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The Fernvale and Viola limestones of south-central Oklahoma are exposed in the Arbuckle and Wichita mountains, and are found in the subsurface in much of the Mid-Continent region. Fernvale (Richmond) and Viola (Trenton) strata are chiefly limestone with varying amounts of chert, dolomite, detrital quartz sand, and minor sedimentary constituents.
Isopachous and lithologic characteristics of these Ordovician formations indicate that the strata were deposited in the northwest-trending Arbuckle geosyncline having a broad platform in the Seminole area. The Fernvale limestone is thickest in a northwesterly trend extending from the Wapanucka graben to the Pauls Valley anticline, whereas the Viola limestone is thickest in a northwesterly trend approximately parallel and corresponding with the zone of maximum sedimentation in pre-Viola basins of deposition in the Arbuckle geosyncline.
The Fernvale limestone contains abundant detrital quartz sand where thickest and the crystallinity ranges from very coarse on the platform to medium in the basin. The Viola limestone is predominantly dolomitic on the platform, cherty on the platform slope, and siliceous in the basin. Fernvale and lower Viola limestones are essentially planar-bedded in the area of maximum deposition, whereas the upper and middle Viola beds are irregularly bedded in most of south-central Oklahoma.
Regionally correlatable lithologic characteristics of the Viola limestone indicate that the formation is divisible into four principal members of which the uppermost and the lowermost wedge out north-eastward from the basin to the platform. With a few local exceptions, such as the Franks and Wapanucka grabens, all members of the Viola limestone thicken regularly from the platform to the basin.
The Viola limestone of the Arbuckle facies is areally divisible into six sub-facies to which the following names are applied: Seminole, McAlester, Fitts, Wapanucka, Tishomingo, and Ardmore. Though there are significant differences in Viola sections of the six stratigraphic provinces, their boundaries are gradational and any suite of Viola cuttings or surface samples may be assigned to its correct province through lithologic characteristics alone, without the aid of specific location on a map.
Fig. 1. Composite stratigraphic diagram of south-central Oklahoma.
Isopachous studies of the Fernvale and Viola limestones show that the orogenies which caused the folds and faults of the Arbuckle Mountains, and adjacent subsurface anomalies, had no effect upon the original Fernvale-Viola basins during the deposition of those strata.
The significant lack of correlation between Viola production and maximum intercrystalline porosity, or location of the wedge-edges of the members, suggests that productive Viola reservoirs are present in localities of fortuitous orogenic shattering rather than in broad areas where maximum porosity may occur because of lithogenesis of the carbonate sediments.
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