Abstract: Regional Fracture Patterns in NW Louisiana: Evidence for Two-Phases of Compressional Tectonics

WASHINGTON, PAUL A.
Northeast Louisiana Univ., Monroe, LA

Coals, shales, and occasional well-cemented silts and sands exposed in northwestern Louisiana commonly exhibit multiple fracture systems. Preliminary mapping of the fracture patterns suggests that the fractures generally fall into four orientation groups that form two orthogonal fracture systems with consistent overprinting relations. Furthermore, each orthogonal fracture system consists of one fracture orientation that morphologically resembles a penetrative shortening fabric, and a perpendicular extension joint system. The regionally consistent orientation, morphologic characteristics, and overprinting relations across large areas of northwestern Louisiana suggest that these fracture systems are caused by regional tectonic events. The later set parallels folds and thrusts in the Kilmichael dome, Mississippi.

Both fracture systems are found throughout the area, with the expected lithologic controls but no apparent stratigraphic control. The earlier fracture system includes close-spaced subplanar fractures striking approximately 320Þ and widely spaced planar fractures striking approximately 050Þ. The former strongly resemble strike-parallel shortening fractures found in plateau regions adjoining fold-and-thrust belts, whereas the latter strongly resemble the associated cross-strike extension joints. The later fracture system consists of pervasive, widely-spaced, curviplanar fractures striking approximately 275Þ, and locally developed closely-spaced sub-planar fractures striking approximately 020 degrees. The curviplanar fractures are planar, except where they curve into and terminate against well-developed fractures of the 320-Þ set. The closely-spaced fractures commonly appear to be an incipient cleavage, and cause pencilling in thicker bedded shales.

AAPG Search and Discovery Article #90932©1998 GCAGS/GCS-SEPM Meeting, Corpus Christi, Texas