ABSTRACT

The Ouachita Mountains reasonably are a product of a collision between the North American plate and a plate to the southeast, Llanoria, which might have been a part of South America or acted as a separate unit. This landmass apparently was subsided beneath the Cretaceous Gulf Coast sediments and was partly broken up during post-Paleozoic opening of the Gulf of Mexico. Thrust faulting from this collision is not of the magnitude previously believed, with only two of the faults in the frontal zone extending beneath 10,000 feet. Displacement on these faults is not in the tens of miles as previously postulated, but appears to be only 10-15 miles on the major fault, the Ti Valley. Most faults are merely breaks at the crests of isoclinal folds. However, additive displacement of faults in the thrust zone may exceed 100 miles as translation of more southerly blocks would increase. A major portion of the deformational forces were taken up in Ordovician and Carboniferous shales as they were tectonically squeezed. This collision is an example of thin skinned tectonics.

The presence of an ancestral Ouachita uplift is indicated by seismic data and interpretation of previous studies of the Johns Valley boulders. This ancestral Ouachita uplift, known as Bengalia, may have started upwards at the end of Viola time as stresses between North America and Llanoria changes from extensional to compressional when Llanoria began to move northward. Bengalia continued rising until the late Mississippian. A system of growth faults may have formed southeast of Bengalia during Stanley time to compensate for the tremendous amount of sediment the approaching Llanorian landmass was feeding into the system. Bengalia may have subsided basinward at this time. Llanoria continued to move northward, overriding a southward dipping subduction zone and colliding with the mid-continent plate in the Atoka and later time.

The resulting structural configuration favors hydrocarbon accumulation. Exploration is beginning in the area at this time. Reserves along the 125 mile mountain belt in Arkansas and Oklahoma could be tremendous.