shale and chert exposed on Shriver Ridge, Cumberland, Md. (Swartz and others, 1913, p. 91). In Pennsylvania the Shriver was raised to the rank of formation by Cleaves (1939, p. 96-97). He considered it to be of late Early Devonian age (1939, p. 109). It comprises the cherty and siliceous limestone and shale in the lower part of the Oriskany Group.

Distribution and thickness.-The Shriver Chert has been traced from Maryland into south-central and central Pennsylvania (Cleaves, 1939, p. 96). In the Delaware Water Gap, Swartz and Swartz (1941, p. 1161, 1180) discovered a siliceous shale 54 feet thick between the Oriskany Sandstone and the calcareous Port Ewen Shale. Because it is similar in lithology and stratigraphic position, it is probably the Shriver Chert. In this quadrangle the strata tentatively assigned to the Shriver Chert are about 128 feet thick.

Lithology. In the Richards 1 well the sequence assigned to the Shriver Chert consists of an upper 28-foot-thick unit of silty and cherty limestone, a middle 20-foot-thick chert sequence, and a lower 80-foot-thick shale and limestone sequence.

The lower 80 feet of the Shriver is made up of dark-gray very silty very siliceous finely granular limestone interbedded with dark-gray very limy shale. The limestone and shale grade downward into the very limy shale of the underlying strata. A few brachipod fragments were found in the well cuttings from near the base of the formation. The insoluble residue is a dark-gray aggregate of silt particles that is cherty in part.

The chert of the middle unit is brownish gray to dark gray, silty, very limy, and dense to finely granular. Interbeds of medium-gray mottled siliceous limestone grade into a thin dark-gray shale at the base of the unit. The insoluble residue ranges from hard siliceous silt to dense chert.

The upper unit consists of finely granular to finely crystalline medium-gray to brownish-gray silty and cherty limestone. The upper 18 feet of the unit contains some limestone beds that are nearly 50 percent chert and has a few calcareous siltstone partings near the top. The chert has a conchoidal fracture and is light brownish gray and subtranslucent to translucent; but when it is immersed in water, it shows "cloudy" limestone inclusions. The lower 10 feet of his unit. is a siliceous silty finely granular limestone that contains a few brachiopods.

Stratigraphic relations.-In some areas of the State, the Shriver Chert apparently rests unconformably on strata of Helderberg age, but in other areas there is evidently no hiatus (Cleaves, 1939, p. 109; Willard, 1952, p. 76). In the Ransom quadrangle the boundary of the Shriver Chert with the underlying strata is apparently gradational.

ESOPUS SHALE

Name and age.-Darton (1894, p. 209–210) named rocks exposed near Esopus, N.Y., and along Esopus Creek the Esopus Shale. Previously, this shale had been referred to as the Cauda-galli Grit by Vanuxem (1842, p. 127–130) in his New York reports; the equivalent strata exposed in Pennsylvania was referred to as the Post-Meridian grits by Rogers (1858, p. 107). According to Willard (1939, p. 145– 146) the Esopus Shale is of early Middle Devonian age. The Esopus Shale was assigned an Early Devonian age by Oliver (1964).

Distribution and thickness.-The Esopus Shale can be seen in outcrops throughout the eastern and northeastern parts of Pennsylvania. West of the Susquehanna River the Esopus has not been recognized in surface exposures, apparently because it has lost its lithologic character. The strata that in central Pennsylvania occupy virtually the same stratigraphic position as the Esopus have been referred to as the Needmore Shale (Willard, 1939, p. 149). The maximum thickness of the Esopus in outcrops in the eastern part of the State is about 300 feet in Monroe County (Swartz and Swartz, 1941, pl. 1). The thickness of the formation, as determined from cuttings from the Richards 1 well, is about 465 feet.

Lithology. In the Richards 1 well the Esopus is subfissile mediumdark-gray to grayish-black nonlimy to very limy shale that contains minor amounts of mica and pyrite. The upper 35 feet of the formation seems to be composed of as much as 50 percent calcite, which suggests a gradational contact between Esopus and the overlying Onondaga Limestone. The upper 300 feet of strata contains a few brachiopods. The insoluble residue of the more limy beds is a lightto dark-gray aggregate of silt.

Stratigraphic relations.-The Esopus Shale reportedly (Willard, 1939, p. 154) rests disconformably on strata of Oriskany age. A disconformity may be present in the strata penetrated by the Richards 1 well, for the change from limestone to shale is abrupt.

MIDDLE DEVONIAN SERIES

ONONDAGA LIMESTONE

Name and age.-Hall (1839, p. 293) applied the name Onondaga to the gray crinoidal limestone in New York that underlies the Seneca Limestone and overlies the Oriskany Sandstone. Later, Emmons (1846, p. 174) included in the Onondaga Limestone the strata previously referred to as the Seneca Limestone, Selenurus Limestone, and Corniferous Limestone. He thus included in the formation all the strata between the Marcellus and the Oriskany. In northeastern Pennsylvania the calcareous rocks that lie at approximately the same

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stratigraphic position as the Onondaga Limestone in New York were designated the Selinsgrove Lower Limestone by I. C. White (1883, p. 80). Willard (1939, p. 142-144), in his study of formations of Devonian age of Pennsylvania, raised the Onondaga to group status and included in it the Buttermilk Falls Limestone and Esopus Shale of the eastern part of the State and the Selinsgrove Limestone and Needmore Shale of the central part of the State. The name Buttermilk Falls Limestone was proposed earlier by Willard (1938, p. 14) for the cherty limestone exposed at Buttermilk Falls on Marshall Creek, Monroe County, Pa. These strata are early Middle Devonian in age (Willard, 1939, p. 137-141). The Onondaga Limestone of this report is a formation that includes all strata from the top of the Esopus Shale to the base of the Hamilton Group. It includes strata previously assigned to the Buttermilk Falls Limestone and the Selinsgrove Limestone of Willard (1939).

Distribution and thickness.-The cherty facies of the Onondaga Limestone is confined to eastern Pennsylvania (Willard, 1939, p. 144). In central and southern Pennsylvania the limestone loses its cherty characteristic and is named the Selinsgrove Limestone (White, I. C., 1883; Willard, 1939). Lesley (1892, p. 1171) reported a maximum thickness of 250 feet for the Onondaga, and Willard (1939, p. 144) reported a maximum of 200 feet for his Buttermilk Falls Limestone, which is the Onondaga of this report. In the Richards 1 well the Onondaga Limestone is about 315 feet thick.

Lithology. In the Richards 1 well the Onondaga Limestone consists of medium-gray to medium-dark-gray micaceous silty clayey limestone that is cherty in part (pl. 2). The strata can be subdivided into three lithologic zones. The lower 155 feet consists of mediumdark-gray limestone that grades into the underlying Esopus Shale. Traces of pyrite are present in this lower unit. The insoluble residue from the limestone is light- to dark-gray silt.

The middle 120 feet of the formation consists of medium-gray to medium-dark-gray limestone, interbedded in part with as much as 20 percent medium-gray slightly limy chert. The insoluble residue from the limestone is mostly clay and fine silt; however, some coarse silt is present near the base. Light-gray nonlimy soft finely granular bentonite beds, some of which contain light-brown mica, are present in the upper 90 feet of the middle unit.

The upper 40 feet of the formation is interbedded clayey limestone and grayish-black very limy shale, which is probably transitional with the overlying Marcellus Shale. This upper unit also contains grayishblack limy chert, a trace of pyrite, and a few gastropods.

Stratigraphic relations.-On the basis of determinations made from the well cuttings, the Onondaga Limestone seemingly grades to or

intertongues with the underlying Esopus Shale. In outcrops, also, a similar relationship has been reported (Willard, 1939, p. 145-146).

HAMILTON group

The name Hamilton Group was applied by Vanuxem (1842, p. 150) to strata exposed near Hamilton, N.Y., that extend from the top of the Marcellus Shale to the base of the Tully Limestone. Later, most investigators referred to these strata as the Hamilton Formation, and a few revised the group to include the Marcellus Shale. Cooper (1930, p. 116-134) redefined the Hamilton as a group that included the Marcellus Shale as the lowest of its four formations. For eastern Pennsylvania, Willard and Cleaves (1933, p. 757) used this classification, but for central Pennsylvania, Willard (1935b, p. 202) divided the Hamilton Group into the Marcellus and Mahantango Formations because he was unable to recognize some of the equivalents to Cooper's New York units. In the Ransom quadrangle about 2,165 feet of strata penetrated by the Richards 1 well is assigned by the authors to the Hamilton Group, which is divided, from the base upward, into the Marcellus Shale and Mahantango Formation (pl. 2).

MARCELLUS SHALE

Name and age.-Hall (1839, p. 295) proposed the name Marcellus Shales for rocks exposed near Marcellus, N.Y. Rogers (1858, p. 107, 138-140) recognized the Marcellus in Pennsylvania and designated it the Cadent Lower Black Slate-the lowest member of his Cadent Series. The Pennsylvania Second Geological Survey introduced both the numerical system, VIIIb (fig. 2), and the terminology used in New York-the Marcellus Formation-for these rocks and divided the Marcellus into three members: a lower black shale, a middle gray slate, and an upper black shale (Lesley, 1892, p. 1203-1208). Willard (1939, p. 168-173) retained the name Marcellus in his study of the Middle Devonian and divided the formation into four members. In the Ransom area these members cannot be recognized; but, because the rock is of a single lithologic type, it is referred to as the Marcellus Shale.

On the basis of both age determinations of fossils and stratigraphic position, the Marcellus Shale of Pennsylvania is correlated with the Marcellus of New York and is considered to be the lowest formation of the Hamilton Group of Middle Devonian age (Willard, 1939, p. 167-168).

Distribution and thickness.-The Marcellus Shale crops out in continuous and discontinuous narrow bands in the folded belt from northeastern to south-central Pennsylvania. Willard (1939, p. 168)

reported that the Marcellus Shale is the most widespread unit of Devonian age in the State.

In Pennsylvania the formation ranges in thickness from 50 to 880 feet (Willard, 1939, p. 169). It is about 790 feet thick in the Ransom quadrangle (pl. 2).

Lithology. In the Richards 1 well the Marcellus Shale consists of dark-gray to grayish-black slightly limy to very limy shale that is micaceous, silty, and soft in part, and that contains traces of pyrite and white vein calcite. The lower part contains traces of silty limestone. The upper 400 feet is ver fossiliferous, containing a crinoid, gastropod, and pelecy pod fauna.

Willard (1939, p. 168) found that the middle gray slate of the Marcellus VIIIb of the Pennsylvania Second Geological Survey is, in outcrops, the most extensive of the three units described and that gray shaly sandstone is the predominant lithology for the formation in eastern Pennsylvania. Willard (1939, p. 173) reported that the formation in central Pennsylvania contains a sequence of fine- to coarsegrained sandstone and shale.

Stratigraphic relations.-Willard (1935b, p. 199–200; 1939, p. 148, 172) reported a disconformity at the contact between the Marcellus and the underlying Onondaga Limestone in central Pennsylvania and a thin sandstone in the basal part of the Marcellus in some areas. In the rest of the State the two formations appear to interfinger, and the contact appears to be transitional. In the Richards 1 well in the Ransom area, the contact is probably transitional; an alternating shale and limestone sequence about 40 feet thick was arbitrarily included in the Onondaga as a result of the contact being placed at the top of the youngest limestone bed in this sequence.

MAHANTANGO FORMATION

Name and age.-The name Mahantango Formation was proposed by Willard (1935b, p. 202, 205) for strata between the Marcellus Shale and the Tully Limestone exposed along Mahantango Creek in Juniata and Snyder Counties, Pa., where these strata are lithologically indivisible.

The Mahantango Formation is of late Middle Devonian age. According to Willard (1935b, p. 205), the Mahantango in central Pennsylvania contains about the same fauna as equivalent strata in Maryland, but it contains a less diverse fauna than the equivalent strata in New York.

Distribution and thickness.-The Mahantango Formation crops out in narrow zigzag bands and belts from northeastern Pennsylvania southwestward to the south border of the State, along part of the