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A High-Resolution Seismic Assessment of Faulting in the Louisiana Coastal Plain
Harry H. Roberts1, Robert A. Morton2, and Angelina Freeman3
1Coastal Studies Institute, Louisiana State University, Baton Rouge, Louisiana 70803
2U.S. Geological Survey, 10100 Burnet Rd., Austin, Texas 78758
3Department of Oceanography and Coastal Sciences, Howe Russell (3rd Floor, Old Building), Louisiana State University, Baton Rouge, Louisiana 70803
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 “Have some of these faults recently intersected and offset the modern surface?” To help answer this question, high-resolution subbottom profiles (chirp sonar) and vibracores were collected along known growth-fault trends and 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 7-12 m (23-39 ft) into the shallow subsurface with a bed resolution of ~10 cm (3.9 in). 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), and possible faulting in Bay Sainte Elaine, Lake Boudreaux, Lake de Cade, Sister Lake, and Lake Mechant. Our surveys produced mixed and surprising results. A linear marshwater contact in Bay Saint Elaine did not correlate with stratigraphic offsets in the shallow subsurface. However, deformation (undulations) of near-surface stratigraphy in Adams Bay and stratigraphic offsets in Lake Washington suggest recent movement on the Empire Fault resulting in a well-defined arcuate marsh-water contact. Small stratigraphic offsets and sags were observed in Sister Lake, Lake Mechant, and possibly Lake de Cade. These features are interpreted as the result of localized deformation associated with differential compaction and other local processes, but they could reflect nearsurface responses to deep fault movement. Our datasets do not image deep enough to convincingly connect the near-surface deformation with deep-cutting faults. A major conclusion from the study is that linear or arcuate marsh-water contacts, frequently attributed to faulting, are not necessarily correlated with offsets in shallow subsurface stratigraphy and therefore with evidence of fault movement.
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