The Bridport Sand Formation is a

formation Formation may refer to: Linguistics * Back-formation, the process of creating a new lexeme by removing or affixes * Word formation, the creation of a new word by adding affixes Mathematics and science * Cave formation or speleothem, a secondary ...

of Toarcian ( Early Jurassic) age found in the Worcester and Wessex Basins of central and southern England. It forms one of the reservoir units in the Wytch Farm oilfield in Dorset. The sandstone is very-fine grained to fine-grained and contains regular narrow bands that are

calcite Calcite is a Carbonate minerals, carbonate mineral and the most stable Polymorphism (materials science), polymorph of calcium carbonate (CaCO3). It is a very common mineral, particularly as a component of limestone. Calcite defines hardness 3 on ...

- cemented and more resistant to weathering, giving it a characteristic banded appearance at outcrop, such as in the cliffs between Bridport and

Burton Bradstock Burton Bradstock is a village and civil parish in Dorset, England, approximately southeast of Bridport and inland from the English Channel at Chesil Beach. In the 2011 Census the parish had a population of 948. The village lies in the Br ...

in Dorset. It is named for Bridport 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, particularly around Yeovil.

Extent and thickness

The Bridport Sand Formation is known from exposures in Dorset, Somerset and Gloucestershire 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 The lithology of a rock unit is a description of its physical characteristics visible at outcrop, in hand or core samples, or with low magnification microscopy. Physical characteristics include colour, texture, grain size, and composition. Lit ...

. Three separate

facies In geology, a facies ( , ; same pronunciation and spelling in the plural) is a body of rock with specified characteristics, which can be any observable attribute of rocks (such as their overall appearance, composition, or condition of formatio ...

have been recognised; the Bridport Sands, the ferruginous (iron-rich) facies and the calcareous 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 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 limestones cemented with goethite, ferroan calcite and dolomite. 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

claystone Mudrocks are a class of fine-grained siliciclastic sedimentary rocks. The varying types of mudrocks include siltstone, claystone, mudstone, slate, and shale. Most of the particles of which the stone is composed are less than and are too sm ...

s and

siltstone Siltstone, also known as aleurolite, is a clastic sedimentary rock that is composed mostly of silt. It is a form of mudrock with a low clay mineral content, which can be distinguished from shale by its lack of fissility.Blatt ''et al.'' 1980, p ...

s. 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