(JBD & RHS)
North Vernon Limestone,
Type locality, reference section, synonyms, and use of name: The name Silver Creek Hydraulic Limestone was given (Siebenthal, 1901b, p. 345-346) to gray dense massive argillaceous dolomitic limestone exposed along Silver Creek northwest of the Falls of the Ohio, Clark County, Ind. The same rocks had previously been called the Hydraulic Limestone, Cement Beds (Rock), and Waterlime. In 1915 the rank of this unit was reduced to member by Butts (p. 120), who further changed the name to the Silver Creek Limestone Member and assigned it to the Sellersburg Limestone. In a further history of nomenclatural changes for North Vernon rocks, this unit came to be considered in the 1970 compendium (Shaver and others) as the Silver Creek Member in the lower part of the North Vernon Limestone. (See the North Vernon article.)
A reference section was designated by Burger and Patton (1970, p. 167) as the approximately 14 feet (4.3 m) of typical Silver Creek rocks exposed in the Louisville Cement Co. quarry at Speed (Clark Military Grants 131 and 132), Clark County, Ind.
Description: Characteristic Silver Creek lithology has been noted above, but in some places chert is present. The only other conspicuous variation in this lithology is owed to the facies relationship that the Silver Creek has with the Speed Member of the North Vernon. Where both these members are seen in single exposures, the Silver Creek overlies the Speed, but the contact is a transitional one involving gray granular shaly fossiliferous limestone of the Speed type interbedded with rocks of the Silver Creek type, for example, in a quarry exposure at Sellersburg (Patton and Dawson, 1955, p. 16).
The Silver Creek is about 15 feet (4.6 m) thick at the Falls of the Ohio in southern Clark County (Dawson, 1941, p. 9), but the Speed is absent, apparently by nondeposition of its lithologic type. Northward, however, the Speed appears and thickens as the Silver Creek thins, so that in southern Scott County the Silver Creek reaches a virtual depositional zero as the Speed attains nearly full occupancy of the lower North Vernon (Patton and Dawson, 1955, p. 21-22). North of here only traces of Silver Creek lithology have been noted in the lower North Vernon rocks usually designated only as the Speed. Maximum reported thickness of the Silver Creek is 26 feet (8 m) in central Clark County (Whitlatch and Huddle, 1932, p. 375). The Silver Creek has not been defined in the subsurface.
As noted above, the Silver Creek overlies the Speed Member conformably in some places elsewhere it overlies the Jeffersonville unconformably. It underlies the Beechwood Member (North Vernon) unconformably.
Correlation: During an early period of study, Silver Creek beds were assigned to or correlated with the Helderberg Group (Lower Devonian of New York) on the basis of their characteristics as a waterlime (properties enabling its use in hydraulic-cement manufacture). In a middle period, however, the Silver Creek was variably assigned to or correlated with the Hamilton Group (upper Middle Devonian of New York) on the basis of its macrofossils (for example, by Cooper and Warthin, 1936, p. 376). The specific stratigraphic position was said to be the Skaneateles Formation (upper lower Hamilton see Rickard, 1975, pl. 3). The later conodont studies by Orr (1971, p. 20) have supported this correlation, as the upper Silver Creek rocks have conodonts of the Icriodus latericrescens latericrescens Zone (early Givetian in age) in common with the Skaneateles. (See, however, the discussion under "Speed Member" for other opinion on age.)
Other midwestern units that are closely or exactly correlative with the Silver Creek include: the Speed Member, southern Indiana as already noted; a lower part of the Traverse Formation, northern Indiana; a lower part of the Laughingly Formation, Illinois; and a part of the section in southern Michigan and northwestern Ohio ranging from the Dundee and Delaware Limestones into the lower part of the Traverse Group. (See also Stumm, 1964; Meents and Swann, 1965; Doheny, Droste, and Shaver, 1975, fig. 3; and Shaver and others, 1985.)
In Kentucky this unit has been recognized only in the Louisville area (south of the Falls) (Powell, 1970, p. 26; Burger and Patton, 1970, p. 167) under the name Silver Creek. This unit has sometimes been recognized in name combination and rank in a stratigraphic scheme intended to apply to both Kentucky and Indiana but not in accord with that used here. (For example, see Conkin and Conkin, 1972, fig. 9, and Conkin, Conkin, and Lipchinsky, 1976, fig. 10, and 1980, fig. 2.)