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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
Shetland Group, Northern North Sea
The term Jorsalfare Formation was introduced by Isaksen & Tonstad (1989) for a unit of calcareous mudstones with interbedded argillaceous limestones that lay between the Kyrre Formation and the Ekofisk, Lista, Våle, Maureen & Ty formations. The Jorsalfare Formation was extended into the UK Sector by Johnson & Lott (1993; Table 2). The Jorsalfare Formation was informally designated 'Formation' E of the Shetland Group by Deegan & Scull (1977; Table 1). It has also been informally designated the 'Shetland Marl' in several published accounts describing oilfields in the East Shetland Basin (Abbotts, 1991).
Named after Sigurd "Jorsalfare" Magnusson, a Norwegian king (A.D. 1103-1130).
Norwegian Sector: The formation generally consists of mudstones interbedded with thin limestone beds. The mudstones are light to medium grey, often calcareous. The limestones are white to light grey, fine-grained, occasionally sandy and dolomitic.
UK Sector: The Jorsalfare Formation consists of mudstones with interbedded argillaceous chalky limestones. The mudstones are pale grey to grey and usually calcareous. The limestones are argillaceous, chalky, white to pale grey, but locally red brown, fine-grained and occasionally sandy and dolomitic. In the East Shetland Basin, the proportion of limestone relative to mudstone generally decreases to the north and east (Johnson & Lott, 1993).
Norwegian Sector: The formation is 286 m thick in the type well (25/1-1), 145 m in well 35/3-2 and 365 m in well 24/9-1.
UK Sector: In contrast to the underlying formations of the Shetland Group, the Jorsalfare Formation displays only minor lateral thickness variation, and forms a blanket-like layer generally about 300-400 m thick, but up to 580 m in the Magnus Trough (Johnson et al., 1993).
Norwegian Sector: The formation is present in the Viking Graben and on the Tampen Spur. Its boundaries towards the Jorsalfare Formation in the Viking Graben, the Hardråde Formation on the Horda Platform and the Tor Formation on the Utsira High are illustrated in Figure 32b of Isaksen & Tonstad (1989).
The main characteristics that can be used to distinguish the three formations are:
a. The Jorsalfare Formation contains shales with thin limestone beds, which are usually no thicker than 5 m.
b. The Tor Formation is dominated by limestones, and has a negligible shale content.
c. The Hardråde Formation contains thick limestone beds (10-60 m), as well as shales, except in the Troll area where it is thin and may consist of only a single bed of limestone or marly limestone. Separation of these three formations may be difficult in transitional areas.
UK Sector: The Jorsalfare Formation is widely distributed in the Viking Graben, Beryl Embayment and East Shetland Basin.
WGS84 coordinates: 59° 53' 17.40'' N 2° 4' 42.70'' 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: 2711-2997 m (8894-9832.5 ft) below KB (Isaksen & Tonstad, 1989, p.30, fig.33). One core (17 m) in the middle of the formation and another (4 m) at the base.
35/3-2: 1665 to 1520 m Lat. 61°51'05.98"N Long. 03°46'28.22"E (Figure 34 in Isaksen & Tonstad (1989)). No cores.
24/9-1: 3117 to 2752 m Lat. 59°16'09.48"N Long. 01°47'31.18"E (Figure 35 in Isaksen & Tonstad (1989)). No cores.
| 3/12-2: 2134-2448.5 m (7002-8033 ft) 3/29-1: 2628-2925 m (8622-9596 ft) 16/3a-1: 2847-3251.5 m (9340-10668 ft) 211/13-7: 1809.5-2126 m (5936-6975 ft) |
Lat. 60º 39’ 27.6”N Lat. 60º 06’ 46.5”N Lat. 58º 52’ 53.2”N Lat. 61º 31’ 17.1”N |
Long. 01º 20’ 31.5”E Long. 01º 44’ 21.9”E Long. 01º 31’ 29.4”E Long. 01º 33’ 25.8”E |
Norwegian Sector: The upper boundary may be towards the Lista, Våle & Ty Formations of the Rogaland Group. When the upper boundary is towards the shale of theLista Formation it is usually characterised by an upward increase in gamma-ray intensity and a distinct drop in velocity (Figure 34 in Isaksen & Tonstad (1989)). When it is towards the Våle Formation it does not show the same distinct drop in velocity and increase in gamma-ray intensity, because the overlying lithology consists of limestones or calcareous mudstones (Figure 35 in Isaksen & Tonstad (1989)). Where the upper boundary is towards the Ty Formation it is identified as a change to sandstone (Figure 33 in Isaksen & Tonstad (1989)).
UK Sector: In the Beryl Embayment, North Viking Graben and East Shetland Basin, the top of the Jorsalfare Formation is usually marked by a downward passage from calcareous mudstones of the Maureen Formation to increasingly calcareous mudstones (e.g. 211/13-7, 3/12-2). In some sections the basal mudstones of the Maureen Formation are replaced by thinly bedded sandstone or reworked limestone (e.g. 9/19-7Z, Panel 2, Johnson & Lott, 1993).
In the South Viking Graben, the top of the Jorsalfare Formation is normally characterized by a downward change from argillaceous chalky limestones and interbedded calcareous mudstones of the Ekofisk Formation to cleaner limestones. It is marked by a downward decrease in gamma values and an increase in velocity (e.g. 16/3a-1).
Norwegian Sector: The lower boundary is defined by a decrease in gamma-ray intensity and an increase in velocity, reflecting an increase in calcareous content from the Kyrre Formation into the Jorsalfare Formation (Figures 33 and 34 in Isaksen & Tonstad (1989)). In the Tampen Spur area, however, the boundary may be difficult to identify due to small differences in calcareous content. The lower boundary may be unconformable above the Jurassic sequences (e.g. in the Gullfaks area).
UK Sector: The base of the Jorsalfare Formation is normally characterized by a downward change from argillaceous chalky limestones and calcareous mudstones, which are characteristically red brown, to grey mudstones of the Kyrre Formation.
The top of the Jorsalfare Formation in the UK Sector is usually marked on wireline logs by a downward increase in velocity (Johnson & Lott, 1993). Where thinly bedded sandstone or reworked limestone occurs at the base of the Maureen Formation the boundary is usually marked by a downward change from more variable to more consistent gamma and sonic responses. Where relatively pure reworked limestone rests unconformably on calcareous mudstones of the Jorsalfare Formation (e.g. 3/29-1) a downward increase in gamma values and a decrease in velocity is recorded.
The base of the Jorsalfare Formation is normally characterized on wireline logs by a downward increase in gamma values and a decrease in velocity.
In the UK Sector the Pseudotextularia elegans biomarker is present at the top of the formation. Here planktonic species such as Globigerinelloides spp., Heterohelix spp. and Rugoglobigerina spp. dominate (over 90% of the fauna). The Reussella szajnochae biomarker may be recognized, although it is rare and patchily distributed in the upper part of its range. Towards the middle part of the formation, assemblages dominated by non-calcareous agglutinated foraminifera replace those dominated by planktonic taxa. This occurs at a level equivalent to the Gavelinella spp. / B. miliaris biomarker in the Chalk facies. The R. szajnochae acme biomarker is present at the J1 /J2 boundary, where it is accompanied by the FDO of Tritaxia capitosa. King et al. (1989) considered this to be within the uppermost part of Formation 'D' (sensu Deegan & Scull, 1977), but the definition used here places it in the basal part of the Jorsalfare Formation.
The following dinoflagellate cyst biomarkers occur within the Jorsalfare Formation: Palynodinium grallator, Odontochitina operculata and Senoniasphaera protrusa. The FDOs of Trithryrodinium suspectum and several species of Chatangiella and Isabelidinium mark the Campanian/Maastrichtian boundary within the Shetland Group (Costa & Davey, 1992).
Late Campanian to Maastrichtian in the Norwegian Sector and Mid Campanian to Maastrichtian in the UK Sector.
The Jorsalfare Formation is laterally equivalent to the uppermost Flounder Formation (Shetland Group) and Tor Formation (Chalk Group) of the Central North Sea and South Viking Graben and is time-equivalent with theHardråde Formation of the Shetland Group (Figure 6 in Isaksen & Tonstad (1989)).
Foraminiferal assemblages in the Jorsalfare Formation are mainly dominated by planktonic forms, indicating outer sublittoral and bathyal environments with open marine circulation. However, non-calcareous agglutinants are dominant in the lower and middle parts of the formation (early Maastrichtian) indicating a phase of more restricted circulation (King et al., 1989).
An informal two-fold subdivision of the Jorsalfare Formation was recognized by Johnson & Lott (1993). At the base of the formation, Jorsalfare unit 1 (J1) is a thin unit of relatively low gamma, high velocity, calcareous mudstones and argillaceous chalky limestones, which are characteristically reddened. Jorsalfare unit 2 (J2) comprises the bulk of the formation and consists of pale grey mudstones with interbedded argillaceous chalky limestones. Usually, unit J2 displays an upward increase in calcareous content (3/12-2, Panel 2, Johnson & Lott 1993).