CONTRASTING SEDIMENTATION RATES WITHIN AN ABANDONED LOUISIANA DELTA

Sediment cores were taken in two ecologically and geographically distinct sites within the abandoned LaFourche lobe of the Mississippi Delta (active 2000-300 ybp) to investigate differences in the depositional environments and sedimentation rates in relation to past deltaic migration events. The Fourchon core was collected in a back barrier lagoon mangrove environment whereas the Cocodrie cores were collected in a Spartina marsh environment. The main differentiating factor between these sites is their relative distance from the head of Bayou Lafourche, the abandoned Mississippi distributary. Radionuclide analysis using ²¹⁰Pb and ¹³⁷Cs was conducted to estimate sediment accumulation rates of 0.82-0.94cm/yr (Fourchon) and 0.43-0.44cm/yr (Cocodrie). In addition, cores were analyzed for density, water and organic content and grain size. Distinct lithological changes were found in each core. In the Fourchon core, a transition from massive clay to parallel laminated and cross-bedded sand was observed. At Cocodrie, a distinct peat layer was found between two organic mud layers. These abrupt changes in lithology, which occurred approximately 300 ybp according to sedimentation rates, are thought to be associated with the transition of the Lafourche delta complex from a regressive to a transgressive phase of the delta cycle.