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Seatearth
Seatearth is a British coal mining term, which is used in the geological literature. As noted by Jackson,〔Jackson, J.A., 1997, ''Glossary of geology'', 4th ed. American Geological Institute, Alexandria. ISBN 0-922152-34-9〕 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, i.e. 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 the fossil roots and exhibits noticeable development soil structures. It often has been noticeably altered due to 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 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.〕 of underclays, underclays and seat earths typically exhibit features characteristics 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 molds, 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 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, which 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. ISBN 0-8137-2216-0〕〔Ober, E.G.., and S.G. Driese, 2003, (''The palehydrologic history of coal underclays based upon Pennsylvanian paleosols in eastern Tennessee'' ). Geological Society of America Abstracts with Programs v. 35, no. 6, p. 601〕
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