File:Iron oxide-travertine dripstone (Ohio Caverns, western Ohio, USA) 5 (30303665964).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.

The general term for all secondary mineral deposits occurring in caves is speleothem. Between 200 and 300 different minerals have been reported to occur in various speleothems around the world. The most common speleothem minerals are calcite (CaCO3), aragonite (CaCO3), and gypsum (CaSO4·2H2O). Calcitic speleothem is given the rock name travertine.

Varieties of cave travertine are named based on morphology and origin. The most common type is dripstone, which includes stalactites, stalagmites, and columns. Other varieties include flowstone, knobstone (a.k.a. coralloids), helictites, shelfstone, rimstone, cave pearls, frostwork, etc.

The speleothem structures shown above are composites of iron oxide (= dark-colored upper areas) and travertine (= light-colored lower areas). Ohio Caverns is relatively rich in iron oxide - the source 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 precipitated, stained the cave's limestone surfaces, and in places produced iron oxide speleothem. Yellowish to yellowish-brown colors are likely limonite. Reddish colors are likely limonite and/or hematite.

I can't be sure without a sample, but the dark-colored speleothem may be entirely iron oxide (no calcareous content), which is rare, but it is at least highly ferruginous travertine. These are stalactites, which are a type of dripstone. Stalactites are attached to cave ceilings or upper walls and form by mineral precipitation from dripping water. Precipitation of iron oxides occurred first (= older). Once the supply of leached iron from the overlying Ohio Shale was depleted, calcium carbonate was precipitated (= younger). The slender, cylindrical structures are soda straw stalactites.

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: