The BGS Lexicon of Named Rock Units — Result Details

Millstone Grit Formation (Offshore) [See Also MG]

Computer Code: MIGR Preferred Map Code: notEntered
Status Code: Full, pending ratification
Age range: Namurian Stage (CN) — Namurian Stage (CN)
Lithological Description: The Millstone Grit Formation is composed of variable proportions of grey, white and brown sandstone, and grey or dark grey, partly carbonaceous mudstone and siltstone. Some sections additionally contain rare thin coal seams and seatearths. The sandstones are commonly fine to medium grained, but there are also coarser, occasionally conglomeratic beds and very fine grained beds. Mudstone-dominated intervals, exceptionally up to 100 m thick, occur in many sections, sometimes including thin beds of siltstone and sandstone. Leeder et al. (1990) described the principal Namurian lithofacies and their wireline log characteristics in well 48/3-3. There, and across most of the southern North Sea, the Millstone Grit Formation contains many sharp-based sandstone units over 10 m thick and exceptionally up to 80 m thick; such sandstones either have sharp tops or grade upwards into mudstone. These sandstones are commonly stacked, with component sandstone beds sometimes separated by thin argillaceous intervals. Towards the base of the Millstone Grit Formation in well 48/3-3, argillaceous, poorly sorted sandstone beds with sharp tops occur, each of which grades downwards into mudstone (Leeder et al. 1990). Wireline-log responses for the Millstone Grit Formation can be used to identify lithological cycles comparable in scale and style to those that characterize the Millstone Grit Group in northern England (refer to Collinson 1988). In the North Sea, such cycles are commonly about 50 m thick, but their thickness ranges from less than 10 m to hundreds of metres. Most cycles have mudstone intervals containing marine bands at their base. The marine bands are typically associated with high or very high gamma-ray response, and they occur generally within intervals of low sonic velocity. However, they are best identified using spectral gamma logs as they are enriched in uranium.
Definition of Lower Boundary: The Millstone Grit Formation rests conformably on basinal mudstones of the Bowland Shale Formation in wells 41/24a-2, 48/3-3 and 48/23-3, the boundary being defined by the base of the lowest significant sandstone bed. The lithofacies transition is abrupt in 48/3-3 and 48/23-3, with only occasional sandstone stringers occurring below the boundary. Even where the transition is more gradational (e.g. 41/24a-2), the boundary is marked by a sharp downhole decrease in velocity. The Millstone Grit Formation continues northwards beyond the limit of the Bowland Shale Formation to rest directly on Yoredale facies (Yoredale Formation) along the flanks of the Mid North Sea High. Here, the formational boundary is defined as the top of the highest marine limestone bed, and is generally more than 100 m above the Dinantian/Namurian boundary. In well 39/7-1, the boundary occurs within a secondarily reddened section about 200 m below the base-Permian unconformity, and is placed at the uppermost blocky gamma-ray trough and corresponding sonic spike, interpreted to be a diagenetically altered limestone. Strata above this horizon display the typical Millstone Grit cyclicity. The Millstone Grit Formation also continues southwards beyond the Bowland Shale Formation to rest on Dinantian platform carbonates (Zeeland Formation) or on folded Lower Paleozoic basinal-marine sediments (e.g. in well 47/29a-1) along the flank of the London-Brabant Massif. The Zeeland/Millstone Grit formational boundary is a sharp lithofacies transition in well 52/5-11X.
Definition of Upper Boundary: In large parts of the northern offshore area, the Millstone Grit Formation subcrops Lower Permian lacustrine and sabkha facies (Silverpit Formation), the boundary being defined by an abrupt facies transition. Along the axes of Variscan anticlines farther south, the formation is locally overlain by Lower Permian desert sandstones (Leman Sandstone Formation). Where Westphalian strata are also preserved beneath the base-Permian unconformity, the Millstone Grit Formation is overlain by coal measures (the sandy Caister Coal Formation or argillaceous Westoe Coal Formation). In such areas, the formational boundary is defined with reference to the basal cluster of coal seams in the Upper Carboniferous succession. Specifically, the base of the coal measures in an individual well is defined along the base of the lowest coal seam in the basal cluster of seams. As so defined, the top of the Millstone Grit Formation is easily identified in most long sections (e.g. 44/12-1, 48/12b-4), but is less clear where the Westphalian A strata contain relatively few coal seams (e.g. 48/3-3).
Thickness: 1308 m in well 43/25-1.
Geographical Limits: The Millstone Grit Formation is widespread between the London-Brabant Massif and the Mid North Sea High. An outlier of the formation has also been proved at the northeastern flank of the Mid North Sea High, in northern Quadrant 39. Thickness of the Millstone Grit Formation is likely to be highly variable because of the varying structural setting of deposition. The maximum drilled thickness so far has been 1308 m in well 43/25-1. By comparison, the 'Millstone Grit' of England has a maximum thickness of c. 1225 m in Wharfedale, northern England (Ramsbottom et al. 1978).
Parent Unit: Whitehurst Group (WHHU)
Previous Name(s): none recorded or not applicable
Alternative Name(s): Millstone Grit Group
Reference Section  North Sea well 41/20-1: 2520.5-3451 m TD (8269-11322 ft TD) below KB. 
Reference Section  North Sea well 44/12-1: 3785.5-4630 m TD (12420-15190 ft TD). 
Reference Section  North Sea well 48/3-3: 3932-4374 m (12900-14351 ft). 
Reference Section  North Sea well 48/12b-4: 2996.5-4017 m TD (9831-13179 ft TD). 
Holdsworth, B K and Collinson, J D. 1988. Millstone Grit cyclicity revisited. In: Besly, B M and Kelling, G (eds.) Sedimentation in a synorogenic basin complex: the Upper Carboniferous of Northwest Europe, 132-152. Blackie & Son, London. 
Leeder, M, Raiswell, R, Al-Biatty, H, McMahon, A and Hardman, M. 1990. Carboniferous stratigraphy, sedimentation and correlation of well 48/3-3 in the southern North Sea Basin; integrated use of palynology, natural gamma/sonic logs and carbon/sulphur geochemistry. Journal of the Geological Society, London 147, 287-300. 
Cameron, T D J. 1993. Carboniferous and Devonian of the Southern North Sea. In: Knox, R W O'B & Cordey, W G (eds.) Lithostratigraphic nomenclature of the UK North Sea. (British Geological Survey, Nottingham). 
1:50K maps on which the lithostratigraphical unit is found, and map code used:
E055 E065 E064 E080