ASSOCIATED CLAY AND IRON DEPOSITS IN EASTERN OHIO AND PENNSYLVANIA

Sedimentary deposits of clay and iron that were commonly mined from Cambrian to Permian age rocks crop out from eastern Ohio to eastern Pennsylvania. A map of the locations of many historic iron and clay mines reveals that the clay deposits, many of which contain iron ores, are developed in carbonate units and as underclays beneath coal beds. The carbonate beds may be within undeformed, conformable, Pennsylvanian and Permian stratigraphic sections or in deformed beds that in places are adjacent to a major fault.

Outcrops and core logs used to construct a cross section from Muskingum County, Ohio to Westmoreland County, Pennsylvania, demonstrate the development of clay and iron ore within or adjacent to several carbonate horizons. A prominent example of a siliceous iron-rich layer is the Buhrstone ore developed above the marine Vanport limestone in Lawrence, Beaver, Butler, Clarion, Armstrong, and Jefferson Counties in Pennsylvania. In central Pennsylvania, residual iron limonite, called brown iron ore, crops out within variegated, kaolinitic clay formed from early to middle Paleozoic limestone beds. Along the flanks of South Mountain in Franklin County, Pa. both iron nodules and masses and the enclosing clay derived from the Tomstown limestone have been extensively mined.

Early studies suggested that these deposits were formed at the time of deposition or by later interaction with groundwater. The role of reactive fluids that in places appear to have moved along stratigraphic boundaries and faults has not been considered. The South Mountain deposits, along with similar deposits in Berks, Centre, Cumberland, Huntingdon, Lancaster, Mifflin, and York Counties, and the deposits in Pennsylvanian formations farther west that must have formed under similar conditions are likely co-genetic.