CONTRIBUTIONS TO THE GEOLOGY OF KENTUCKY
Geologic features related to the location and distribution of mineral resources in Kentucky include the Cincinnati arch, the Appalachian and Illinois basins, the Mississippi Embayment, the Ohio River Valley, faulting, erosional unconformities, and paleovalleys.
The economic importance of Carboniferous rocks has been summarized in a recent report by Rice, Sable, and others (1979, p. F24-F28). All of Kentucky's coal production is from Pennsylvanian strata; Mississippian age rocks are the principal petroleum reservoirs and sources of limestone and fluorspar; and natural gas has been obtained mainly from Devonian and Mississippian rocks.
The following discussion of the mineral resources of Kentucky is presented by the age of the rocks in which they occur.
Dolomitic limestone of the Oregon Formation and the Tyrone Limestone were once used as building stones for central Kentucky residences and commercial and public buildings. Because the dense, finely crystalline dolomitic limestone of the Oregon Formation could take a high polish, it was sometimes referred to in trade circles as "Kentucky River marble."
Thin-bedded limestones of Late Ordovician age also were sources of building stones in central Kentucky. Referred to by local builders as "creekstone" and "fieldstone," these micrograined to coarsely crystalline, locally fossiliferous gray limestones have been used in a rough state for veneer, flagging, and numerous farm fences. The beds of limestone are usually separated by partings or layers of shale, thus allowing the stone to be obtained in thin, irregular slabs. Stone came from dozens of small quarries, creekbeds, and weathered slabs scattered across agricultural land.
Ordovician rocks are also targets for oil and gas exploration. Production has been obtained, principally in south-central Kentucky, from stratigraphic correlatives of the Leipers and Lexington Limestones and carbonates of the High Bridge Group, as well as the deeper Knox Dolomite, which is not exposed at the surface. (The deepest production in the State, in terms of both depth and geologic age, is from a single well in Cambrian sandstone at a depth of 7,598 ft in the Mavity Pool in Boyd County, northeastern Kentucky.)
The outcrop of Middle Ordovician rocks coincides roughly with the area referred to by some writers as the Central Kentucky mineral district (Jolly and Heyl, 1964). Faults and related fractures and joints associated with the Lexington and Kentucky River fault systems are the focal points for numerous vein deposits of barite, calcite, fluorite, galena, and sphalerite. The veins are generally thin and the ore is commonly a mixed variety. Limited mining has taken place. Very little mineralization has been noted in rocks of Late Ordovician age in central and north-central Kentucky, but a few mineral veins are exposed in Late Ordovician carbonates on the Cumberland saddle portion of the Cincinnati arch in south-central Kentucky.
Fertility of the residual soils in the Inner Bluegrass region is enhanced by the presence of phosphate in the Lexington Limestone. This area is the locale of the horse-farm industry of central Kentucky. Phosphatic rock was mined on a small scale approximately 60 years ago, but the deposits are limited and land is more valuable for agricultural, industrial, and residential uses.
Upper Ordovician strata of north-central Kentucky, particularly the Kope Formation, contain soft, easily deformed shale. Oversteepened slopes, artificial cuts, and highway fill containing quantities of shale are subject to slump and sliding.
The Louisville Limestone and the Laurel Dolomite, which crop out on the west side of the Cincinnati arch, principally in Bullitt, Jefferson, Nelson, and Oldham Counties, have been sources of stone for aggregate, building stone, and agricultural limestone. On the east side of the arch, the carbonate rocks are too thin, too soft, or too impure for construction stone.
Shales of the Crab Orchard Formation on the east side of the Cincinnati arch and of the Osgood Formation on the west side of the arch are located near potential markets but appear to offer little possibility for structural clay products. The presence of calcium carbonate, gypsum, or soluble salts cause abrupt overfiring, bloating, scumming, and short firing ranges. Crab Orchard shales in northern Lewis County near the Ohio River appear to be relatively free of these deleterious materials and offer the greatest possibility of any of the Silurian deposits.
Hematitic iron ore occurs in the upper part of the Brassfield Formation in the Rose Run area of eastern Bath County. Although the Brassfield is exposed on both flanks of the Cincinnati arch, the only minable iron deposits were in the Rose Run area. Mining took place in the late 1800's and early 1900's. A drilling project by the U.S. Bureau of Mines on four tracts in eastern Bath County indicated that ore beds were not continuous and that the iron ore can be considered as a reserve for future use only when the high-grade and medium-grade ores and more extensive low-grade ores of other districts have been exhausted (Muir, 1950).
Sphalerite, occurring as sparsely distributed nodules and fracture and cavity fills, has been recognized in the Bisher Dolomite in Lewis County, northeastern Kentucky (Fohs, 1912). Deposits disclosed to date appear to have little, if any, commercial potential.
This geologic unit is classified as a low-grade oil shale. When fresh Devonian shales are heated to 900oF, they give off a gas similar to natural gas and an oil not much different from conventional crude oil. Assays by the modified Fischer retort method conducted by the Institute for Minerals and Mining Research, Lexington, indicate from 0 to more than 20 gallons of oil per ton of shale.
The Devonian black shales are also known to be radioactive, typically containing 15 to 30 parts per million uranium (Provo and others, 1977). The shales are generally silty and nonplastic and are not used for ceramic purposes, but they have been used locally for fill and for a base course for road construction.
Petroleum has been produced from Devonian carbonates and sandstones at a number of localities in Kentucky, some from the Boyle Dolomite (Limestone) and Jeffersonville Limestone and some from formations that are not exposed at the surface. A conspicuous unconformity is present beneath Devonian strata along the flanks of the Cincinnati arch. On the outcrop, Devonian rocks may rest on various formations in the Silurian or Upper Ordovician. The hiatus, due to erosion or nondeposition or both, may represent as much as 250 to 300 ft of stratigraphic section. Recognition of these unconformable relations is essential to planning exploration and development programs for several potential resources. Some of the older oil production attributed to the Devonian may in reality have been from the Silurian.
Generally, the carbonate rocks of Devonian age in Kentucky are too thin for exploitation for construction stone, although the Jeffersonville and Sellersburg Limestones have been quarried with subjacent Silurian strata at a few sites on the west side of the Cincinnati arch.
In the late 1800's, iron ore was mined from the Boyle Dolomite near Preston, southern Bath County. The deposit was reportedly exhausted before the end of the century (McFarlan, 1943, p. 435).
Small deposits of high-alumina, halloysite-type clay are present locally at the Silurian-Devonian unconformity, but to date commercial quantities have not been identified.
Principal uses of the limestone are in the construction, agricultural, and cement industries, but Mississippian limestones have been used for flux stone, rock dust for underground coal mines, and lime.
At one time, Kentucky supported an active building-stone industry, the largest production coming from Mississippian-age limestones and sandstones or siltstones. The Girkin Limestone of western Warren and northeastern Logan Counties was quarried in large blocks at several sites for subsequent cutting and sculpturing. The Farmers Member of the Borden Formation was the most widely used sandstone for building purposes in Kentucky. The principal outcrop area of the Farmers is in Rowan and Lewis Counties, northeastern Kentucky. Although large reserves of the building stones remain, there is little or no production at the present time. However, small flagstone quarries in Chesterian sandstones (mainly Hardinsburg) have operated intermittently in recent years in western Kentucky.
It is estimated that 70 percent of the petroleum and 40 percent of the natural gas produced in Kentucky have been from Mississippian rocks (L.R. Ponsetto, oral commun., 1982). Stratigraphic and structural traps characterize the occurrences. The principal reservoirs are sandstones of Chesterian age and porous zones in Meramecian-age limestones, and their stratigraphic equivalents. Petroleum and natural gas from Lower Mississippian rocks have been obtained from the Berea Sandstone, the Farmers Member of the Borden Formation, and limestones associated with the Fort Payne Formation.
Chesterian sandstones along the southeastern rim of the Eastern Interior (Illinois) basin are bitumen impregnated at a number of localities. Largest deposits appear to be in the Big Clifty Member of the Golconda Formation. The tar sands have been mined at several sites for paving and road-surfacing material. Much attention is currently being given to these deposits for their potential as an alternate source of fuel raw material.
At other localities around the Illinois basin, high-silica Chesterian sandstones were quarried in the past for foundry, glass, and molding sands.
More than 3 million tons of finished fluorspar and smaller quantities of zinc, lead, and barite have been recovered from vein deposits in Mississippian strata in the complexly faulted western Kentucky fluorspar district. The area has been the second largest producer of fluorspar in the United States (Trace, 1974a). The highest grade and thickest vein deposits occur where one fault wall is in the Ste. Genevieve Limestone, although ore does occur where the walls are in the St. Louis or lower Chesterian units (Trace, 1979).
Mississippian shales are used, or have been used, for the manufacture of face brick, drain, quarry, and roofing tile, and lightweight aggregate. The largest production has come from the New Providence Member (and its stratigraphic equivalents) of the Borden Formation.
Limonitic and sideritic concretionary iron ores from the Borden Formation were mined on a limited scale in
the 1800's for smelting at small, local furnaces (McFarIan, 1943, p. 435, 436).
Subsurface stratigraphic records in northwestern Kentucky indicate the presence of gypsum and anhydrite
deposits in small restricted basins in the lower St. Louis Limestone at potentially minable depths (McGrain
and Helton, 1964). Deposits of commercial thickness have not yet been demonstrated in Kentucky.
The Mammoth Cave region, situated in the outcrop area of Mississippian carbonates, is classic in geologic literature because probably no part of the United States rivals this area for the number of well-known large and spectacular caverns and underground solution features. The Mammoth Cave-Flint Ridge cave system in Mammoth Cave National Park has more than 200 mi of measured and charted passageways and has been visited by millions of tourists. Hundreds of smaller, less known eaves are present in the Mississippian outcrop area on the east side of the Cincinnati arch between Carter and Wayne Counties.
Clay shales of the Borden Formation and the Chesterian Series become plastic when wet and make unstable foundations for roads and various structures constructed on them. Oversteepened slopes and fill containing quantities of these shales are subject to slump and slide.
The eastern Kentucky coal field covers 11,000 sq mi and is part of the larger Appalachian coal-bearing region. It contains both high- and low-sulfur coals, which have been used for steam-plant fuels and metallurgical coke. Principal production has been from the Elkhorn and Hazard coal zones in the Breathitt Formation. Pike and Harlan Counties have been the largest producers.
The western Kentucky coal field is in the southeastern portion of the Illinois basin and covers 6,400 sq mi in northwestern Kentucky. The coals are predominantly a higher sulfur variety and most of the production is used by electric utilities. Principal production has been from beds in the Carbondale Formation, and Muhlenberg and Hopkins Counties are the leading producers.
Coal-field clays and shales are another important resource. In Kentucky they are used for face and fire brick, field, floor, and sewer tile, and flue linings. The Olive Hill Clay Bed of Crider (1913b) in the basal part of the Breathitt Formation in northeastern Kentucky was the main source of refractory raw material for a once-major firebrick industry in that part of the State. Although Pennsylvanian shales in Kentucky are not used for the manufacture of lightweight aggregate, some of the shales possess excellent bloating characteristics and are present in areas where other natural aggregates are not present. A high-alumina, halloysite-type clay is present at the Mississippian-Pennsylvanian erosional unconformity in Hart County; deposits are small and exploitation has been minor.
Pennsylvanian rocks have been sources of petroleum and natural gas for many years in Kentucky, but the supplies have not been large, contributing an estimated 5 percent or less of the State's total. Principal reservoir rocks have been quartzose sandstone strata of the Lee Formation in eastern Kentucky and the Caseyville Formation of western Kentucky.
Large deposits of bitumen-bearing, basal Pennsylvanian sandstones are present near the southeastern tip of the western Kentucky coal field, and smaller deposits are present in eastern Kentucky on the Paint Creek anticline and on the western edge of Carter County. Occurrences are associated with faults, paleovalleys, and stratigraphic traps. Development to date has been from surface and near-surface deposits for almost exclusive use as a paving material, but there has been considerable interest recently in the sandstones' potential as an alternate source of energy raw material.
Pennsylvanian sandstones are being used, or have been used, for construction aggregate, flagstone, rough building stone, and miscellaneous industrial sand and in the manufacture of ferrosilicon.
During the 19th century, block and kidney iron ores of the Breathitt Formation and "limestone ores" found at the Mississippian-Pennsylvanian unconformity were used by a number of small iron furnaces in northeastern, east-central, and western Kentucky.
Weathering of massive, cliff-forming sandstones of Early Pennsylvanian age has produced some of the most rugged and picturesque landscape features of the Commonwealth. Precipitous cliffs, craggy pinnacles, rocky gorges, rock shelters, natural bridges, cascading waterfalls, and other natural features abound in the areas of outcrop of these rocks and have been the focal points for the development of a number of State and National parks and other recreational areas.
Decomposed kimberlite-like material from the intrusive igneous plugs of Elliott County has yielded a variety of minerals. The U.S. Bureau of Mines has identified garnet, tourmaline, hornblende, magnetite, ilmenite, chromite, titanite, rutile, actinolite, olivine, staurolite, quartz, mica, feldspar, zircon, topaz, monazite, xenotime, and fluorite (M.V. Denny, written commun., 1963). Quantitative determinations to evaluate the deposit as a possible source of industrial abrasives and rare earths have not been made.
The igneous dikes of western Kentucky are of economic interest because of zinc concentration at their intersections with mineralized veins. Examples are the Commodore and Old Jim mines in Crittenden County and the Hutson mine in Livingston County.
The Tuscaloosa Formation, the basal unit of the Cretaceous sediments along the eastern edge of the Mississippi Embayment area of western Kentucky, consists largely of well-rounded, waterworn, nonmarine pebbles and boulders derived from Mississippian and Devonian cherts. Gravels from the Tuscaloosa were once used for roads, railroad ballast, and other construction and building purposes in several western Kentucky counties, but because of the presence of deleterious chert they are not currently used in significant quantities for concrete aggregate, being replaced by crushed limestone. Attempts have also been made to use the Tuscaloosa for fluxing silica, but the results have not always been satisfactory.
Deposits of soft, earthy, very fine, tripoli-like material of variable thickness occur at the Paleozoic Cretaceous unconformity and in the lower part of the Tuscaloosa Formation. Apparently derived from the disintegration of Devonian and Mississippian cherts, the tripolitic earth has a silica content lower than commercial tripoli but may be considered a reserve when richer deposits elsewhere are depleted.
Iron-bearing minerals, commonly limonite, are sometimes found at the contact of Tuscaloosa gravels and Mississippian carbonates. The iron deposits, which appear to be a bog ore developed on the pre-Cretaceous erosional unconformity, were the source of raw material for several furnaces that were active in Trigg, Lyon, and Livingston Counties a century ago (McFarIan, 1943, p. 436). The deposits are not considered an economic resource at this time.
Silica sands from the MeNairy Formation have been mined and processed for use in foundries and
metallurgical industries, for construction and use as abrasives, in the manufacture of glass, and for
miscellaneous industrial and recreational purposes. Unprocessed, or pit-run, sand from both Cretaceous and
Tertiary deposits is used for subfoundation fill and building-site preparation.
Since the early 1900's, western Kentucky has been a leading producer of ball clay, which is used in the
manufacture of whiteware, sanitary ware, artware, enameling, structural products, and refractories and in
plasticizers and fillers (Olive, 1980). Most of the pits have been in the Claiborne Formation (Eocene) in
Graves County and adjacent counties. Numerous undeveloped clay deposits in the Mississippi Embayment
sediments have potential for structural clay products, stoneware, and low-heat-duty refractories and for the
manufacture of white portland cement.
Another well-known clay-bearing formation in western Kentucky is the Porters Creek Clay (Paleocene). Because of poor drying and firing characteristics, the Porters Creek is generally not suited for ceramic products, but its absorbent properties make it a potential source of raw material for floor-sweep compounds, animal litter, insecticides, and fungicides (McGrain, 1965).
Clay from the "Irvine Formation" (high-level fluvial deposits) in east-central Kentucky has been the source of raw material for a small but well-known pottery in Madison County for many years.
Brown chert gravels of continental deposits, referred to as "Lafayette gravel," blanket most of the upland areas of the eight-county Jackson Purchase region, cap hills and ridges of the adjacent Mississippian Plateau, and occur as small, discontinuous, high-level fluvial or high-terrace deposits on upland surfaces near some of Kentucky's larger rivers. These gravels have been mined from hundreds of pits for fill, base material for paved roads, and road metal for secondary roads. Lesser amounts have been used as mulch at the base of buildings and as roofing gravel. The content of lightweight and porous chert has rendered the gravel unsatisfactory for most current specifications for cement concrete.
Sands underlying the central and western parts of the Jackson Purchase region contain sufficient ground water to supply all domestic and many industrial and public needs. Maximum yields of individual wells may be in excess of 500 gallons of water per minute.
Other potential economic resources in the Mississippian Embayment sediments include lignite and heavy minerals.
Sand and gravel are obtained from the valley of the Ohio River from land-based pits in fluvioglacial and alluvial deposits and from the channels of the Ohio and Mississippi Rivers by floating operations from bars, islands, and other channel and bedload deposits in the streams themselves. Combined, these sources constitute more than three-fourths of the sand and gravel produced in Kentucky; floating dredge operations provide the largest amount.
Most of the active, inactive, and abandoned land-based sand and gravel pits along the Ohio River between Ashland in northeastern Kentucky and Henderson in northwestern Kentucky are in glacial-outwash terraces of Wisconsinan age, discontinuously located (within Qg) along the meandering valley. The outwash deposits have been mapped variously as Wisconsinan outwash, terrace deposits, fluviatile deposits, older alluvium, and alluvium. By judicious use of 1:24,000-scale geologic quadrangle maps, a geologist can determine areas that might be underlain by potentially commercial sands and gravels.
Sands and gravels are also found in numerous stream channels within the State. Although used locally for fill and for surface material for public and private secondary roads, neither the size of the deposits nor the quality of materials classify them as major sources of construction aggregates. The composition and nature of the deposits are varied, reflecting the diversity of rock types through which the streams flow. Chert and fragments of soft siltstone are common constituents of stream gravels in much of Kentucky, and because of this the gravels are generally excluded from use in concrete aggregate. Deposits at any one locality are not large, but supplies are replenished as floods bring rock debris from headwaters.
Alluvial and residual clays were the earliest sources of brick clay in Kentucky and have been used, or are being used, at scattered localities for the manufacture of brick, field tile, and portland cement.
The Ohio River Valley, which for 664 mi marks most of the northern and western boundaries of Kentucky, is one of the most important ground-water provinces in the State. The alluvial and fluvioglacial deposits of the valley constitute one of the largest and most used aquifers in the Commonwealth, as is attested by the urban and industrial development from near Ashland, where the Ohio River first touches Kentucky, to Wickliffe, at the river's mouth. Pumping capacity of individual wells may be as great as 800 gallons of water per minute.
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