EVIDENCE FOR COMPRESSIONAL LATE HOLOCENE TECTONICS IN NORTHERN LOUISIANA

Evidence of Holocene tectonic activity in northern Louisiana includes locally uplifted alluvial surfaces and geomorphically youthful escarpments. The actively uplifting features appear to trend east-west, but the primary escarpments in the area trend approximately 030 and 325. Plateau-style fracture sets (parallel and perpendicular) are associated with each of these directions. The age of the escarpments is as yet undetermined, although they appear to offset post-Oligocene strata. Pencilling paralleling each of escarpment trends has been found in the Eocene and Oligocene strata, indicating that they are compressive in origin. Subsurface structures associated with the largest 030 escarpment also suggest thrusting. Relative timing based on both outcrop-based fracture relations and geomorphology of the escarpments indicates the tectonic event creating the 325-striking features predated the tectonic event creating the 030-striking feature. Surface exposures of thrust faults striking approximately east-west have suggested that the currently active uplifts are contractional, but a lack of control on the age of the faulting has discouraged linkage. Recently, an 090-striking thrust zone with uplifted and folded Pleistocene gravels containing Ozark-derived clasts has been discovered spatially associated with a locally uplifted alluvial surface that is so recent that it still impounds a lake. Subsurface data also indicates north-south shortening and is directly linked to active differential vertical motion of various parts of the Ouachita River alluvial plain. Thus, we can confirm that the Late Holocene tectonic activity in northern Louisiana is associated with north-south shortening.