Pennsylvanian System

Type section and use of name in Indiana: The name Colchester Coal was introduced by Worthen (1868, p. 11) for exposures in McDonough County, Ill. Wanless (1956, p. 10) designated as the type section exposures near Colchester (secs. 12 and 13, T. 5 N., R. 4 W.), McDonough County. The name Colchester Coal Member (of the Linton Formation) was adopted in Indiana (Wier and Gray, 1961) for the coal designated Coal IIIa by Ashley (1909, p. 55-57). In Pike and Warrick Counties this unit was previously known as the Velpen Coal (Fuller and Ashley, 1902, p. 2), but the name Velpen is now used only for the overlying limestone. A reference core drilled in Survey drill hole 306 by the Indiana Geological Survey in Pike County contains Colchester coal from 140.6 to 141.6 feet (42.8 to 43.2 m) (Hasenmueller and Ault, in preparation).

Description and correlation: The Colchester Coal Member is a thin (0.1 to 3 feet < 0.1 to 0.9 m) but widespread coal that is bright banded and commonly displays a thin medial shale parting. It overlies a persistent underclay, 1 to 4 feet (0.3 to 1.2 m) thick. It is overlain by the Mecca Shale Member, a black missile shale that is 1 to 7 feet (0.3 to 2.1 m) thick. The Colchester may lie from a few inches to 65 feet (20 m) above the Seelyville Coal Member but generally lies 20 to 40 feet (6 to 12 m) above the Seelyville. The Colchester coal lies 5 to 50 feet (1.5 to 15 m) below the Survant Coal Member.

In Indiana the Colchester is traceable on outcrop from Spencer County on the Ohio River to the Vermilion River in northern Vermillion County. It is an excellent and widespread marker on geophysical logs in most of the Illinois Basin. The name Colchester is now used for this coal in Illinois and in the subsurface in Kentucky, but correlation with surface exposures in Kentucky is controversial and not definitely established (Jacobson and others, 1985).