USING HOLOCENE RELATIVE SEA-LEVEL DATA FOR HIGH-PRECISION MEASUREMENT OF TECTONIC SUBSIDENCE RATES IN THE MISSISSIPPI DELTA

It is commonly believed that the most complete Quaternary stratigraphic record of the Mississippi River is to be found in the Mississippi Delta, due to relatively rapid tectonic subsidence rates involving a combination of lithospheric flexure and growth-faulting. However, very little quantitative data on late Quaternary tectonic subsidence rates for this area are available. We have collected relative sea-level data covering the past 8500 years from three study areas in different sections of the Mississippi Delta, to assess whether significant differential crustal movements occur. Our sea-level index points were obtained from basal peat that accumulated during the initial transgression of the pre-existing, consolidated Pleistocene basement, thus ruling out the role of compaction of Holocene strata. The study areas differ in their distance to the present shoreline; in addition, a presumed major growth-fault system may be located between two of them. The rationale of our analysis is that given spatially uniform eustatic and glacio-hydro-isostatic signals, any difference between relative sea-level curves from the three study areas can be attributed to differential tectonic subsidence rates. The extremely favorable conditions for sea-level research on the US Gulf Coast (largely due to the low tidal range) and the long time span of observation allow us to calculate tectonic movements with exceptionally high accuracy and precision.

Our results show that differential crustal movements among the three study areas have been on the order of ~0.1 mm/yr, values that are statistically indistinguishable. We compare our new evidence with a recently published compilation of relative sea-level data from the Caribbean, to a large extent based on data from areas that are widely believed to be tectonically very stable (e.g., Florida, Bahamas, Belize). All our sea-level index points nearly coincide with the Caribbean data, showing that considerable parts of the Mississippi Delta may be surprisingly tectonically stable.