| Computer Code: |
VALH |
Preferred Map Code: |
notEntered |
| Status Code: |
Full |
| Age range: |
Berriasian Age (KR)
— Aptian Age (KP) |
| Lithological Description: |
Interbedded calcareous mudstones, chalky mudstones and thin limestones. Local sandstones and conglomerates also occur in the Central and northern North Sea. The mudstones and chalky mudstones are soft to firm, blocky, locally sandy and silty, micromicaceous, glauconitic, glauconitic and carbonaceous. They are generally pale to dark grey but locally black, grey-green and red-brown. Thin, but widespread units of black, laminated, non-calcareous mudstone are developed at two levels ('Munk Marl Bed' and 'Fischschiefer'). Limestones are hard to soft, microcrystalline or argillaceous and locally microlaminated or sandy; white to pale grey, but locally tan and yellow-orange or red-brown. Locally, particularly on palaeohighs, where condensation occurs, lithologies are predominantly limestones, argillaceous limestones and chalky mudstone. Sandstones are fine- to coarse- grained, calcareous and locally pass into conglomerates (some sandstones have been given formal status: Devil's Hole Sandstone, Scapa Sandstone, Sloop Sandstone, Yawl Sandstone members). In some areas of the Southern North Sea, bentonite horizons occur. In the Central and Northern North Sea the formation has been divided into seven informal units. |
| Definition of Lower Boundary: |
A sharp downward change from pale grey and grey interbedded calcareous mudstones, chalky mudstones and limestones (Valhall Formation) to dark brown-grey, olive-grey or black, variably calcareous to non-calcareous, organic-rich mudstones (Kimmeridge Clay Formation). This corresponds with a marked downward increase in gamma-ray values and a decrease in velocity on wireline logs. Locally sandstones of the Valhall Formation rest directly on the Kimmeridge Clay Formation (e.g. Scapa Sandstone Member at 14/19-9 and 14/29a-2). On some structural highs the Valhall rests on older strata. Locally, on the margins of the Central Graben (e.g. 29/24-1), calcareous sandstones (Devil's Hole Sandstone Member) rests on less calcareous sandstone of the Fulmar Formation and the boundary is shown by a downward decrease in velocity on wireline logs.
In some parts of the Southern North Sea, the boundary is defined by the downward change from dark, calcareous mudstones to sandstones (Spilsby Clay Formation) causing a downward decrease in gamma-ray values and an increase in velocity on wireline logs. Various definitions and interpretations of the unit have been published and included in company reports. The present definition follows Johnson and Lott, 1993. |
| Definition of Upper Boundary: |
In the Central and Northern North Sea, the upper boundary is a downward change from dark grey, non-calcareous mudstones (Carrack Formation) to chestnut brown, brick-red, greyish red and pale grey calcareous mudstones, chalky mudstones and limestones. Red stained planktonic foraminifera are also a characteristic element of the upper Valhall Formation. On wireline logs there is a sharp downward increase in velocity and density, together with a local small downward decrease in gamma-ray values (e.g. wells 15/23-6A and 15/23a-8).
In the Southern North Sea the upper boundary is defined by a downward change from dark grey to brown, non-calcareous mudstones (Carrack Formation) to pale to dark grey chalky mudstones (Valhall Formation). There is a corresponding downward decrease in gamma response and increase in velocity.
In the Outer Moray Firth and South Viking Graben there is a downward change from sandstone (Britannia Sandstone Formation) to calcareous and chalky mudstones (Valhall Formation) (e.g. Well 15/28a-3) and in the Inner Moray Firth there is also a downward change from sandstone (Wick Sandstone Formation) to the mudstones of the Valhall Formation (e.g. wells 13/14-1, 13/17-1 and 13/24-1).
Various definitions and interpretations of the unit have been published and included in company reports. The present definition follows Johnson and Lott, 1993. |
| Thickness: |
Up to 1000 m (? possibly 1800 m). |
| Geographical Limits: |
Central and Northern North Sea: Quadrants 6-9, 11-23, 26-30.
Southern North Sea: Quadrants 35-39, 41-44, 47-49, 51-54. |
| Parent Unit: |
Cromer Knoll Group (CRKN)
|
| Previous Name(s): |
none recorded or not applicable
|
| Alternative Name(s): |
none recorded or not applicable
|
| Stratotypes: |
| Reference Section |
North Sea well 14/04- 1 at 1329-1754.5 m depth (Johnson and Lott, 1993). |
| Reference Section |
North Sea well 14/20- 8 at 2771.5-3058 m (Johnson and Lott, 1993). |
| Reference Section |
North Sea well 16/12b- 6 at 4126.5-4367.5 m (Johnson and Lott, 1993). |
| Reference Section |
North Sea well 29/03- 1 at 3401-3859 m (Johnson and Lott, 1993). |
| Reference Section |
North Sea well 42/13- 1 at 839-1020 m (Lott and Knox, 1994). |
| Reference Section |
North Sea well 48/17- 1 at 402-559 m (Lott and Knox, 1994). |
| Reference Section |
North Sea well 48/22- 3 at 430-553.5 m (Lott and Knox, 1994). |
| Reference Section |
North Sea well 49/25a- 5 at 1853-1957 m (Lott and Knox, 1994). |
| Reference(s): |
| Johnson, H and Lott, G K. 1993. 2. Cretaceous of the Central and Northern North Sea. In Knox, R W O'B and Cordey, W G (eds.) Lithostratigraphic nomenclature of the UK North Sea. British Geological Survey, Nottingham. |
| Waters, C N, Gillespie, M R, Smith, K, Auton, C A, Floyd, J D, Leslie, A G, Millward, D, Mitchell, W I, McMillan, A A, Stone, P, Barron, A J M, Dean, M T, Hopson, P M, Krabbendam, M, Browne, M A E, Stephenson, D, Akhurst, M C, and Barnes, R P. 2007. Stratigraphical Chart of the United Kingdom: Northern Britain. (British Geological Survey.) |
| Waters, C N, Smith, K, Hopson, P M, Wilson, D, Bridge, D M, Carney, J N, Cooper, A H, Crofts, R G, Ellison, R A, Mathers, S J, Moorlock, B S P, Scrivener, R C, McMillan, A A, Ambrose, K, Barclay, W J, and Barron, A J M. 2007. Stratigraphical Chart of the United Kingdom: Southern Britain. British Geological Survey, 1 poster. |
| Lott, G K and Knox, R W O'B. 1994. 7. Post-Triassic of the Southern North Sea. In: Knox, R W O'B and Cordey, W G (eds.) Lithostratigraphic nomenclature of the UK North Sea. British Geological Survey, Nottingham. |
| Deegan, C E and Scull, B J. 1977. A standard lithostratigraphic nomenclature for the Central and Northern North Sea. Report of the Institute of Geological Sciences, 77/25; NPD Bulletin No.1. |
| Harker, S D, Gustav, S H and Riley, L A. 1987. Triassic to Cenomanian stratigraphy of ther Witch Ground Graben. In: Brooks, J and Glennie, K W (eds.) Petroleum Geology of North West Europe, 809-818. [Graham & Trotman, London] |
| Rhys, G H. 1974. A proposed standard lithostratigraphic nomenclature for the southern North Sea and an outline structural nomenclature for the whole of the (UK) North Sea. Report of the Institute of Geological Sciences, 74/8. |
| Riley, L A, Harker, S D, and Green, S C H. 1992. Lower Cretaceous palynology and sandstone distribution in the Scapa Field, UK North Sea. Journal of Petroleum Geology, 15, 97-110. |
| Lott, G K, Ball, K C, and Wilkinson, I P. 1985. Mid Cretaceous stratigraphy of a cored borehole in the western part of the Central North Sea Basin. Proceedings of the Yorkshire Geological Society, 45, 235-248. |
| Lott, G K, Fletcher, B N, and Wilkinson, I P. 1986. The stratigraphy of the Lower Cretaceous Speeton Clay Formation in a cored borehole off the coast of north-east England. Proceedings of the Yorkshire Geological Society, 46, 39-56. |
| Lott, G K, Thomas, J E, Riding, J B, Davey, R J, and Butler, N. 1989. Late Ryazanian black shales in the Southern North Sea Basin and their lithostratigraphical significance. Proceedings of the Yorkshire Geological Society, 47, 321-324. |
| 1:50K maps on which the lithostratigraphical unit is found, and map code used: |
| none recorded or not applicable |
