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Greensand orgreen sand is asand orsandstone which has a greenish color. This term is specifically applied to shallowmarine sediment that contains noticeable quantities of rounded greenish grains. These grains are calledglauconies and consist of a mixture of mixed-layerclay minerals, such assmectite andglauconite. Greensand is also loosely applied to any glauconitic sediment.[1][2][3]
Greensand forms inanoxicmarine environments that are rich inorganic detritus and low insedimentary input.[1] Having accumulated in marine environments, greensands can befossil-rich, such as in the late-Cretaceous deposits ofNew Jersey.[4]
Important exposures are known from both northern and westernEurope,North America,southeastern Brazil andnorth Africa. Well known and important greensands are the Upper and Lower Greensands ofEngland and occur withinEocene andCretaceous sedimentary strata underlying the coastal plains ofNew Jersey andDelaware. Although greensand has been found throughoutPhanerozoic and LatePrecambrian sedimentary deposits, it appears to be most common in Eocene,Cambrian, and Cretaceous sedimentary deposits.[1][2]
InBrazil, greensand refers to a fertilizer produced from glauconitic siltstone units belonging to theSerra da Saudade Formation, Bambuí Group, ofNeoproterozoic/Ediacaran age. The outcrops occur in the Serra da Saudade ridge, inAlto Paranaíba region,Minas Gerais.[5] It is a silt-clay sedimentary rock, laminated, bluish-green, composed ofglauconite (40–80%), potassium feldspar (10–15%),quartz (10–60%),muscovite (5%) and minor quantities ofbiotite (2%),goethite (<1%),titanium andmanganese oxides (<1%), barium phosphate andrare-earth elements phosphates (<1%).
Enriched levels of potash haveK2O grades between 8% and 12%, thickness up to 50 m (160 ft) and are associated to the glauconitic levels, dark green in color. Glauconite isauthigenic and highly mature. The high concentration of this mineral is related to a depositional environment with a low sedimentation rate. The glauconitic siltstone has resulted from a high level flooding event in theBambuí Basin. The sedimentary provenance is from supracrustal feldsic elements on a continental margin environment with an acidic magmatic arc (foreland basin).
InGreat Britain, greensand usually refers to specific rock strata ofEarly Cretaceous age. A distinction is made between the Upper Greensand and Lower Greensand. The term greensand was originally applied byWilliam Smith to glauconitic sandstones in the west of England and subsequently used for the similar deposits of theWeald, before it was appreciated that the latter are actually two distinct formations separated by theGault Clay.[6] The Upper Greensand was also once known as either the "Malm" or "Malm Rock Of Western Sussex".[7]
Both Upper and Lower Greensand outcrops appear in the scarp slopes surrounding theLondon Basin and the Weald. Prominent seams are to be found in theVale of White Horse, inBedfordshire, inKent,Surrey, theSouth Downs National Park,[8] elsewhere inHampshire, theIsle of Wight, and theJurassic Coast inDorset. Some minor seams are found further west in Devon in theBlackdown andHaldon hills.
The soil of the greensand is quite varied, ranging from fertile to fairly sterile. On the fertile soilschestnut and stands ofhazel andoak are common, whileScots pine andbirch colonise the poorer soils.[9] TheseGreensand Ridges are popular long distance walking routes, for instance theGreensand Way in Kent.
The Lower Greensand (known as the Woburn Sand north of theLondon Basin) is ofAptian age. In the Weald the Lower Greensand consists of four deposits which are partlydiachronous: the Atherfield Clay 5–15 m (15–50 ft) thick, the Folkestone Beds 20–80 m (60–250 ft) thick; the Hythe beds 20–110 m (60–350 ft) thick and the Sandgate Beds 2–37 m (5–120 ft) thick.[10] Although it appears both north and south of the London Basin it is not present everywhere beneath theChalk Group which underlies the basin; the Gault lies directly on erodedJurassic orDevonian rocks under much of the area.
The Upper Greensand is ofAlbian age. It represents a sandylithofacies deposited in areas of stronger currents than theGault Clay. Like the Lower Greensand it is not present beneath the whole of the London Basin, apparently passing laterally into Gault clay east of a line betweenDunstable andTatsfield and of uncertain extent to the east of London.[11][12]
Outcrops of the Upper Greensand occur in the southwest of England including theBlackdown Hills and East Devon Plateau and theHaldon Hills, remnants of a once much wider extent.[13]
The green color of greensand is due to variable amounts of themineralglauconite, anironpotassiumsilicate with very lowweathering resistance; as a result, greensand tends to be weak and friable. It is a common ingredient as a source ofpotassium inorganicgardening andfarmingfertilisers. Greensand glauconite is used as a water softener for its chemical-exchange properties. Greensand coated with manganese oxide (called manganese greensand) is used in well water treatment systems to remove dissolved (reduced) iron and manganese with the addition of an oxidant, usually potassium permanganate, under controlled pH conditions.[14] It is also used as a type of rock for stone walls in areas where greensand is common.
In Roman times in Britain, coarse grits derived from the lower greensand were used to line the inner surface ofmortars (grinding bowls) produced inOxfordshire pottery kilns.[15]
Recently, glauconitic greensand has become a popular organic soil amendment. The porous properties of glauconite greensand allows for the absorption of water and minerals, making irrigation and nutrient delivery much more efficient (seesoil conditioner).[citation needed] Greensand can be used to absorb excess water in clay-rich soils and to prevent water loss in sandy soils.
The dictionary definition ofgreensand at Wiktionary| International | |
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