Upper Oligocene
T. Eidvin, F. Riis, E. S. Rasmussen & Y. Rundberg, 2013. New layout 2021
Vestbakken Volcanic Province, western Barents Sea
In well 7316/5-1, a fine-grained section is given an unspecified Late Oligocene to Early Miocene age, but Upper Oligocene sediments have not been proved by biostratigraphy. There is no distinct hiatus or unconformity recorded locally on seismic data at this level. Seismic data show that the thickness increases towards Knølegga Fault, and it is suggested that the succession is eroded and/or condensed due to tectonic movements along the structure. It is likely that an expanded succession of Oligocene and Lower Miocene sediments was deposited and is now preserved in the basin between the drilled structure and the Knølegga Fault (Map 2, Profile 15, Profile 15 Map and Eidvin et al. 1998b). Expanded sections are also expected towards the oceanic crust in the west. At this time, sea-floor spreading activity was established to the west of the well area, promoting sedimentation in locally developed open marine basins formed between highs generated by the continuing structural readjustments to the new tectonic regime. A global eustatic lowering of sea level as a result of the initiation of the Antarctic glaciations (Vågnes et al. 1992, Zachos et al. 2001) may also have exerted an influence on the Oligocene successions, although it is not possible to quantify the relative magnitude of these events (Eidvin et al. 1998b).
Norwegian Sea and its continental shelf
Seismic data indicate that the progradation of the Molo Formation along the inner continental shelf of the Norwegian Sea (Map 1) continued in the Late Oligocene, but no well has so far been drilled in a position where Upper Oligocene sediments from the Molo Formation could be identified. Upper Oligocene sediments are recorded in thin shaly sections in distal positions in wells 6507/12-1 and 6609/11-1 (Fig. 1 and Fig. 2). In the Møre Basin the Upper Oligocene was investigated in well 6305/5-1, and the lower part of the Upper Oligocene in wells 6305/4-1and 6404/11-1 and the gravity core 49-23 (Fig. 6). In the Vøring Basin, the Upper Oligocene is present at DSDP Sites 336 and 338 and ODP Site 643 according to Talwani et al. (1976) and Eldholm et al. (1989). Biostratigraphical, lithological and seismic data of the Upper Oligocene successions suggest a continuation of the Early Oligocene setting with a fairly deep-marine depositional environment dominated by hemipelagic sedimentation on the central shelf areas and pelagic sedimentation (biogenic ooze) in the Møre and Vøring basins.
North Sea
In the Late Oligocene, there was a large input of sandy sediments from the Shetland Platform into the northern North Sea. Most sediments were laid down in the southern Tampen area (Map. 1). Farther south, Upper Oligocene sandy deposits are recorded below the Skade Formation in the Frigg Field area, i.e., within the area of the Hutton sand according to Gregersen & Johannessen (2007, Map 1). Sediment transport from the Scandes in this period was directed mainly towards the Norwegian-Danish Basin, where the sandy Vade Formation was deposited. The progradation of sediments towards the west ceased at the Early/Late Oligocene transition, except in the northernmost Nordfjord area (Map 1).
Northern North Sea
The western sands (quadrants 30, 34 and 25) shale out to the east and are sourced from the Shetland Platform (Profile P6 and Profile P7), while the eastern sands (blocks 35/3 and 36/1) appear to be sourced from the Nordfjord area, West Norway (Profile P9). In this study, the sandy section has been analysed in wells 34/10-17, 35/3-1, 36/1-2 and 25/1-8 S. Due to intense sand injections and mud diapirism, it is difficult to study the lithostratigraphy and depositional environment of the western sands in seismic data. Mud diapirs in the Oligocene-Miocene section cover a large area in the Viking Graben (Løseth et al. 2003), and the distribution of massive diapirism seems to coincide with the central and outer parts of the Oligocene and Miocene sandy systems (NPD, 2011).
Several mechanisms have been proposed to explain the sand injection activity and diapirism (Løseth et al. 2003), but will not be discussed further here. However, some observations based on interpretation of regional 3D seismic data are of importance in understanding the distribution of the Oligocene and Miocene sediments. Our observations are compatible with the observations of Løseth et al. (2003).
Mud diapirs typically contain sand intrusions which can commonly be recognised in seismic data as V-shaped reflections (“V-brights”, Løseth et al. 2003). Care must be taken in the biostratigraphic analysis to avoid misinterpretation of intrusive sands as in situ. Mud diapirism has taken place in different phases, which are interpreted in seismic data by correlation of the sediments between diapirs and sediments deformed by diapirism. Within the diapir area, there are local basins where a full Miocene section is preserved, implying that the deposition of Miocene sediments in the northern North Sea was partly controlled by a pre-existing topography created by diapirism. Later diapirism and erosion took place in the Miocene and Late Pliocene and will be discussed below.
Between 60º and 61ºN east of well 34/10-17, the top of the Upper Oligocene (unit UH-2 of Rundberg & Eidvin 2005) is defined by a moderate- to high-amplitude, semi-continuous seismic reflector in the eastern part of the basin (Map 1 and Profile 7). The top of the unit also corresponds to a diagenetic horizon characterised by the transition from opal-A to opal-CT-rich mudstones. This siliceous mudstone is thought to be present locally within the northern North Sea, particularly between 60º and 61ºN, according to Rundberg & Eidvin (2005). In the basin centre, the exact position of the top of the Oligocene is sometimes difficult to detect seismically since there is no depositional break between the uppermost Upper Oligocene (unit UH-3 of Rundberg & Eidvin 2005) and the Lower Miocene (unit UH-4 of Rundberg & Eidvin 2005).
Farther west, unit UH-2 is strongly disturbed by diapirism, and it is difficult to map the top. It is probably best defined at about 60º45'-61ºN, as illustrated in Profile 7. Along this profile, the seismic reflector defining the top of the high-density zone can probably be correlated to discontinuous, high-amplitude seismic events farther to the west. These events define the top of a thick sandy interval, which is penetrated in wells 34/10-17 and 34/10-23 (not analysed for this study). The sands make up a gross thickness of about 400 m in block 34/10 (Profile 7). In well 34/10-17, the fossil data confirm that the sands belong to the sequence and have not been injected. The Lower-Upper Oligocene sands are unnamed in the Norwegian sector of the northern North Sea (Fig. 3).
The thick sandy interval in block 34/10 shows that there must have been a significant input of sand from the Shetland Platform. Erosional products from the uplifted areas of the East Shetland Platform have probably been transported eastwards by river systems, with delta progradation and gravity flow transport towards the Statfjord Field-Tampen area and the Frigg area. In seismic sections, this sandy system is illustrated by wedging of the Oligocene strata, Profile 6 and Profile 7.
Sands continued to be derived from an easterly source (Nordfjord), during the Late Oligocene, in the Agat Discovery area (Map 1 and Profile 9). Most of these sands are contemporaneous with sands of the Statfjord Field and Frigg Field areas (Fig. 1).
In wells in the northern Tampen area the Upper Oligocene consists of silty mudstones. In the well 25/10-2, from the southern Viking Graben, the Upper Oligocene is dominated by clay, but thin sand beds are quite common throughout. In the other wells from the southern Viking Graben (Map 1), the unit contains mainly fine-grained material (see also Eidvin & Rundberg 2001 and 2007, Rundberg & Eidvin 2005).
Northern Central Graben
In the northern Central Graben there is no well-defined seismic reflector close to the Oligocene-Miocene boundary; this is similar to the situation in the southern Viking Graben. However, a number of seismic reflectors are visible on seismic profiles and if guided by a tie from the Danish sector and key wells it is possible to map the boundary. In well 2/4-C-11 the Upper Oligocene comprises mostly clay with intercalations of silt, sand and some limestone stringers (see also Eidvin et al. 1999 and 2000). These deposits were laid down in a deep- marine setting beyond the deltaic slope.
Norwegian-Danish Basin
Transport of coarser clastics from Fennoscandia into the Norwegian-Danish Basin continued in the early Late Oligocene (Vade Formation). In well 2/2-2 (Map 1, Profile P2, Jarsve et al. work in progress), in the deepest part of the basin, sand (glauconitic, biotitic and quartzose) is common but not dominant. The sands were probably transported by gravity flows. The sediments of the Vade Formation in well 11/10-1, from the marginal part of the basin, are dominated by quartzose sand. The sand is rich in mollusc fragments and was probably deposited in a shallow shelfal environment. It is coarsening upwards and the uppermost part is coarse. Corresponding sediments of the Hordaland Group in well 9/12-1, also from the marginal part of the basin, are rich in mollusc fragments and lignite (Map 1, Jarsve et al. work in progress), and this unit was also probably deposited in a shallow shelfal environment. Sand is not frequent in the lower, main part, but glauconitic sand is quite common in the upper part. The uppermost part of the Upper Oligocene has been reported from the well Nini-1 in the Danish sector by Sliwinska et al. (2010, Map 1 and Profile P2). In the central and eastern Norwegian-Danish Basin, mud characterises the Branden Formation which shows a coarsening-upward trend containing slightly more sand in the upper part. The formation was deposited in deep water in front of a southward prograding slope system. An unconformity separates the Branden Formation from the overlying, latest Late Oligocene, Brejning Formation (Rasmussen et al. 2010). The Brejning Formation is composed of glauconitic clay in the lower part, but is more sand-rich in the upper part. Locally, the Brejning Formation displays thin, commonly bioturbated, sand layers. Cross-bedding has been recognised in this formation in the easternmost part of Denmark. The Brejning Formation was deposited mainly in a water depth of more than 200 m, but the cross-bedded sand and microfauna indicate shallow-water conditions locally on the Ringkøbing-Fyn High at the end of the Oligocene.