The BGS Lexicon of Named Rock Units — Result Details

Plenus Marls Member

Computer Code: PLMA Preferred Map Code: notEntered
Status Code: Full
Age range: Cenomanian Age (KE) — Cenomanian Age (KE)
Lithological Description: Northern Province: Thinly bedded dark-coloured calcareous mudstone (marl) overlain by rubbly limestone and calcareous silty beds followed by a higher calcareous mudstone siltstone (marl). Southern Province: Interbedded coloured marl and chalk. In the Southern Province, the Plenus Marls unit was defined by Jefferies (1963) and it is now treated at member level. It marks the basal unit of the Holywell Nodular Chalk Formation. The Plenus Marl Member in the Northern Province, as defined in Sumbler (1999), included beds A to H of Dodsworth (1996) and thus contained the Black Band. The term was redefined by Hopson (BGS Research Report RR/05/02) to include only beds A and B of Dodsworth, with the higher beds C to H forming the newly proposed formal term Black Band Member. On this basis the Plenus Marl Member as defined in the Northern Province is the direct equivalent of the member as described for the Southern Province. No formal subdivisions are recognized, but the unit has been finely divided into eight beds of widespread geographical distribution in the Southern Province by Jefferies (1963).
Definition of Lower Boundary: The Lower Boundary is defined in:- The Northern Province as an uneven erosion surface that may be stained with iron minerals and glauconite, at the top of a succession of marly chalk (the Ferriby Formation). This member and the newly proposed Black Band Member, generally form a topographical slack at outcrop, which facilitates the mapping of the base of the Welton Formation, and can also be recognised from its higher gamma-ray geophysical log signature in boreholes. In the Southern Province as conformable beneath the lowest marl on the underlying Zig Zag Chalk Formation. Sequence constitutes the lowest member of the Holywell Nodular Chalk Formation and the White Chalk Subgroup.
Definition of Upper Boundary: The upper boundary is defined in:- the Northern Province as conformable at the base of the Black Band ss (the "Main Black Band" of Wood et al., 1997), the lowest unit of the Black Band Member. In the Southern Province as conformable at the bedding plane above the highest marl and below the hard nodular limestone of the Melbourn Rock Member.
Thickness: Northern Province: About 1.15m at the type-site but to as little as 0.25m elsewhere. Southern Province: Generally in the range 1 to 1.5m, but known to be up to 3.0m at the Holywell Cafe cliff section in Eastbourne.
Geographical Limits: Widespread within the Southern, Transitional and Northern provinces (see additional entry under PLMN) and offshore in the North Sea area. Forms the most characteristic downhole geophysical signature inflecture in the whole of the Chalk Group. In the Northern Province the distribution is not known in detail but it is suspected that this member forms part of the succession formerly identified as the Black Band in boreholes and seismic sections.
Parent Unit: Holywell Nodular Chalk Formation (HCK)
Previous Name(s): Flixton Member (-700)
Belemnite Marls (-4475)
Plenus Marl [Obsolete Name And Code: See PLMA And PLMN] (PLNM)
Variegated Beds (Lower Part) (-3614)
Plenus Marls (-455)
Variegated Beds (-60)
Alternative Name(s): none recorded or not applicable
Stratotypes:
Type Section  Merstham Greystone Limeworks in Surrey. Southern Province type section in old quarry. 
Type Section  Melton Ross Quarry, Lincolnshire. Northern Province type section. See entry under PLMN. 
Reference Section  Melton Bottom upper quarry. This is the stratotype for the whole Welton Chalk Formation. 
Reference Section  South Ferriby Quarry, Lincolnshire. 
Reference(s):
Wood, C J and Mortimore, R N. 1995. An anomalous Black Band succession (Cenomanian-Turonian boundary interval) at Melton Ross, Lincolnshire, eastern England and its international significance. Berliner Geowissenschaftliche Abhandlungen, Reihe E, Vol.16, 277-287. 
Jefferies, R P S. 1963. The stratigraphy of the Actinocamax plenus Subzone (Turonian) in the Anglo-Paris Basin. Proceedings of the Geologists' Association, Vol.74, 1-33. 
Dodsworth, P. 1996. Stratigraphy, microfossils and depositional environments of the lowermost part of the Welton Chalk Formation (late Cenomanian to early Turonian, Cretaceous) in eastern England. Proceedings of the Yorkshire Geological Society, Vol.51, 45-64. 
Sumbler, M G. 1999. The stratigraphy of the Chalk Group in Yorkshire and Lincolnshire. British Geological Survey Technical Report WA/99/02. 
Wood, C J, Batten, D J, Mortimore, R N and Wray, D S. 1997. The stratigraphy and correlation of the Cenomanian-Turonian boundary interval succession in Lincolnshire, northern England. Freiberger Forschungsheft, Reihe C, Vol. 468, 333-346. 
Hopson, P M. 2005. A stratigraphical framework for the Upper Cretaceous Chalk of England and Scotland, with statements on the Chalk of Northern Ireland and the UK Offshore Sector. British Geological Survey Research Report RR/05/01 102pp. ISBN 0 852725175 
1:50K maps on which the lithostratigraphical unit is found, and map code used:
E237