Seatearth is a British coal mining term, which is used in the

geological Geology () is a branch of natural science concerned with Earth and other astronomical objects, the features or rocks of which it is composed, and the processes by which they change over time. Modern geology significantly overlaps all other E ...

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 Coal is a combustible black or brownish-black sedimentary rock, formed as rock strata called coal seams. Coal is mostly carbon with variable amounts of other elements, chiefly hydrogen, sulfur, oxygen, and nitrogen. Coal is formed when ...

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 Clay is a type of fine-grained natural soil material containing clay minerals (hydrous aluminium phyllosilicates, e.g. kaolin, Al2 Si2 O5( OH)4). Clays develop plasticity when wet, due to a molecular film of water surrounding the clay par ...

or other fine-grained sediment, either immediately underlying or forming the floor of a

seam. Underclay typically contains the fossil roots and exhibits noticeably developed

soil structure Soil structure describes the arrangement or the way of soil in the solid parts of the soil and of the pore space located between them. It is determined by how individual soil granules clump, bind together, and aggregate, resulting in the arrangem ...

s. It often has been altered by

weathering Weathering is the deterioration of rocks, soils and minerals as well as wood and artificial materials through contact with water, atmospheric gases, and biological organisms. Weathering occurs ''in situ'' (on site, with little or no movement) ...

. Underclays, which occur within Carboniferous coal measures, commonly contain

Stigmaria ''Stigmaria'' is a form taxon for common fossils found in Carboniferous rocks. They represent the underground rooting structures of coal forest lycopsid trees such as '' Sigillaria'' and '' Lepidodendron''. These swamp forest trees grew to ...

n 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 In the geosciences, paleosol (''palaeosol'' in Great Britain and Australia) is an ancient soil that formed in the past. The precise definition of the term in geology and paleontology is slightly different from its use in soil science. In geol ...

. 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 seat earths typically exhibit features characteristics of

soil profile A soil horizon is a layer parallel to the soil surface whose physical, chemical and biological characteristics differ from the layers above and beneath. Horizons are defined in many cases by obvious physical features, mainly colour and texture. ...

development. Depending on the specific underclay, these soil features can include some combination of pedogenic

slickensides In geology, a slickenside is a smoothly polished surface caused by frictional movement between rocks along a fault. This surface is typically striated with linear features, called slickenlines, in the direction of movement. Geometry of slickens ...

, pedogenic ped structures, illuviated clay pore fillings, different types of pedogenic microfabrics, rhizocretions,

caliche Caliche () is a sedimentary rock, a hardened natural cement of calcium carbonate that binds other materials—such as gravel, sand, clay, and silt. It occurs worldwide, in aridisol and mollisol soil orders—generally in arid or semiarid regions ...

nodules, root molds, and

soil horizons A soil horizon is a layer parallel to the soil surface whose physical, chemical and biological characteristics differ from the layers above and beneath. Horizons are defined in many cases by obvious physical features, mainly colour and texture. ...

. 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

smectite A smectite (from ancient Greek ''σμηκτός'' smektos 'lubricated'; ''σμηκτρίς'' smektris 'walker's earth', 'fuller's earth'; rubbing earth; earth that has the property of cleaning) is a mineral mixtures of various swelling sheet sil ...

s and hydroxy-interlayer

vermiculite Vermiculite is a hydrous phyllosilicate mineral which undergoes significant expansion when heated. Exfoliation occurs when the mineral is heated sufficiently, and commercial furnaces can routinely produce this effect. Vermiculite forms by the we ...

, will have occurred. In poorly developed paleosols, as seen in the soil profiles of modern poorly developed soils, called "

Inceptisols Inceptisols are a soil order in USDA soil taxonomy. They form quickly through alteration of parent material. They are more developed than Entisols. They have no accumulation of clays, iron oxide, aluminium oxide or organic matter. They have an ...

", of modern

river delta A river delta is a landform shaped like a triangle, created by deposition of sediment that is carried by a river and enters slower-moving or stagnant water. This occurs where a river enters an ocean, sea, estuary, lake, reservoir, or (more ra ...

s and floodplains, there might not exist any noticeable alteration of the underclay. These studies demonstrate that a

paleosol In the geosciences, paleosol (''palaeosol'' in Great Britain and Australia) is an ancient soil that formed in the past. The precise definition of the term in geology and paleontology is slightly different from its use in soil science. In geolo ...

, 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 underclayGardner, 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 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

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 vast majority of fireclay that comprises underclay.

Flint clay

Another clay associated with coal beds is a smooth,

flint Flint, occasionally flintstone, is a sedimentary cryptocrystalline form of the mineral quartz, categorized as the variety of chert that occurs in chalk or marly limestone. Flint was widely used historically to make stone tools and sta ...

-like refractory clay or mudstone composed dominantly of

kaolin Kaolinite ( ) is a clay mineral, with the chemical composition Al2 Si2 O5( OH)4. It is an important industrial mineral. It is a layered silicate mineral, with one tetrahedral sheet of silica () linked through oxygen atoms to one octahedral ...

, called "flint clay". Flint clay breaks with a pronounced conchoidal fracture and resists slaking in water. Flint clay can be either

detrital Detritus (; adj. ''detrital'' ) is particles of rock derived from pre-existing rock through weathering and erosion.Essentials of Geology, 3rd Ed, Stephen Marshak, p G-7 A fragment of detritus is called a clast.Essentials of Geology, 3rd Ed, Stephe ...

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, such as

peat Peat (), also known as turf (), is an accumulation of partially decayed vegetation or organic matter. It is unique to natural areas called peatlands, bogs, mires, moors, or muskegs. The peatland ecosystem covers and is the most efficient ...

, accumulating within swamps and

mire A mire, peatland, or quagmire is a wetland area dominated by living peat-forming plants. Mires arise because of incomplete decomposition of organic matter, usually litter from vegetation, due to water-logging and subsequent anoxia. All types ...

s. 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 Volcanic ash consists of fragments of rock, mineral crystals, and volcanic glass, created during volcanic eruptions and measuring less than 2 mm (0.079 inches) in diameter. The term volcanic ash is also often loosely used to refer ...

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,

ganister A ganister (or sometimes gannister ) is hard, fine-grained quartzose sandstone, or orthoquartzite,Jackson, J. A., 1997, ''Glossary of geology'', 4th ed. American Geological Institute, Alexandria. used in the manufacture of silica brick typically ...

s are found within Carboniferous and other

sedimentary Sedimentary rocks are types of rock that are formed by the accumulation or deposition of mineral or organic particles at Earth's surface, followed by cementation. Sedimentation is the collective name for processes that cause these particles ...

strata independent of coal beds. Thus, as in case of fireclays, not all ganisters are seatearths. Ganisters are

indurated Friability ( ), the condition of being friable, describes the tendency of a solid substance to break into smaller pieces under duress or contact, especially by rubbing. The opposite of friable is indurate. Substances that are designated hazardous, ...

, fine-grained quartzose

sandstones Sandstone is a clastic sedimentary rock composed mainly of sand-sized (0.0625 to 2 mm) silicate grains. Sandstones comprise about 20–25% of all sedimentary rocks. Most sandstone is composed of quartz or feldspar (both silicates) b ...

which 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 silicrete, or by

diagenetic Diagenesis () is the process that describes physical and chemical changes in sediments first caused by water-rock interactions, microbial activity, and compaction after their deposition. Increased pressure and temperature only start to play a ...

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

silcrete Silcrete is an indurated (resists crumbling or powdering) soil duricrust formed when surface soil, sand, and gravel are cemented by dissolved silica. The formation of silcrete is similar to that of calcrete, formed by calcium carbonate, and ferr ...

s.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 The Okavango Delta (or Okavango Grassland; formerly spelled "Okovango" or "Okovanggo") in Botswana is a swampy inland delta formed where the Okavango River reaches a tectonic trough at an altitude of 930–1,000 m in the central part of the en ...

Botswana Botswana (, ), officially the Republic of Botswana ( tn, Lefatshe la Botswana, label= Setswana, ), is a landlocked country in Southern Africa. Botswana is topographically flat, with approximately 70 percent of its territory being the Kalaha ...

.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

{{reflist Sedimentary rocks