Long-term subsidence and compaction rates: a new model for the Michoud area, south Louisiana

The stratigraphic record of the Michoud area in East New Orleans, Louisiana was examined to address hypotheses developed from recent geodetic studies, which contend faulting, as opposed to shallow sediment compaction, is the primary mechanism driving land-surface subsidence in south Louisiana. Testing these hypotheses involved constructing a structural cross section using well logs, chronostratigraphic data, and fault picks, so as to evaluate differential motion along specific faults through time. Employing ages and corrected depths for three key subsurface horizons, long-term (Middle Miocene – Present) time-averaged subsidence rates were calculated: rates range from -0.140 mm/yr to -0.177 mm/yr. Long-term subsidence rates are incompatible with subsidence rates determined from geodetic studies: geodetically derived subsidence rates range from -14.2 mm/yr to -23.0 mm/yr for the Michoud area, which are two orders of magnitude greater than long-term subsidence rates. However, considering the difference in scale of resolution of respective techniques used to calculate subsidence rates, caution is advised when comparing rates. In addition, mean long-term compaction rates, which are a component of total subsidence, for strata residing above the Middle Miocene Bigenerina humblei horizon were calculated using a standard decompaction technique: rates range from -0.0704 mm/yr to -0.0914 mm/yr, which are two orders of magnitude less than geodetically derived, pre-Holocene strata compaction rates (-4.6 mm/yr). In conclusion, the findings from this research suggest reactivation of local faults, including any recent movement of the Michoud Fault, is a transient phenomenon. Furthermore, the discrepancy between mean long-term compaction rates from this study and pre-Holocene strata compaction rates derived geodetically raises questions into the interpretations and/or accuracy of the geodetic data for the Michoud area, and therefore, the subsidence rates determined from such data.