Millstone Grit Formation DCGM


Premise Unit, defined by Van Adrichem Boogaert and Kouwe (1995). Cameron (1993) formally introduced this name for the Southern North Sea area. Information on this unit from the Dutch continental shelf is still scanty. The range of the formation can be assessed only using observations from the adjoining UK sector. For further reading, see descriptions of Collinson (1988) , Holdsworth (1988) , Steele () , Leeder (1990) Collinson (1993) and Cameron (1993) .
Derivatio nominis The name Millstone Grit was introduced by J. Whitehurst in 1778, for a succession of coarse-grained sandstones and interbedded mudstones, which are found in many outcrops throughout the Central Pennines of England. Four reference sections have been designated for the UK Southern North Sea Basin by Cameron (1993) .
Type section Location map See figure (pdf)
  Well A14-01 (pdf)
  Location N 55°12’01.1
E 03°36’38.3
  Depth 1609 to 1717 m
  Length 108 m along hole
Additional section Location map See figure (pdf)
  Well Nagele-1 (pdf)
  Location N 52°37’10.0
E 05°44’23.3
  Depth 2598 to 2708 m
  Length 110 m along hole
Definition In the UK offshore (summarised after Cameron (1993) ): succession of interbedded grey, white and brown sandstones, and dark-grey, partly carbonaceous mudstones and siltstones. Individual sandstone beds are more than 10 m thick, but stacked beds can amount up to 80 m. The sands are mainly fine- to medium-grained, with occasional gravelly intercalations. Sandstone beds can have sharp tops and bottoms, or grade into mudstones. Mudstone-dominated intervals with thin silt- and fine-grained sandstone intercalations can be up to 100 m thick. The formation consists of stacked coarsening-upward cycles, generally starting with a basal marine interval. In the Dutch reference section, some 1 - 4 m thick argillaceous limestone beds and tuff layers are intercalated. Sandstone beds are up to 17 m thick here.
Upper Boundary The Millstone Grit Formation reflects a depositional regime, intermediate between the basin facies of the Epen Formation and the delta-plain facies of the Klaverbank Formation. As a result of the north-to-south regressive development of the Namurian-Westphalian in the Silverpit Basin area, the two formations are partial age equivalents. The Ubachsberg Formation only rarely displays intercalated massive sandstones ( Klaverbank Member). The Klaverbank Formation contains coal seams.
Lower Boundary The boundary with the underlying Epen Formation has been placed at the base of the lowermost sandstone interval which is more than 10 m thick. The top has been drawn at the base of the sandstone bed immediately underlying the first definite coal seam, or the base of that coal seam where such a sandstone is not developed (Cameron (1993) ).
Correlation The Millstone Grit Formation is the thicker, usually coarser-grained equivalent in the UK and northwestern Dutch offshore Cameron (1993) .
Distribution At the moment (1995), well A14-01 is the only released Dutch well in which this formation was encountered. Information from the adjoining UK sector of the North Sea implies that this unit extends southwards until the UK part of the London-Brabant Massif (e.g. well 53/12-02; Cameron (1993) ). The formation thins to the south and east, as it grades into the Epen
Age Mostly Namurian B/C, but locally extending into the earliest Westphalian A. Near the Mid North Sea High possibly even including part of the Namurian A. The individual sandstone intervals have been attributed to regressive phases during the Namurian B and C, separated by specific marine bands (Ramsbottom (1979) ; Ross (1988) ; Holdsworth (1988) ; Leeder (1990) ; Collinson (1988) , Cameron (1993) ). The age of the base is determined by the timing of the first incursion of sandstones. This in turn is related to the overall regressive development of the Geul Subgroup from the north and northwest. The top of the member is also diachronous, as a consequence of the variable first occurrence of peat (coal) deposition. In UK quadrant 41 this commenced in the Namurian B, but in block 48/03 it started during the Westphalian A Leeder (1990) ;Cameron (1992) ). In many UK wells the first coal seam is found 50 - 100 m above the Namurian-Westphalian boundary (Cameron (1993) ).
Depositional Setting Most of the sandstones in this formation have been deposited by sheet-delta systems (Collinson (1988) ). The sandstones have been interpreted as delta-front deposits: mouth bars, and distributary-channel fills. Their amalgamation during delta progradation has resulted in sheets of great lateral continuity. Intercalated fines reflect periods of higher sea/lake level, and were deposited as prodelta turbidites, or settled from suspension. Marine bands represent brief marine incursions.
References See References Upper Carboniferous

Van Adrichem Boogaert, H.A. & Kouwe, W.F.P., 1993-1997. [Stratigraphic unit]. In: Stratigraphic Nomenclature of the Netherlands.
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