| Computer Code: |
SVRT |
Preferred Map Code: |
notEntered |
| Status Code: |
Full |
| Age range: |
Cenomanian Age (KE)
— Cenomanian Age (KE) |
| Lithological Description: |
The Svarte Formation consists of calcareous mudstones with interbedded argillaceous chalky limestones. In the North Viking Graben and East Shetland Basin, the proportion of limestone within the formation generally decreases northwards and away from contemporary high structures. The mudstones are typically medium to pale grey and occasionally micaceous, glauconitic and pyritic. The argillaceous chalky limestones are mainly white to medium grey and locally sandy. |
| Definition of Lower Boundary: |
The base of the Svarte Formation is normally marked by a downward change from calcareous mudstones or argillaceous chalky limestones to less calcareous sediments of the undifferentiated Cromer Knoll Group. It corresponds to a downward increase in gamma values and a decrease in velocity. |
| Definition of Upper Boundary: |
The top of the Svarte Formation is marked by a downward change from mudstones of the Macbeth Formation (Black Band) to argillaceous chalky limestones or calcareous mudstones (e.g. 3/29-1 and 9/10c-2). It is marked on wireline logs by a decrease in gamma values and an increase in velocity. Where the Black Band is not easily distinguished, the log break associated with the boundary is not as conspicuous (e.g. 3/8b-10). In the north of the East Shetland Basin, the Svarte Formation is overlain by undivided Shetland Group mudstones (e.g. 211/26-4). |
| Thickness: |
In the UK sector, the formation ranges up to about 250 m thick (e.g. 210/15b-4). |
| Geographical Limits: |
The Svarte Formation occurs in the North Viking Graben, East Shetland Basin and Magnus Trough. It is, however, absent over contemporary structural highs. Although seismic evidence suggests onlap of the formation onto these structures, condensation and intra-Cretaceous erosion may also be a limiting factor (Johnson et al., 1993). |
| Parent Unit: |
Shetland Group (STLN)
|
| Previous Name(s): |
none recorded or not applicable
|
| Alternative Name(s): |
none recorded or not applicable
|
| Stratotypes: |
| Type Section |
Norwegian Sector North Sea well N25/1-1 (Isaksen and Tonstad, 1989, p.27, fig.33): 3807-3995 m (12490-13107 ft) below KB. |
| Reference Section |
North Sea well 3/29- 1: 3778-3932 m (12395-12900 ft) (Johnson and Lott, 1993). |
| Reference Section |
North Sea well 3/01- 1: 3081.5-3176.5 m (101100-10421 ft) (Johnson and Lott, 1993). |
| Reference(s): |
| 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. |
| Isaksen, D and Tonstad, K. 1989. A revised Cretaceous and Tertiary lithostratigraphic nomenclature for the Norwegian North Sea. NPD-Bulletin No.5 |
| Johnson, H, Richards, P C, Long, D, and Graham, C C. 1993. United Kingdom offshore regional report: the geology of the northern North Sea. (London: HMSO for the British Geological Survey.) |
| King, C, Bailey, H W, Burton, C A, and King, A D. 1989. Cretaceous of the North Sea. In: Jenkins, D G and Murray, J W (eds) Stratigraphical atlas of fossil foraminifera, 372-417. Ellis Horwood Limited, Chichester, for the British Micropalaeontological Society. |
| 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.) |
| Ritchie, J D, Gatliff, R W and Riding, J, 1996. 1. Pre - Tertiary lithostratigraphy. In: Stratigraphic Nomenclature of the UK North West Margin. British Geological Survey, Nottingham. |
| 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. |
| 1:50K maps on which the lithostratigraphical unit is found, and map code used: |
| none recorded or not applicable |
