Seatearth
Encyclopedia
Seatearth is a British coal mining term, which is used in the geological
literature. As noted by Jackson, 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.
, 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 structure
s. 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 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 studiesof 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 delta
s 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
Fire clay
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. 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.
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.
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 sediment
s altered in place after deposition as beds within acid, organic sediment
s, 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.
s 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.
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. The modern equivalent of ganisters have been observed in the process of being formed in the Okavango Delta
of Botswana
.
Geology
Geology is the science comprising the study of solid Earth, the rocks of which it is composed, and the processes by which it evolves. Geology gives insight into the history of the Earth, as it provides the primary evidence for plate tectonics, the evolutionary history of life, and past climates...
literature. As noted by Jackson, a seatearth is the layer of sedimentary rock underlying a coal
Coal
Coal is a combustible black or brownish-black sedimentary rock usually occurring in rock strata in layers or veins called coal beds or coal seams. The harder forms, such as anthracite coal, can be regarded as metamorphic rock because of later exposure to elevated temperature and pressure...
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 clayClay
Clay is a general term including many combinations of one or more clay minerals with traces of metal oxides and organic matter. Geologic clay deposits are mostly composed of phyllosilicate minerals containing variable amounts of water trapped in the mineral structure.- Formation :Clay minerals...
, or other fine-grained sediment, either immediately underlying or forming the floor of a coal
Coal
Coal is a combustible black or brownish-black sedimentary rock usually occurring in rock strata in layers or veins called coal beds or coal seams. The harder forms, such as anthracite coal, can be regarded as metamorphic rock because of later exposure to elevated temperature and pressure...
seam. Underclay typically contains the fossil roots and exhibits noticeable development soil structure
Soil structure
Soil structure is determined by how individual soil granules clump or bind together and aggregate, and therefore, the arrangement of soil pores between them...
s. It often has been noticeably altered due to weathering
Weathering
Weathering is the breaking down of rocks, soils and minerals as well as artificial materials through contact with the Earth's atmosphere, biota and waters...
. Underclays, which occur within Carboniferous
Carboniferous
The Carboniferous is a geologic period and system that extends from the end of the Devonian Period, about 359.2 ± 2.5 Mya , to the beginning of the Permian Period, about 299.0 ± 0.8 Mya . The name is derived from the Latin word for coal, carbo. Carboniferous means "coal-bearing"...
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 being altered by plant activity and soil forming processes
Pedogenesis
Pedogenesis is the science and study of the processes that lead to the formation of soil ' and first explored by the Russian geologist Vasily Dokuchaev , the so called grandfather of soil science, who determined that soil formed over time as a consequence of...
and are either in whole or part buried soils, called paleosols. As documented in various detailed studiesof 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
Cutans
Cutans are the modification of the soil texture, or soil structure, at natural surfaces in soil materials due to illuviation. Cutans are oriented deposits which can be composed of any of the component substances of the soil material. Cutans are common features in soil and represent focuses of...
, different types of pedogenic microfabrics, rhizocretions, caliche
Caliche
Caliche may refer to:*Caliche, a hardened deposit of calcium carbonate.*Caliche slang, a collection of slang words unique to Salvadoran Spanish....
nodules, root molds, and soil horizons. In the better-developed paleosols, significant alteration of the mineralogy
Mineralogy
Mineralogy is the study of chemistry, crystal structure, and physical properties of minerals. Specific studies within mineralogy include the processes of mineral origin and formation, classification of minerals, their geographical distribution, as well as their utilization.-History:Early writing...
, i.e. leaching and translocation of alkali and alkaline earth elements and the kaolinitization of smectites and hydroxy-interlayer vermiculite
Vermiculite
Vermiculite is a natural mineral that expands with the application of heat. The expansion process is called exfoliation and it is routinely accomplished in purpose-designed commercial furnaces. Vermiculite is formed by weathering or hydrothermal alteration of biotite or phlogopite...
, will have occurred. In poorly developed paleosols, as seen in the soil profiles of modern poorly developed soils, called "Inceptisols
Inceptisols
Inceptisols are a soil order in USDA soil taxonomy. They form quickly through alteration of parent material. They are older than entisols. They have no accumulation of clays, Iron, Aluminum or organic matter. They have an Ochric or Umbric horizon and a cambic subsurface horizon....
", of modern river delta
River delta
A delta is a landform that is formed at the mouth of a river where that river flows into an ocean, sea, estuary, lake, reservoir, flat arid area, or another river. Deltas are formed from the deposition of the sediment carried by the river as the flow leaves the mouth of the river...
s and floodplains, there might not exist any noticeable alteration of the underclay. These studies demonstrate that a paleosol
Paleosol
In the geosciences, paleosol can have two meanings. The first meaning, common in geology and paleontology, refers to a former soil preserved by burial underneath either sediments or volcanic deposits , which in the case of older deposits have lithified into rock...
, 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
Swamp
A swamp is a wetland with some flooding of large areas of land by shallow bodies of water. A swamp generally has a large number of hammocks, or dry-land protrusions, covered by aquatic vegetation, or vegetation that tolerates periodical inundation. The two main types of swamp are "true" or swamp...
, in which a layer of peat accumulated that later became the overlying coal, modified the paleosol to create an underclay
Fire clayFire clayFire clay is a term applied to a range of refractory clays used in the manufacture of ceramics, especially fire brick.High grade fire clays can withstand temperatures of 1775°C , but to be referred to as a "fire clay" the material must withstand a minimum temperature of 1515°C...
Underclay, which consists of siliceous refractory clay rich in hydrous aluminum silicates, is also called "fireclayFire clay
Fire clay is a term applied to a range of refractory clays used in the manufacture of ceramics, especially fire brick.High grade fire clays can withstand temperatures of 1775°C , but to be referred to as a "fire clay" the material must withstand a minimum temperature of 1515°C...
" (fire clay). Just as not all underclays are fireclays, not all fireclays are underclays. Within Carboniferous
Carboniferous
The Carboniferous is a geologic period and system that extends from the end of the Devonian Period, about 359.2 ± 2.5 Mya , to the beginning of the Permian Period, about 299.0 ± 0.8 Mya . The name is derived from the Latin word for coal, carbo. Carboniferous means "coal-bearing"...
and other coal
Coal
Coal is a combustible black or brownish-black sedimentary rock usually occurring in rock strata in layers or veins called coal beds or coal seams. The harder forms, such as anthracite coal, can be regarded as metamorphic rock because of later exposure to elevated temperature and pressure...
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.
Flint clay
Another clayClay
Clay is a general term including many combinations of one or more clay minerals with traces of metal oxides and organic matter. Geologic clay deposits are mostly composed of phyllosilicate minerals containing variable amounts of water trapped in the mineral structure.- Formation :Clay minerals...
associated with coal beds is a smooth, flintlike refractory
Refractory
A refractory material is one that retains its strength at high temperatures. ASTM C71 defines refractories as "non-metallic materials having those chemical and physical properties that make them applicable for structures, or as components of systems, that are exposed to environments above...
clay
Clay
Clay is a general term including many combinations of one or more clay minerals with traces of metal oxides and organic matter. Geologic clay deposits are mostly composed of phyllosilicate minerals containing variable amounts of water trapped in the mineral structure.- Formation :Clay minerals...
or mudstone composed dominantly of kaolin, called "flint clay". Flint clay
Clay
Clay is a general term including many combinations of one or more clay minerals with traces of metal oxides and organic matter. Geologic clay deposits are mostly composed of phyllosilicate minerals containing variable amounts of water trapped in the mineral structure.- Formation :Clay minerals...
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
Coal
Coal is a combustible black or brownish-black sedimentary rock usually occurring in rock strata in layers or veins called coal beds or coal seams. The harder forms, such as anthracite coal, can be regarded as metamorphic rock because of later exposure to elevated temperature and pressure...
-bearing strata
Stratum
In geology and related fields, a stratum is a layer of sedimentary rock or soil with internally consistent characteristics that distinguish it from other layers...
accumulated. Authegenic flint clays consist of sediment
Sediment
Sediment is naturally occurring material that is broken down by processes of weathering and erosion, and is subsequently transported by the action of fluids such as wind, water, or ice, and/or by the force of gravity acting on the particle itself....
s altered in place after deposition as beds within acid, organic sediment
Sediment
Sediment is naturally occurring material that is broken down by processes of weathering and erosion, and is subsequently transported by the action of fluids such as wind, water, or ice, and/or by the force of gravity acting on the particle itself....
s, i.e. peat
Peat
Peat is an accumulation of partially decayed vegetation matter or histosol. Peat forms in wetland bogs, moors, muskegs, pocosins, mires, and peat swamp forests. Peat is harvested as an important source of fuel in certain parts of the world...
, 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
Volcanic ash
Volcanic ash consists of small tephra, which are bits of pulverized rock and glass created by volcanic eruptions, less than in diameter. There are three mechanisms of volcanic ash formation: gas release under decompression causing magmatic eruptions; thermal contraction from chilling on contact...
by acid waters after it accumulated as thin beds within peat
Peat
Peat is an accumulation of partially decayed vegetation matter or histosol. Peat forms in wetland bogs, moors, muskegs, pocosins, mires, and peat swamp forests. Peat is harvested as an important source of fuel in certain parts of the world...
swamps or mires.
Ganister
In addition to underclays, ganisterGanister
A ganister is hard, fine-grained quartzose sandstone, or orthoquartzite, used in the manufacture of silica brick typically used to line furnaces...
s also occur as seatearths. Like fireclays, they also found within Carboniferous and other sedimentary
Sedimentary rock
Sedimentary rock are types of rock that are formed by the deposition of material at the Earth's surface and within bodies of water. Sedimentation is the collective name for processes that cause mineral and/or organic particles to settle and accumulate or minerals to precipitate from a solution....
strata
Stratum
In geology and related fields, a stratum is a layer of sedimentary rock or soil with internally consistent characteristics that distinguish it from other layers...
independent of coal
Coal
Coal is a combustible black or brownish-black sedimentary rock usually occurring in rock strata in layers or veins called coal beds or coal seams. The harder forms, such as anthracite coal, can be regarded as metamorphic rock because of later exposure to elevated temperature and pressure...
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.
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. The modern equivalent of ganisters have been observed in the process of being formed in the Okavango Delta
Okavango Delta
The Okavango Delta , in Botswana, is the world's largest inland delta. It is formed where the Okavango River empties onto a swamp in an endorheic basin in the Kalahari Desert, where most of the water is lost to evaporation and transpiration instead of draining into the sea...
of Botswana
Botswana
Botswana, officially the Republic of Botswana , is a landlocked country located in Southern Africa. The citizens are referred to as "Batswana" . Formerly the British protectorate of Bechuanaland, Botswana adopted its new name after becoming independent within the Commonwealth on 30 September 1966...
.