Structures Associated with Downsag and Basin Development: 

Examples from the Gulf of Mexico Province and Other Environments

James J. Willis¹, Daniel A. Ruberg², and Kristen M. Willis^1,3

¹Odyssey International, LLC, 7190-C Cemetery Hwy., St. Martinville, Louisiana  70582

²Fugro GeoServices, Inc., 200 Dulles Dr., Lafayette, Louisiana  70506

³Department of Geology, University of Louisiana at Lafayette, P.O. Box 44530, Lafayette, Louisiana  70504

ABSTRACT

Downsag and basin development in scenarios typified by dominantly vertical motion, but not precluding possibility of significant lateral motion as well, routinely results in a characteristic suite of associated structural fabrics.  A number of factors control the mode, style, and location of accommodating structures.  Axisymmetry of structural fabrics is characteristic of circular basins emplaced without significant regional differential horizontal stresses or overall lateral translation—circumferential normal faults generally define the entire outer flexural margin, and radial folds and/or thrust faults may occur in the central region.  Some examples of circular salt withdrawal minibasins record central compression manifested by pinching off of a central diapiric salt stock to form an “ice cream cone” shaped salt mass (e.g., Calcasieu Lake Salt Dome, Louisiana, and La Rue Salt Dome, Texas).  Circular basins emplaced within a strong regional stress environment exhibit a bilateral symmetry of structural fabrics—e.g., marginal normal faults occur more prevalently where the local radial minimum horizontal stress subparallels the direction of the regional minimum horizontal stress.  Elliptical basins typically exhibit bilateral symmetry, whether emplaced within or without a strong regional stress, though if present the regional stress may emphasize or deemphasize certain fabrics.  Some basins, including many salt-withdrawal basins, exhibit significant lateral translation—e.g., preferential collapse on one side of the basin may translate to compression on the opposite side, with potential for linking strike-slip (wrench) systems.  The causal mechanism of the downsag itself—e.g., salt withdrawal, salt or carbonate dissolution, differential compaction, etc.—affects its spatial and temporal development.  Topographic or bathymetric expression often yields a depocenter, in which the added weight of sediments may further contribute to the basin forming process itself, especially in salt withdrawal or compaction related basins.  Understanding the similarity as well as the diversity between basinal downsags ultimately yields a greater predictability of both structural and stratigraphic components, including better prediction of favorable reservoir systems and traps.

Willis, J. J., D. A. Ruberg, and K. M. Willis, 2009, Structures associated with downsag and basin development:  Examples from the Gulf of Mexico Province and other environments:  Gulf Coast Association of Geological Societies Transactions,  v. 59, p. 813-846.

AAPG Search and Discover Article #90093 © 2009 GCAGS 59th Annual Meeting, Shreveport, Louisiana