The BGS Lexicon of Named Rock Units — Result Details

Lawrenny Cliff Formation

Computer Code: LWC Preferred Map Code: notEntered
Status Code: Full
Age range: Emsian Age (DE) — Emsian Age (DE)
Lithological Description: The Formation consists of very fine- to coarse-grained, grey-green sandstones 46.1%), extraformational conglomerates (18.6%), interlaminated fine- to medium-grained to very fine-grained sandstones and red-brown siltstones (22.5%), red-brown siltstones (10.4%) and intraformational conglomerates (2.4%). The rocks are arranged predominantly in 0.4 to 11.0m-thick, fining-upward rhythms. Abundant exotic detritus in the extraformational conglomerates characterize the Formation and differentiate it from the underlying Mill Bay Formation. The Formation is prediminantly grey-green (68.1%), the remainder (mainly siltstones) are red-brown or red-purple; reddish colouration increases upwards in the Formation.
Definition of Lower Boundary: The base of the Formation is a conformable, gradational junction with the Mill Bay Formation. It lies 139.91m above the base of the section in the eastern section of Mill Bay (Mill Bay East) measured by Thomas et al, (2006), at the base of the first extraformational conglomerate, which is 0.09m thick and contains over 10% of exotic clasts of the total clast population. It lies above an irregular erosion surface with up to 0.07m relief, and forms part of a 0.48m thick unit of interbedded sandstones and extraformational conglomerates (see Thomas et al, 2006, Figure 14B).
Definition of Upper Boundary: The upper boundary is a conformable transition from the grey-green sandstones and conglomerates at the top of the Formation into the red-brown, coarser extraformational conglomerates of the overlying New Shipping Formation. The actual boundary is not exposed.
Thickness: 210-250m
Geographical Limits: The Formation occupies most of the southern half of the Lawrenny peninsula [c. SN 015 065] south of the Benton Fault. It also crops out on the west side of the Daugleddau estuary in a small area north of Williamston Pill [SM 995 065] to the Benton Fault [SM 995 075] northwest of Benton Castle in the core of the Lawrenny Syncline. It is also present in a small, fault-bounded outlier at the southern end of Williamston Cliff [SN 002 054]. The main outcrop lies south of the Lawrenny peninsula, south of the Carew River and east of Cosheston [SN 00 04] from near Paskeston [SN 025 035] north to the Carew River near Mount Pleasant [SN 016 059].
Parent Unit: Cosheston Group (COB)
Previous Name(s): Cosheston Group (COB)
Alternative Name(s): none recorded or not applicable
Partial Type Section  The formation is named from Lawrenny Cliff, which forms the western margin of the Lawrenny peninsula, Milford Haven, Pembrokeshire. South of the Benton Fault, Lawrenny Cliff is divided into northern, central and southern sections. The central section ('LC-C') is the partial type section for the formation (Thomas et al, 2006, Figure 17), exposing all but the basal 10% of the formation. Thomas et al. 2006. 
Partial Type Section  The eastern sector of Mill Bay ('Mill Bay East') provides a stratotype for the base of the formation and the lowermost beds (see Thomas et al, 2006, Figure 14B). 
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. 
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