Geology of Black Knob Ridge, Oklahoma

Abstract

Black Knob Ridge is situated at the west end of the Ouachita Mountains, near Atoka, and about 15 miles east of the east end of the Arbuckle Mountains. It is the westernmost area that reveals the facies of pre-Pennsylvanian rocks characteristic of the Ouachita Mountains. The strata in Black Knob Ridge dip eastward beneath the Pennsylvanian Stanley shale, which is overlain by the conformable sequence of the Jackfork sandstone, Johns Valley shale, and Atoka formation, all of which are of Pennsylvanian age. At the south end of Black Knob Ridge the Paleozoic strata are overlapped by beds of the Lower Cretaceous Trinity sand of the Gulf Coastal Plain, which dip gently southward. On the west side of Black Knob Ridge, south of Stringtown, the Caney shale, Springer formation, and toka formation of the facies or sequence characteristic of the Arbuckle Mountains are exposed. Northwest of Black Knob Ridge, north of Stringtown, undifferentiated shales and sandstones of Pennsylvanian age are exposed. The rocks of Black Knob Ridge are composed of clastic siliceous sediments together with some siliceous limestones. They are almost identical in lithologic and faunal characteristics with rocks of the same age in the Potato Hills, Oklahoma, and are only slightly different from rocks of the same age in McCurtain County, Oklahoma, and the Arkansas part of the Ouachita Mountains. The rocks differ greatly in both lithologic character and fauna from rocks of the same time interval in the Arbuckle Mountains, but pronounced similarity between rocks at several horizons in Black Knob Ridge and rocks of equivalent age in the Arbuckle Mountains suggests that the basin of deposition of the rocks of Black Knob Ridge and the similar rocks farther east in the Ouachita Mountains was connected at least intermittently with the basin of deposition of the rocks characteristic of the Arbuckle Mountains. The greater abundance of limestone in the Bigfork chert in Black Knob Ridge than in the areas of its occurrence farther east suggests that the change from the Bigfork chert of the Ouachita Mountains to the Viola limestone of the Arbuckle Mountains was gradual within the basin of deposition; however, later northwestward thrusting of the rocks of Black Knob Ridge has moved them closer to the areas of deposition of the rocks of the Arbuckle Mountains and has therefore accentuated the two different sequences, one typically developed in the Arbuckle Mountains, and the other in the Ouachita Mountains. Black Knob Ridge lies at the west side of the Oklahoma structural salient of the Ouachita Mountains and is made up of a series of rocks that dip steeply eastward and End_Page 1------------------------------ are bounded on the north and west by the Ti Valley and the combined Ti Valley and Choctaw faults. Along these faults the strata in Black Knob Ridge have been thrust northwestward 20 miles or more. The presence of several minor thrust faults and folds in Black Knob Ridge indicates that the movement along the Ti Valley fault was not entirely normal to the strike, but was in part parallel with the strike in a northerly direction. The overthrusting has moved the Black Knob Ridge rocks westward to within 15 miles of the Arbuckle Mountains over strata that, prior to the thrusting, were exposed in an uplift near the present position of Black Knob Ridge. The rocks in this uplift are similar to the sequence of the Arbuckle Mountains and are thus dissimilar to the overthrust rocks which belong o the sequence of the Ouachita Mountains.