The Bridport Sand Formation is a formation of

Toarcian The Toarcian is, in the ICS' geologic timescale, an age and stage in the Early or Lower Jurassic. It spans the time between 182.7 Ma (million years ago) and 174.1 Ma. It follows the Pliensbachian and is followed by the Aalenian. The Toarcian ...

(

Early Jurassic The Early Jurassic Epoch (geology), Epoch (in chronostratigraphy corresponding to the Lower Jurassic series (stratigraphy), Series) is the earliest of three epochs of the Jurassic Period. The Early Jurassic starts immediately after the Triassic-J ...

) age found in the

Worcester Worcester may refer to: Places United Kingdom * Worcester, England, a city and the county town of Worcestershire in England ** Worcester (UK Parliament constituency), an area represented by a Member of Parliament * Worcester Park, London, Engla ...

and

Wessex Basin The Wessex Basin is a petroleum-bearing geological area located along the southern coast of England and extending into the English Channel. The onshore part of the basin covers approximately 20,000 km2 and the area that encompasses the Englis ...

s of central and southern England. It forms one of the

reservoir A reservoir (; from French ''réservoir'' ) is an enlarged lake behind a dam. Such a dam may be either artificial, built to store fresh water or it may be a natural formation. Reservoirs can be created in a number of ways, including contro ...

units in the

Wytch Farm Wytch Farm is an oil field and processing facility in the Purbeck district of Dorset, England. It is the largest onshore oil field in western Europe. The facility, taken over by Perenco in 2011, was previously operated by BP. It is located in ...

oilfield in

Dorset Dorset ( ; archaically: Dorsetshire , ) is a county in South West England on the English Channel coast. The ceremonial county comprises the unitary authority areas of Bournemouth, Christchurch and Poole and Dorset. Covering an area of , ...

. The sandstone is very-fine grained to fine-grained and contains regular narrow bands that are calcite- cemented and more resistant to weathering, giving it a characteristic banded appearance at outcrop, such as in the cliffs between

Bridport Bridport is a market town in Dorset, England, inland from the English Channel near the confluence of the River Brit and its tributary the Asker. Its origins are Saxon and it has a long history as a rope-making centre. On the coast and wit ...

and Burton Bradstock in Dorset. It is named for

and has previously been known as the Midford Sand(s), Cotteswold Sands, Yeovil Sands and Upper Lias Sand(s). It forms a locally important

aquifer An aquifer is an underground layer of water-bearing, permeable rock, rock fractures, or unconsolidated materials ( gravel, sand, or silt). Groundwater from aquifers can be extracted using a water well. Aquifers vary greatly in their characteris ...

, particularly around Yeovil.

Extent and thickness

The Bridport Sand Formation is known from exposures in Dorset,

Somerset ( en, All The People of Somerset) , locator_map = , coordinates = , region = South West England , established_date = Ancient , established_by = , preceded_by = , origin = , lord_lieutenant_office =Lord Lieutenant of Somerset , lord_ ...

and

Gloucestershire Gloucestershire ( abbreviated Glos) is a county in South West England. The county comprises part of the Cotswold Hills, part of the flat fertile valley of the River Severn and the entire Forest of Dean. The county town is the city of Gl ...

and from the subsurface in numerous boreholes. It is also present offshore in the Portland–South Wight Basin, a sub-basin of the Wessex Basin. It reaches a maximum thickness of over 135 m in the Kimmeridge-3 well, in the onshore part of the Portland–South Wight sub-basin. It thins northwards from Dorset onto the Mendips axis, where it is missing. It reappears to the north of the axis.

Lithology

The formation shows significant lateral variations in lithology. Three separate facies have been recognised; the Bridport Sands, the ferruginous (iron-rich) facies and the

calcareous Calcareous () is an adjective meaning "mostly or partly composed of calcium carbonate", in other words, containing lime or being chalky. The term is used in a wide variety of scientific disciplines. In zoology ''Calcareous'' is used as an ad ...

facies. The Bridport Sands facies consists of very-fine to fine-grained sandstones. They vary in colour from light grey to yellowish grey and contain varying amounts of silt and clay grade material. They are locally

glauconitic Glauconite is an iron potassium phyllosilicate (mica group) mineral of characteristic green color which is very friable and has very low weathering resistance. It crystallizes with a monoclinic geometry. Its name is derived from the Greek () me ...

and often calcareous. This facies is found in the main part of the Wessex Basin where the formation is at its thickest. The ferruginous facies consists of

limestone Limestone ( calcium carbonate ) is a type of carbonate sedimentary rock which is the main source of the material lime. It is composed mostly of the minerals calcite and aragonite, which are different crystal forms of . Limestone forms whe ...

s cemented with

goethite Goethite (, ) is a mineral of the diaspore group, consisting of iron(III) oxide-hydroxide, specifically the "α" polymorph. It is found in soil and other low-temperature environments such as sediment. Goethite has been well known since ancient t ...

, ferroan calcite and

dolomite Dolomite may refer to: *Dolomite (mineral), a carbonate mineral *Dolomite (rock), also known as dolostone, a sedimentary carbonate rock *Dolomite, Alabama, United States, an unincorporated community *Dolomite, California, United States, an unincor ...

. This facies is developed in the thinner sequence to the northeast of the Wessex Basin interbedded with the Bridport Sands facies. The calcareous facies consists of interbedded sandy limestones and very fine-grained sandstones, with some interbeds of calcareous claystones and siltstones. This facies is best developed to the east and south of the Portland–South Wight sub-basin. In the Wessex Basin, the lowest 21 metres are recognised as the Down Cliff Clay Member, which consists of "uniform blue-grey fine sandy clay that weathers to a yellowish clay".

References

{{Reflist Geology of Dorset Jurassic England Toarcian Stage Jurassic System of Europe