The BGS Lexicon of Named Rock Units — Result Details

Codden Hill Chert Formation

Computer Code: COH Preferred Map Code: notEntered
Status Code: Full
Age range: Arundian Substage (CJ) — Brigantian Substage (CX)
Lithological Description: Lower half (Tawstock Formation of Jackson, 1991) comprises thin- to medium-bedded pale and dark grey siliceous mudstones, shaly mudstones and cherts. Upper half (Hearson and Rubble Hills formations of Jackson, 1991) comprises thin- to medium-bedded limestones, turbidites and mudstones with streaky laminated cherts in the uppermost part. Two thin tuffs are present and Pectinoid bivalves are common in the upper half. Facies variations mapped on Sheet 293 (Barnstaple) by Edmonds et al (1985) have been shown by Jackson (1991) to be illusory and due to faulting resulting in stratigraphic omissions. Similarly a facies recognised by Prentice (1960) in the Fremington area and dominated by limestones and mudstones has been shown by Jackson (1991) to be the upper half of the Formation with the lower half thrust out.
Definition of Lower Boundary: The lower boundary is taken at the incoming of thin-bedded dark grey siliceous mudstones and shaly mudstones of the Codden Hill Chert Formation above the dark grey slightly siliceous mudstones of the Doddiscombe Formation (Unit D of the Pilton Beds of Prentice 1960). The Doddiscombe Formation has not been mapped on Sheet 293 (Barnstaple by Edmonds et al (1985), being included in the Pilton Mudstone Formation and/or Codden Hill Chert Formation. Further investigation is needed.
Definition of Upper Boundary: West of South Molton, the upper boundary is taken at the incoming of dark grey to black pyritic mudstones within thin siltstones of the overlying Dowhills Mudstone Formation above the streaky laminated cherts and mudstones of the Codden Hill Chert Formation. The boundary is taken at the top of the highest chert bed. The Dowhills Mudstone Formation (Limekiln Beds of Prentice, 1960), was included in the Crackington Formation by Edmonds et al. (1985) on Sheet 293 (Barnstaple). Further investigation is needed.
Thickness: 190m
Geographical Limits: Between Fremington and South Molton, north Devon (SS 70 28).
Parent Unit: Teign Valley Group (TEVY)
Previous Name(s): Codden Hill Group (-804)
Bydown Chert [Obsolete Name And Code: See COH] (BYDC)
Mount Ararat Chert Formation [Obsolete Name And Code: Use COH] (ACH)
Meldon Chert Formation [Obsolete Name And Code: Use COH] (MCF)
Newton Chert Formation [Obsolete Name And Code: Use COH] (NCH)
Chert Beds (*642)
Codden Hill Beds (-3317)
Codden Hill Chert (-3318)
Tawstock, Hearson And Rubble Hills Formation (-3319)
Combe Wood Chert [Obsolete Name And Code: See COH] (CWC)
Fire Beacon Chert Formation [Obsolete Name And Code: Use COH] (FCF)
Alternative Name(s): none recorded or not applicable
Stratotypes:
Partial Type Section  Codden Hill Quarry at western end of Codden Hill, Bishop's Tawton, north Devon. Quarry exposes about 20m of bedded cherts with thin siliceous shaly partings and beds in the middle of the Formation. Neither the lower or upper boundary of the Formation is seen. 
Reference(s):
Edmonds, E A, Whittaker, A, and Williams, B J. 1985. Geology of the country around Ilfracombe and Barnstaple. Memoir of the British Geological Survey, Sheets 277 and 293 (England and Wales). 
Prentice, J E. 1960. The Dinantian, Namurian and Lower Westphalian rocks of the region of southwest of Barnstaple, north Devon. Quarterly Journal of the Geological Society, London, Vol.115, 261-290. 
Edmonds, E A, Williams, B J and Taylor, R T. 1979. Geology of Bideford and Lundy Island. Memoir of the Geological Survey of Great Britain, Sheets 292 with 275, 276, 291, and part of 308 (England and Wales). 
Jackson, P C. 1991. The Dinantian stratigraphy of northwest Devon. Proceedings of the Yorkshire Geological Society, Vol.48, 447-460. 
Ussher, W A E. 1887. The Culm of Devonshire. Geological Magazine, Vol.24, 10-17. 
1:50K maps on which the lithostratigraphical unit is found, and map code used:
none recorded or not applicable