The BGS Lexicon of Named Rock Units — Result Details

Cosheston Group
Computer Code: COB Preferred Map Code: notEntered
Status Code: Full
Age range: Lochkovian Age (DO) — Emsian Age (DE)
Lithological Description: Predominantly grey-green very fine- to coarse-grained, plant-bearing sandstones, red-brown and purple-brown siltstones; some intraformational conglomerates in the lower part (Llanstadwell and Mill Bay formations); extraformational conglomerates in the upper part (Lawrenny Cliff and New Shipping formations), becoming dominant at the top of the Group (New Shipping Formation). The rocks are arranged predominantly in fining-upward rhythms.
Definition of Lower Boundary: The base of the group is coincident with the base of the Llanstadwell Formation. This is placed at the base of the lowest primary-coloured green bed at the western end of the railway cutting in the naval depot at Newton Noyes, Pembrokeshire [SM 9150 0530]. This is a 0.32m thick greenish grey, coarse grained, parallel-laminated siltstone 21.7m above the base of the section logged by Thomas et al (2006), which lies conformably on red mudstones of the underlying Gelliswick Bay Formation (formerly Red Marl Group).
Definition of Upper Boundary: The upper boundary is not seen, the highest (New Shipping) Formation being unconformably overlain by strata of the Avon Group (formerly the Lower Limestone Shales) of early Carboniferous age. This junction is seen on the shore on the east side of the Carew River [SN 03852 04105] southsoutheast of New Shipping Point. It is a scour surface with up to 2cm relief cut in medium- to coarse-grained purple sandstone overlain by a bed of quartz grit up to 4cm thick with shell fragments.
Thickness: To 1800m
Geographical Limits: The Group comprises an area of 27 square km in the Milford Haven area, south Pembrokshire. It mainly crops out in the Burton Anticline, between the Benton Fault in the north [c.SM 97 08] to near Cosheston [SN 003 034] in the south, and from Llanstadwell [SM 95 05] in the west to New Shipping [SN 04 04] in the east. A smaller outlier to the west of the main outcrop extends from Newton Noyes [SM 92 05] in the west to Wear Point [SM 940 043] in the east.
Parent Unit: Lower Old Red Sandstone (LORS)
Previous Name(s): Cosheston Beds (-801)
Alternative Name(s): none recorded or not applicable
Stratotypes:
Partial Type Section  Partial type section in the railway cutting in the naval depot at Newton Noyes, Pembrokshire., where the base and lowermost 96.35m of the group (and of the Llanstadwell Formation) are exposed. Cantril et al. (1916), Thomas et al. (2006). See Thomas et al, 2006, Figure 7 for a detailed log of thesection. 
Partial Type Section  Partial type section at Burton Cliff of the lowermost quarter of the Mill Bay Formation. The section lies on the northeast side of Burton Hawn, where the base of the Formation [SM 09067 05258] and succeeding 30m are exposed. Thomas et al, 2006, Figure 9. 
Partial Type Section  The eastern sector of Mill Bay ("Mill Bay East") provides a stratotype for the base and the lowermost beds of the Lawrenny Cliff Formation. See Thomas et al, 2006, Figure 14B. 
Type Area  Milford Haven area. 
Partial Type Section  Sections in Lawrenny Cliff, which forms the western margin of the Lawrenny peninsula, Milford Haven, Pembrokshire. South of the Benton Fault, Lawrenny Cliff is divided into northern, cenral and southern sections. The central section ("LC-C") is the partial type section for the Lawrenny Cliff Formation, exposing all but the basal 10% of the Formation. Thomas et al, 2006, Figure 17. 
Partial Type Section  Partial type section at New Shipping exposing about 80% of the New Shipping Formation, in discontinuous low cliffs and rock platforms on the northern shore of the Carew River between New Shipping Point [SN 0335 0463] and 200m southsouthwest of New Shipping Farm [SM 0388 0410]. See Thomas et al. 2006, Figure 19A for a representative 32m portion of the section. 
Partial Type Section  Shore section east of Hobbs Point, Milford Haven, Pembrokshire, where 235.39m of beds of the Llanstadwell Formation are exposed. See Thomas et al, 2006, Figure 8 for a detailed log of the section). Thomas et al, 2006. 
Partial Type Section  Partial type section in Mill Bay of the uppermost threequarters of the Mill Bay Formation. It is situated on the southern shore of the Daugleddau estuary 3.5km northeast of Pembroke Dock, Milford Haven. Mill Bay is split into western and eastern sectors by a 50m wide central inlet. The section on the western sector (Mill Bay West) begins near the low water mark [c.SM 99645 05062]. The eastern sector (Mill Bay East) starts a few metres below high water mark on the northeast side of the central inlet [SN 0026 0491] and exposess the uppermost 23m of the Mill Bay Formation and the junction with the overlying Lawrenny Cliff Formation). Thomas et al. 2006. Figure 14B provides a graphic log of the section. 
Reference(s):
Thomas, R G, Williams, B P J, Morrissey, L B, Barclay, W J and Allen, K C. 2006. Enigma variations: the stratigraphy, provenance, palaeoseismicity and depositional history of the Lower Old Red Sandstone Cosheston Group, south Pembrokeshire, Wales. Geological Journal, Vol.41, 481-536. 
Strahan, A, Cantrill, A, Dixon, E E L, Thomas, H H and Jones, O T. 1914. The geology of South Wales. Part XI. Haverfordwest. Memoir of the Geological Survey, England and Wales. 
Cantrill, T C, Dixon, E E L, Thomas, H H, Jones, O T. 1916. The geology of the South Wales Coalfield, Part XII: the country around Milford. Memoir of the Geological Survey, England and Wales. 
1:50K maps on which the lithostratigraphical unit is found, and map code used:
none recorded or not applicable
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