Suggestions for an update of the lithostratigraphic nomenclature for the post Eocene succession
T. Eidvin, F. Riis, E. S. Rasmussen & Y. Rundberg, 2013. New layout 2021
The lithostratigraphy of the post-Eocene succession in the Norwegian North Sea was established by Isaksen & Tonstad (1989).
Within the Hordaland (upper part) and Nordland groups, only two major sandy sections have been defined (Skade and Utsira formations) whereas the remaining lithologies have remained undifferentiated. Eidvin & Rundberg (2007) suggested some changes to this, and we have amended and developed this further and extended the area to the successions offshore Mid and North Norway (nomenclature of Dalland et al. 1988).
Central and northern North Sea
Eidvin & Rundberg (2007) suggested using the name Lark Formation, introduced by Knox & Halloway (1992), for mudstones of Oligocene to early Mid Miocene age (upper part of the Hordaland Group). The Lark Formation has already been used informally by industry consultants and stratigraphic workers in the Norwegian North Sea. In accordance with international lithostratigraphic rules and recommendations (Nystuen 1986, Salvador 1994), formation names which already have been defined in parts of the basin should also be used for equivalent strata across national borders. We therefore also propose to extend the usage of the term Lark Formation into Norwegian waters. Its original type section will remain unchanged, but we suggest the use of the well 2/4-C-11 succession from 2789 to 1664.7 m as the well reference section in Norwegian waters. Below 2789 m there is a hiatus down to Lower-Middle Eocene. A description of the interval from 2472 to 1664.7 m is available under the hyperlink to well 2/4-C-11. The lower part has been investigated by Eidvin et al. (1995, unpublished report).
Eidvin & Rundberg (2007) suggested that, in the northern North Sea, the Lark Formation should comprise the Skade Member (downgraded from formation) and an unnamed member (see their figure 27). We suggest that we do not downgrade the Skade Formation and at the same time keep the Vade Formation, in the central and eastern North Sea. For the unnamed sandstone unit in the northern Viking Graben, we suggest using the name Ull Formation after an Æsir (“god”) in Norse mythology (Fig. 9).
Isaksen & Tonstad (1989) introduced the term Skade Formation for sands that were thought to be Oligocene in age. Eidvin & Rundberg (2007), however, showed that these sands are dominantly Early Miocene (Fig. 3 and Fig. 9). Knox & Halloway (1992) adopted the term Skade Formation but used it for shallow-marine sands in the UK sector. Isaksen & Tonstad (1989) suggested that the sands were deposited in deeper marine settings in Norwegian waters. Eidvin & Rundberg (2007) interpreted the Skade Formation sands to be deposited by gravity processes (turbidites) mainly in upper bathyal water depths in the area they studied in the southern Viking Graben. The Skade sections in wells 25/2-10 S and 25/1-8 S (analysed for the current publication) contain common mollusc fragments and lignite, and have been deposited in shallower water close to, or parts of, a delta. According to the mapping of Gregersen & Johannessen (2007), these wells are situated in the distal part of the Hutton sand (see Fig. 10 and Profile P6). Hutton sand is an informal term used in British sector by several oil companies to describe all sands above the Lower Eocene Balder Formation in the Northern North Sea (British Geological Survey 2000). According to Bater (2003) and Gregersen & Johannessen (2007), the Hutton sand constitutes the entire sediment succession from the Upper Oligocene/Lower Miocene to Upper Pliocene and reaches eastwards into quadrants 30 and 25 in the Norwegian sector (see figure 5 in Gregersen & Johannessen 2007, Fig. 10 and Eidvin et al. 2013a). In some areas, parts of the Hutton sand continue into the Skade Formation, but we prefer not to use the term Hutton sand in Norwegian waters. More work need to be done on this relationship.
According to Isaksen & Tonstad (1989) the well type section for the Skade Formation is in well 24/12-1 from 1007 to 851 m, and the well reference section is 15/9-13 from 1224 to 1143 m. These wells were investigated by Eidvin & Rundberg (2007), and they found that the sandy unit in well 15/9-13 was younger than the Skade Formation in the other wells that they had studied, and dated the unit to Mid Miocene and belonging to the base of the Nordland Group. For the current publication we have recorded sandy sections of Mid Miocene age in four other wells and here suggest a name for this unit (see the Nordland Group below). We suggest well 16/1-4 from 1170 to 916 m as the new well reference section for the Skade Formation.
Isaksen & Tonstad (1989) introduced the term Vade Formation for sands that were suggested to be Late Oligocene in age. We have investigated the distal part of these sandy sections in well 2/2-2 and the proximal part in well 11/10-1. We found that in well 2/2-2 the unit is of early Late Oligocene age and that the unit in well 11/10-1 is of late Early to early Late Oligocene age (the top was not seen). According to Isaksen & Tonstad (1989) the well type section is 2/2-1 from 2172 to 2100 m and the well reference section is the nearby 2/3-2 from 1855 to 1795 m. They interpreted the sand in the wells to be deposited in a shallow-marine environment. Our interpretation of the fossil assemblages and the sediments of the Vade unit in the nearby well 2/2-2 indicate deposition by gravity processes in upper bathyal to outer neritic water depths. It is also obvious from seismic interpretation that wells 2/2-1, 2/2-2 and 2/3-2 are all situated central in the basin containing distal deposits (Profile P2, Jarsve et al. work in progress). On the contrary, the Vade unit in well 11/10-1 contains much coarser sand than in 2/2-2, mollusc fragments and a foraminiferal fauna with a more shallow-water affinity. We interpret the Vade unit in well 11/10-1 to have been deposited in middle to inner neritic water depths (see also Profile P2 and Jarsve et al. work in progress). Since wells 2/2-2 and 11/10-1 have both been investigated thoroughly by lithological, micropalaeontological and strontium isotope analyses, we suggest using 2/2-2 from 2060 to 1950 m as the new well type section (representing distal deposits) and 11/10-1 from 500 to 304.8 m as the well reference section (representing proximal deposits).
We would here like to introduce the Ull Formation as a new formation in the Hordaland Group. The Ull Formation represents erosional products from uplifted areas of the western part of the Fennoscandian Shield, i.e. the area close to the present day Nordfjord and the East Shetland Platform (Map 1 and Fig. 9) during Early to Late Oligocene. We have investigated these sandy sections in wells 34/10-17, 35/3-1 and 36/1-2. Well 34/10-17 contains deposits shed from the East Shetland Platform and wells 35/3-1 and 36/1-2 contain, respectively, distal and proximal deposits shed from the Fennoscandian Shield. The Lower Oligocene is recorded only in well 36/1-2, and this part contains a very coarse-grained sand-rich succession characterised by rounded, sub-rounded and sub-angular pebbles and mollusc fragments. The succession was probably deposited as debris-flow and/or shallow- marine sediments. The successions in wells 34/10-17, 35/3-1 and the upper part of 36/1-2 are dominated of medium- to fine-grained sand with variable clay and silt content. These sediments are probably all of turbiditic origin and were laid down during the Late Oligocene. We suggest well 36/1-2 from 940 to 620 m as the well type section (representing proximal deposits) and 35/3-1 from 1100 to 740 m as the well reference section (representing distal deposits).
According to Isaksen & Tonstad (1989), the term Nordland Group was introduced by Deegen & Scull (1977) for Middle Miocene to recent claystones in the North Sea Cenozoic Basin. According to their definition, the Nordland Group contains only one formation, namely the sandy Utsira Formation in the Viking Graben area.
The Utsira Formation represents a huge sedimentary depositional system in the northern North Sea (about 450 km long and 90 km wide) comprising one large sandy depocentre (250-300 m thick in the southern Viking Graben) and one much smaller centre (80-100 m thick in the northern Viking Graben). These areas are separated by a central area (Frigg area) where the unit is very thin and contains only Early Pliocene deposits (well 25/2-10 S and Fig. 11). Just to the west of this well the Utsira Formation pinches out (see Profile P6). To the north, in the Tampen area, the Utsira Formation is represented by a thin glauconitic unit overlying Oligocene and Lower Miocene strata deposited close to the Miocene-Pliocene transition (Eidvin & Rundberg 2001, Eidvin 2009, see Fig. 3, Fig. 11 and figures 4a and 7d in Rundberg & Eidvin 2005). We suggest subdividing the Utsira Formation into three members, viz. Southern Utsira Member, Northern Utsira Member and a Glauconitic Utsira Member in the northernmost part (Fig. 3, Fig. 9 and Fig. 11). In the same way as the Skade Formation, in the western part of quadrants 30 and 25 in the Norwegian sector, the Utsira Formation merges with parts of the Hutton sand (see figures 5 and 6A-C in Gregersen & Johannessen 2007, Fig. 11, Profile P6 and Eidvin et al. 2013c).
Rundberg & Eidvin (2005) pointed out an obvious correlation conflict between the Utsira and Skade formations in the type wells of Isaksen & Tonstad (1989, see figure 6 in Rundberg & Eidvin 2005) and showed the need for a revision of the base of the Utsira Formation. Eidvin & Rundberg (2007) suggested an adjustment of the base of the Utsira in the type well 16/1-1 from 1064 to 815 m, and noted also that after such a revision the well 16/1-1 is no longer the appropriate choice as the type well for the sandy system. It penetrates only thin sand and does not represent a typical succession of the sandstones of the Utsira Formation. Consequently, we suggest well 24/12-1 from 730 to 495 m as the new well type section (Profile 5, also the well reference section for the Southern Utsira Member). We suggest well 30/6-3 from 750 to 680 m as the well reference section for the Northern Utsira Member and 34/4-6 from 1250 to 1210 m as the well reference section for the Glauconitic Utsira Member.
According to Eidvin & Rundberg (2007), a revision of the base of the Utsira Formation might lead to some uncertainty about the dominantly fine-grained section above the Mid Miocene unconformity and below the Utsira sands. These sediments represent the basal part of the Nordland Group and occur as an infilling unit within the southern Viking Graben. They contain a Bolboforma assemblage with the important index fossils B. badenensis and B. reticulata which show that the age is slightly more than 14 to about 11.7 Ma (Middle Miocene, also vertified by Sr analyses). Except for a part of the Middle Miocene section in well 15/9-13, Eidvin & Rundberg (2007) recorded mainly fine-grained sediments in the wells they investigated in quadrants 15 and 24 in the southern Viking Graben (see wells 25/10-2, 24/12-1, 16/1-2, 16/1-4 and 15/12-3). They suggested that this basal section of the Nordland Group in the Viking Graben should be given the status of a formation, but did not suggest a name (see their figure 27). For the current publication, we have investigated this unit in wells 30/5-2 and 30/6-3 in the northern Viking Graben and 25/1-8 S and 25/2-10 S in the southern Viking Graben (Fig. 12). In wells 30/5-2, 30/6-3 and 25/2-10 S, the Middle Miocene contains mainly sandy sediments. The age is to a large extent based on the occurrence of the index fossils B. badenensis and B. reticulata. Sand also dominates the Middle Miocene section in well 25/1-8 S, but this was probably at a very shallow-marine site and oceanic forms such as Bolboforma did not enter the area. In this well the age is based mainly on correlation of benthic and planktonic foraminifera and a number of Sr analyses. It may be difficult to distinguish these sands from the sands of Utsira above and Skade below. However, since these sandy sections are clearly younger than the Skade Formation and older than the Utsira Formation in the type section areas in quadrants 16 and 24, we here introduce the name Eir Formation, after an Æsir (“god”) in Norse mythology, for these units in the Norwegian sector as a new formation in the Nordland Group (Fig. 9). We suggest well 25/2-10 S from 630 to 520 m as the well type section and 30/5-2 from 920 to 760 m as the well reference section. In the same way as the Skade and Utsira, this unit also merges with parts of the Hutton sand in the western part of quadrant 25. As mentioned above, wells 25/1-8 S and 25/2-10 S are situated in the distal part of the Hutton sand area according to Gregersen & Johannessen (2007, Fig. 12 and Eidvin et al. 2013b). However, we suggest using the term Eir Formation for this unit in Norwegian waters.
The very conspicuous Upper Pliocene prograding complex and the overlying Pleistocene deposits constitute the uppermost part of the Nordland Group of Isaksen & Tonstad (1989). These sediments consist mainly of clay-rich glacial diamictons. In the lower part of the succession gravity sands that were deposited in front of the oblique clinoforms are common (Eidvin & Rundberg 2001). Løseth et al. (2012) interpret some of these units to be extrusive sands sourced from Paleocene-Eocene turbidites. The sands may be difficult to distinguish from the Utsira sands below (informally called Tampen Spur sandstones by Robertson Resarch 1996 and Eidvin & Rundberg 2001, see also Eidvin 2009 and Eidvin & Øverland 2009). The uppermost part of the Nordland Group also includes the large Peon gas discovery in the basal Pleistocene sands of block 35/2 (Carsten 2005, Eidvin 2005). Dalland et al. (1988) introduced the name Naust Formation for the Upper Pliocene, including the Pleistocene section. As these sediments are similar in genetic origin, reflecting the glaciation history and the main uplift of Fennoscandia, it could be appropriate to extend the usage of the term Naust Formation to the North Sea; this has in fact been a common practice by industry consultants and stratigraphic workers for a long time. Eidvin & Rundberg (2007) argued against this, but we consider that the usage has been so common that an extension of the term to the North Sea will be most convenient and appropriate. We suggest that the original type section on the Norwegian Sea shelf should remain unchanged, but we suggest the use of well 15/12-3 from 900 to 200 m as the well reference section for the North Sea (see figure 9a in Eidvin & Rundberg 2007). We also suggest regarding the Tampen Spur and Peon sandy sections as formal members within the Naust Formation in the northern North Sea. We suggest well 34/4-7 from 1090 to 1050 m as the well type section for the Tampen Spur Member (see also figure 11b in Eidvin & Rundberg 2001) and well 34/4-6 from 1150 to 1130 m as the well reference section (see also figure 10 in Eidvin & Rundberg 2001). We suggest the use of 35/2-1 from 618 to 573 m as the well type section for the Peon Member (Eidvin 2005) and 35/2-2 from 609 to 581 m as the well reference section.
Offshore Mid Norway
Hordaland Group and Brygge Formation
The lithostratigraphy of the post-Eocene succession offshore Mid Norway was introduced by Dalland et al. (1998). As for the North Sea, according to Isaksen & Tonstad (1989) the terms Hordaland Group and Nordland Group, introduced by Deegan & Scull (1977), were also used for offshore areas outside Mid Norway. Dalland et al. (1998) introduced the Brygge Formation for fine-grained deposits of Early Eocene to Early Miocene age within the Hordland Group. We suggest no changes for the well type section of the Brygge Formation.
Nordland Group, Naust, Molo and Kai formations
Within the Nordland Group, Dalland et al. (1998) defined the Kai Formation for fine-grained sediments of Middle to Late Pliocene age, and the Naust Formation for the uppermost Late Pliocene to Pleistocene succession containing mainly glacial diamictons. We suggest no changes for the well type section of the Naust Formation.
The Molo Formation is a sand-dominated unit on the middle/inner part of the shelf extending from the coast off Møre (63°15’N) to Lofoten (67°50’N, Map. 1, Profile P10, P11, P12, P13 and Eidvin & Riis (2013). It has a unique seismic signature, and represents a prograding system comprising fairly steep clinoforms (Eidvin et al. 2007). Eidvin et al. (1998a) investigated sidewall cores of the Molo Formation in well 6610/3-1 (in its northern part) and gave an Early Oligocene age for the unit based on benthic foraminiferal and dinoflagellate cyst correlations and strontium isotope analyses. Later, T. Eidvin and M. Smelror investigated sidewall cores of the same formation in well 6510/2-1 (in middle part of the formation, Map 1). Based on the same kind of analyses they suggested an Early Miocene age for the formation in that well. Eidvin et al. (2007) investigated ditch-cutting samples of the Molo Formation in wells 6407/9-5, 6407/9-2 and 6407/9-1 (in its southern part, Map 1) and based on the same kind of analyses they suggested a Late Miocene to Early Pliocene age for the unit in those wells.
Eidvin et al. (2007) interpreted the Oligocene fossils in well 6610/3-1 and the Early Miocene fossils in well 6510/2-1 to be reworked and suggested a post Mid Miocene age for the whole of the Molo Formation. They interpreted the Molo Formation to be the proximal equivalent to the deeper marine Kai Formation. However, interpretation of new seismic data for the current paper indicates that the northern proximal part of the Molo Formation is as old as Early Oligocene and that the formation contains younger sediments towards the west and south (Map 1, Profile P10, P11, P12 and P13). We now believe that the recorded index fossils in wells 6610/3-1 and 6510/2-1 are not reworked, and that the Molo Formation is the proximal equivalent to both the Brygge and the Kai formations (Fig. 4 and Fig. 5). Eidvin et al. (2007) suggested well 6610/3-1 from 555 to approximately 349 m (the upper part was not sampled or logged) as the well type section and 6407/9-5 from 787 to 670 m as the well reference section. For the current publication we suggest well 6510/2-1 from 480 to 441 m (the upper part was not sampled or logged) also as the well reference section.
Eidvin et al. (2007) investigated also the Kai Formation, by use of lithological, micropalaeontological and strontium isotope analyses, in a number of wells, and several more were investigated for the current publication (Map 1 and Fig. 4). As a result of this new, thorough investigation we think it is appropriate to choose new type and reference sections among these wells. We suggest well 6507/12-1 from 1495 to 1340 m as the new well type section and well 6508/5-1 from 1358 to 1165 m as the new well reference section.
Sotbakken Group and Torsk Formation
For the Trømsøflaket offshore northern Norway, Dalland et al. (1998) introduced the term Sotbakken Group and Torsk Formation for sediments of Late Paleocene to Oligocene age and chose well 7119/12-1 from 810 to 465 m as the type well section and 7120/12-1 from 725 to 462 m as the well reference section. In our inspection of the biostratigraphic reports for these wells, we could not find evidence of any Oligocene sediments. Reliable evidence of Oligocene and Miocene deposits, in any Barents Sea well, were first recorded in well 7316/5-1 (Lower Oligocene and Lower Miocene, see also Eidvin et al. 1994, 1998b) from the Vestbakken Volcanic Province on the western margin of the Barents Sea (Map 2, Profile P15 and Fig. 5). Ryseth et al. (2003) described a more complete post-Eocene succession in well 7216/11-1S, with Upper Oligocene, Middle and Upper Miocene sediments, in the Sørvestnaget Basin on the southwestern Barents Sea Margin (Map 2 and Fig. 5). A thin succession of Lower Miocene sediments was also recorded in the shallow stratigraphic borehole 7316/06-U-01 (Map 2, Vestbakken Volcanic Province, Sættem et al. 1994). It is natural to choose new well type and reference sections for the Sotbakken Group and Torsk Formation from these two wells. We suggest 7216/11-1S from 4215 to 2346 m as the new type section and 7316/5-1 from 3751.5 to 948 m for the well reference section (only the description of the Middle Eocene to Lower Miocene part is available under the hyperlink to this well).
Nordland GroupDalland et al. (1988) used the term Nordland Group (introduced by Deegan & Scull (1977) for the North Sea) for sediments of Late Pliocene to Pleistocene/Holocene age and chose well 7119/12-1 from 465 to 225 m as well reference section. However, in our inspection of the biostratigraphic reports for this well, we could not find any evidence of Upper Pliocene sediments, but only Pleistocene. Reliable evidence of Upper Pliocene deposits, in any Barents Sea well, was first recorded in wells 7117/9-1 and 7117/9-2 on the Senja Ridge (Map 2), and documented in Eidvin & Riis (1989) and Eidvin et al. (1993). Wells 7316/5-1, 7216/11-1S, the shallow stratigraphic borehole 7316/03-U-01 (Vestbakken Volcanic Province, Sættem et al. 1992, Mørk & Duncan 1993) and ODP Hole 986D (southwestern Svalbard margin, Eidvin & Nagy 1999, Smelror 1999) are the only other wells/boreholes with well documented evidence of Upper Pliocene deposits in the Barents Sea. Since well 7216/11-1S also contains thin sections of both Middle and Upper Miocene deposits (Ryseth et al. 2003), we suggest the interval from 2346 to 361 m as the new reference section for the Nordland Group (see also Knies et al. 2009).
We suggest extending the usage of the term Naust Formation within the Nordland Group also to the Barents Sea. These sediments are very similar in both genetic origin and age to equivalent sediments in the northern North Sea and on Norwegian Sea shelf. We suggest well 7117/9-1 from 1180 to 550 m as the well reference section for the Naust Formation in the Barents Sea area (Eidvin & Riis 1989, Eidvin et al. 1993).