FAULT-SLIP HISTORY OF THE DELACROIX ISLAND FAULT SYSTEM AND ITS EFFECT ON HOLOCENE SALT MARSHES OF THE MISSISSIPPI RIVER DELTA PLAIN

An in-depth study of the Delacroix Island producing field, in Plaquemines Parish, LA, illustrates the fault-slip evolution of the main east-trending Delacroix Island fault during the last 13 million years and its effect on Holocene geomorphology. Well log correlations and 3-D seismic interpretation of 23 subsurface bio-stratigraphic horizons across the fault reveal a range of stratigraphic thickening and displacement. Wells across the fault, with well-log curve data to as shallow as 31m below the surface, were used to calculate interval thicknesses between the bio-stratigraphic markers. Isopach maps, created with these interval thicknesses, depict two styles of stratigraphic thickening both of which indicate differential subsidence across the fault. Plots of stratigraphic interval thickening, sediment accumulation and depth versus displacement were utilized to assess the history of fault-slip motion. Through these analyses, as well as a reconstruction of burial history, a correlation was found between positioning of ancient Miocene depocenters over the Delacroix Island area and a large fault movement event. This adds weight to the theory that sediment loading causes fault-slip events. A lineation on the modern marsh surface coincident with the surface projected fault plane suggests that fault movement has continued during the Holocene. Historic satellite imagery (last ~ 60 years) of the field depicts continuous wetland loss on the downthrown side of the surface fault trace, suggesting that recent and continued fault movement may be contributing to marsh submergence.