It has been long held that the area of southwest Mississippi and southeast Louisiana is the site of modern uplift associated with structural arching. This ~east-west trending arch, termed the “Southern Mississippi uplift”(sMU), is bound on the north by the Mississippi salt basin and to the south by southward dipping sediment packages of the Gulf coastal plain. Jurkowski et al. (1984, J. Geophys. Res.), based on historical geodetic leveling data, showed that the crest of the sMU is rising at a rate of 3-4 mm/yr relative to Jackson, MS. The authors were careful not to claim that “uplift” was relative to a datum.

A new analysis of National Geodetic Survey first-order leveling and National Ocean Service tide gauge data from the Mississippi and Louisiana coastal region indicates that the absolute movement of all benchmarks is downward with respect to sea level. The arching of the Southern Mississippi uplift and coastwise tilting of Pleistocene strata are apparently the result of differential subsidence, with the southern flank of the sMU sinking at a faster rate (>-10 mm/yr) than the crest (~-6 mm/yr). The geometry of subsidence is complex in detail, suggesting both local and regional mechanisms. Regional mechanisms include flexure due to the surface load of the modern Mississippi delta and buried loads of older sediment packages and isostatic adjustments due to subsurface salt flowage. Local mechanisms include synsedimentary normal faulting, sediment compaction, fluid withdrawal, etc.).