SILURIAN

(After Foerste, 1931, p. 173.) * Centerville (Foerste, 1931).-The type section is at Centerville, Ohio, and consists of 2½ feet of indurated clay formerly regarded as the top of the Elkhorn. The fauna consists of a number of Silurian forms associated with others of an Ordovician aspect. Foerste correlated it provisionally with the Edgewood of Illinois. This horizon has not been identified in Kentucky but is known at West Union, Ohio, six miles north of the Ohio River.

The Silurian outcrops in a narrow band on the eastern and western margin of the Blue Grass. Along the southern margin it is overlapped from both the east and west by the Mid-Devonian limestone (Boyle-Jeffersonville), which, along the axis of the Arch in Boyle County, rests on the Ordovician as low as the Tate. Locally also the Boyle limestone is missing and the Ohio shale rests on the pre-Devonian.

EAST SIDE OF CINCINNATI ARCH

Brassfield.¹—The Brassfield is the only one of the Silurian formations which has been identified on both sides of the Arch. It is Orton's Ohio Clinton, named by Foerste (1906) and correlated with the Cataract (Alexandrian). The type section along the Louisville and Atlantic Railroad between Brassfield and Panola in Madison County, Kentucky shows:

 

Feet  Ferruginous limestone............................................... 1.5 Horizon with large crinoid beads and Whitfieldella subquadrata Irregularly bedded limestone with very little clay.................... 2 Clay, forming from one-half to three-fourths of the section, interbedded with thin limestone.............................................. 2.3 Limestone, irregularly bedded with thin clay partings................ 8.2 Very massive bluish limestone, apparently unfossiliferous............ 6 20.0

Northward it thickens to 40 or even 50 feet in Adams and Highland counties, Ohio, and then thins westward into Indiana. In the southern Blue Grass the Brassfield thins westward and is eliminated from the section near Stanford, where the Devonian overlaps on the Ordovician. A small outlying remnant occurs on Scrub Grass Creek, 3 miles southwest of Mitchellsburg in western Boyle County, Kentucky. Small discontinuous areas occur along the Cumberland River about 15 miles west of Burnside and on Fishing Creek west of Somerset. On the west of the Arch the Brassfield has a thickness of about 5 feet in Trimble and Oldham counties, 3 feet east of Louisville (7 feet locally), and somewhat more in Nelson and western Marion counties.

The formation consists of medium to coarsely crystalline grey to pink dolomitic limestone, commonly sandy in appearance and weathering brown. The lower part is usually massive. Above, it is relatively thin bedded, is associated with shale, and is more fossiliferous. Large characteristic crinoid "beads" one-half to 1 inch in diameter, many with a cogwheel (scalloped) outline, occur near the top. This is overlain by the zone of Whitfieldella subquadrata. Whitfieldella quadrangularis occupies the same horizon in Adams County, Ohio. The top layers are often ripple marked. The layers above the crinoidal zone are frequently ferruginous from Clay City northward, and near Olympia in Bath County carry the Clinton type of flaxseed (oolitic) hematite ore. The lower Brassfield is very cherty from Bath County northward, and a cherty phase is also developed near Bardstown. On Jeptha Knob in Shelby County Silurian dolomites, resting with angular unconformity on the Ordovician, contain lenses of breccia showing a mixture of Brassfield and Ordovician fossils.

The Brassfield is rather sparingly fossiliferous in the southern area of outcrop in central Kentucky, but is quite fossiliferous in Ohio and Indiana and adjoining parts of Kentucky. The zone of large crinoid "beads" and the overlying Whitfieldella beds are characteristic of the uppermost layers in the eastern area of outcrop. The rather thin-bedded zone below has yielded the greater part of the fauna in which brachiopods, bryozoans, and corals are most common. The more common and consistently present forms include:

Camarotoechia neglecta Dalmanella elegantula Leptaena rhomboidalis Orthis flabellites Platystrophia daytonensis Platystrophia reversata Sowerbyella transversalis

Strophonella daytonensis Rhinopora verrucosa Enterolasma caliculum Zaphrentis celator var. daytonensis Cyclonema daytonense Calymene niagarensis Calymene vogdesi

It is a widely distributed formation with fauna of southern origin and is known from Oklahoma, Arkansas, and Alabama through to Indiana, Illinois, western New York, and Ontario (Foerste, 1935). At Birmingham, Alabama that part of the Red Mountain formation below the Big seam, including the Irondale seam and other seams of "Clinton ore," is Brassfield (Butts, 1927).

"Approximately the same horizon is exposed by the Sexton Creek limestone in southwestern Illinois and adjacent part of Missouri, and by the Kankakee limestone in northeastern Illinois. In southern Ontario the faunas most similar to the Brassfield of Ohio are those of the Cabot Head shale and of the underlying Manitoulin dolomite, the bryozoans being more common in the shale though most species are common to both" (Foerste, 1935, p. 189).

Plum Creek Shale (Foerste, 1906).—The Plum Creek is a local shale formation with a little interbedded limestone and about 5 feet thick known from the vicinity of Indian Fields (Clark County) north to Clay City in Powell County. Both to the north and south it loses its identity, as limestone becomes more common. Fossils are few.

Oldham limestone² (Foerste, 1906).—The Oldham is thin to medium and unevenly bedded magnesium limestone interbedded with blue shale. Many surfaces show fucoidal markings. Selenite crystals occur in some of the shales.

The formation is less well-marked southwest of Berea because of the lack of fossils and change in lithology. It has been traced north to near Owingsville. At Ribolt, 10 miles west of Vanceburg in Lewis County, the limestone overlying the Brassfield is regarded as Dayton (Foerste, 1935).

In its typical development the top of the limestone is characterized by abundant Stricklandinia norwoodi. Foerste (1935) called attention to the occurrence of the Pentamerus zone (Dayton) above the Stricklandinia zone at Birmingham, thus suggesting the relative ages of the two. They are unknown in the same section in Kentucky. Thickness, 10 to 15 feet.

Alger (Foerste, 1906).—The remainder of the Crab Orchard is composed of the Lulbegrud shale, Waco limestone, and Estill clay. To the north of Powell and Clark counties the Waco limestone disappears as a recognizable unit and the undifferentiated shale succession was referred to by Foerste as the Alger. The upper part of these shales is fossiliferous, and has since been differentiated as the Ribolt.

Lulbegrud clay (Foerste, 1906).—A blue unfossiliferous shale about 13 feet thick in exposures on Lulbegrud Creek, where it forms the boundary between Powell and Clark counties. At Crab Orchard it is only 6 feet thick. Its northern extent and differentiation from the Estill clay is determined by the northern limit of the Waco limestone. Ground water in it is often impregnated with epsom salts, and selenite crystals are not uncommon.

Waco limestone (Foerste, 1906).—A magnesian limestone ranging up to 10 feet thick. The base is marked by a one- to two-foot heavy layer succeeded by interbedded thin limestones and shale. Miller (1919) mentioned radial fucoid markings as characteristic. It has not been differentiated in the section except in Madison, Clark, Powell, and Estill counties.

The Waco is quite fossiliferous. The fauna consists largely of corals and resembles the Dayton in this respect. It does not resemble other known Clinton faunas. Characteristic forms include Strombodes mamillaris var. distans, S. granulosus, Favosites gothlandicus, Lindstromia lingulifera, Heliolites spongiosus, and Meekopora bassleri.

Estill clay (Foerste, 1906).—The Estill Clay consists of nonfossiliferous blue shales, which locally attain a thickness of 100 feet. At the type locality near Estill Springs the thickness is 56 feet. In the absence of the Waco limestone it is part of the undifferentiated Alger. From Clark and Madison counties south it is overlain by the Devonian.

Ribolt (Foerste, 1931).—In Lewis County a clay shale at least 100 feet thick overlies the Dayton limestone. Thin indurated layers, often only one-fourth inch thick, are interbedded with the shale in the upper part. For this upper part with its distinctive Mastigobolbina typus fauna the name Ribolt was proposed. For the lower shales the term Alger is retained. The fauna, characteristic of the middle part of the upper Clinton is well known in Virginia, West Virginia, Maryland, and Pennsylvania (Foerste, 1931). It is 30 feet thick in Lewis County, is known to be present in Ohio, but not farther south in Kentucky.

Bisher (Foerste, 1917).—The Bisher is typically developed in Adams and Highland counties, Ohio; is known in Lewis County, Kentucky, and in outcrop as far south as the northern edge of Fleming County. It is an impure grey even-bedded rather fine-grained granular dolomite, which weathers to a yellow and brown "sandy" rock. Fossils are commonly present as moulds. There are interbedded crinoidal layers. A fossiliferous zone about 10 feet above the base is characterized by Whitfieldella cylindrica. Associated with it are Stropheodonta plana, Schuchertella proserri, Spirifer radiatus, Atrypa elongata, Camarotoechia roadsi, Dalmanites limulurus var. brevicaudatus, and Bumastus ioxus.

Lilley (Foerste, 1917).—This is a medium- to coarse-grained rather soft grey dolomite, much of it highly crinoidal. It weathers brown. Bedding planes are rather uneven and hackley. Many layers contain fragments of Cladopora which is distinctive. Corals occur in considerable numbers, particularly at the base. Halysites and a small Stromatoporoid are common. Holophragma calceoides is common in a shale zone near the top.

A number of corals including Coenites verticillatus, Cystiphyllum niagarense, Omphyma verrucosa, Plasmopora follis, and Strombodes striatus suggest a Louisville (Lockport) age. It is not known in outcrop in Kentucky but has been identified (McFarlan, 1938d) as an oil producing formation in the Irvine oil field, Estill County. Diamond cores have shown a thickness of 43 feet, but there is no reason to believe the formation was drilled through.

Peebles (Foerste, 1929).—The Peebles has been traced south to the Ohio River in Adams County and adjoining parts of Kentucky. It is unknown in outcrop farther south but has been recognized in drill cores from the Irvine field, Estill County, where it is an outstanding producer of petroleum, and with the Lilley constitutes the "Corniferous" pay (McFarlan, 1938d). Here it has a known thickness up to 30 feet. The abundant asphaltic residue in the Peebles at the Basic Products Quarry east of Peebles, Ohio, is in keeping with this.

It is fine-grained hard grey dolomite weathering light grey. A rather characteristic feature is a texture involving an irregular patchy distribution of fine to dense and somewhat coarser grained patches that are petroliferous, In the eastern end of the Irvine oil pool a sandstone facies is developed in the lower part.

The formation is highly fossiliferous and has been referred to as the Pentamerus zone. The upper part contains a Guelph assemblage including Megalomus canadensis, Trimerella ohioensis, Pycnostylus guelphensis and various gastropods (Foerste, 1935). Halysites is common as are also Stromatoporoids. It has been regarded as Cedarville but was correlated by Foerste (1931, 1935) with the Guelph and would thus overlap the Cedarville.

UPPER SILURIAN

The Cayugan is unrepresented in the exposed section in Kentucky, nor is it shown in the drill cores of the Irvine pool mentioned above. However, the limestone section between the Ohio shale and Crab Orchard thickens eastward from a few feet to 500 and 600 feet in Magoffin and Lawrence counties. With the presence of the Cayugan and Helderberg in the section where these beds again come to the surface at Big Stone Gap, Virginia, the Cayugan would seem to be present under the Eastern Coal Field. Ballard (1938) has recently recognized both Cayugan and Helderberg in the subsurface section in Wolfe County and eastward, thus completing the picture of the Mid-Devonian overlap. He also described a sandstone (Big Six oil sand) at the base of the Cayugan, which he regarded as erosional debris occupying the position normally held by the Niagara dolomite.3 It has a known range in thickness up to 60 feet and is overlain by dolomite with anhydrite scattered through it.

WEST SIDE OF CINCINNATI ARCH

Brassfield.—Described on page 35.

Osgood (Foerste, 1897).—The Osgood formation includes the beds between the well-defined Brassfield below and Laurel above.

In Jefferson County Butts recognized four divisions with a total thickness of about 30 feet.

(d) Upper shale, 2 to 3 feet of soft, fairly fissile, greenish grey shale,
(c) Five to 8 feet of fine-grained magnesian limestone in one-foot layers,
(b) Twelve to 20 feet of coarse, lumpy, grey, calcareous and magnesian shale,
(a) About 2 feet of limestone much like that of the Brassfield but resting disconformably on it.

The divisions are persistent. The Osgood itself is known from Tennessee to southern Indiana, and in Kentucky may be traced south into Nelson County.

Fossils are present but not to the same extent that they are present in the region of the type locality in Indiana. The rather limited fauna that Butts found in Jefferson County includes:

Enterolasma caliculum Favosites cristatus Striatopora sp.? Holocystites cf. parvulus Eucalyptocrinus caelatus? Stephanocrinus gemmiformis Hallopora elegantula Pachydictya crassa

Atrypa reticularis Dalmanella elegantula Leptaena rhomboidalis Orthis flabellites Rhipidomella hybrida Rhynchotreta cuneata var. americana Sowerbyella transversalis Spirifer niagarensis Spirifer radiatus Diaphorostoma niagarense Dawsonoceras annulatum Calymene niagarensis

The Osgood seems to be the equivalent of the Rochester of New York. "The nearest approach to the Osgood formation on the east side of the Cincinnati anticline appears to be the Bisher formation, but faunally this Bisher formation is related more closely to the Rochester of New York, Pennsylvania, and Maryland than to the Osgood of Indiana." (Foerste, 1931, p. 178).

Laurel (Foerste, 1896).—The Laurel is a bluish to light grey medium fine-grained heavy-bedded dolomitic limestone. Analysis shows more than 30 per cent magnesian carbonate. The upper part is in evenly bedded layers about 1 foot thick, which are particularly suitable for building purposes. The lower layers are less even and less compact. It outcrops southward as far as southwestern Nelson County. Thickness, 35 to 45 feet.

Few fossils are found in the Kentucky Laurel. The species listed below are the known forms that have been found in Nelson and Jefferson counties (Butts, 1915; Foerste, 1931).

Striatopora gorbyi Eucalyptocrinus crassus Stephanocrinus gemmiformis Atrypa reticularis Camarotoechia whitii Dalmanella elegantula Schuchertella subplana Spirifer (Delthyris) crispus Uncinulus stricklandi

Diaphorostoma niagarense Dawsonoceras annulatum Calymene niagarensis Dalmanites limulurus

The formation reappears in outcrop in Sumner and Wayne counties, Tennessee. Near St. Paul, Indiana, an upper 5- to 15-foot zone is much more fossiliferous. The crinoids show a closer relationship to the Beech River division of the Brownsport of western Tennessee than to the Racine of northern Illinois and southwestern Wisconsin (Foerste, 1931). One-third of the genera are listed from the Silurian of Gotland and England. Dalmanites limulurus, an excellent Rochester guide fossil, occurs in the lower part.

Waldron shale (Elrod, 1883).—The Waldron is a highly calcareous and magnesian shale, greenish grey, coarse-textured, nonfissile, and breaks into irregular pieces or lumps. On weathering it disintegrates readily into a greenish clay. It is richly fossiliferous in the Waldron, Indiana, region and again near Newsom, Tennessee, but sparingly so in Kentucky. Thickness, 10 to 12 feet.

Two-thirds of the fauna is composed of brachiopods, bryozoans, and crinoids. The rather meager fauna found in Jefferson and Nelson counties includes (Butts, 1915; Foerste, 1931):

Dictyonema sp. Pachydictya crassa Atrypa reticularis Camarotoechia (?) acinus Camarotoechiar (Stegerhynchus) neglecta Dalmanella elegantula Homoeospira evax

Rhynchotreta cuneata var. americana Spirifer (Delthyris) crispus var. simplex Spirifer (Eospirifer) eudora Spirifer (Eospirifer) radiatus Calymene sp.? Calymene breviceps

The formation is typically developed in southeastern Indiana, western Kentucky, and the northern part of western Tennessee. Its distribution in western Kentucky is essentially the same as that of the Osgood and Laurel, and it is overlain disconformably by the Louisville limestone. In central Tennessee over 200 feet of limestone intervene between the Waldron and the Louisville equivalent.

Foerste (1935) noted a similar fauna in the Liston Creek formation of northeastern Indiana and in the Massie clay of Ohio.

Louisville limestone (Foerste, 1897).—The formation is typically developed in Jefferson and Oldham counties, Kentucky, and consists of 40 to 100 feet of massive grey fine-grained low magnesian limestone. From the overlying massive Jeffersonville limestone it differs both in texture and fauna. Two layers near the top, known to quarrymen as the "Blue Ledges," are high magnesian limestones analyzing 25.30 and 29.76 per cent respectively. The formation may be traced south to southwestern Nelson County, where it is overlapped by the Devonian. It reappears in outcrop in Sumner County, Tennessee.

Most of the known fauna comes from the vicinity of Louisville and is made up dominantly of corals and brachiopods, which come principally from the upper 8 feet. Pate and Bassler (1908) correlated it with the Lobelville division of the Brownsport of Tennessee. Foerste regarded the Louisville and Brownsport seas as southern invasions. "The surmise here adopted is that the Louisville is older than the Cedarville, the latter being the Ohio phase of the Racine" (Foerste, 1931, p. 184). Among the more common and distinctive forms are:

Alveolites louisvillensis Amplexus davisi Amplexus shumardi Anisophyllum unilargum Calceola (Rhizophyllum) attenuatus Calceola (Rhizophyllum) corniculum Cladopora reticulata Coenites verticillatus Cyathophyllum radicula Cystiphyllum niagarense Eridophyllum rugosum Eridophyllum sentum Favosites favosus Favosites forbesi Favosites niagarensis Halysites catenularia Heliolites interstinctus Heliolites megastoma Lindstromia (?) herzeri Lyellia papillata Omphyma verrucosa Plasmopora follis Ptychophyllum ipomoea Ptychophyllum stokesi Streptelasma spongaxis Stombodes mamillaris Strombodes striatus var. pentagonus Thecia major Caryocrinites indianensis Anastrophia internascens Atrypa calvini Atrypa reticularis var. niagarensis

Camarotoechia (?) acinus Camarotoechia (?) indianensis Camarotoechia pisa Clorinda ventricosa Coelocaulus petila Conchidium knappi Conchidium littoni Conchidium nettelrothi Conchidium nysius Conchidium tenuicostatum Cyrtia exporrecta Cyrtia exporrecta var. myrtia Dalmanella elegantula Gypidula knotti Gypidula (Sieberella) nucleus Gypidula (Sieberella) uniplicata Hebertella (Glyptorthis) rugiplicata Meristina maria Orthis nettelrothi Orthostrophia (Schizoramma) nisis Pentamerus oblongus var. cylindricus Poleumita rugaelineata Reticularia dubia Rhipidomella hybrida Rhynchonella rugicosta Rhynchospira (?) helena Rhynchotreta cuneata var. americana Stricklandinia (?) louisvillensis Spirifer crispus Spirifer (Eospirifer) rostellum Uncinulus stricklandi Whitfieldella nitida Wilsonia saffordi Wilsonia saffordi var. depressa

PLATE VII
FIG. 1. Peebles dolomite (Guelph) overlain by the Greenfield dolomite (Cayugan) with the Olentangy and Chattanooga above. Basic Products Quarry east of Peebles, Ohio. The Peebles, unknown in outcrop in Kentucky except near the Ohio River in Lewis County, is the upper part of the important Irvine pay ("Corniferous") of Estill County. The Peebles here in outcrop is quite petroliferous.	FIG. 2. Lilley dolomite (= Louisville limestone) with the Peebles above. Quarry 1 mile east of Lynx and about 10 miles east of West Union, Adams County, Ohio. The Lilley is also an important reservoir rock comprising the lower part of the "Corniferous" of the Irvine pool.
FIG. 3. Disconformable contact of the Louisville and Jeffersonville limestones, Louisville. The upper few feet of the Louisville are cherty and quite fossiliferous with abundant Halysites catenularia, Strombodes striatus, Cladopora reticulata, etc.	FIG. 4. Disconformable contact of the Louisville and Jeffersonville limestones, Louisville, Kentucky.

Plate VIII.—Silurian sections

Footnotes

¹ Belfast.—The name was applied by Foerste (1896) to 3 to 6 feet of bluish argillaceous and arenaceous, usually massive limestone underlying the Brassfield in Highland County, Ohio. It is known as far south as Hillsboro in Fleming County, Kentucky. The same writer has since (1931) shown it to be a local phase of the basal Brassfield.
² Indian Fields.—The name was given by Foerste (1906) to the succession of beds included in the Plum Creek and Oldham formations. In 1931 he dropped the term and referred the Plum Creek shale to the Medinan.
³ This is another sandstone as the Big Six occupies a lower stratigraphic position.