The BGS Lexicon of Named Rock Units — Result Details

Stretton Group

Computer Code: YST Preferred Map Code: notEntered
Status Code: Pending Upgrade
Age range: Ediacaran Period (AD) — Ediacaran Period (AD)
Lithological Description: Interbedded purple and green mudstones, siltstones, sandstones and subordinate conglomerates, with several tuff units. The suffix 'Eastern Longmynd' is superfluous and it is also preferable not to ascribe a geographical extent to this division, given the uncertainties over its relationship to the much more extensive Wentnor Group that overlies it (i.e. whether their boundary is unconformable or not). It is possible that the term Stretton Group will become obsolete in the future, as there is already a formation of this name (the Stretton Shale Fmn). However, the term 'Stretton Group' is currently used and understood widely (eg. Pauley 1990), and it is suggested that this is retained as an informal name.
Definition of Lower Boundary: Uncertain. Taken by some at base of Helmeth Grit Member (Cobbold 1935; Greig et al, 1968), but the latter was included by Pauley (1990) within the Ragleth Tuff Formation. The relationship of the latter to the Uriconian Volcanics is uncertain as all contacts are faulted.
Definition of Upper Boundary: Traditionally taken at the top of the Portway Formation by those who considered there to be an unconformity with the overlying Wentnor Group at this level (e.g. Greig et al, 1968). However, Pauley (1990) considered that no such unconformity exists, and the succession was one of prograding alluvial facies.
Thickness: 2700-4000 m thick.
Geographical Limits: Welsh Borderlands, Shropshire, Powys.
Parent Unit: Longmyndian Supergroup (LO)
Previous Name(s): Stretton Series (-3152)
Alternative Name(s): none recorded or not applicable
Stratotypes:
Type Area  The Longmynd, Shropshire. 
Reference(s):
Cobbold, E S. 1901. The geology of the Church Stretton district. 1-115 in Church Stretton vol 1. Campbell-Hyslop, C W (Editor). No. 1. (Shrewsbury: L Wilding). 
Cobbold, E S. 1925. Unconformities in south Shropshire. Proceedings of the Geologists' Association, Vol. 36, 364-367. 
Lapworth C and Watts W W. 1910. Shropshire. 739-769 in Geology in the field; the Jubilee Volume of the Geologists' Association (1858-1908). Monckton H W and Herries R S (editors). (London: Edward Stanford). 
Pauley J C. 1990. Sedimentology, structural evolution and tectonic setting of the Late Precambrian Longmyndian Supergroup of the Welsh Borderland, UK. 341-351 in the Cadomian Orogeny. D'Lemos R S, Strachan R A and Topley C G (editors). Geological Society of London Special Publication No. 51. 
Pauley J C. 1991. A revision of the stratigraphy of the Longmyndian Supergroup, Welsh Borderland, and of its relationship to the Uriconian volcanic complex. Geological Journal, Vol. 26, 167-183. 
Greig D C, Wright, J E, Hains, B A and Mitchell, G H. 1968. Geology of the country around Church Stretton, Craven Arms, Wenlock Edge and Brown Clee. Memoir of the Geological Survey, Sheet 166 (England and Wales). (London: H.M.S.O.). 379pp. 
Cobbold, E S and Whittard, W F, 1935. The Helmeth Grits of the Caradoc Range, Church Stretton; their bearing on part of the Pre-Cambrian succession of Shropshire. Proceedings of the Geologists' Association, Vol.46, 348-359. 
Pauley J C. 1990. The Longmyndian Supergroup and related Precambrian sediments of England and Wales. 5-27 in Avalonian and Cadomain geology of the North Atlantic. Strachan R A and Taylor G K (editors). (Glasgow: Blackie). 
1:50K maps on which the lithostratigraphical unit is found, and map code used:
E180