Subsidence and Sea-Level Change along the Northern Gulf of Mexico: Response of Mississippi River to the Last Glacial Cycle, and the Flexural Ups and Downs of Mississippi Delta

Mike Blum
Louisiana State University ([email protected])

Subsidence and sea-level change in the Mississippi delta region have seen renewed interest after Hurricanes Katrina and Rita. Subsidence and sea-level change include contributions from a number of interrelated processes, which operate over a variety of spatial and temporal scales. This presentation discusses a new component to land-surface dynamics in the delta region, and along the adjacent Gulf of Mexico shoreline, a cyclical flexural response to excavation of lower Mississippi valley sediments by meltwaters during the last deglaciation, when sea level was relatively low, and valley filling during Holocene sea-level rise.
Recent studies of the lower Mississippi valley provide a new and more detailed view on valley evolution in response to glaciation, deglaciation and meltwater routing, and global sea-level change. These studies contribute to our understanding of the subsidence and sea-level change because they constrain the thickness and lateral extent of sediments that were removed during the last glacial period and subsequently replaced during the Holocene, as well as provide a chronology for excavation and filling. Results of 1D steady-state and 3D visco-elastic models show the volume of sediments removed and replaced was sufficient to induce large-scale flexural uplift of the delta region, followed by flexural subsidence. Amplitudes of uplift and subsidence range from 12 m in the valley center to 9 m at the valley margins, and dissipate to negligible values only over distances of >100 km along the adjacent Gulf of Mexico shoreline.
This high-frequency, cyclical flexural signal has a number of implications for the analysis of subsidence patterns, as well as spatially varying views on sea-level change along the Gulf of Mexico shoreline. Moreover, incision and aggradation is a common response of large rivers to cyclical climate and sea-level change: cyclical, high-amplitude flexural uplift and subsidence should therefore be an important component in large fluvial-deltaic systems elsewhere, today and in the stratigraphic record.

AAPG Search and Discovery Article #90073 © 2007 AAPG Foundation Distinguished Lecturer Series 2007-2008