Subsidence is leading to the slow inundation of communities and wetlands of Louisiana, Mississippi, Texas, and Alabama (United States) by the Gulf of Mexico. The prevailing paradigm considers subsidence to be the result of young sediment compaction and/or consolidation and human activities. This paper describes the results of a test of this theory based on an examination of historic motions of benchmark in the Michoud area of Orleans Parish, Louisiana. This methodology allowed for an assessment of vertical change at different levels over time relative to a precise vertical datum (North American Vertical Datum of 1988, NAVD88). Data do not support the current theory on the origins of subsidence; they demonstrate that tectonic causes dominate in the study area. During 1969– 1971 and 1971–1977, tectonism was responsible for −16.9 mm/yr and −7.1 mm/yr of subsidence, respectively. These contributions account for 73% and 50% of the total subsidence during these intervals. The change in deep subsidence is attributed to renewed motion along a large normal fault (Michoud fault). Over the same time intervals, intermediate depth subsidence due to compaction of Pleistocene to middle Miocene strata was constant (−4.6 mm/yr). Similarly, subsidence due to shallow processes, i.e., sediment compaction and groundwater offtake, was −1.5 mm/yr and −2.5 mm/yr. Subsidence associated with petroleum extraction was not a factor due to the lack of local production.

Keywords: tectonic subsidence, Louisiana, Gulf of Mexico, Michoud fault