The BGS Lexicon of Named Rock Units — Result Details

Loggerheads Limestone Formation

Computer Code: LGHL Preferred Map Code: LoL
Status Code: Full
Age range: Asbian Substage (CR) — Asbian Substage (CR)
Lithological Description: Thickly bedded, massive, pale grey shelly limestones (packstones and grainstones), locally mottled and pseudobrecciated, arranged in shoaling upwards cycles capped by calcretes, hummocky palaeokarstic surfaces and associated thin bentonitic clay seams (palaeosols) and rare coals. Locally dolomitised and with scattered chert nodules. The Loggerheads Limestone Formation records late Asbian platform carbonate deposition on the North Wales Dinantian shelf. Each cyclic sequence records a shoaling upwards unit developed in response to transgressive and regressive movements in sea level. Many regressions culminated in emergence of the platform surface and the formation of calcrete and karstic dissolution features. During these periods of emergence, wind blown volcanic ash accumulated on the platform surface to form thin bentonitic soils.
Definition of Lower Boundary: Top of highest porcellanous limestone of underlying Leete Limestone Formation.
Definition of Upper Boundary: Base of lowest sequence of thinly interbedded dark-grey limestones and mudstones of overlying Cefn Mawr Limestone Formation.
Thickness: To 275m
Geographical Limits: North Wales between Anglesey [SH5080] and Oswestry [SJ2723]; including the Corwen Outlier [SJ0542].
Parent Unit: Clwyd Limestone Group (CLWYD)
Previous Name(s): Middle White Limestone (-2900)
Llandulas Limestone Formation [Obsolete Name And Code: Use LGHL] (LNDL)
Great Orme Limestone Formation [Obsolete Name And Code: Use LGHL] (ORL)
Llandulas Limestone (*419)
Penmon Limestone Formation [Obsolete Name And Code: Use LGHL] (PEL)
Eglwyseg Limestone (*73)
Treborth Formation [Obsolete Name And Code: Use LGHL] (TREB)
White Limestone, Middle [Obsolete Name And Code: Use LGHL] (MDWL)
Alternative Name(s): Great Orme Limestone
Eglwyseg Limestone
Llandulas Limestone
Stratotypes:
Partial Type Section  Base exposed in scarp east of River Alyn, in the Leete Country Park at Loggerheads; taken above highest porcellanous limestone of the underlying Leete Limestone Formation. Somerville, 1979. 
Reference(s):
Morton, G H 1876-8. The Carboniferous Limestone and Millstone Grit of North Wales. Proceedings of the Liverpool Geological Society. Vol.3, 190-201. 
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. 
Davies, J R, Wilson, D and Williamson, I T. 2004. Geology of the country around Flint. Memoir for 1:50 000 Geological Sheet 108. 
Somerville, I D, 1979. Cyclotherms in the early Brigantian (lower D2) limestones east of the Clwydian Range, North Wales. Geological Journal, Vol.14, 69-87. 
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. 
1:50K maps on which the lithostratigraphical unit is found, and map code used:
E108 E096 E093 E094 E095 E107 E120 E121 E137