The BGS Lexicon of Named Rock Units — Result Details

Pentre Chert Formation

Computer Code: PECH Preferred Map Code: notEntered
Status Code: Full
Age range: Pendleian Substage (CE) — Pendleian Substage (CE)
Lithological Description: Banded glassy cherts and siliceous (cherty) mudstones with subordinate thin siltstone and silicified crinoidal limestone beds, and scattered beds of pebbly quartzose sandstone, arranged in thickening-upwards cycles. Petrographic studies suggest that recrystalised radiolaria tests and rod-shaped sponge spicules are locally abundant in some of the cherts. The Pentre Chert Formation represents an offshore to basinal early Namurian facies deposited across the northern sector of the former Dinantian limestone platform, in a setting isolated from coarser grade siliclastic contamination, but in which sponges and radiolaria where abundant. The formation also exhibits the effects of extensive secondary silicification.
Definition of Lower Boundary: Conformable or disconformable contact between the chert lithologies of the Pentre Chert Formation and mudstones of the Teilia Formation around Prestatyn, or, to the south, with limestones of the Cefn Mawr Limestone Formation.
Definition of Upper Boundary: Chert/mudstone contact taken as the base of the Bowland Shale Formation.
Thickness: To 180m
Geographical Limits: Northeast Wales between Prestatyn (SJ 09 83) and Halkyn Mountain (SJ 21 69).
Parent Unit: Craven Group (CRAV)
Previous Name(s): Chert Beds (*642)
Halkyn Formation [Obsolete Name And Code: See Revision Data] (HAF)
Alternative Name(s): none recorded or not applicable
Type Section  Pentre Quarry and the Gronant Borehole (drilled in floor of quarry), Gronant. SJ08SE/22 (SJ 0950 8279). Together the quarry and borehole provide a continuous section through the basal 83m of the formation. Warren et al., 1984. 
Warren, P T, Price, D, Nutt, M J C and Smith, E G. 1984. Geology of the country around Rhyl and Denbigh. Memoir for 1:50 000 geological sheets 95 and 107 and parts of sheets 94 and 106. 
Davies, J R, Wilson, D and Williamson, I T. 2004. Geology of the country around Flint. Memoir for 1:50 000 Geological Sheet 108. 
1:50K maps on which the lithostratigraphical unit is found, and map code used: