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(From NPD Bulletin no. 5)
updated to follow: Stratigraphic Guide to the Cromer Knoll, Shetland and Chalk Groups of the North Sea and Norwegian Sea. Felix M. Gradstein & Colin C. Waters (editors), Mike Charnock, Dirk Munsterman, Michelle Hollerbach, Harald Brunstad, Øyvind Hammer & Luis Vergara (contributors). Newsletter on Stratigraphy, vol 49/1 pp71-280, 2016
The term Svarte Formation was introduced by Isaksen & Tonstad (1989) for a unit of calcareous mudstones with interbedded argillaceous chalky limestones lying between the Rødby and Blodøks formations in the Norwegian Northern North Sea. The Svarte Formation was extended into the UK North Viking Graben, East Shetland Basin and Magnus Trough by Johnson & Lott (1993). The Svarte Formation was informally designated 'Formation' A of the Shetland Group by Deegan & Scull (1977).
Named after Halvdan Svarte, King of Ringerike, Norway about A.D. 850.
Norwegian Sector: The formation generally consists of mudstones interbedded with limestones. Sandstones occur in the Agat region. The content of limestones relative to mudstones is generally lower in the northern than in the southern part of the Viking Graben. The mudstones are medium to light grey, often calcareous, occasionally micaceous, glauconitic and pyritic. The limestones are mainly white to medium grey, argillaceous or sandy. The sandstones are clear to light grey and often cemented by calcite.
UK Sector: The Svarte Formation consists of calcareous mudstones with interbedded argillaceous chalky limestones (Johnson & Lott, 1993). 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.
Norwegian Sector: In the type well 25/1-1, in the Viking Graben, the formation is 188 m thick. In the reference sections the thickness is 240 m in well 35/3-2, and 188 m in well 24/9-1.
UK Sector: The formation ranges up to about 250 m thick (e.g. 210/15b-4, Johnson & Lott, 1993). The formation can be over 57 m thick in the Magnus Trough of the Northern North Sea, thinning and pinching out over the basin-margin highs (Johnson et al., 1993).
Norwegian Sector: The formation is present in the Viking Graben, and north of the Tampen Spur towards the Marulk Basin. It is, however, lacking on structural highs such as the Lomre Terrace (e.g. Norwegian wells 35/8-1 and 35/8-2).
UK Sector: The Svarte Formation occurs in the North Viking Graben, East Shetland Basin and Magnus Trough (Johnson & Lott, 1993). 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).
WGS84 coordinates: Lat. 59º 53’ 17.5”N Long. 02º 04’ 42.7”E
UTM coordinates: 6639470.41 N 448427.57 E
UTM zone: 31
Drilling operator name: Elf Petroleum Norge AS
Completion date: 22.07.1971
Status: P & A
Interval of type section & thickness in type well: 3807-3995 m (12490-13107 ft) below KB (Isaksen & Tonstad, 1989, p.27, fig.33). No cores.
35/3-2: 3447 to 3207 m Lat. 61°50'5.98"N Long. 03°46'28.22"E (Figure 34 in Isaksen & Tonstad (1989)). No cores.
24/9-1: 3992 to 3804 m Lat. 59°16'09.48"N Long. 01°47'31.18"N (Figure 35 in Isaksen & Tonstad (1989)). No cores.
3/29-1: 3778-3932 m (12395-12900 ft) Lat. 60º 06’ 46.5”N Long. 01º 44’ 21.9”E
3/1-1: 3081.5-3176.5 m (10110-10421 ft) Lat. 60º 56’ 43.5”N Long. 01º 09’ 31.9”E
Norwegian Sector: The upper boundary is generally easily located, and is characterised by an increase in gamma-ray intensity and a distinct decrease in velocity from the Svarte Formation up into the Blodøks Formation (Figure 33 in Isaksen & Tonstad (1989)). This is caused by lower carbonate content in the Blodøks Formation.
UK Sector: 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 & 9/10c-2 Johnson & Lott, 1993). In the north of the East Shetland Basin, the Svarte Formation is overlain by undivided Shetland Group mudstones (e.g. 211/26-4).
Norwegian Sector: The lower boundary shows a general upward decrease in gamma-ray intensity, and an increase in velocity from the Cromer Knoll Group into the Svarte Formation (Figure 34 in Isaksen & Tonstad (1989)). This is due to a higher content of carbonate in the Svarte Formation.
UK Sector: 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 (Johnson & Lott, 1993).
The upper boundary in the UK Sector is marked on wireline logs by a decrease in gamma values and an increase in velocity into the Svarte Formation. Where the Black Band is not easily distinguished, the log break associated with the boundary is not as conspicuous (e.g. 3/8b-10). The base of the formation corresponds to a downward increase in gamma values and a decrease in velocity (Johnson & Lott, 1993).
The calcareous microfauna in the UK Sector is dominated by planktonic foraminifera (over 90%), particularly species of Hedbergella. The Rotalipora cushmani biomarker is well represented in the south, but becomes increasingly difficult to recognise to the north. The Lingulogavelinella ciryi inflata / Rotalipora reicheli biomarker is present in the middle part of the formation. The Axopodorhabus albianus nannofossil biomarker occurs at the top of the formation, with the Gartnerago theta, G. nanum and Biscutum constans acme biomarkers occurring lower down in the formation (Johnson & Lott, 1993).
The Apteodinium granulatum dinoflagellate cyst biomarker occurs at the top of the formation, with the Epelidosphaeridia spinosa and Ovoidinium verrucosum subsp. verrucosum biomarkers occurring lower down. A downward influx of peridiniacean (deflandroid) dinoflagellate cysts occurs in the upper part of the formation, including Chatangiella spp., Isabelidinium magnum and Trithyrodinium suspectum (Costa & Davey, 1992).
Cenomanian.
The Svarte Formation is laterally equivalent to the Hidra Formation in the Central Graben and Central North Sea and with the informal "formation A" of Deegan & Scull (1977), (Figure 6 in Isaksen & Tonstad (1989)).
The microfossil assemblages in the Svarte Formation are rich in planktonic foraminifera and indicate marine, sublittoral and bathyal conditions with access to open oceanic circulation (King et al., 1989).