Lower Oligocene

T. Eidvin, F. Riis, E. S. Rasmussen & Y. Rundberg, 2013. New layout 2021

Forlandsundet, Svalbard

The Forlandssundet basin is a partly fault-bounded structure trending north-south along the west coast of Spitsbergen. The western border fault of the Forlandsundet basin is exposed on Prins Karls Forland (Map 2), and consists of graben-parallel normal faults stepping down into the interior of the basin. Adjacent Cenozoic fanglomerates locally rest unconformably on basement rocks. The syntectonic Cenozoic fill is deformed by both extensional and oblique-contractional structures. The eastern fault margin of the Forlandsundet graben is intermittently exposed at the base of Sarstangen and Kaffiøyra. This boundary lies along the northern strike continuation of the Svartfjella-Eidembukta-Daumannsodden Lineament (SEDL), a major fault zone of Carboniferous rocks intensely deformed during a complex history of Cenozoic motion (Gabrielsen et al. 1992, Kleinsphen & Teyssier 1992, Lepvrier 1992, Bergh et al. 1999).

According to Bergh et al. (1999), there are two Cenozoic units of very different character and likely different age exposed on Sarstangen at Forlandsundet; an eastern unit exposed mainly along the interior moraine margin, and a western unit that is exposed mainly along the southern shore of Sarstangen. The differences in coloration, lithification, grain size and proximity to source, and deformation history are consistent with two units of different age. Large clasts of the eastern unit are found in the western succession, clearly indicating that the western unit is younger. A greater degree of lithification and structural complexity of the eastern complex is also consistent with it being the older one. Slip-linear data and other structural observations are consistent with a history beginning with sinistral, then dextral motions along the easternmost border faults, followed by orogen-perpendicular extension. Such a history likely records the change from transpressional to transtensional plate motions between northeast Greenland and Svalbard (Bergh et al., 1999).

This age relationship, however, is opposite to that interpreted by Gabrielsen et al. (1992). Considering the geological setting and the plate history and limited paleontological data, Bergh et al. (1999) concluded that the older unit might well be Eocene and the younger Oligocene.

An exploration well drilled at Sarstangen in 1974 penetrated about 1000 m of Cenozoic rocks and terminated in crystalline basement (total length 1135.5 m, Harland 1997, Skotte 2007, Brugmans 2008). Reports and samples from this drillhole were not available for this study. The thickness of the Cenozoic section recorded implies that the lowermost Cenozoic rocks in the basin may not be represented by the outcrop samples.

Feyling-Hanssen & Ulleberg (1984) described two outcrops of Oligocene deposits in Balanusviken, Sarsbukta, on the east side of Forlandsundet (western sequence according to Berg et al. 1999). At Balanuspynten, in the southern end of Balanusviken, they described an 8 m-thick section of micaceous claystone. At Konglomeratodden, in the northern end of Balanusviken, they recorded a 7 m-thick section with beds of varying lithology including different kinds of conglomerates, sandstone and sandy clay. The micaceous clay was not found at Konglomeratodden (see figure 1 under Forlandsundet).

On the basis of benthic, calcareous, foraminiferal assemblages in nine samples, these authors divided the sections into two zones: the overlying Asterigerina guerichi Zone and the underlying Bolivina cf. antiqua Zone. Feyling-Hanssen & Ulleberg (1984) assigned a transitional Early to Late Oligocene age to this entire section, and indicated an age equivalent to Subzones NSB 7a to NSB 8a according to King (1989). However, Manum & Throndsen (1986) proposed an age not younger than Late Eocene for the Balanusviken (Forlandsundet) sections based on dinoflagellate cysts. Eidvin et al. (1998b) performed Sr isotope analyses on foraminiferal tests from four of these samples and new dinoflagellate cyst analyses in two samples. These gave ages close to and slightly younger than the Early/Late Oligocene boundary (based on the time scale of Berggren et al. 1995) for the western sequence. In this study, that result has now been revised to slightly older ages of latest Early Oligocene by use of the look-up table presented by Howard & McArthur (1997, see figure 2 under Forlandsundet). Eidvin et al. (1998b) concluded that most of the recorded dinoflagellates identified by Manum & Throndsen (1986) are older Paleogene forms reworked into Oligocene strata.

Vestbakken Volcanic Province, western Barents Sea

Well 7316/5-1 (Map 2, Profile 15 and Profile 15 Map) penetrates the western margin of a local fault bounded terrace which subsided in the Oligocene and Miocene due to movements on the Knølegga Fault zone which bounds the terrace to the east. A thick wedge of glacigenic sediments overlies the Oligocene-Miocene section with an erosional contact.

Based on analyses of benthic foraminifera, dinoflagellate cysts and Sr isotopes, a 108 m-thick unit of Lower Oligocene sediments and 22 m of Lower Oligocene to Lower Miocene deposits were recorded in well 7316/5-1. The Lower Oligocene consists of silty clay. Upper Middle Eocene and Upper Eocene sediments are absent in the well. East of the well location, the hiatus is expressed as an angular unconformity truncating older Middle Eocene units (Fig. 1 and Profile 15). The unconformity was a result of uplift and rotation of the terrace due to movement along the boundary faults. The sediments in the drilled structure were consequently raised above the wave base, resulting in a period of mild erosion and non-deposition. This appears to have occurred in the period between 45 and 34 Ma, to some extent pre-dating, but also overlapping, the supposed initiation of the northwesterly direction of relative plate motion in the Norwegian – Greenland Sea (Anomaly 13 time, 37-36 Ma, Eidvin et al. 1998b). Regional mapping indicates that along the western Barents Sea margin, a seismic sequence boundary of approximately this age can be defined, where the overlying Lower Oligocene succession is onlapping the underlying, more strongly faulted and folded Eocene sequence. The Oligocene onlap was described by Ryseth et al. (2003) in well 7216/11-1S in the Sørvestsnaget Basin (Map 2). Movements of this age have also been recorded in the Vøring Basin (Brekke 2000), suggesting that the shift in direction of the plate motion was reflected in the sedimentary record over a large region.

Norwegian Sea and its continental shelf

Along the inner continental shelf of the Norwegian Sea north of 66°N, progradation of sandy coastal plains and deltas started in the Early Oligocene (Molo Formation, Map 1). This unit has been analysed in well 6610/3-1 and contains coarse, rust-stained sand with pebbles in the upper part and mica-rich, medium sand in the lower part (see also Profile P12 and Profile P13). Farther west, in the shelf areas, the Lower Oligocene sediments are generally thin. In places, the section is below seismic resolution, or is totally missing (Henriksen et al. 2005), such as in the wells 6508/5-1 and 6609/5-1 on the Nordland Ridge and well 6607/5-1 on the Utgard High (Fig. 6, Map 1, Fig. 1 and Fig. 2). The Lower Oligocene is present in the DSDP Site 336 and ODP Site 643 in the Vøring Basin (Talwani et al. 1976, Eldholm et al. 1989), in wells 6305/5-1 and 6404/11-1 in the Møre Basin, in well 6407/9-1, 6407/9-2, 6407/9-5, 6507/12-1 and 6609/11-1 on the Trøndelag Platform and in wells 6610/2-1 S on the Nordland Ridge (Fig. 6, Map 1). The top is eroded in wells 6407/9-1, 6407/9-2, 6407/9-5 (Draugen Field, Trøndelag Platform), 6610/2-1 S (Nordland Ridge) and 6404/11-1 (Storegga slide area, Møre Basin, Fig. 1, Fig. 2 and Fig. 6). These sections are overall fine-grained and are characterised by semi-transparent and parallel seismic facies patterns (Henriksen et al., 2005). In the outher part of the Vøring Plateau, the sediments were rearranged by countouritic currents (Laberg et al. 2005b). Biostratigraphical, lithological and seismic data suggest a quite deep-marine depositional environment dominated by hemipelagic sedimentation on the shelf areas and pelagic biogenic sedimentation in the Møre and Vøring basins. The base of the Lower Oligocene has been investigated only in well 6610/2-1 S, and lies unconformably on the Middle Eocene. The sandy, deltaic, Lower Oligocene section in well 6610/3-1 can be tied to its distal equivalent in 6610/2-1 S by 3D seismic data (Profile P13).

North Sea

In the Norwegian sector of the North Sea, the Lower Oligocene sediments that were derived from the Shetland Platform in the west are predominantly fine grained and located in a distal setting. Progradation from the Scandes mountains continued offshore West Norway, but sandy sediments have been recorded locally only in the northernmost part. The Early Oligocene marks the onset of large-scale progradation of sediments southwards from the Scandes into the Norwegian-Danish basin (Jarsve et al. work in progress).

Northern North Sea

Based on well log correlation, Rundberg & Eidvin (2005) presented an outline of the Lower Oligocene of the northern North Sea in their figure 7a (unit UH-1 and UH-2). However, our new investigation of wells 34/10-17, 35/3-1, 36/1-2 and 31/3-1 (Map 1, Profile 8 and Profile 9) shows that only unit UH-1 and the proximal, eastern part of unit UH-2 (recorded in well 36/1-2, Fig. 1, Fig. 2 and Profile 9) are of Early Oligocene age. The Lower Oligocene part of UH-2 in well 36/1-2 (Agat Discovery area) includes a very coarse sand. The lower half of the Lower Oligocene sandy section is rich in rounded, sub-rounded and sub-angular pebbles and mollusc fragments. There is a break below the Lower Oligocene in well 36/1-2. Gradstein et al. (1992) recorded a break between the Lower Oligocene and Middle Eocene in wells 34/8-1 (Tampen area) and 16/1-1 (southern Viking Graben).

Most parts of the Lower Oligocene sandy section in the Agat Discovery area were interpreted to be of turbiditic origin by Rundberg (1989), but the lower part probably also contains some debris flow and/or shallow-marine deposits (Profile 9). It is likely that the deposits are erosional products derived from the western part of the Fennoscandian Shield in the present -day Nordfjord area. Farther south towards the Troll Field area, a distinct wedge of organic-rich mudstones strikes parallel to the western coast of Norway (Map 1). This wedge has a slightly older Early Oligocene age than 36/1-2 (investigated in well 31/3-1, Fig. 1 and Profile 7). In seismic sections, the seismic unit UH-1 wedge appears to represent the last stage of regional progradation of Paleogene sediments from the Fennoscandian Shield. It is onlapped by a younger Oligocene sequence, and the whole section was slightly rotated during later tectonic events.

Central North Sea, Northern Central Graben

There is no well-defined seismic reflector and no obvious lithological change close to the Lower/Upper Oligocene boundary. The Oligocene in well 2/4-C-11 comprises mostly clay with some components of silt and sand. The Central Graben was located in a distal position relative to the Oligocene progradation from Scandinavia. According to Eidvin et al. (1995) the Lower Oligocene lies unconformably on the Lower Eocene.

Norwegian-Danish Basin

The Lower Oligocene of the Norwegian-Danish Basin was investigated in well 2/2-2 from the deep central part of the basin and in wells 11/10-1 and 9/12-1 from the marginal part (Map 1, Fig. 1, Profile 2, Jarsve et al. work in progress). The base of the Lower Oligocene was not studied in these wells, but was investigated in the well Nini-1 (Danish sector) by Sliwinska et al. (2010, Map 1 and Profile 2). In the marginal, northeastern part of the basin, the Lower Oligocene succession is locally very thick, approximately 800 m. The succession is dominated by fine-grained sediments of the Hordaland Group except for the uppermost part of well 11/10-1 which is quite sandy. This sandy section constitutes the lower part of the Vade Formation, and can be interpreted as the beginning of transport of coarser clastics into the Norwegian-Danish Basin. The main part of the sandy Vade Formation was deposited during the Late Oligocene (Fig. 1 and Jarsve et al. work in progress). This was also the case for the Oligocene sandy sections in the northern North Sea (off Nordfjord). Our data imply that the sediment progradation from Scandinavia was directed mainly to the west and northwest until the Early Oligocene and towards the south and southwest later in the Oligocene and Miocene (Map 1).