File:"Liesegang banding" in sandstone (lower Pottsville Group, Lower Pennsylvanian; Irish Ridge East roadcut, near Trinway, Ohio, USA) 2 (24510813588).jpg

"Liesegang banded" sandstone in the Pennsylvanian of Ohio, USA.

The Pottsville Group is a Pennsylvanian-aged cyclothemic succession in eastern Ohio that contains nonmarine shales, marine shales, siltstones, sandstones, coals, marine limestones, and chert ("flint"). The lower Pottsville dates to the late Early Pennsylvanian. The upper part dates to the early Middle Pennsylvanian. The Lower-Middle Pennsylvanian boundary is apparently somewhere near the Boggs Member (?).

The Pottsville sandstone sample shown here has thin bands of iron oxide staining. Such banding is often present in porous rocks, particularly sandstones and pebbly sandstones. These have been almost universally referred to as “Liesegang banding”, representing precipitation lines of iron-rich minerals (e.g., hematite, limonite, goethite, etc.) at & along groundwater chemical interfaces. But, according to Neil Wells of Kent State University, the original concept of Liesegang banding (Liesegang, 1896) does not match up with what is seen in the rock record (see Wells et al., 2003).

True Liesegang banding refers to parallel bands of precipitate formed by diffusion along a single chemical gradient during one event. What's seen in the rock record often consists of sets of irregularly concentric iron bands, with different sets of bands quite frequently oriented in different directions, and showing cross-cutting and dissolution of older sets. Iron banding in the rock record is clearly the result of numerous precipitation events over long periods of time by moving groundwater (Wells et al., 2003). Iron oxide mineralization along these bands appears to be induced by the presence of either a redox interface (change from reducing to oxidizing conditions in the groundwater) or a pH interface (change in acidity).

Because Neil Wells is the first (apparently) to point out that what geologists have been calling Liesegang banding really isn’t, a renaming seems to be in order. It was jokingly suggested in 2003 that the iron banding discussed above be called “Wells Banding”. I’m all for that.

Stratigraphy: lower Pottsville Group, upper Morrowan Stage, upper Lower Pennsylvanian

Locality: Irish Ridge East Outcrop - roadcut along the northern side of the eastbound entrance ramp from Rt. 60 to Rt. 16 (southeastern portion of Rt. 16-Rt. 60 interchange), north-northwest of the towns of Trinway & Dresden, northern Cass Township, northwestern Muskingum County, eastern Ohio, USA (~40° 09’ 13.56” North latitude, ~82° 01’ 29.37” West longitude)

References cited:

Liesegang, R.E. 1896. Ueber einige Eigenschaften von Gal-lerten [On some properties of gelatin]. Naturwissenschaftliche Wochenschrift 11: 353-362. (see also: Liesegang, R.E. 1945. Geologische Bänderungen durch Diffusion und Kapillarität [Geologic banding by diffusion and capillarity]. Chemie der Erde, Zeitschrift der Chemischen, Mineralogie, Petrographie, Geologie und Bodenkunde 15: 420-423.)