Discussion
T. Eidvin, F. Riis, E. S. Rasmussen & Y. Rundberg, 2013. New layout 2021
Implications for the development of the North Sea and the southern Scandes Mountains
In Map 1 it is possible to correlate the locations of the described Oligocene to Lower Pliocene depocentres with the topography and present drainage of Scandinavia. In the topographic map in Map 1, the shift from greenish to brownish colour takes place at about 800 m elevation. The highest peaks of the South Scandes Dome exceed 2000 m. Here, red lines show generalised water divides, separating major drainage systems. Extensive river capturing has taken place, in particular in the northern part of the dome where the present water divide has moved to the southeast (yellow line, Map 1). It is believed that the paleo-drainage which had developed in the crystalline basement rocks would, to a large extent, have controlled the present drainage, and one hypothesis for a rapid significant movement of the water involved in tectonic movements of the South Scandes Dome.
Our compilation shows that the Oligocene sediments in the northern North Sea were sourced from the western part of the South Scandes Dome (blue broken arrows, Map 1). Offshore West Norway, the sedimentation continued from the Eocene and terminated in the Early Oligocene. In the northernmost North Sea, sedimentation continued throughout the Oligocene. In the Late Oligocene, a much larger volume of clastic sediments was transported to the Norwegian-Danish Basin. This depositional pattern is consistent with a water divide located far to the northwest and west on the South Scandes Dome. At the transition to the Early Miocene, there was a shift in depocentres and nearly all the clastic sediment transport was now apparently directed towards the south (blue unbroken arrows in Southeast Norway and Sweden, Map 1). This situation continued to the Late Miocene, when the paleo-Sognefjorden valley was probably sourcing the sandy deltaic deposits that are found in well 35/11-1 north of the Troll Field (Map 1, Profile P8 and Fig. 11). Compared with the Utsira Formation, the volumes are negligible. Large-scale erosion and sediment progradation from the western part of the South Scandes Dome did not take place until the onset of glaciations in the Late Pliocene. The Mid Miocene angular unconformity appearing in the seismic data offshore West Norway suggest a rotation of the inner shelf which could be related to tectonic uplift of the South Scandes Dome.
Throughout the Oligocene and Miocene, the water depth in the Viking Graben was gradually shallowing, probably because of the high sedimentation rates and deltaic progradation in the Frigg area. In the huge delta system of the Frigg area, sedimentation was affected by changes in relative sea level, but there is no direct evidence of Mid Miocene tectonism in our data. Løseth & Henriksen (2005) interpreted the erosional surface in the northernmost North Sea to indicate subaerial erosion caused by Mid Miocene uplift. However, our new data presented above indicate that the area where the erosion surface occurs was located in a distal position relative to the Middle Miocene delta and the corresponding shelfal depocentre penetrated by wells 30/5-2 and 30/6-3. This depocentre postdates the Mid Miocene compressional tectonism. We suggest as an alternative explanation that the erosional event may have been caused by an increased marine circulation and vigorous current erosion due to the subsidence of the Greenland-Scotland Ridge (Laberg et al. 2005b). The abrupt Mid Miocene climatic cooling at approximately 14 Ma (Fig. 5, Zachos 2001) may have intensified the oceanic circulation system.
Another aspect which is important is the change in the North Sea Basin physiography that developed during Mid and Late Miocene time, in which the northern North Sea gradually became shallower and narrower due to Oligocene sedimentary infilling and Miocene tectonic uplift of the south Scandes Dome. The resultant basin physiography of the North Sea with a shallow threshold to the north may have been ideal for the formation of strong tidal current regimes. Such currents may have swept across the uplifted Viking Strait (term proposed in Galloway 2002) and caused vigorous erosion of the relatively shallow sea floor. The tidal effect would probably have increased as the strait became shallower. The fact that the hiatus is largest to the north and decreases southwards is in accordance with this model (Rundberg & Eidvin 2005). It should also be taken into account that the Lower and Middle Miocene sections, which were deposited in the western part of the northernmost North Sea, were so thin that even a small amount of erosion could have created a large stratigraphic hiatus.
A major transgression in the Early Pliocene created accommodation space for huge volumes of glacigenic sediments sourced by the Scandes Mountains. In the Late Pliocene, in the Frigg area, there was a new phase of sandy deltaic aggradation. The shallowest, Upper Pliocene, sand sequence of the Frigg delta is in direct communication with the Utsira-Skade aquifer system (NPD 2011).
The Utsira Formation and the western (youngest) part of the Molo Formation (Map. 1, Profile P10 and Profile P12) postdate the Mid Miocene tectonism in the North Atlantic. The Molo Formation was derived from the central part of the Scandes Mountains, where the present drainage system seems to be controlled by longitudinal valleys and by Mesozoic fault blocks and fractures.
The age and depositional environments of the Kai, Utsira and Molo formations
Mid Miocene compression formed large anticlines, synclines and elongated domes in the deep Norwegian Sea and the outer part of the shelf. The age of this event is constrained to Middle Miocene by seismic evidence from the wells on the Ormen Lange dome.
Based on an assumption that the Molo Formation is everywhere younger than the Middle Miocene, Løseth & Henriksen (2005) argued that a compression phase caused a major regression along the Norwegian margin between 62º and 69ºN during the Mid to Late Miocene. This interpreted regression forced the coastline of the syn-tectonic Kai Formation 50-150 km seaward of the present coastline. The regression should also have lifted the many intra-basin highs in the Norwegian Sea above sea level. Their idealised palaeogeography for the Late Miocene (figure 15 in Løseth & Henriksen 2005) shows a situation where most of the Norwegian Sea continental shelf, northern North Sea including the Viking Graben area and large parts of the compressional dome structures in the Norwegian Sea were dry land. Furthermore, they suggested that a stress reduction at the end of the Miocene resulted in a subsidence of approximately 400 m near the coast. Subsequently, the sandy Molo Formation and its assumed southern equivalent, the sandy Utsira Formation, were built out (their figure 16). Løseth & Henriksen (2005) assigned a late Mid to Late Miocene age to the Kai Formation and an Early Pliocene age to the Molo and Utsira formations. Their model implies that a postulated major regression should have caused the development of shallow-marine deltas and sand-rich fans in Mid to Late Miocene time. The existence of such deposits, older than the Molo and Utsira formations, has not yet been proven.
Based on biostratigraphy and seismic correlation, Eidvin et al. (2007) interpreted the Kai, Molo and Utsira formations to have been deposited mainly contemporaneously during the Late Miocene and Early Pliocene. They showed, however, that the oldest part of the Kai Formation (late Middle Miocene) is slightly older than the oldest part of the Utsira Formation (see also Rundberg & Eidvin 2005 and Eidvin & Rundberg 2007). Additional investigations of wells and boreholes in the present publication support the latter findings. The most important correlative tool for this interpretation is that of the Bolboforma assemblages. We recorded Bolboforma assemblages in the Kai, Molo and Utsira formations that enabled us to correlate shelfal fossil assemblages with short-range, deep-ocean Bolboforma zones which are calibrated with nannoplankton and paleomagnetic data. However, no Bolboforma were recorded in the Utsira Formation in the northwestern part of the North Sea (wells 34/4-7, 34/7-1 and 34/7-2). In this area, only the youngest part of the Utsira Formation is present, consisting of a thin glauconitic sand bed which probably drapes over the main Utsira Formation towards the east (Profile P8).
According to the deep-sea record, Spiegler & Müller (1992) and Müller & Spiegler (1993) described a B. fragori/B. subfragori Zone from sediments with an age of 11.7-10.3 Ma (earliest Late Miocene). We recorded B. fragori or B. subfragori assemblages at the base or in the lower part of the Utsira Formation in wells 34/8-3A (Tampen area), 35/11-1 (Sogn Graben), 25/10-2 (Viking Graben) and 24/12-1 (Viking Graben). The same assemblages were recorded at the base or near the base of the Kai Formation in wells 6508/5-1, 6609/5-1, 6609/11-1, 6507/12-1 (Trøndelag Platform), 6305/5-1 (Møre basin) and in the cored borehole 6704/12-GB1 (Gjallar Ridge).
According to the deep-sea record, Spiegler & Müller (1992) and Müller & Spiegler (1993) described a B. badenensis/B. reticulata Zone from deposits with an age slightly older than 14 to 11.7 Ma (Middle Miocene). In the southern Viking Graben, we recorded a B. badenensis/B.reticulata assemblage in a number of wells in the fine-grained deposits at the base of the Nordland Group, just below the Utsira Formation and just above a distinct base Middle Miocene seismic reflector. However, in wells 24/12-1 and 15/12-3 the uppermost part of the B. badenensis/B.reticulata assemblage is within the lowermost part of the Utsira Formation. On the Norwegian Sea continental shelf, we recorded the same assemblage at the base of the Kai Formation in well 6507/12-1. In the distal part of the Kai Formation we recorded the B. badenensis/B.reticulata assemblage in borehole 6403/5-GB1 (Møre Basin) and a Bolboforma compressispinosa-B. badenensis assemblage in borehole 6704/12-GB1 (Møre Basin). The ages indicated by the Bolboforma correlations are confirmed by Sr analyses in a number of wells and boreholes, especially from the southern Viking Graben, but also from the Norwegian Sea (see also Eidvin et al. 2013c). Unfortunately, except for a few samples, the Bolboforma tests are usually too small and too few to provide sufficient CaCO3 for Sr isotope analyses. Consequently, calcareous foraminifera, and in sandy sections mollusc fragments, are used for Sr analysis.
Eidvin et al. (1998a) investigated sidewall cores of the Molo Formation in well 6610/3-1 (in its northern part, Map 1) 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 the 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 interpreted a Late Miocene to Early Pliocene age for the unit in those particular 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 the new seismic data and correlation with the well 6610/2-1 S, for the current publication, support the view 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. 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 of both the Brygge and the Kai formations (Map 1, Profile P10, Profile P11, Profile P12 and Profile P13, see also Eidvin & Riis 2013).
Profile 13 shows that there is a good seismic correlation between the proximal sandy Lower Oligocene sediments in well 6610/3-1 and their fine-grained distal equivalent in 6610/2-1 S. Map 1 and Profile P13 also show that the Molo progradation in this area covers a more than 20 km-wide, coast-parallel belt where at least three different stages of progradation can be defined seismically. Farther south in well 6510/2-1 and towards the Draugen area, the Molo progradational belt is much narrower and its seismic character can be correlated with the younger, western part of the 6610/3-1 profile (Profile P13). These seismic observations are compatible with the biostratigraphic analysis. The oldest part of the Molo Formation is located offshore from the northern Scandes dome and could have been sourced through a paleo-drainage in Vestfjorden.
The Mid to Late Miocene regression model of Løseth & Henriksen (2005) implies that large parts of the Kai Formation on the continental shelf of the Norwegian Sea should have been deposited in shallow water depths in an inner to middle shelf environment. Also, on at the intra-basin highs in the Norwegian Sea, a shallow-water depositional environment should have prevailed during deposition of the Kai Formation (see their figures 15 and 16). However, our biostratigraphical record contradicts such a model. Even in the most marginal wells, where we have investigated the Kai Formation, including 6609/11-1 and 6507/12-1 (Trøndelag Platform), we recorded a fine-grained sediment rich in pelagic microfossils including planktonic foraminifera, Bolboforma, radiolaria and diatoms immediately above the Mid Miocene unconformity. No inner-shelf benthic foraminifera are recorded. In well 6305/5-1, situated on the Ormen Lange dome in the Møre Basin (Map 1, Fig. 6), we recorded an approximately 30 m-thick section of Upper Miocene Kai Formation lying unconformably on Upper Oligocene Brygge Formation. The Kai Formation in this well contains mainly pelagic ooze, and all of the recorded microfossils indicated deposition in deep water. There is no indication that this intra-basin high could have been close to the sea surface. It should be noted that these sediments were deposited on a structural dome which formed a positive relief on the sea floor.
Regional seismic mapping on the Trøndelag Platform is consistent with the biostratigraphic interpretation, and provides some additional information about the paleogeography. The thickness of the Kai Formation is greatest in the central part of the Trøndelag Platform and decreases towards the Molo Formation to the east and towards the Nordland Ridge to the west. In particular, the central part of the Nordland Ridge (the Sør High) was affected by Miocene compression and formed a dome structure which is onlapped by the Kai Formation. The central Trøndelag Platform can be regarded as a very wide and shallow syncline between the Nordland Ridge and the central Scandes Mountains. West of, and restricted to, the Sør High, there are conspicuous internal structures within the Miocene succession which can be interpreted as reworking by contourites (Fig. 13).
In summary, our interpretation is that most of the Kai Formation is a distal equivalent to the youngest, western part of the Molo Formation in the Norwegian Sea continental shelf and the Utsira Formation in the Møre Basin. We believe that only the eastern part of the Norwegian continental shelf was submerged during the Late Miocene and that the paleo-coastline is marked by the Molo Formation. Since no shallow-marine fossil assemblages were recorded in the lower part of the Kai Formation, immediately above the Mid Miocene unconformity, the observed erosional features probably occurred in quite deep water. In their seismic investigation of the Kai Formation in the Norwegian Sea, Laberg et al. (2005ab) showed that the sediments have been redistributed by contour currents. Intra Kai Formation prograding clinoforms were interpreted as parts of deltas by Løseth & Henriksen (2005), but similar seismic signatures are also made by along-slope flowing currents according to Laberg et al. (2005ab). The structures west of the Sør High are a good example. If these structures should be interpreted as deltaic, they would imply a sedimentary transport from west to east. Such an interpretation is incompatible with the deep-water ooze deposits in the Kai Formation in the deep Norwegian Sea. Contour currents, compressional structures and deposition of thick ooze layers in the deep sea may be important factors to explain the thinning and local pinching-out of the Kai Formation towards the distal part of the Molo Formation. It should also be remembered that the Mid Miocene compression resulted in the formation of large anticlines and synclines rather than just regional uplift.
The erosional features observed in the northernmost North Sea towards the Møre Basin were interpreted by Løseth & Henriksen (2005) as subaerial erosion, whereas the biostratigrapic analysis presented here is consistent with submarine erosion since no shallow-water assemblages were found in the relevant wells. Seismic mapping shows no indications of any regional tectonic structures which could have uplifted the area north of the Utsira Formation delta by several hundred metres in the Mid Miocene and then have subsided rapidly in the Late Miocene. Considering that the whole development of the Utsira Formation delta from the Oligocene to the Late Pliocene is consistent with a quiet tectonic regime, such inferred uplift and subsidence affecting its northern part is regarded as unlikely.
Oligocene to Pliocene along the Barent Sea margin
From the Sørvestsnaget Basin on the Barents Sea margin, Ryseth et al. (2003) recorded Middle and Upper Miocene deposits in well 7216/11-1S (Map 2), and stated that uppermost Middle Miocene deposits lie unconformably on the Upper Oligocene in that well. They also reported a small break between the Upper Eocene and the Lower Oligocene (their figure 3). The Oligocene to Miocene stratigraphy in well 7216/11-1S is based mainly on analyses of dinoflagellate cysts since no Oligocene calcareous benthic index foraminifera are recorded, as in well 7316/5-1, and the recorded agglutinated foraminifera are of long-range forms (Ryseth et al. 2003, T. Eidvin personal investigation).
The stratigraphical and regional interpretations of Ryseth et al. (2003) deviate somewhat from ours in that they indicate an absence of any Miocene deposits in the Vestbakken Volcanic Province and in well 7316/5-1 (Map 2). In their figure 11 they show that Miocene deposits pinch out on a high in the Knølegga Fault Zone south of the Vestbakken Volcanic Province. Also, they extend the Oligocene deposits, recorded in the Vestbakken Volcanic Province and the Sørvestsnaget Basin, towards the southwest and onto the Senja Ridge. They interpreted the interval 1180-1020 m in well 7117/9-1 and 1120-960 m in well 7117/9-2 to be of Oligocene age (their figure 10). A similar age for these intervals was also given by different biostratigraphical industry consultants soon after the wells were drilled in 1982 to 1983. This led several authors of published literature to conclude that the lower boundary of the large sedimentary fan along the Barents Sea margin is of Oligocene to Miocene age. However, a re-dating of these wells was performed at the Norwegian Petroleum Directorate in 1988, which concluded that these sections are Late Pliocene glacial deposits (Eidvin & Riis 1989, Eidvin et al. 1993). This result was later supported by different authors, working in several independent disciplines, including Mørk & Duncan (1993), Sættem et al. (1992, 1994) and Faleide et al. (1996). It appears that Ryseth et al. (2003) have used the original age interpretations, carried out by the industry consultants, in their regional interpretations, since no new re-dating of these wells is referred to in their paper. A profile through the Senja Ridge based on a seismic section (Fig. 14) shows the erosional contact between the Eocene ooze sediments and the glacigenic section.