U.S. Geological Survey banner and link to main Web site

CONTRIBUTIONS TO THE GEOLOGY OF KENTUCKY


SILURIAN SYSTEM

By Warren L. Peterson

GENERAL STATEMENT

The Silurian rocks crop out in Kentucky in narrow arcuate belts on the east and west flanks of the Cincinnati arch and in small isolated areas mostly in south-central Kentucky (fig. 3). The Silurian rocks are of marine origin and are composed of dolomite and shale and minor amounts of limestone and chert, with a total thickness ranging from 0 to 300 ft. The basal contact is a minor erosional unconformity in much of the outcrop area; elsewhere it is probably conformable or paraconformable. The upper contact is a regional erosional unconformity, and on the crest of the Cincinnati arch the entire Silurian section has been removed. The large variation in thickness is caused by erosion along this unconformity. The original thickness of the Silurian strata is not known, as the upper part has been everywhere removed.

Diagram showing area of outcrop of Silurian strata in Kentucky

FIGURE 3.--Area of outcrop of Silurian strata in Kentucky (shaded).

During the cooperative mapping program, five Silurian formations were recognized on the west side of the Cincinnati arch and three on the east. Only the basal unit, the Brassfield Dolomite, is present on both sides.

For the purposes of discussion, the outcrop area is separated into three parts: (1) the western outcrop belt, (2) the eastern outcrop belt, and (3) other exposures.

WESTERN OUTCROP BELT

The five formations in the western outcrop belt, from oldest to youngest, are (1) Brassfield Dolomite, (2) Osgood Formation, (3) Laurel Dolomite, (4) Waldron Shale, and (5) Louisville Limestone. On the State geologic map, the lower three are combined as one map unit (Slb) and the upper two as another (Slw). The Brassfield Dolomite is of Early Silurian age, and the other formations are Middle Silurian.

The total thickness of the Silurian within this outcrop belt ranges from 0 to about 180 ft, as a result of erosion along the superjacent unconformity. The rocks are thickest west of Mount Washington and thinnest at the southern end of the outcrop belt, where they pinch out. Because of this erosion, the Louisville Limestone is at the top of the Silurian section throughout the northern two-thirds of the outcrop belt, whereas older formations occur at the top in the southern part of the belt. This description of the western outcrop belt has been abstracted from a recent summary by Peterson (1981).

Brassfield Dolomite.--South of Mount Washington, the Brassfield Dolomite is thin-bedded gray dolomite that contains nodules and discontinuous beds of chert, which probably constitute less than 5 percent of the formation, and a few thin beds of shale in the upper few feet. Glauconite is common in both the dolomite and the chert. Fossils, which occur in both the dolomite and the chert, are sparse to abundant and are generally indistinct and poorly preserved; they include brachiopods, crinoid columnals, horn corals, and colonial corals. The thickness ranges from about 10 to about 30 ft where the entire formation is present.

North of Mount Washington, the Brassfield is much thinner and is composed of limestone, interbedded dolomite, and minor chert; here it has been called the Brassfield Formation. The limestone, which accounts for more than 50 percent of the formation, contains abundant crinoid columnals, brachiopods, bryozoans, and solitary and colonial corals. The dolomite is finely crystalline and unfossiliferous. The bedding tends to be irregular and disrupted. Glauconite is sparse to common in both the limestone and the dolomite. The Brassfield in this area ranges in thickness from 0 to about 10 ft but is commonly 2 to 3 ft thick.

The basal contact is an erosional unconformity of small relief, probably less than a few feet, over most of the western outcrop belt, but it may be conformable in a small area near Bardstown (Peterson, 1981).

Osgood Formation.--The Osgood Formation is composed of greenish-gray clay shale and gray clayey dolomite. Shale makes up about 90 percent of the unit, is commonly dolomitic, and weathers with a blocky fracture to a plastic clay; less commonly it is crudely fissile. The dolomite occurs in thin interbeds in the shale and is concentrated near the top and at the base. Where the entire unit is present the Osgood ranges in thickness from about 10 to about 50 ft. It is thickest near Bardstown and thinnest in the northern part of the outcrop belt. The basal contact appears to be gradational with the underlying Brassfield Dolomite through a few inches to a foot or so by interlayering of shale and dolomite south of Mount Washington, and is commonly sharp and apparently conformable to the north. Rexroad (1967), however, concluded on the basis of missing conodont zones that the Osgood and underlying Brassfield are separated by an unconformity.

Laurel Dolomite.--The Laurel Dolomite is composed 95 percent or more of gray dolomite with minor greenish-gray shale and sparse gray limestone. The Laurel is divisible into six remarkably persistent and distinct subunits throughout most of the outcrop belt (Peterson, 1981). The subunits are (1) a thinly interbedded dolomite and shale at the base, (2) a thin shale, (3) a vuggy-weathering dolomite, (4) a smooth weathering dense dolomite, (5) a thin-bedded dolomite, and (6) a dolomite oolite at the top. Fossils, which are sparse to abundant, are commonly strongly dolomitized and poorly preserved. Recognizable forms include crinoid stems and columnals, brachiopods, and trilobites. The Laurel has a minimum thickness of about 40 ft; maximum thickness ranges from about 55 ft in the northern half of the outcrop belt to 65 ft in the southern half, where it is overlain by Waldron Shale and thus was not eroded before deposition of the Devonian rocks. The basal contact with the underlying Osgood Formation is gradational by interlayering of shale and dolomite through several inches to as much as 3 ft.

Waldron Shale.--The Waldron Shale is composed of greenish-gray shale and minor gray dolomite; probably at least 95 percent is shale. The shale is dolomitic and weathers with angular fracture or crude fissility, eventually producing a plastic clay. The dolomite is clayey and occurs in irregular masses, lumps, and thin discontinuous beds. Fossils, which are sparse in both the shale and the dolomite, include brachiopods, crinoid columnals, gastropods, and bryozoans. The Waldron Shale ranges in thickness from 9 to 15 ft where overlain by the Louisville Limestone and not eroded prior to deposition of the Devonian sediments. The basal contact with the underlying Laurel Dolomite is conformable and sharp.

Louisville Limestone.--The Louisville Limestone is mostly thin-bedded gray dolomitic limestone and gray calcitic dolomite, commonly in lumpy or irregular beds. Shale, in partings and very thin beds, constitutes a few percent, and very sparse chert is present in nodules and thin layers. South of La Grange, most of the Louisville is finely crystalline calcitic dolomite; the sparse fossils are dolomitized and include crinoid columnals, brachiopods, horn corals, and colonial corals. North of La Grange, the Louisville is mostly a dolomitic limestone containing abundant, well-preserved whole and fragmental fossils, including horn corals, colonial corals, stromatoporoids, brachiopods, bryozoans, and crinoid columnals. The upper contact of the Louisville is a regional erosional unconformity or paraconformity. The maximum thickness, about 95 ft, occurs in a small area about 10 mi southwest of Mount Washington; it is thinner to the north and south and is completely removed below the unconformity in the southern part of the western outcrop belt. The basal contact with the Waldron Shale is conformable and generally sharp.

EASTERN OUTCROP BELT

Three formations were recognized in the eastern outcrop belt during the Kentucky mapping project: (1) Brassfield Dolomite (Lower Silurian), (2) Crab Orchard Formation (Lower and Middle Silurian), and (3) Bisher Dolomite (Middle Silurian). On the State geologic map, the two lower formations are combined as a single map unit (Scb), forming a sequence with dolomite at the base, clay shale at the top, and a thin gradational zone of interbedded dolomite and shale in between. The Bisher (Sb) is shown separately.

The Silurian rocks overlie Ordovician rocks with apparent conformity, although faunal studies have suggested that the uppermost Ordovician is not present (McDowell, 1983, p. 19). No evidence of erosion is discerned except for a conglomerate at the base of the Silurian composed of underlying Ordovician rocks near Berea (Weir, 1967). Rexroad (1967) has suggested that a hiatus exists at the base because of the thinness of the lowest Silurian conodont zone. The upper contact of the Silurian system is everywhere a regional erosional unconformity.

The total thickness of the Silurian within the outcrop belt ranges from 0 to 300 ft. It is generally thickest at the north and thinnest at the south end. This thickness range is the result of erosion along the superjacent unconformity.

Most of the information used in the following description of the Silurian rocks in the eastern outcrop belt has been abstracted from a recent summary by McDowell (1983); some is taken from various geologic quadrangle maps. McDowell has slightly revised the terminology used during the mapping program and on the State geologic map (fig. 4), but the nomenclature used herein is that shown on the map.

Brassfield Dolomite.--The Brassfield Dolomite is composed of thick- to thin-bedded gray dolomite with minor interbedded greenish-gray shale and, locally, chert nodules and lenses. The shale is more abundant in the upper part. Fossils are sparse to abundant and include crinoid columnals, brachiopods, horn corals, colonial corals, trilobites, pelecypods, and bryozoans. Gordon and Ettensohn (1984) have described the depositional environment of the Brassfield in eastern Kentucky.

The top of the Brassfield is distinct only south of Owingsville, Bath County, where the overlying Crab Orchard Formation is dominantly shale. North of Owingsville, dolomite beds are more abundant in the lower part of the Crab Orchard, and the top of the Brassfield, as characterized by fossils (the "bead bed" of some authors; see Gordon and Ettensohn, 1984, p. 105) at the type section, is not at a lithologic contact. Consequently, in the northern part of the outcrop belt a contact was mapped at the top of the dominantly dolomitic lower part of the Silurian section and the unit was labeled "Lower part of Crab Orchard Formation and Brassfield Formation" without specifying the top of the Brassfield. McDowell (1983) has subsequently modified the stratigraphic nomenclature to match the mappable units (fig. 4).

Chart showing recent revisions in Silurian nomenclature in Kentucky

FIGURE 4.--Recent revisions in Silurian nomenclature in Kentucky.

As shown in figure 4, the Brassfield Formation was reduced in rank to member, and a new formation, the Drowning Creek, was erected to reflect the units shown on the geologic quadrangle maps (McDowell, 1983, p. 7-11). The Drowning Creek Formation, described from exposures in Estill County, includes at the top dolomite members of the Crab Orchard Formation as used during the mapping program: Oldham Member south of Bath County and Dayton Dolomite Member to the north (see fig. 4). This revision separates the Silurian of eastern Kentucky below the Bisher Dolomite into two mappable units of lithologic homogeneity: predominantly dolomite below and predominantly shale above (McDowell, 1983, p. 18).

The Brassfield ranges in thickness from about 9 to 28 ft within the areas where the top can be definitely located and where it has not been truncated by the unconformity at the top of the Silurian.

Crab Orchard Formation.--The Crab Orchard Formation, as used during the mapping program, is composed mainly of greenish-gray clay shale with minor amounts of dolomite, which occur mostly near the base. In the southern part of the outcrop belt, south of Owingsville, the Crab Orchard is composed of five members, which are, from oldest to youngest, (1) Plum Creek Member, greenish-gray clay shale ranging from 2 to 12 ft in thickness, (2) Oldham Member, gray fossiliferous dolomite interbedded with greenish-gray clay shale, which ranges in thickness from 3 to 15 ft, (3) Lulbegrud Shale Member, greenish-gray clay shale with minor amounts of dolomite which ranges in thickness from 2 to 21 ft, (4) Waco Member, interbedded greenish gray clay shale and lesser fossiliferous dolomite which range from about 2 to 13 ft in thickness, and (5) Estill Shale Member, mainly greenish-gray clay shale, with a few brownish-gray, inch-thick, dolomite beds and lenses, which ranges in thickness from 60 to 170 ft. The Plum Creek, Oldham, and Lulbegrud Shale Members are not readily identifiable north of Owingsville, where the Waco Member pinches out.

The basal beds of the Waco reappear 2 to 3 mi northward, as the Dayton Dolomite Member, where they form the top of the dominantly dolomitic basal Silurian map unit now named the Drowning Creek Formation. The Estill Shale Member forms the upper part of the Crab Orchard in the northern part of the eastern outcrop belt.

The Crab Orchard Formation is conformable with the underlying Brassfield Formation, and the contact south of Owingsville is sharp or gradational through a few inches.

Bisher Dolomite.--The Bisher Dolomite (Sb) is a thick- to thin-bedded gray dolomite with locally abundant brachiopods, trilobites, gastropods, bryozoans, crinoid columnals, and scattered algal heads. The unit is everywhere truncated by the unconformity at the top of the Silurian. It has a maximum thickness of about 100 ft at the Ohio River and thins to pinch-out about 10 mi to the south; it reappears locally southward beyond the pinch-out as thin erosional remnants. The Bisher is conformable on the underlying Crab Orchard Formation with a sharp contact.

OTHER EXPOSURES

Outside the two main outcrop belts, that is, along the crest of the Cincinnati arch, the Silurian rocks are commonly missing (fig. 3) and Ordovician rocks are overlain unconformably by Devonian rocks. In some small areas, however, erosional remnants are exposed. West of the axis of the arch, these Silurian outliers are similar to the rocks found in the western outcrop belt, but commonly only the lowermost part of the section is preserved. The Brassfield Dolomite is preserved on the top of Jeptha Knob, located 20 mi southeast of La Grange, and the succeeding Silurian rocks up through the lower part of the Louisville Limestone may be present as well but are too poorly exposed to identify (Cressman, 1975). In a small area 6 mi southeast of Scottsville, Silurian rocks up through the Laurel Dolomite, and probably also through the Waldron Shale and the lower part of the Louisville Limestone, are preserved (Nelson, 1962). A section of Brassfield Dolomite 15 ft thick is present in an isolated outcrop in Boyle County (Moore, 1978). East of the axis of the arch, near Somerset, Silurian remnants are similar to the rocks in the eastern outcrop belt, but only the Brassfield and the lowermost part of the Crab Orchard Formation are exposed (Thaden and Lewis, 1965).

CORRELATIONS ACROSS THE JESSAMINE DOME

The Brassfield Dolomite is the basal Silurian unit on both the east and west sides of the Jessamine dome. The Osgood Formation, particularly in the vicinity of Bardstown, is a greenish-gray clay shale similar to the shales in the Crab Orchard Formation; these units may have originally been continuous across the Jessamine dome. The base of the Laurel Dolomite and the base of the Bisher Dolomite have been correlated across the dome by Berry and Boucot (1970).

DEPOSITIONAL HISTORY

The Silurian section is entirely marine. The dolomite and limestone are composed largely of whole and fragmented skeletons of animals that grew in warm, shallow, mildly agitated seas. These remains were deposited near where they lived. Fluctuations in the supply of terrigenous elastic material probably account for the shale units between the carbonate units; the source of these sediments was to the east (Freeman, 1951). The remarkable continuity of so many minor units indicates that the sea floor must have been generally stable throughout Silurian time and that a slow rise in sea level or a subsidence of the basin must have prevailed in order to accommodate the sediment buildup.

TECTONICS

Although the sea floor must have been generally stable during deposition of the Silurian sediments, some tectonic activity occurred during Silurian time. In the vicinity of Bardstown, the Brassfield Dolomite and the Osgood Formation thicken southward across a west-northwest-trending monocline; a small east-northeast trending syncline in which Brassfield sediments are thickened occurs about 10 mi south of La Grange (Peterson, 1981, p. 22, 23). These two structural features, which indicate movement during or before deposition, trend at large angles to the Cincinnati arch and do not appear to be related to that trend. The Silurian formations on the west side of the Jessamine dome do not appear to change systematically in thickness in outcrop, but those on the east flank of the Jessamine dome thicken eastward into the Appalachian basin. Apparently the east flank of the arch was subsiding more rapidly than the west flank during deposition of the Silurian sediments. Sometime after deposition of the Silurian sediments, downwarping occurred on both the east and west flanks and the Silurian sediments were eroded off the crest of the arch prior to the deposition of the Devonian sediments, producing a profound unconformity.

REGIONAL RELATIONS

The relationship of the Silurian stratigraphic units in Kentucky and adjacent states on the north, west, and east have been diagramed by McDowell (1983, fig. 18). Regional correlations of Silurian formations for the continental interior have been described by Berry and Boucot (1970). The five Silurian formations exposed on the west side of the Cincinnati arch in Kentucky extend north into southern Indiana, where the Osgood Formation and the Laurel Dolomite are considered members of the Salamonie Dolomite (French, 1967). Farther north, younger Middle and Upper Silurian formations which do not occur in outcrop in Kentucky are preserved above the Louisville Limestone (Berry and Boucot, 1970). On the eastern side of the arch, the Silurian formations extend north in outcrop into Ohio. The stratigraphic relations are shown by McDowell (1983, fig. 18). Silurian rocks are exposed on the west and south flanks of the Nashville dome in Tennessee but have been removed by pre-Devonian erosion on the east flank. All five Silurian formations occurring on the west flank of the Cincinnati arch in Kentucky are identified in northern Tennessee (Wilson, 1975), but in Tennessee the Louisville Limestone is called the Lego Limestone. Farther to the southwest in Tennessee, Upper Silurian limestones and shales of the Wayne and Brownsport Groups, which do not crop out in Kentucky, are preserved at the top of the Silurian section (Wilson, 1949).

NEXT   ||   PREVIOUS   ||   Return to STRATIGRAPHY


This page is https://pubs.usgs.gov/prof/p1151h/silurian.html
Created 01-09-01
Maintained by Eastern Publications Group Web Team