Ganister

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As defined in Jackson (1997), a ganister is hard, fine-grained quartzose sandstone, or orthoquartzite, used in the manufacture of silica brick typically used to line furnaces. Ganisters are cemented with secondary silica and typically have a characteristic splintery fracture. Cornish miners originally coined this term for hard, chemically and physically inert silica-cemented quartzose sandstones, commonly, but not always found as seatearths within English Carboniferous coal measures. This term is now used for similar quartzose sandstones found typically as seatearths in the Carboniferous coal measures of Nova Scotia, the United States, and the Triassic coal-bearing strata of the Sydney Basin in Australia (Retallack 1977, 2001; Gibling and Rust 1992).

Where a ganister underlies coal as a seatearth, it typically is penetrated by numerous root traces. These root traces typically consist of carbonaceous material. Ganisters that contain an abundance of fossil roots, which appear as fine carbonaceous, pencil-like streaks or markings, are called “pencil ganisters”. In other cases, the root traces consist of fine, branching nodules, called “rhizoliths”, which formed around the roots before they decayed (Jackson 1997; Rettallack 1977, 2001).

From detailed studies of ganisters, Gibling and Rust (1992), Perciveil (1982, 1983), Retallack (1977), and other geologists have concluded that the typical ganister is the silicified surface horizons, i.e. E horizon, of a buried soil, called “paleosol”, developed in sandy sediments. These and other studies have found ganisters to contain abundant evidence of having once been the upper horizon of a soil, which has developed in loamy or sandy sediments. The evidence includes some combination of carbonized roots and rootlets, rhizoliths, illuvial clay cutans, silcrete-like silica cements, and the leaching and alteration of the sandy sediments by weathering and plants. These studies argue that the destruction of easily weathered minerals, i.e. feldspar, within the surface horizon of a soil by soil-forming processes is what creates the quartz-rich nature of ganisters. The silica-cementation that creates a ganister typically results from dissolution of plant opal within a soil profile and its redeposition as silica cement within it. The formation of ganisters within modern soils have been observed by McCarthy and Ellery (1995) in the Okavango Delta of Botswana (Rettallack 2001).

[edit] References

Gibling, M.R., and B.P. Rust, 1992, Silica-cemented paleosols (ganisters) in the Pennsylvanian Waddens Cove Formation, Nova Scotia, Canada in K.H. Wolf and G.V. Chilingarian, George, eds., Diagenesis, III. Developments in Sedimentology 47:621-655 ISBN 0-444-88516-1

Jackson, J.A., 1997, Glossary of geology, 4th ed. American Geological Institute, Alexandria. ISBN 0-922152-34-9

McCarthy, T.S. and W.N. Ellery, 1995, Sedimentation on the distal reaches of the Okavango Fan, Botswana, and its bearing on calcrete and silcrete (ganister) formation, Journal of Sedimentary Research A65(1):77-90.

Perciveil, C.J., 1982, Paleosols containing an albic horizon: examples from the upper Carboniferous of northern Britain in V.P. Wright, ed., pp. 87-111, Paleosols: Their Recognition and Interpretation. Princeton, Princeton University Press ISBN 0-691-08405-X

Percival, C.J., 1983, The Firestone Sill Ganister, Namurian, northern England—the A2 horizon of a podzol or podzolic palaeosol, Sedimentary Geology 36(1):41-49.

Retallack, G.J., 1977, Triassic palaeosols in the upper Narrabeen Group of New South Wales. Part II: Classification and reconstruction Journal of the Geological Society of Australia. 24(1):19-35.

Retallack, G.J., 2001, Soils of the Past, 2nd ed. New York, Blackwell Science. ISBN 0-632-05376-3