A High-Resolution Seismic Assesment of Faulting in the Louisiana Coastal Plain

Land loss in Louisiana's coastal plain has been the focal point of intense scientific and management discussions for the last several decades. Recently, fault movement has been suggested as a significant component of coastal plain subsidence and land loss. Onshore oil and gas activities over the last half century have confirmed the existence of numerous deep-cutting subsurface faults beneath the coastal plain. The question is, do some of these faults intersect and offset the modern surface? To help answer this question, high resolution subbottom profiles (chirp sonar) and vibracores were collected in areas where land loss patterns, linear marsh-water contacts in particular, have been interpreted as the surface expressions of faults. The chirp sonar methodology produces clear images of the stratigraphy 6-10 m into the shallow subsurface with a bed resolution of ~ 10 cm. So, even small stratigraphic offsets can be detected with this acoustic system. Data were collected to evaluate the Empire Fault (Adams Bay and Lake Washington area), Lake Boudreaux, Lake de Cade, Lake Barre, Bay Sainte Elaine, Sister Lake, and Lake Mechant. Our surveys produced mixed and surprising results. Linear marsh-water contacts (Adams Bay-Lake Washington and Bay Saint Elaine) did not correlate with stratigraphic offsets in the shallow subsurface. However, “rumpling” of near-surface stratigraphy occurs in Adams Bay (Empire Fault area). Small offsets and stratigraphic sags were observed in Sister Lake, Lake Mechant, and Lake Barre. These features are interpreted as the result of localized deformation associated with differential compaction and other local processes. A major conclusion from the study is that linear or arcuate marsh-water contacts, frequently attributed to faulting, are not clear indicators of offsets in the shallow subsurface stratigraphy.