QUATERNARY TECTONICS OF NORTHEASTERN LOUISIANA – GEOMORPHIC CONTROLS ON STRUCTURE

Northeast Louisiana has experienced at least two non-coaxial deformation events during late Quaternary time. The earlier of these deformational events involved ESE-verging thrusting and created the prominent C-O-W and Macon escarpments; this appears to be no longer active. The later event involves N– and S-verging thrusts and has caused many valley gradation anomalies throughout the area; this appears to be active. Geomorphic slope and its accompanying confining stress gradient appear to control the location and timing of deformation.

Dating of escarpment surfaces is difficult, but erosional degradation patterns along the C-O-W and Macon escarpments suggest that much of the activity occurred during late Pleistocene to early Holocene time. Stabilization probably coincided with the infilling of the late Pleistocene Mississippi valley, suggesting that this deformation was, in part, driven by the high geomorphic slopes and resulting high confining stress gradients along the western valley margins. Present geomorphic slopes are insufficient to allow thrusting in this direction.

Along the Ouachita River north of Monroe, an anticline created a natural lake that existed until late Holocene time (c. 3000 BP). Evidence of late Holocene thrusting on the south side of this anticlinal crest abounds, but most of these structures appear to be inactive. On the north side of the anticlinal crest, on the other hand, there has been deformation more than 4 m of differential vertical motion of the former lake bed since the draining of the lake, and the deformation appears to be active. This transfer of motion from the south flank to the north flank of the anticlinal crest can be attributed to the loss of the water weight within the Ouachita valley.