The BGS Lexicon of Named Rock Units — Result Details

Minera Formation

Computer Code: MRF Preferred Map Code: notEntered
Status Code: Full
Age range: Brigantian Substage (CX) — Brigantian Substage (CX)
Lithological Description: Interbedded limestones and calcareous sandstones. Limestone components compare with those of the underlying Cefn Mawr Limestone Formation and include both thick-bedded to massive pale grey limestones [packstones and grainstones], and sequences of thinly interbedded dark grey limestone [packstones and wackstones] and mudstone arranged in shoaling upwards cycles with the calcareous sandstones typically forming the upper parts of the cycles. The sandstones include oolitic units composed of radial fibrous ooids with sand-grain cores. Chert beds and nodules are common at some levels, and coral biostromes are present locally. The Minera Formation records late Brigantian, mixed carbonate-siliciclastic shallow marine deposition at a time of increased supply of siliciclastic detritus to the North Wales Dinantian platform. Each cycle represents a shoaling upwards sequence formed in response to transgressive and regressive movements in sea level. The diagnostic calcareous sandstones principally record the progradation of nearshore siliciclastic facies over transgressive off-shore limestone facies. Many regressions culminated in emergence of the platform surface, but, since sandstones commonly occupy the tops of the cycles, calcrete and karstic dissolution features, which are a common feature in the Cefn Mawr Limestone Formation, are only poorly developed in the Minera Formation.
Definition of Lower Boundary: Base of the lowest, laterally extensive and mappable sandstone above the Cefn Mawr Limestone Formation, but diachronous as the formation passes laterally into the Cefn Mawr Limestone Formation in North Flintshire.
Definition of Upper Boundary: Top of the highest laterally extensive and mappable limestone beneath thick, sandstone dominated sequences of the Cefn-y-fedw Sandstone Formation.
Thickness: c.50m in the type area, thickening to 180m north of the Bala Lineament.
Geographical Limits: Northeast Wales between Oswestry [SJ2825] and Halkyn Mountain [SJ1970]
Parent Unit: Clwyd Limestone Group (CLWYD)
Previous Name(s): Arenaceous Limestone (-4804)
Sandy Limestone: Arenaceous Limestone; Sandy Passage Beds (-4203)
Alternative Name(s): none recorded or not applicable
Stratotypes:
Type Area  Crags and stream sections on the north flank of Ruabon Mountain west of Wrexham Brown, 1960 
Reference(s):
Davies, J R, Wilson, D and Williamson, I T. 2004. Geology of the country around Flint. Memoir for 1:50 000 Geological Sheet 108. 
Brown, W W M, 1960. The petrology and palaeoecology of certain Carboniferous Limestone regions of North Wales. Unpublished PhD thesis, University of Wales, Aberystwyth. 
Wedd, C B, Smith, B, King, W B R and Wray, D A. 1929. Geology of the country around Oswestry. Memoir of the Geological Survey of Great Britain, Sheet 137. 
Wedd, C B, Smith, B and Wills, L J. 1927. The geology of the country around Wrexham, Part 1. Lower Palaeozoic and Lower Carboniferous rocks. Memoir of the Geological Survey of Great Britain. [London: HMSO.] 179pp. 
Morton, G H 1876-8. The Carboniferous Limestone and Millstone Grit of North Wales. Proceedings of the Liverpool Geological Society. Vol.3, 190-201. 
1:50K maps on which the lithostratigraphical unit is found, and map code used:
E121 E137