ABSTRACT

Vertical warping and lateral migration of river valleys has long been recognized as evidence of neotectonic activity of the Monroe Uplift in northeastern Louisiana, but the direct cause of this activity has remained elusive. Further detailed analysis of the geomorphology reveals that the uplift is localized along east-west striking zones, with differential uplifts of up to 10 ft (3 m) since the final draining of Paleolake Monroe about 5000 years ago. In addition, small-displacement thrust faults with roughly the same strike and associated plateau-type fracture systems have recently been found (in outcrop) cutting Claiborne Group (Middle Eocene) strata in north-central Louisiana and cutting Eocene to Pleistocene strata in southern Arkansas.

Analysis of structural cross-sections through the Monroe Gas Field indicates that the Monroe Gas Rock is cut by thrust faults -- primarily north verging near the north edge of the field and primarily south verging near the south edge of the field. Vertical offsets on these thrusts range up to 100 ft (30 m) along the south edge of the uplift and up to 50 ft (15 m) along the north edge. The primary thrusts in the subsurface can be linked to geomorphically-defined recent uplifts of the surface. The central portion of the Monroe Uplift, therefore, appears to be the result of thrust-related uplift of the central portions of this thrust structure. The uplift is apparently related to an actively growing anticlinal uplift in southern Arkansas that has caused the Ouachita valley to migrate southward during the late Quaternary.