Ordovician System

Type locality and use of name: The Fort Atkinson Limestone was named by Calvin (1906, p. 60, 98) for Fort Atkinson in eastern Iowa and was designated as the middle formation of the Maquoketa Group. It has been widely recognized in Illinois, where it is 15 to 40 feet (5 to 12 m) thick (Templeton and Willman, 1963; Kolata and Graese, 1983). Gutstadt (1958a, p. 76) recognized the unit in northwestern Indiana but did not name it Gray (1972b, fig. 5 and p. 20-21) extended the name Fort Atkinson into Indiana along with names for other formations in the Maquoketa Group.

Description: The Fort Atkinson Limestone is restricted in Indiana to subsurface usage and is sufficiently distinct to be recognized throughout much of the northern, central, and western parts of the state. Over much of this area it is close to 50 feet (15 m) thick. It includes light-colored, coarsely crystalline limestone and dolomite, mainly in its upper part, and gray argillaceous limestone and calcareous shale, mainly in its lower part. Its wire-line log signature is distinctiveCa sharp series of peaks and shoulders in the upper one-third of the formation, tapering in the lower two-thirds to an indefinite lower boundary.

Over most of its extent, the Fort Atkinson Limestone is overlain, probably conformably, by the Brainard Shale (Ordovician). In northeastern Indiana, however, the overlying rocks are appropriately assigned to an extended Whitewater Formation (Ordovician), and in a few places in central northern Indiana the Ordovician-Silurian contact descends to the upper part of the Fort Atkinson, so that in those places it is overlain disconformably by the Sexton Creek Limestone (basal Silurian). The Fort Atkinson overlies the Scales Shale (Ordovician), probably conformably, throughout its extent.

Correlation: According to Willman and others (1975, p. 86), the Fort Atkinson Limestone of Illinois is equivalent to the Waynesville Formation of former Indiana usage (in part the Waynesville Shale Member of Hay, 1981). Subsurface work by Gray (1972b) and unpublished work by John B. Droste (oral communication, 1983) substantiate this correlation.