The principal exposures of the Upper Devonian Shell Rock Formation occur along the Shell Rock River in north-central Iowa. Three members, the Mason City, Rock Grove and Nora are recognized. Except for the Upper Devonian State Quarry Limestone in northern Johnson County, the Shell Rock Formation has the smallest areal extent of any formation in Iowa and attains a maximum thickness of only 65 feet.
The Shell Rock Formation lies unconformably between the Middle Devonian Cedar Valley Limestone and the Upper Devonian Lime Creek Formation. Outliers of Cretaceous sand stones and shales lie unconformably upon the Shell Rock Formation along its eastern outcrop margin.
The formation consists of a series of argillaceous limestones, dolomites and shales, and prominent coralline and stromatoporoid biostromes. Stromatoporoid biostromes predominate in the upper part of the formation and indicate successive transgressions of the late Shell Rock sea. During the time of their formation these biostromes extended over shallow banks or shoals and covered a larger area than the lower beds. A discussion of the changes in the physical and mineralogical characteristics of each member as observed in the outcrop section and in the subsurface section outside the type area is included.
Dense limestone (micrite) beds and underlying dolomite beds in the vicinity of Mason City, Iowa, have been correlated by previous workers as facies equivalents of the lower two members of the Shell Rock Formation. However, collation of surface and subsurface data reported here demonstrates that these beds correlate with the underlying Cedar Valley Limestone and that the depositional basin of the Shell Rock Formation was more restricted than previous workers thought.