The extensive exploration activities on the Norwegian Shelf the past 15 years have significantly increased our knowledge of the Cenozoic succession, and a revision of the existing formal lithostratigraphy is appropriate and needed. New and high-resolution stratigraphic data from exploration wells, seismic well ties, and new data from regional seismic 2D and 3D surveys assists the revision of the previous published Norwegian Cenozoic lithostratigraphies published by Deegan & Scull (1977), Dalland et al. (1988) and Isaksen & Tonstad (1989). Improved seismic-, well log- and biostratigraphy, including restudies of the Cenozoic formations type wells have led to a better understanding of the regional and temporal distribution of these units. An illustrative example concerns the Utsira and Skade formations of Isaksen & Tonstad (1989), which where thought to span the Middle-Late Miocene and Late Oligocene time intervals, respectively. New age-determinations have proved that the Utsira unit is of Late Miocene to Early Pliocene age, while the Skade unit has been reassigned from its previously Paleogene position, to belong in the Miocene. In addition, we now have a better understanding of the presence and extent of the ?mid-Miocene hiatus? between the Skade and Utsira units. The stratigraphic gap between the units increase rapidly northward from a minimal time gap in the south Viking Graben to 15-20 m.y. in the northernmost North Sea. The hiatus marks the boundary between the uppermost Hordaland Group (Oligocene and Lower Miocene) and the overlying lowermost Nordland Group (Middle Miocene to Lower Pliocene).
In addition to provide the new age information, we here further propose that both units are given Member status; the Utsira Member being assigned to the Kai Formation and the Skade Member to the Lark Formation. The Kai Formation has consequently been extended from the original type area in the Norwegian Sea to embrace the lithological similar Middle Miocene-Early Pliocene deposits of the Northern North Sea. This extension is a consequence of the obvious need for unifying the formal lithostratigraphy as the gaps between the established (and previously separated) are being filled. Originally Deegan & Scull (1977) subdivided the Middle Miocene to Recent strata of the central and northern North Sea into the Nordland Group. Subsequently, Dalland et al. (1988) defined the Nordland Group to also to cover the Miocene to Recent strata Off Mid-Norway and on the Barents shelf.
In the North Sea the Nordland Group incorporates the Utsira Member, and in the Norwegian Sea the group is subdivided into the Kai Formation, with the Molo Member, and the overlying Naust Formation.
No regional formational subdivision has been presented for the Nordland Group in the Barents Sea. Several Quaternary and Holocene units have been defined by Vorren et al. (1978), Hald and Vorren (1984). Informal lithostratigraphic units have been described in the Bjørnøya West area by Sættem et al. (1994).
In the Norwegian Sea the group consists of alternating claystone, siltstone and sandstone of the Kai Formation (Worsley et al. 1988), and thick sandstones with pebbles comprising the Molo Member (Gustavson & Bugge 1995, Eidvin et al. in press).
In the Barents Sea the group consists of sands and clays which grade into sandstones and claystones, the sand content increasing upwards. Cobbles and boulders of quartzite, granite and different metamorphic rocks occur with clay in the upper parts of the group. The clay is grey to greyish green, soft to firm, blocky, noncalcareous, and in parts silty (Worsley et al. 1998).
The frequent ice-rafted material found in the Nordland Group consists of both sedimentary rocks typical for the Barents Shelf and crystalline rock fragments (Eidvin et al. 1993). A core from the Bjørnøya West area has recovered more than 150 m of Upper Pliocene gravity flow sandstone and siltstone with basaltic ash and lapilli (Mørk & Duncan 1993; Sættem et al. 1994).
The base of the group coincides with the Oligocene/Miocene unconformity. In the reference well this is defined by a decrease in interval transit time and gamma ray readings, and a lithologic change from claystones in the underlying Sotbakken Group to the basal sand/sandstones of this group. In wells where claystones are found above the base, the boundary is not easily identified by wireline logs. In such cases, minor lithological variations have to be relied upon; the claystones of the Nordland Group are generally softer and darker than those of the Sotbakken Group (Worsley et al 1988).
The group is often drilled with return to seabed only (seabed 30" casing) (Worsley et al. 1988; Eidvin et al. 1993; Ryseth et al. 2003). As a consequence, especially where there is no marked lithological variation, it is often difficult to pick the base of the group, particularly when it is thin and poorly developed. This is the case for most wells located north of 71°15'N and east of 20°20'E (Worsley et al. 1988).
In the Norwegian Sea well 6407/2-1 the Nordland Group is 1288 m (Worsley et al. 1988), and the group
In the Barents Sea the group thins from approximately 250 m in the southern wells to less than 100 m in northernmost locations in the Hammerfest Basin. Westernmost wells drilled over the Senja Ridge in Block 7117/9 show thicknesses of over 700 m (Worsley et al. 1988), while seismic data suggest that the Upper Pliocene and younger deposits in the central part of the wedge exceed 3 km in thickness (Eidvin et al. 1994, Eidvin et al. 2000). Similar thicknesses are seen in the Sørvestsnaget Basin and in the Bjørnøya West area (Sættem et al. 1994; Ryseth et al. 2003).
The Nordland Group is present throughout the Mid-Norwegian shelf, but the lower part is not present on the crest of the Nordland Ridge (Worsley et al. 1988). The sand content may vary locally, but there is no significant regional variation.
On the western Barents Shelf the Nordland Group is represented over the whole of Tromsøflaket (Worsley et al. 1988) and along the western margin of the Barents Shelf. The most continuous sequence is recovered over and to the west of the Senja Ridge (Eidvin et al. 1993), in the Sørvestsnaget Basin (Ryseth et al. 2003) and in the Bjørnøya West area (Sættem et al. 1994; Eidvin et al. 1998).
In the Norwegian Sea the lower boundary of the Nordland Group corresponds to the transition from brownish claystones of the Brygge Formation to grey claystones of the Kai Formation, or from the Brygge Formation to overlying sandstones of the Molo Member. Comparable to what is found in the North Sea area this boundary usually corresponds to a stratigraphic break, where Lower-Middle Miocene strata are missing.
In the Sørvestsnaget Basin in the Barents Sea the lower boundary of the Nordland Group corresponds to the transition between Upper Oligocene brown to grey mudstone of the Sotbakken Group and overlying Middle Miocene silty mudstone of the Nordland Group (Ryseth et al. 2003). Eastwards on the Barents shelf the Nordland Group rest directly on Middle Eocene claystones of the Torsk Formation.
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