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

Abstract

This study examined the stratigraphic record of the Michoud area in East New Orleans, Louisiana to address questions concerning the magnitude of, and processes leading to land-surface subsidence. The hypotheses under review are based on recent geodetic studies, which challenge the widely held position that modern subsidence is primarily a function of shallow sediment compaction. Testing these hypotheses involved constructing a structural cross section of the Michoud area 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 to Present) time-averaged subsidence rates were calculated: rates range from -0.140 to -0.177 m/kyrs (-0.140 to -0.177 mm/yr). Long-term subsidence rates are incompatible with those derived from geodetic studies: geodetically derived subsidence rates (-14.2 to -23.0 mm/yr) for the Michoud area are two orders of magnitude greater than long-term subsidence rates. Considering the scale of resolution of respective techniques, caution is advised when comparing respective subsidence rates. Nevertheless, the new subsurface, structural model for the Michoud area suggests reactivation of local faults, including any recent movement of the Michoud Fault, is a transient phenomenon that is likely related to rapid Quaternary sediment loading. It is reasonable to compare mean long-term compaction rates, which is a component of total subsidence, derived from this research to geodetically derived compaction rates of pre-Holocene strata. Using a standard decompaction technique, mean long-term compaction rates for strata residing above the Middle Miocene Bigenerina Humblei horizon were calculated: rates range from -0.0704 to -0.0914 m/kyrs (-0.0704 to -0.0914 mm/yr), which are two orders of magnitude less than geodetically derived, pre-Holocene strata compaction rates (-4.6 mm/yr). The findings of this research, particularly the discrepancy between mean long-term compaction rates derived in this study and pre-Holocene 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.