The BGS Lexicon of Named Rock Units — Result Details

Llanstadwell Formation

Computer Code: LLSW Preferred Map Code: notEntered
Status Code: Full
Age range: Lochkovian Age (DO) — Pragian Age (DP)
Lithological Description: Predominantly grey-green very fine- to medium-grained, plant bearing sandstones, with some intraformational conglomerates, red-brown and purple-brown siltstones; the facies are mainly arranged in fining-upwards cyclothems; up to 20% of the formation is affected by soft sediment deformation.
Definition of Lower Boundary: The base of the Formation is placed at the base of the lowest primary-coloured green siltstone at the western end of the railway cutting in the naval depot at Newton Noyes, Pembrokshire [SM9150 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 of the Formation is placed at the base of an 11.8m-thick multistorey complex of interbedded grey-green intraformational conglomerates and sandstones with a few siltstones at the base of the overlying Mill Bay Formation. The boundary at Burton Cliff [SM 98967 05258] is placed northeast of Burton Hawn at a low-relief scour surface by the lowermost intraformational conglomerate of the Mill Bay Formation (see Thomas et al, 2006, Figure 9 for a detailed log of the beds above and below the boundary).
Thickness: 435 to 550m
Geographical Limits: Confined to the northern side of the Daugleddau estuary, Milford Haven, except for a small area east of Hobbs Point [SM 971 043] on the south side of the estuary from Llanion [SM 97 04] to Waterloo [SM 98 04]. On the north side, an outcrop extends from west of Newton Noyes [SM 92 05] to the east of Weir Point [SM 940 043]; the main outcrop extends from Llanstadwell [SM 950 050] eastwards to near Burton [SM 990 052], and from there northnorthwestwards around the axis of the Burton Anticline to the Benton Fault east of Rosemarket [c. SM 960 080]. (See Thomas et al, 2006, Figure 1).
Parent Unit: Cosheston Group (COB)
Previous Name(s): Burton Cliff Formation (BRCL)
Lower Cosheston Division (-4164)
Alternative Name(s): Burton Cliff Palynological Association
Hobbs Point Palynological Association
Stratotypes:
Partial Type Section  Shore section east of Hobbs Point, Milford Haven, Pembrokeshire, where 235.39m of beds are exposed. See Thomas et al, 2006 Figure 8 for a detailed log of the section. 
Partial Type Section  Named from the village of Llanstadwell, Milford Haven, Pembrokeshire. This partial type section is the railway cutting in the naval depot at Newton Noyes, Pembrokeshire [SM 9150 0530], where the lowermost 96.35m are exposed. Cantrill et al, (1916); see Thomas et al, 2006, Figure 7 for a detailed log of the section. 
Reference(s):
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. 
Thomas, R G. 1978. The stratigraphy, palynology and sedimentology of the Lower Old Red Sandstone Cosheton Group, southwest Dyfed, Wales. PhD Thesis, University of Britstol. 
Thomas, R G, Williams, B P J, Morrissey, L B, Barclay, W J and Allen, K C. 2005. Enigma variations: the stratigraphy, provenance, palaeoseismicity and depositional history of the Lower Old Red Sandstone Cosheston Group, south Pembrokshire, Wales. Geological Journal, Vol.41, 481-536. 
1:50K maps on which the lithostratigraphical unit is found, and map code used:
none recorded or not applicable