thesis

Abstract

The Mississippi River Delta is currently facing existential threats from declining sediment loads, sea-level rise, and localized subsidence leading to land loss. Most documented land loss has been focused on the deltaic wetlands; however, this is also taking place in the subaqueous portions of the delta. Submarine retreat has been previously documented in deeper parts of the delta front via offshore sediment transport, taking place through active seafloor features like bottleneck slides and gullies. Yet, the export of sediment through these instability features remains understudied in interdistributary bays, which bridge gaps between the fluvial hydrology and sediment dispersal to the delta front. Recent river-channel surveys reveal that sediment supply into East Bay may have increased from previous years through cuts in Southwest Pass and South Pass banks. If retained, this sediment could be helpful in maintaining the nearshore submarine foundations of the delta; however, the timescales of sediment retention, or reworking by waves and currents is unknown. This study used ⁷Be, ²¹⁰Pb, and ¹³⁷Cs radioisotope geochronology of sediment cores to estimate how much of the sediment delivered to East Bay is being retained and the styles of sediment reworking on decadal to centennial timescales. Results from this study show that seabed instability features like bottleneck slides, collapse depressions, and gullies act as effective sediment traps, based on seasonal and decadal-to-centennial sediment deposition and accumulation trends. The resulting sediment budgets show a decline in sediment accumulation rates from the period before 1953 to after 1953. Additionally, seabed slopes derived in this study and previously documented high suspended sediment concentrations point to the presence of wave-enhanced gravity flows and pure gravity-driven flows that limit sediment retention in this area. This initial analysis indicates that this subaqueous portion of the delta is not likely to become emergent to subaerial exposure. This work provides insight on the processes driving delta retrogradation, which can help the energy industry assess risks to aging infrastructure within East Bay and can assist coastal management decisions that could impact navigation and sediment diversion projects.

Date

12-12-2025

Recommended Citation

Portocarrero Cortez, Giancarlo J., "Sedimentary Controls on Subdelta Formation and Mass Transport in an Interdistributary Bay of the Mississippi River Delta: East Bay, Louisiana" (2025). LSU Master's Theses. 6276.
https://repository.lsu.edu/gradschool_theses/6276

Committee Chair

Samuel J. Bentley