TEMPORAL SCALES OF SEDIMENT ACCUMULATION, MASS WASTING, AND STRATIGRAPHY ACROSS THE MISSISSIPPI RIVER DELTA FRONT

Author

Jeffrey E. Duxbury, Louisiana State University and Agricultural and Mechanical CollegeFollow

Degree

Doctor of Philosophy (PhD)

Department

Geology & Geophysiscs

Document Type

Dissertation

Abstract

The Mississippi River Delta Front (MRDF) is a subaqueous apron of rapidly deposited, weakly

consolidated sediment extending from the subaerial portions of the Mississippi River’s Birdsfoot Delta, historically known for sediment transport via mass wasting. To better understand the spatial and temporal dynamics of processes influencing the MRDF, 29 cores ranging from 5.8 to 8.71 meters in length were collected from the South Pass (SP) and Southwest Passes (SWP). These cores targeted specific depositional environments, including the undisturbed topset apron, medial mudflow gully, mudflow lobe, and prodelta. The cores were analyzed along their full lengths for gamma density, grain size, X-radiography, resin infused thin sections, and geochronology using ²¹⁰Pb / ¹³⁷Cs radionuclides. Analyzing sediment for ²¹⁰Pb stair-step profiles and gamma density fluctuation reveals mass movement and deposition within gully flows up to 6 m deep, aligning with Keller et al. (2017). Mudflow gully event return periods in the SWP and SP are decadal (10.7 years) and multi-decadal (66.7 years), respectively with 210 Pb inventories and focusing factors center around the SWP gully, indicating preferential sediment loading in proximal environments and distal transport via mudflow gullies. Elements of microfabrics, including sediment grain composition, biogenic activity, biogenic debris, and sediment structures, display distinct depositional mechanisms not limited to specific environments or study sites. These microfabrics aid paleodelta reconstruction and apply to regions of sediment instability in ancient systems. Budget estimations via 137 Cs geochronology (~9.4 x 10⁶ mt y⁻¹ km⁻²) across the 306 km² study area represent 12% of the river load minus channel storage at Belle Chasse, LA, and 31% of the sediment discharge for SW Pass, South Pass, and Pass a Loutre (2008–2010, Allison et al., 2012). Extrapolated to the 2000–2500 km² MRDF region, sediment accumulation accounts for ≤77–98% of Belle Chasse’s suspended discharge. This study serves as an initial step toward a more thorough sediment mass balance for the Mississippi Birdsfoot delta (both subaerial and subaqueous), which should incorporate sediment accumulation rate (SAR) estimates for the delta platform’s wetland and cores from regions that have not yet been analyzed geochronologically.

Date

1-13-2025

Recommended Citation

Duxbury, Jeffrey E., "TEMPORAL SCALES OF SEDIMENT ACCUMULATION, MASS WASTING, AND STRATIGRAPHY ACROSS THE MISSISSIPPI RIVER DELTA FRONT" (2025). LSU Doctoral Dissertations. 6672.
https://repository.lsu.edu/gradschool_dissertations/6672

Committee Chair

Dr. Sam Bentley