Sherwood M. Gagliano1
(1) Coastal Environments, Inc, Baton Rouge, LA
Abstract: Fault movement and the 20th Century transgression of the Mississippi River Deltaic Plain
The primary process driving massive erosion along Louisiana’s deltaic coast during the 20th century has been marine invasion of lowlands on the surface of sinking fault-bounded blocks. Fault-induced (geotechnical) subsidence is the sinking of blocks bounded by deep-seated linear growth faults and circular fault patterns circumscribing collapse structures. Fault movement occurs in response to geosynclinal down-warping, sediment loading, salt depletion at depth, compaction, and continental margin gravity slumping. All surface features on some subsiding blocks (including wetlands, natural levee ridges, barrier islands, highways, and flood protection levee) are affected.
Rates of block movement have been determined from radiometric dating of buried deposits, sequential land level data, and tide gauge records. The highest subsidence and land loss rates in the region occur on blocks on the down-thrown sides of the Theriot, Golden Meadow, and Forts fault systems. Land loss on these blocks accounts for more than 60 percent of the total 100 year loss in the Deltaic Plain.
These blocks and others are subject to short-term changes in subsidence rates, as fault movement is episodic. An inferred rate increase occurred in the 1890’s, initiating the 20th Century Transgression, and rates have increased significantly since the early 1960’s, resulting in accelerated land loss and barrier island deterioration.
AAPG Search and Discovery Article #90914©2000 AAPG Annual Convention, New Orleans, Louisiana