The BGS Lexicon of Named Rock Units — Result Details

Mill Bay Formation

Computer Code: MBY Preferred Map Code: notEntered
Status Code: Full
Age range: Lochkovian Age (DO) — Pragian Age (DP)
Lithological Description: Coarse- to very fine-grained green-grey sandstones (68.2%), interlaminated/thinly interbedded medium- to very fine-grained sandstones and red-brown siltstones (14.9%), intraformational conglomerates (9.7%) and red-brown siltstones (7.3%). All three sandstone categories in the Cosheston Group - litharenite, sublitharenite and subarkose - are present, but the last is restricted to this Formation. c.30% of the Formation is affected by synsedimentary deformation structures.
Definition of Lower Boundary: The base of the Formation is conformable on the underlying Llanstadwell Formation. It is defined at Burton Cliff northeast of Burton Hawn [SM 98967 05258], at the base of an 11.8m-thick multistorey complex of green-grey, interbedded intraformational conglomerates, horizontally or low-angle parallel-laminated F/M to VF sandstones and a few thin siltstones, (see Thomas et al, 2006, figures 9 and 10). The base (at 110.34m above the base of the section measured by Thomas et al, 2006) is a low relief (?0.2m) scour surface overlain by a 0.05m-thick intraformational conglomerate.
Definition of Upper Boundary: The upper boundary is a conformable, transitional one with the overlying Lawrenny Cliff Formation. The 139.91m-thick section at Mill Bay (east) starts a few metres below the high water mark on the northeast side of Mill Bay inlet, at the base of a 1.59m-thick, fine-grained, horizontally and low-angle parallel-laminated sandstone [SN 0026 0491], and ends in the Lawrenny Cliff Formation, this being the only section that exposes the boundary (see Thomas et al, 2006, Figure 14B), which intersects the high water mark 60m southwest of the quay [SN 00570 05260].
Thickness: 540 to 600m
Geographical Limits: On and adjacent to the shores of the Daugleddau estuary, Milford Haven, Pembrokeshire around the axial trace of the Burton Anticline. This encompasses an area around Cosheston village [SN 00 04] from Cosheston Pill [SM 995 035] northwards to Mill Bay [SN 00 05], and on the west side of the Daugleddau from east of Burton [SN 000 055] northwest to the Burton Fault in the Houghton [SM 983 073] - Williamson [SM 990 070] area (see Thomas et al., 2006, Figure 1).
Parent Unit: Cosheston Group (COB)
Previous Name(s): Lower Cosheston Division (-4164)
Alternative Name(s): none recorded or not applicable
Stratotypes:
Partial Type Section  Mill Bay, from which the unit is named, provides a type section for the uppermost threequarters of the 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 exposes the uppermost 23m of the Formation and the junction with the overlying Lawrenny Cliff Formation. See Thomas et al, 2006, Figure 14B. 
Partial Type Section  Burton Cliff provides a type section for the lowermost quarter of the Formation. The section lies on the northeast side of Burton Hawn, where the base of the Formation [SM 98967 05258] and succeeding 30m are exposed. Thomas et al, 2006, Figure 9. 
Reference(s):
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