Liesegang banding in quartzite in the Precambrian of Wisconsin, USA.

The Baraboo Ranges of southern Wisconsin are dominated by a hard, erosion-resistant Precambrian metamorphic unit called the Baraboo Quartzite. These rocks were originally marine sandstones and have been subjected to metamorphism and structural folding. Original sedimentary structures are preserved, such as cross-bedding and ripple marks. Baraboo Quartzites vary in color from pinkish to dark reddish to grayish. During metamorphism, quartz overgrowths formed over the original quartz sand grains. Long-term, modern weathering can result in original sand grains being released.

This unit has economic significance - it has been quarried historically and in modern times. The quartzite is broken down into gravel-sized pieces for use as railroad ballast and erosion-control rip-rap.

The lines, or "layers", in the quartzite shown above do not represent sedimentary bedding or lamination or any other sedimentary structures. This thin, closely-spaced banding is called "Liesegang banding". It usually occurs as irregular, reddish or brownish or orangish-brown iron oxide banding in porous rocks, particularly sandstones and pebbly sandstones. These features are 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. However, 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 oxide 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.

The "Liesegang banding" seen above is unusual in that it is in quartzite, a hard, tight, non-porous metamorphic rock. Before metamorphism, the quartzite was sandstone. The banding was acquired before alteration to quartzite.

Stratigraphy: Baraboo Quartzite, upper Paleoproterozoic, ~1.7 Ga

Locality: Tumbled Rocks Trail, northwestern margin of Devil's Lake, Devil's Lake State Park, northern part of the South Range of the Baraboo Ranges, southeast of the town of Baraboo, eastern Sauk County, southern Wisconsin, USA (43° 25' 34.34" North, 89° 44' 06.56" West)

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.)