Summary and conclusions

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

Based on an extensive study of biostratigraphic and strontium data from 47 wells and boreholes from the entire Norwegian continental shelf, one outcrop from northwestern Svalbard, one ODP borehole off Scoresby Sund (East Greenland), two stratigraphic boreholes from onshore Denmark, and both published and new data from a number of other boreholes and outcrops from Denmark,

we have presented an improved chronology of Oligocene to Lower Pliocene strata. Emphasis has been placed on sandy deposits. The deposits of the Norwegian continental shelf and onshore Denmark are correlated to the deep-sea record. Most wells and boreholes have been integrated with wire-line log and seismic data.

Our findings show that during late Early to Late Oligocene time, sediments in the northernmost part of the North Sea Basin were sourced from the northwestern part of the South Scandes Dome, which was topographically high throughout the Paleogene. In the northeastern part of the northern North Sea off Nordfjord, sandy gravity-flow sediments were deposited. Farther south off Hordaland and Sogn and Fjordane, a distinct wedge of organic-rich mudstones was formed along the coast (Early Oligocene). Deltaic complexes prograded southwards into the Norwegian-Danish Basin (Vade Formation and the sand-rich part of the Lark Formation, Dufa and Freja members). In the Late Oligocene there is a large input of sandy sediments from the Shetland Platform into the northern North Sea. Most of the sediments were laid down in the southern Tampen area. Farther south, Upper Oligocene deposits are recorded below the Skade Formation in the Frigg Field area, i.e. within the area belonging to the Hutton sand according to Gregersen & Johannessen (2007). Along the inner continental shelf of the Norwegian Sea a pronounced progradation of coastal plains and deltas from the Northern Scandes Dome started in the Early Oligocene north of 66°N (Molo Formation). Mainly argillaceous sediments were deposited elsewhere in the central and northern North Sea, on the Norwegian Sea continental shelf and on the Barents Sea margin. Mainly pelagic ooze was deposited in the Norwegian Sea. Conglomerates, sandstones and sandy clay were deposited in the Forlandsundet basin in northwestern Svalbard. Climate was probably cold temperate during the Early to early Late Oligocene and warm temperate to subtropical during the latter part of Late Oligocene. 

At the end of the Oligocene, prominent polar ice caps built up primarily in Antarctica which resulted in a global sea-level fall. Contemporaneous with this event, tectonism occurred along the Sorgenfrei-Tornquist Zone and on former graben structures in the southern North Sea. Associated with this, deltas (Ribe Group) prograded southwards during the Early Miocene, and covered large parts of the present-day Jutland area in Denmark. In the northern North Sea, there was a marked shift in depocentres. The thick Oligocene depocentre in the Tampen area was abandoned, and sand-rich sediments of the Lower Miocene Skade Formation were deposited in the Viking Graben. North of 60ºN, in the North Sea, fine-grained Lower Miocene sediments are present only in the central basin and absent at the margins to the west and east. The Skade Formation thus represents a southern shift in coarse clastic influx to the basin from the East Shetland Platform, relative to Oligocene time. These deposits are turbiditic in origin and were probably deposited in quite deep parts of the shelf. The Skade sections in blocks 25/1 and 25/2 contain common mollusc fragments and lignite, and have probably been deposited in shallower water close to, or as parts of, a delta. According to the mapping of Gregersen & Johannessen (2007), these wells are situated in the distal part of the Hutton sand area. Farther east, fine-grained, distal sediments were deposited. The out-building of the Molo Formation along the inner continental shelf of the Norwegian Sea continued in the Early Miocene over a larger area than in the Oligocene, possibly as far south as to 64ºN. A distal, fine-grained, thin Lower Miocene wedge is recorded on the Trøndelag Platform. Mainly pelagic ooze was laid down in the Norwegian Sea.

Tectonic movements in the Norwegian Sea culminated at the Early to Middle Miocene transition, and the deposition of the Skade Formation sands was followed by progradation of a large delta in the Frigg area. In the Norwegian Sea, major compressional structures were formed.

Middle Miocene sediments in the northern North Sea represent the basal part of the Nordland Group. Seismic data show that in the Mid Miocene a significant delta structure was formed in the Frigg Field area, and throughout the Middle Miocene the delta-front prograded to the east. Middle Miocene units in block 25/2 (southern Viking Graben) and in the depocentre in blocks 30/5 and 30/6 in the northern Viking Graben are sandy and microfossils indicate an inner to middle shelf environment. South of the Viking Graben, the Middle Miocene sediments are mainly fine-grained. Fine-grained sediments were deposited in parts of the Trøndelag Platform on the Norwegian Sea continental shelf. No data exist for the Mid Miocene development of the Molo Formation. Hiatuses are either minor or absent in the Viking Graben, but the youngest part of the Middle Miocene may be missing in the Ekofisk Field in the Central Graben. Pelagic sedimentation continued uninterrupted in most of the Norwegian Sea. However, hiatuses are present on large dome structures. On the Ormen Lange Dome, for example, a hiatus occurs below the Upper Miocene. Along the western Barents Sea margin there was renewed tectonic activity, and a hiatus is recorded below the Middle Miocene in the Sørvestnaget Basin (Ryseth et al. 2003). In well 7316/5-1 in the Vestbakken Volcanic Province there is an erosional boundary between the Lower Miocene and Upper Pliocene. The climate was probably warm temperate during the Early Miocene and culminated with a subtropical climate in the early Middle Miocene.

In the Late Miocene, a considerable climatic deterioration and a low global sea level resulted in continued out-building of coastal plains and deltas of the Molo Formation along the inner continental shelf of the Norwegian Sea. To the west, clayey and hemipelagic sediments were laid down on the shelf and pelagic ooze on the slope and rise (Kai Formation). During the same period, the northern North Sea formed a narrow seaway between the deeper water of the Møre Basin and the central North Sea. This strait received a large amount of coarse clastic sediments from the East Shetland Platform in the west (Utsira Formation) and locally from the Fennoscandian Shield in the northeast. The Utsira Formation represents a huge sedimentary depositional system in the northern North Sea comparable in volume to the Skade Formation. The Utsira Formation comprises one large sandy depocentre in the southern Viking Graben and a much smaller one in the northern Viking Graben. In the deltaic system of the Frigg Field area, the Utsira Formation is thin or absent due to the fall in sea level. In the northern North Sea Tampen area, the Utsira Formation is represented by a thin glauconitic unit deposited close to the Miocene-Pliocene transition and overlying an erosional surface which cuts into Oligocene and Lower Miocene strata. This glauconitic member is thought to cap the main Utsira Formation sands in the northeastern part of the basin. In late Early Pliocene the climate temporarily improved, and this coincided with a temporary rise in sea level.

The deep-sea record from the Norwegian-Greenland Sea reveals small quantities of ice-rafted material during the Late Eocene to Early Oligocene and the late Mid to early Late Miocene. East Greenland is the likely source of this material. However, the investigation of the large sediment wedge off the Scoresby Sund fjord system shows that the build up of substantial continental ice on Greenland started late in the Late Miocene at approximately 7.5 Ma. There is no evidence for the existence of glaciers on the eastern seaboard of the Norwegian-Greenland Sea before 2.75 Ma.