Seatearth is a British coal mining term, which is used in the geological literature. As noted by Jackson[1], 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.
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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[1]
Underclays typically show considerable evidence of having being 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[2][3][4][5]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[2][6][7]
Underclay, which consists of siliceous refractory clay rich in hydrous aluminum silicates, is also called "fireclay" (fire clay). Just as not all underclays are fireclays, not all fireclays are underclays.[1][8] Within Carboniferous and other coal bearing strata, fireclay quite commonly comprises many underclays. In fact, within Great Britain, underclays, which are 3 to 9 feet (1 to 3 meters) thick, are major source of commercial fireclay deposits. The alteration of sediments by weathering, plants, and other soil processes comprising underclay resulted in the formation of vast majority of fireclay that comprises underclay.
Another clay associated with coal beds is a smooth, flintlike refractory clay or mudstone composed dominantly of kaolin, called "flint clay". Flint clay breaks with a pronounced conchoidal fracture and resists slaking in water.[1]
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. Authegenic flint clays consist of sediments altered in place after deposition as beds within acid, organic sediments, i.e. 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. At least, in case of tonsteins found within coal, the formation of flint clays resulted from the alternation of glass comprising volcanic ash by acid waters after it accumulated as thin beds within peat swamps or mires.[9][10]
In addition to underclays, ganisters also occur as seatearths. Like fireclays, they also found within Carboniferous and other sedimentary strata independent of coal beds. Thus, as in case of fireclays, not all ganisters are seatearths. Ganisters are indurated, fine-grained quartzose sandstones, which can be used in the manufacture of silica brick. It is cemented with secondary silica and has a characteristic splintery fracture.[1][8]
As defined, ganisters can either be created by either the cementation of quartzose by surficial soil-forming processes to form silicrete 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.[11][12][13] The modern equivalent of ganisters have been observed in the process of being formed in the Okavango Delta of Botswana.[14]