Pseudokarst is the subject of caves and underground drainage in natural areas where the bedrock is not carbonate, that is, the chemistry of solution and mechanisms involved in forming caves is not based on carbonate chemistry.

Topics

• Sydney Sandstone Caves
• Iron precipitations in Sydney Sandstone
• Cavernous development in igneous intrusions (non-lava)

Iron Precipitations in Sydney Sandstone

Iron oxides and oxyhydroxides are often seen as red and brown bands and ironstone shapes in the yellow sandstone around Sydney. Some of these are quite decorative, and banded sandstone has been used in the quarrying industry for building facing. The sandstone itself is a quartz-cemented quartz sandstone and contains no carbonate. The variety of ironstone shapes in the sandstone includes:

• Bulk colouring: a variety of yellows, reds, purples etc. depending on the quantity and oxidation state of the iron, usually goethite. Sometimes the bulk colouring corresponds to a particular stratum where the layer is bound by shales.
• Stripes: resembling liesegang rings, and have very little to do with the bedding.
• Tubes and concentric shells: Most likely liesegang rings.
• Joint fillings: colours range from red, orange, brown to black. On a canyoning trip, I saw a filled joint which was filled with a black material which resembled renniform goethite. On another trip, I found a joint filled with massive pyrite.

One of the problems to be solved is identifying the source of iron in the quartz sandstone, however some suggestions include the many mafic dykes and diatremes found throughout the Sydney region. There may be a few episodes of iron migration into the (Triassic) sandstone, as basaltic lava flows in the region (and possibly the dykes) are commonly Tertiary, and the diatremes are often Early Jurassic. Iron migration may also be a more recent phenomenon, caused by vegetation (humic and fulvic acids) allowing iron (ions) to migrate from weathering igneous rocks under low O₂ and low pH conditions. Areas with more neutral pH and higher O₂ levels would include fresh water drainage lines such as creeks, canyons and joints. Here, iron oxyhydroxides can precipitate (usually as goethite).

References

• Pickett J.W. & Alder J.D. 1997. "Layers of Time: the Blue Mountains and their geology", New South Wales Department of Mineral Resources, Sydney 34pp. ISBN 0 7313 0274 5.
• Jirí Adamovic & Václav Cílek, editors, 2002. "Pseudokarst Reports Volume 2: Ironstones", (in Czech and English.) Library of the Czech Speleological Society Volume 37. ISBN 8-85304-75-9.