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Hallie Latham1, Beth Yuvancic1, Juan M. Lorenzo1, Ivor van Heerden1
(1) Louisiana State University, Baton Rouge, LA

Abstract: Neotectonics in the lower Mississippi Valley: High-resolution gravity and seismic characterization of active shallow normal faults

Shallow (< 0-500 m), active, growth faults in the lower Mississippi Valley have been surface-mapped only near urbanized zones, all of which rely exclusively on aquifers for drinking water. The geophysical character of these faults and their rates of movement in the Quaternary remain virtually unknown. Lack of knowledge of these fault systems exposes aquifers to unmanaged, surface contamination. Fault movement may correlate with (1) regional stress induced by sedimentary loads and sea-level rise and/or (2) local stress associated with differential compaction, or (3) extraction of subsurface brine or fresh water.

At the latitude of Baton Rouge, Louisiana, we study a 30 km-long E-W fault system interpreted from the alignment of high topographic gradients., damaged buildings and roads. Three N-S high-resolution gravity profiles and one high-resolution seismic reflection profile sample localities with high (10m), intermediate (6m) and low (1m) scarp relief. Interpreted zones of reduced porosity correlate with 1mGal Bouguer gravity anomaly peaks, tens of meters in wavelength. Half-grabens, ~300 m wide are testable in seismic images (24-fold, 1-s, 1.5-m CDP spacing, 40-Hz geophones).

Seismic surfaces which correlate to existing water-well bore logs permit determination of changes in the rate of fault movement. Stratigraphic patterns across the fault that show acceleration in fault movement above the Pleistocene regional surface are interpreted to result from greater subsidence induced by the depletion of local aquifers. Fault movement would be greatest below the Pleistocene surface as a result of regional sedimentary loads, which are greatest during glacial times.

AAPG Search and Discovery Article #90914©2000 AAPG Annual Convention, New Orleans, Louisiana