Underclay

Seatearth is a British coal mining term that is used in the geological literature. As noted by Jackson,Jackson, J.A., 1997, ''Glossary of geology'', 4th ed. American Geological Institute, Alexandria. a seatearth is the layer of sedimentary rock underlying a coal seam. Seatearths have also been called seat earth, "seat rock", or "seat stone" in the geologic literature. Depending on its physical characteristics, a number of different names, such as underclay, fireclay, flint clay, and ganister, can be applied to a specific seatearth.

Underclay

Underclay is a seatearth composed of soft, dispersible clay or other fine-grained sediment, either immediately underlying or forming the floor of a coal seam. Underclay typically contains fossil roots and exhibits noticeably developed soil structures. It has often been altered by weathering. Underclays, which occur within Carboniferous coal measures, commonly contain stigmarian roots. Synonyms for underclay included seat clay, root clay, thill, warrant, coal clay, and warrant clay

Underclays typically show considerable evidence of having been altered by plant activity and soil-forming processes and are either in whole or in part buried soils, called paleosols. As documented in various detailed studies,Driese, S.G., and E.G. Ober, 2005, ''Paleopedologic and paleohydrologic records of precipitation seasonality from Early Pennsylvanian "underclay" paleosols'', U.S.A., Journal of Sedimentary Research. v. 75, no. 6, pp. 997-1010.Huddle, J.W., and S.H. Patterson, 1961, ''Origin of Pennsylvanian underclay and related seat rocks'', Geological Society of America Bulletin. vo. 72, pp. 1643-1660.Joeckel, R.N., 1995a, ''Paleosols below the Ames marine unit (Upper Pennsylvanian, Conemaugh Group) in the Appalachian Basin, U.S.A.: variability on an ancient depositional landscape'', Journal of Sedimentary Research. v. A65, no. 2, pp. 393-407.Joeckel, R.M., 1995b, ''Tectonic and paleoclimatic significance of a prominent upper Pennsylvanian (Virgilian/Stephanian) weathering profile, Iowa and Nebraska, USA'', Palaeogeography, Palaeoeclimatology, Palaeoecology. v. 118, pp. 159-179. underclays and seatearths typically exhibit features characteristic of soil profile development. Depending on the specific underclay, these soil features can include some combination of pedogenic slickensides, pedogenic ped structures, illuviated clay pore fillings, different types of pedogenic microfabrics, rhizocretions, caliche nodules, root moulds, and soil horizons. In the better-developed paleosols, significant alteration of the mineralogy, i.e. leaching and translocation of alkali and alkaline earth elements and the kaolinitization of smectites and hydroxy-interlayer vermiculite, will have occurred. In poorly developed paleosols, as seen in the soil profiles of modern poorly developed soils, called "inceptisols", of modern river deltas and floodplains, there might not exist any noticeable alteration of the underclay.

These studies demonstrate that a paleosol, which is either developed in or comprises an underclay, largely reflects the effects of plants and other soil-forming processes on the underclay while it formed the ground surface prior to being buried by organic sediments. Plant growth, waterlogging, and other processes that occurred during the development of a mire or swamp, in which a layer of peat accumulated that later became the overlying coal, modified the paleosol to create an underclay.Gardner, T.W., E.G. Williams, and P.W. Holbrook, 1988, ''Pedogenesis of some Pennsylvanian underclays; ground-water, topography, and tectonic controls'' in J. Reinhardt and W.R. Sigleo, eds., Paleosols and Weathering Through Geologic Time: principles and Applications. Geological Society of America Special Paper. no. 216, pp. 81-102. Ober, E.G.., and S.G. Driese, 2003
''The paleohydrologic history of coal underclays based upon Pennsylvanian paleosols in eastern Tennessee''
Geological Society of America Abstracts with Programs v. 35, no. 6, p. 601

Fire clay

Underclay, which consists of siliceous refractory clay rich in hydrous aluminium silicates, is also called fireclay. Just as not all underclays are fireclays, not all fireclays are underclays.United States Bureau of Mines and American Geological Institute, 1996, ''Dictionary of mining And mineral-related terms''. Mines Bureau Special Publication SP 96-1, 2nd ed, United States Bureau of Mines. Within Carboniferous and other coal-bearing strata, fireclay quite commonly comprises many underclays. The alteration of sediments by weathering, plants, and other soil processes comprising underclay resulted in the formation of the vast majority of fireclay that comprises underclay.

Flint clay

Another clay associated with coal beds is a smooth, flint-like refractory clay or mudstone composed predominantly of kaolin, called "flint clay". Flint clay breaks with a pronounced conchoidal fracture and resists slaking in water.

Flint clay can be either detrital or authegenic in origin. Detrital flint clays consist of kaolinite-rich sediments eroded and transported from uplands deeply weathered under tropical climates and redeposited within the coastal plains, in which coal-bearing strata accumulated. Authigenic flint clays consist of sediments altered in place after deposition as beds within acid, such as peat, accumulating within swamps and mires.

Flint clays associated with coal typically occur as thin, laterally continuous layers (bands), called "tonsteins", found within coal beds. In the case of tonsteins found within coal, the formation of flint clays resulted from the alternation of glass comprising volcanic ash by acidic waters after it accumulated as thin beds within peat swamps or mires.Burger, K., and H.H. Damberger, 1985, ''Tonsteins in the Coalfields of Western Europe and North America''. in Compte Rendu 4:433-448, IXICC International Congress on Carboniferous Stratigraphy and Geology, Southern Illinois University Press.Outerbridge, W.F., 2003, ''Isopach map and regional correlations of the Fire Clay tonstein, central Appalachian Basin''. Open-File Report 03-351. United States Geological Survey.

Ganister

Like fireclays, ganisters are found within Carboniferous and other sedimentary strata independent of coal beds. Thus, as in the case of fireclays, not all ganisters are seatearths. Ganisters are indurated, fine-grained quartzose sandstones that can be used in the manufacture of silica brick. They are cemented with secondary silica and have a characteristic splintery fracture.

As defined, ganisters can be created by either the cementation of quartzose by surficial soil-forming processes to form silcrete, or by diagenetic cementation within the subsurface. Detailed studies of ganisters, which occur either as seatearths or elsewhere within coal-bearing strata, have found them to be ancient paleosols, which are equivalent in both physical characteristics and origin to modern silica-cemented soils, called silcretes.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. v. 47, pp. 621-655 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 Percival, C.J., 1983, ''The Firestone Sill Ganister, Namurian, northern England—the A2 horizon of a podzol or podzolic palaeosol'', Sedimentary Geology. v. 36, no. 1, pp. 41-49. Modern formation of ganisters has been observed in the Okavango Delta of Botswana.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. vol. A65, no. 1, pp. 77-90.

References

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Sedimentary rocks