File:Iron-oxide stained limestone cave wall (Ohio Caverns, western Ohio, USA) 7 (30590906180).jpg

Ohio Caverns is the largest cave system in Ohio. It is located in a bedrock knob called Mt. Tabor on the southern side of the Bellefontaine Outlier (= Ohio's elevationally highest area). The cave is hosted in the Middle Devonian Columbus Limestone, which is part of a widespread shallow marine carbonate succession in eastern and midwestern America.

Many caves have walls with earth-tone staining due to iron oxides minerals. Much of Ohio Caverns is similarly stained, often intensely so. Some of the speleothem ("cave formations") in the cave is composed of iron oxide, which is unusual. The source of the abundant iron oxide is a relatively thin cover of Ohio Shale, an Upper Devonian anoxic marine black shale unit that directly overlies the Columbus Limestone at this locality. The Ohio Shale is pyritic, typically in the form of disseminated tiny crystals. In the presence of water, oxidative chemical weathering of pyrite (FeS2 - iron sulfide) in the black shale produces both iron oxide species and some sulfuric acid (H2SO4). The cave was principally dissolved out by carbonic acid in groundwater (as underwater rivers) during the latest Pleistocene and early Holocene, but sulfuric acid probably contributed. After the cave drained (changing from phreatic conditions to vadose conditions), water entering the cave via drips or seeps was relatively rich in dissolved iron oxide. The iron oxide then precipitated and stained the cave's limestone surfaces. Yellow colors are likely limonite. Reddish colors are likely limonite and/or hematite.

Locality: Ohio Caverns, Mt. Tabor, east of the town of West Liberty, northern margin of Champaign County, western Ohio, USA

For a recent technical article on the geology of Ohio Caverns, see: