Holsworthy Group

The Holsworthy Group is a late Carboniferous lithostratigraphic group (a sequence of rock strata) in north and east Cornwall and Devon in southwest England. The name is derived from the Devon town of Holsworthy. The Group comprises (in ascending order i.e. oldest first) the Crackington, Bideford and Bude formations. In the Launceston area the group is represented by the Bealsmill Formation. It was formerly known as the Upper Culm Group.[1]

Holsworthy Group
Stratigraphic range: Namurian to Bolsovian (Carboniferous)
Chevron folded alternations of sandstone and mudstone of the Crackington Formation, Warren Bay, Hartland Quay
TypeGroup
Unit ofCulm Supergroup
Sub-unitsCrackington Formation, Bideford Formation, Bude Formation
UnderliesExeter Group
OverliesTeign Valley Group
Thickness3500 to 4000m
Lithology
Primarymudstones,
Othersiltstones, sandstones, conglomerates
Location
RegionEngland
CountryUnited Kingdom
Extentnorth and east Cornwall to north and east Devon
Type section
Named forHolsworthy

The Crackington Formation is over 1km thick in the Exeter area but amounts to only 250m around Bideford. It consists of mudstones with lesser sandstones and siltstones displaying rhythmic bedding, typical of turbidite deposits.[2] The overlying Bideford Formation is over 1200m thick in places and consists of interbedded sandstones, mudstones and siltstones with a range of different characters including turbidites, channel sandstones, cross-bedded and slumped sandstones and locally contains burrows and rootlets.[3] The Bude Formation is calculated to be around 1.3km thick along the north coast and consists of silty sandstones and mudstones. It is fossiliferous and locally contains ironstones and calcareous nodules.[4] The Bealsmill Formation is at least 120m thick and consists of interbedded sandstones, siltstones and mudstones with some conglomerate beds containing chert, lava and limestone clasts.[5]

References
  1. http://www.bgs.ac.uk/Lexicon/lexicon.cfm?pub=HOWY (BGS on-line lexicon of rock units)
  2. https://www.bgs.ac.uk/lexicon/lexicon.cfm?pub=CKF
  3. https://www.bgs.ac.uk/lexicon/lexicon.cfm?pub=BFF
  4. https://www.bgs.ac.uk/lexicon/lexicon.cfm?pub=BF
  5. https://www.bgs.ac.uk/lexicon/lexicon.cfm?pub=BEAL
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