3D Seismic Evaluation of Fault Control on Quaternary Subsidence Patterns, Rates, and Related Surface Morphology in Southeastern Louisiana

Throughout the past century, the Louisiana coastline has been deteriorating at an alarming rate. Fault induced subsidence has been suggested to be one of the likely drivers of this land loss. We directly addressed the control of faulting on Quaternary delta subsidence using subsurface architectural data. We carried out a systematic process for evaluating fault movement on individual faults in the subsurface by using an industry grade 3D seismic survey to evaluate the interaction of complex fault patterns and shallow substrate (<1 s or 1,000 m). Using amplitude data, as well as coherency and dip algorithms, horizons and faults were mapped in ~ 1400 km2 of data in southeastern Louisiana. Measurements of fault displacement were obtained by determining time offset of synchronous horizons. Down-to-the-basin and counter-regional growth faulting was observed. Both styles of faulting are associated with movement of Jurassic-age salt deposits and display measurable offset at depths as shallow as ~.400 ms (roughly 380 m). Fault displacements increase roughly linearly with burial depth. The displacement data together with long-term age control provide a tentative Quaternary rate for the normal faulting of 0.1 – 0.05 mm/yr.

We have assessed the possible control of subsurface faulting on surface topography using co-registered maps of fault traces and orthophoto quadrangles. This analysis has revealed a number of instances where faulting affects modern surface morphology. Often, the up-thrown sides of growth faults are capped by marshland, while the downthrown blocks are drowned by surface water. Further, the influence of faulting extends to the modern planform of the lower Mississippi River, as data indicate growth faults set the position of a major bend near the river's outlet.