Abstract Major known sphalerite deposits of the Central Tennessee zinc district are localized in upper Lower Ordovician carbonate rocks of the Knox Group along the crest of the Cincinnati Arch, a broad northeast-trending regional uplift with gentle dips of only a few meters per kilometer (Figs. 1 and 2). Some deposits occur at the northeast edge of the Nashville Dome, the southern extension of the Cincinnati Arch (Fig. 3). Other deposits extend north and northeastward into the Cumberland Saddle, which separates the Nashville Dome from the Jessamine Dome of north-central Kentucky. The district occupies the northeast part of the Central Basin, and adjacent parts of the Eastern and Western Highland Rim physiographic provinces of north-central Tennessee and south-central Kentucky (Fig. 2). The crest of the Nashville Dome, from its apex near Murfreesboro, plunges gently northeastward into the Cumberland Saddle. Consequently, dips in the Elmwood-Gordonsville area are toward the northeast at about six meters per kilometer. Pre-Middle Ordovician erosional thinning of the upper Knox Group from west to east across the crest of the Nashville Dome indicates the pre-existence of a paleoarch with a crest east of the present crest of the dome (Stagg and Fischer, 1970; Fischer, 1977). A reconstruction of the crest of the dome as it existed during early Middle Ordovician time is shown in Figure 3, and illustrates this relationship.

Abstract The Cincinnati arch is a continual anticlinal trend approximately parallel with the Appalachian Mountains. There are two independent domes along the crest—Nashville dome in Tennessee and Jessamine dome in central Kentucky. Sedimentary rocks, in an essentially normal sequence, range from Upper Cambrian or possibly older, to the Dunkard Series (Permian), and these rocks contain many units productive of gas and oil. Cambrian .—Trempealeau dolomite, with vuggy porosity locally, produces gas in northeastern Ohio, and oil was discovered in the unit in Morrow County in 1959; the fields are small and disconnected, and accumulation is controlled by post-Cambrian topography. Oil and gas were found recently in wells in the northwest corner of Pennsylvania; although these wells reached Cambrian rocks, they do not produce from a Trempealeau equivalent. In Ohio there have been good shows of oil and gas in crystalline dolomitic zones in Upper Cambrian rocks beneath the Middle Ordovician unconformity. Ordovician .—Gas from seeps in Hancock County, Ohio, was used by the first settlers. The first well, drilled near Findlay to a depth of 1,092 ft in the Trenton Limestone, is considered one of the most important discoveries. The field became known as the Lima-Indiana oil and gas field. The entire Trenton-Black River-Chazy section, 600 -ft thick, is ordinarily termed “Trenton.” Edward Orton determined that the exceptionally great porosity in the upper part, caused by the granular or crystalline character of secondary dolomite, controlled the accumulations. Noncommercial Trenton gas production has been developed in many directions from the Lima-Indiana field. Silurian .—In 1887, near Lancaster, Ohio, gas was discovered at 2,000 ft in a Lower Silurian zone arbitrarily named “Clinton.” Since that time, one of the most valuable gas fields in the world has been developed in Ohio along the western extent of this sandstone body, on the east flank of the Cincinnati arch. On the basis of sandstone thickness and continuity, virgin pressure, and delivery, this large gas field can be divided into three productive areas. In the northern part, production is from elongate lenses which overlap to such an extent that several have been found in most producing wells; in the southern extension of the northern area, the productive unit is a relatively continuous sandstone body averaging 22 ft thick; and in the unconnected eastern productive area, reservoir characteristics are more comparable to the southern than the northern fields. The Upper Silurian Salina Group contains a dolomitic unit, called “Newburg,” which yields gas and some oil in more than 20 small pools in the area of the “Clinton” fields. Accumulations are shown to be controlled by closed structure. Devonian .—The Oriskany Sandstone, which is 100-300 ft below the top of the “Big Lime,” is thought to have been deposited over a post-Silurian erosion surface. It is present as a fairly regular sandstone body in an area comprising 14 counties in eastern Ohio, and is productive of gas and oil in several of them. In Kentucky, Middle Devonian Onondaga Limestone is called “Corniferous.” It is a siliceous, magnesian limestone which locally in central Kentucky yields moderate quantities of gas in several fields, from a very porous zone in the upper part. “Shale gas” has been developed in several commercially important fields in southern Indiana, eastern Kentucky, southeast Ohio, and western West Virginia by fracturing the Upper Devonian brown or black bituminous shale with high explosives. Estimates of 400 million cu ft per well, ultimate yield, have been made. Sandstone, productive of gas and oil, is developed locally in the Devonian shale in western Pennsylvania, certain areas of eastern and southeastern Ohio, and West Virginia. At Macksburg, Ohio, a sandstone 350 ft below the top of the Devonian shale has yielded gas and some oil; this unit is correlated with the Gordon Sand of West Virginia. Mississippian and Pennsylvanian .—Shallow fields which produce from Mississippian and Pennsylvanian rocks were discovered in southeastern Ohio soon after completion of the Drake well. In eastern Ohio, eastern Kentucky, and western West Virginia, at least 15 units in the Mississippian and Pennsylvanian Systems have yielded gas in commercial quantities. Probably an equal number is present in the western part of the province. Of these, the most important are the First and Second Berea Sands in the basal part of the Mississippian. Gas production from none of these has had the commercial importance of reservoirs in older rocks.

Abstract The Big Sinking field is located high on the eastern side of the Cincinnati anticline and only 20 miles or so east of the outcrop of the Chattanooga shale and the Devonian limestone. The Silurian Lockport and Cayugan dolomites thin westward from about 600 feet in Magoffin County to extinction before the outcrop is reached, and the Devonian Hamilton limestone which is present at the outcrop thins eastward, is gone in the main part of the Big Sinking field, and if present at all west of Magoffin County is represented only by thin outliers. About 60 feet of strata are present between the Chattanooga shale and the green Silurian shales on the western edge of the field, perhaps a third of it being Hamilton and the rest Lockport, while 80-100 feet of strata occupy this position in the main part of the field, all of which is Lockport. Pre-Hamilton movements in the Cincinnati arch, with consequent stripping and leaching of the Silurian dolomites together with post-Hamilton erosion, were responsible for the creation of reservoir beds. This was not a case of oil accumulating on the side of a structure, but rather of the structure being responsible for the initial porosity and permeability of the formations lying unconformably below the Chattanooga shale. Silurian, and in some places even Ordovician, beds occupying this unconformable position have produced in various places around the Jessamine dome of the Cincinnati arch. The wells are long-lived and respond readily to repressuring with air and gas, although acidization, perhaps due to the dolomitic character of the reservoir rock, has been less successful.