Dent Group

Dent Group
Stratigraphic range: Ordovician

Steeply-dipping calcareous mudstones of the Dent Group on Timley Knott near Coniston, Cumbria
Type Group
Unit of Windermere Supergroup
Underlies Stockdale Group
Overlies Borrowdale Volcanic Group
Thickness ca. 100 m
Location
Region England
Country United Kingdom
Extent Southern Lake District, northwest Pennines

The Dent Group is a group of Upper Ordovician sedimentary and volcanic rocks in north-west England. It is the lowermost part of the Windermere Supergroup, which was deposited in the foreland basin formed during the collision between Laurentia and Avalonia. It lies unconformably on the Borrowdale Volcanic Group. This unit was previously known as the Coniston Limestone Group or Coniston Limestone Formation and should not be confused with the significantly younger (uppermost Silurian) Coniston Group.[1]

Distribution

The Dent Group is exposed in four areas, the southern Lake District as a narrow strip across the whole width of the outcrop, in the Cautley and Dent inliers, the Cross Fell inlier and the Craven inliers.[2]

Lake District outcrop

In the main Lake District outcrop, the group consists of calcareous siltstones and mudstones of the basal Kirkley Bank Formation,[3] micritic limestones of the Broughton Moor Formation[4] and dark blue-gray shales of the Ashgill Formation.[5] Locally volcanic formations are developed, including the Yarlside Volcanic Formation at the base of the Kirkley Bank Formation and the High Haume Tuff and Appletreeworth Formations at the base of the Ashgill Formations.[6]

Cautley and Dent inliers

In the Cautley and Dent inliers, rocks of the Dent Group occur in two fault bounded strips next to the Dent Fault. In these inliers, calcareous mudstones of the Cautley Mudstone Formation[7] are overlain by the Ashgill Formation. The Cautley Volcanic Member is a unit of volcaniclastic rocks, within the Cautley Mudstone Formation.[6]

Cross Fell inlier

In the Cross Fell inlier, the basal Dufton Shale Formation[8] is followed by the Swindale Limestone Formation[9] and the Ashgill Formation. In the Keisley area, the uppermost part of the Dent Group is represented by the highly fossiliferous Keisley Limestone Formation, exposed in the core of an anticline.[10]

Craven inliers

The Craven inliers are found to the north of the North Craven Fault. In these inliers, calcareous siltstones and muddy limestones of the Norber Formation[11] are overlain by siltstones and mudstones of the Sowerthwaite Formation.[12] The Dam House Bridge Tuff Member forms the basal part of the Sowerthwaite Formation.[6]

Tectonic setting

The early Paleozoic geology of northern England records the convergence and eventual collision of the Avalonia microcontinent with the continent of Laurentia. The underlying Borrowdale Volcanic Group formed in an island arc, caused by the southerly subduction of the crust of the now vanished Iapetus Ocean. At the end of active arc volcanism, the area began to subside, allowing deposition of the shallow marine Dent Group. The final closure of Iapetus occurred during the Devonian Acadian orogeny, with the resulting continental collision causing deformation of the Ordovician rocks, including folding, thrusting and cleavage development.

Depositional environment

The Dent Group was deposited in a shallow water environment.[1]

See also

References

  1. 1 2 "Dent Group". The BGS Lexicon of Named Rock Units. British Geological Survey. Retrieved 14 March 2015.
  2. ↑ Rushton A.W.A. & Ingham J.K. "Northern England". In Fortey R. A Revised Correlation of Ordovician Rocks in the British Isles. Special Reports. 24. London: Geological Society. pp. 33–37. ISBN 9781862390690.
  3. ↑ "Kirkley Bank Formation". The BGS Lexicon of Named Rock Units. British Geological Survey. Retrieved 17 March 2015.
  4. ↑ "Broughton Moor Formation". The BGS Lexicon of Named Rock Units. British Geological Survey. Retrieved 17 March 2015.
  5. ↑ "Ashgill Formation". The BGS Lexicon of Named Rock Units. British Geological Survey. Retrieved 17 March 2015.
  6. 1 2 3 "A stratigraphical framework for the upper Ordovician and Lower Devonian volcanic and intrusive rocks in the English Lake District and adjacent areas" (PDF). Integrated Geoscience Surveys (North) Programme Research Report RR/01/07. British Geological Survey. 2004. Retrieved 28 March 2015.
  7. ↑ "Cautley Mudstone Formation". The BGS Lexicon of Named Rock Units. British Geological Survey. Retrieved 17 March 2015.
  8. ↑ "Dufton Shale Formation". The BGS Lexicon of Named Rock Units. British Geological Survey. Retrieved 17 March 2015.
  9. ↑ "Swindale Limestone Formation". The BGS Lexicon of Named Rock Units. British Geological Survey. Retrieved 17 March 2015.
  10. ↑ "Keisley Limestone Formation". The BGS Lexicon of Named Rock Units. British Geological Survey. Retrieved 19 March 2015.
  11. ↑ "Norber Formation". The BGS Lexicon of Named Rock Units. British Geological Survey. Retrieved 17 March 2015.
  12. ↑ "Sowerthwaite Formation". The BGS Lexicon of Named Rock Units. British Geological Survey. Retrieved 17 March 2015.
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