Silurian System

Type locality, reference sections, and use of name: The Laurel Member was named as the Laurel Formation by Foerste (1896, p. 191) for exposures near Laurel in Franklin County, Ind., but no type section was designated. The Laurel was defined as the unit between the Brassfield Limestone, which was then called the Clinton Limestone, and the Waldron Shale, and so it included rocks later assigned to the Osgood Member. In 1897 (Foerste, p. 217) the Laurel was restricted to beds above the Osgood, and in 1967 (French) the Laurel was reduced to member rank and assigned to the Salamonie Dolomite.

Because the Osgood has an increasing carbonate content to the north and the west, it is difficult to separate the Osgood and the Laurel in the type area of the Laurel. The once numerous quarries near Laurel are now abandoned, and one of the best Laurel sections in the type area is a natural exposure in the northeast bluff of Sanes Creek in the NW¼NE¼ sec. 7, T. 12 N,, R. 12 E. Much farther south where the Osgood-Laurel divisions are clear, an excellent exposure of the Laurel is found along Indiana Highway 62 on the east bluff of Fourteenmile Creek in the S¼S¼ lot 121, Clark's Grant, Clark County.

Description: The Laurel is generally light-gray to tan dense dolomitic limestone that has lenticular and nodular sheet especially in the upper part. Although fossiliferous, recrystallization has destroyed many of the fossils. The member on outcrop ranges from about 27 to 55 feet (8 to 17 m) in thickness, and it thickens northward. The Laurel outcrop belt extends southward from Laurel to the Ohio River, through Kentucky, and into Tennessee. North of the type area and west from the outcrop belt in Indiana, the Osgood loses its distinctive lithology and cannot be separated from the Laurel, so that the two members form an undifferentiated part of the Salamonie Dolomite. In most areas of outcrop, as at Fourteenmile Creek, the Laurel is conformable with the underlying Osgood and the overlying Waldron, but in places pre-Middle Devonian erosion has removed the overlying Silurian beds, and the Jeffersonville Limestone (Middle Devonian) unconformable rests on the Laurel.

Correlation: The Laurel Member is continuous with a mostly upper part of the Salamonie Dolomite, and therefore the upper part of the Laurel is also equivalent to the Limberlost Dolomite Member of the Pleasant Mills Formation. It probably correlates with the Bisher Formation of southern Ohio and adjacent Kentucky, a unit that in British terms is early and middle Wenlockian in age (Niagara in the North American standard) (Rexroad and Kleffner, 1984). It also apparently correlates with the Laurel Limestone, the Euphemia Dolomite, and the Springfield Dolomite of west-central Ohio usage, with upper parts of the Joliet Dolomite of northeastern Illinois, with part of the St. Clair Limestone of southwestern Indiana and southeastern Illinois (Rexroad and Droste, 1982), and with all but the lower carbonate and shale beds of the Laurel of central Kentucky (Rexroad, Gray, and Noland, 1983). Generally, however, the Laurel is nearly synchronous from southeastern Indiana into central Tennessee.