The Arbuckle Mountains are a high range of hills located between Davis and Ardmore, Oklahoma. The structure of the Arbuckles consists of three northwest-southeast trending anticlines known as the Hunton arch, and the Tishomingo and Arbuckle anticlines (U.S. Department of Interior, 1982).
Tapp (1997) provides the following geologic and tectonic chronology of events within the Arbuckles. During the Cambrian Period, (570 to 500 MYA), a large-scale faulting event (a triple junction aulacogen) formed a rift valley known as the Southern Oklahoma Aulacogen.
During the Ordovician Period, (500 to 430 MYA), the rift ceased spreading and a broad, shallow sea began to encroach over the entire region. For the next 200 million years, marine sediments accumulated on the sea floor. The calcium-rich bodies of dead sea organisms also formed thick limestone layers; in some areas, limestone and shale deposits reach thicknesses of up to two miles.
Another major geologic episode in the region occurred during the Permian Era (280 MYA to 225 MYA). During this episode, the region's crust was forced upward, presumably by the Arbuckle Orogeny (uplift event). The force of this upheaval was so great that many of the sandstone, shale, and limestone layers were broken and folded, creating large anticlines. Along with the folding process, this upward thrust formed the Arbuckle Mountains.
The Arbuckle Mountains have been severely denuded as a result of millions of years of erosion. The mountains now appear as a moderately dissected, low plateau (Harp et al., 1976). Regional geologic studies also show CNRA's geology to be dominated by a syncline having a west-northwest plunging graben (Hanson and Cates, 1992).
In general, the geological complexity of the area is reflected by the high frequency of faults and folds. Some of the fault variations seen in the area include dip-slip, strike-slip, and overthrust faults. There are two main faults within this region: the North Sulphur and the South Sulphur Faults. The dominant fault found near CNRA, the North Sulphur Fault, can be traced from eight miles southeast to one mile east of CNRA (Hanson and Cates, 1994). The South Sulphur Fault can be traced two and a half miles southeast of CNRA to where it parallels to the North Sulphur Fault. Despite the availability of information about them, the extent to which these and other faults influence groundwater flow in CNRA is unknown.
Although the area surrounding CNRA is geologically complex, it is situated in a region that is considered to have low seismic activity and has historically been free of high magnitude earthquakes. A plot of regional earthquake epicenters shows that no historical earthquakes have occurred within a 20-mile radius of CNRA. Earthquakes detected outside this 20-mile radius have had magnitudes less than 4.0 on the Richter scale. However, a level of uncertainty about seismic activity in the area remains, given the fact that CNRA is located only 50 miles to the west of the Nemaha uplift. This ridge is composed of a deeply buried granite traversing north-south from Nebraska to Oklahoma (U.S. Department of Interior, 1982). In this uplift is a zone of deep-seated faults, known as the Humboldt fault zone, which has been determined to be seismically active by the Kansas Geological Survey (U.S. Department of Interior, 1982).
There are three primary rock units in the subsurface of CNRA. The predominant surface rock is the Vanoss Formation, consisting of a limestone conglomerate (Cates, 1989). The Simpson Group, comprised of limestone, sandstone, and shale, lies beneath the Vanoss. The deepest and oldest major unit is the Arbuckle Group, which consists of dolomite, limestone, and sandstone.