The BGS Lexicon of Named Rock Units — Result Details

Dark Grey Mudstone Member [Obsolete Name And Code: See MSW]

Computer Code: DMM Preferred Map Code: notEntered
Status Code: Index Level
Age range: Aeronian Age (SA) — Aeronian Age (SA)
Lithological Description: Comprises dark grey anoxic mudstone. Pyritous. Generally sandstone and siltstone poor. Graptolitic hemipelagic mudstone is dominant.
Definition of Lower Boundary: The member is underlain by the dominantly oxic mudstones and siltstones of the Derwenlas Formation. Generally a sharp boundary.
Definition of Upper Boundary: The member is overlain by oxic sandstones, siltstones and mudstones of the remainder of the Cwmsymlog Formation. Generally a sharp boundary.
Thickness: (i) Up to 18-20 metres thick. (ii) 18-20 metres thick in the type section.
Geographical Limits: The unit is located in the south-east and north-east parts of Sheet 149. It is present on the Aberystwyth Sheet 163 as the "Dark Grey Mudstone".
Parent Unit: Cwmsymlog Formation [Obsolete Name And Code: See RHS] (CSF)
Previous Name(s): Monograptus-Sedgwicki Shales (-4000)
Dark Grey Mudstone (-4612)
Alternative Name(s): none recorded or not applicable
Stratotypes:
Type Section  Type locality provides type section. Old coach road, Derwenlas, 335m due west of the old National School. 
Reference Section  The type section is fully representative of the member. 
Reference(s):
Jones, O T, and Pugh, W J, 1916. The geology of the district around Machynlleth and the Llyfnant Valley. Quarterly Journal of the Geological Society, Vol.71, p. 343-385. 
Cave, R and Hains, B A. 1986. Geology of the country between Aberystwyth and Machynlleth. Memoir of the British Geological Survey, Sheet 163 [England and Wales]. [London: HMSO.]. 148pp. 
1:50K maps on which the lithostratigraphical unit is found, and map code used:
none recorded or not applicable