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The Ouachita Mountains of Oklahoma are considered by many as a classical example of large scale, flat overthrusting, and are envisaged as a huge, floating, allochthonous body with a root somewhere farther south.
During extensive geologic mapping between June, 1953, and January, 1956, the authors failed to find large, flat overthrusts in the Ouachita Mountains. On the contrary, the stratigraphic and structural evidence indicates that the mountains are an autochthonous folded system.
The Potato Hills anticlinorium, where the idea of overthrusting was conceived, was mapped in detail. This anticlinorium consists of closely spaced, steep, partly overturned folds. The overturning is both toward the north and, against the supposed overthrusting, toward the south. Some anticlinal limbs have ruptured and steep reverse faults have developed, some of which yield north, others south. All of these faults die along strike, generally in the steep limbs of anticlines.
In the eastern Potato Hills there is the Round Prairie synclinorium which is bounded on opposite sides by faulted, overturned anticlines which face each other. Round Prairie is part of what has been described as a window in a major overthrust. However, the "window" is non-existent. The two border faults are separate, distinct, and die toward the west in unbroken folds. At the eastern end of the "window" the southern fault truncates the northern.
The Choctaw anticlinorium is also autochthonous, as originally determined by Honess (1923). Southward overturning predominates, and slaty cleavage has developed which mostly dips steeply north. This confirms predominant southward yielding and is incompatible with the concept of a great thrust sheet which has moved north.
In the core of the Choctaw anticlinorium the style of deformation changes: steep folds disappear and the cleavage becomes gently inclined. The change is gradual, stratigraphic and structural continuity being maintained. There is no overthrust break and there is no window. This change in structural style with increasing stratigraphic and structural depth suggests that a basal zone of disharmonic shearing-off is approached.
The northern and northwestern border of the mountains has been considered as a great overthrust front with a frontal sole thrust succeeded to the south and east by a number of higher overthrusts. However, none of the structures in this outer zone supports this concept. If flat thrusts were present one would expect strong or uniform northward overturning of folds, and gentle to moderately dipping fault planes. Shearing and strong mechanical rock deformation should be widespread and conspicuous at the overthrusts.
On the contrary, at mapped major overthrusts weak shale sequences are intact and maintain regular bedding despite juxtaposition to mechanically stronger units. The mapped overthrusts are partly steep reverse faults, and partly no faults can be found.
The basis for most mapped "thrusts" apparently has been the tectonic interpretation of the boulder-bearing "Johns Valley shale" as a "friction carpet." However, the boulders of Arbuckle rocks are depositional as are the Ozark region exotics found near Boles, Arkansas. Ulrich (1927) affirmed a depositional origin for these boulders, and the present authors consider his hypothesis of ice rafting as substantially correct. Moreover, though the "Johns Valley shale" customarily is assigned to the interval between the Jackfork group and the Atoka formation, the boulder-bearing shales actually occur as three separate and distinct members within the Jackfork group.
The contrast between the rocks of the Ouachita Mountains and those of the Arbuckle region, though real, is not abrupt. Some units are identical, others differ relatively little, and others differ strongly. Most of the contrasted facies have transitional relationships. Some transitions are gradual; others have been accentuated by shortening resulting from folding and reverse faulting. None exceeds those often encountered in adjacent, connected basins or different parts of the same basin.
Of major importance is the fact that facies changes occur both along and across the structural trend. Moreover, certain units and facies boundaries cross from the Arbuckle region into the Ouachita Mountains. The greatest facies contrast is supposed to be between the Arbuckle limestone and its presumed Ouachita clastic correlatives. However, part of the Ouachita rocks may be younger than the Arbuckle limestone, the customary correlation being open to question. Moreover, below the Ouachita clastic rocks there is a limestone which might correspond with part of the Arbuckle limestone.
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