T. Eidvin, F. Riis, E. S. Rasmussen & Y. Rundberg, 2013. New layout 2021
The Miocene section of the Barents Sea shelf has been preserved only along the western margin in local basins formed by tectonism along the Senja Fracture Zone. Possible Lower Miocene has only been recorded in well 7316/5-1 (Map 2), which penetrated an Oligocene – Lower Miocene section below the base Upper Pliocene erosional surface. In well 7216/11-1S, there is a hiatus below the Mid Miocene unconformity (Ryseth et al. 2003, Map 2). In the Senja Ridge, in wells 7117/9-1 and 7117/9-2, the Upper Pliocene sediments rest on Eocene sediments with an erosional boundary (see also the discussion section).
Norwegian Sea and its continental shelf
The progradation of the Molo Formation along the inner continental shelf of the Norwegian Sea continued into the Early Miocene, and extended farther south than in the Oligocene. The formation is investigated in well 6510/2-1 (Map 1) and consists of glauconitic (dominant) and quartzose sand of late Burdigalian age. It is not clear if there is a hiatus or continuous deposition between the Lower Oligocene and Lower Miocene part of the formation (Fig. 5). In the wells 6407/9-5 and 6407/9-3 (not investigated) in the Draugen Field (Map 1) there is an up to 22 m-thick, fine-grained, hemipelagic section of Lower Miocene sediments. It is wedge shaped and overlies a well defined erosional unconformity (see figure 4 in Eidvin et al. 2007). The stratigraphic gap in well 6407/9-5 ranges from the Lower Oligocene to the Lower Miocene below the wedge, and from the Lower to the Upper Miocene above (Fig. 1). About 50 km northeast of the Draugen Field another wedge with the same stratigraphic position and appearance has been observed. It has not been investigated in any well, but from its geometry it might be considered to be similar to the one in the Draugen Field (see figure 5 in Eidvin et al. 2007). Stratigraphically and agewise, the wedge belongs to the Brygge Formation. The unconformity below the wedge seems to represent more active erosion than at the time of the Mid Miocene unconformity on top. There are few signs of erosional products related to any of the two hiati, which indicates that the Mid Miocene unconformity in this area is more related to non-deposition rather than representing a major episode of erosion (Bugge et al. 2004, Eidvin et al. 2007). Lower Miocene pelagic sediments are also present at several DSDP and ODP sites in the Vøring and Møre basins (Talwani et al. 1976, Eldholm et al. 1989).
In the North Sea, the Oligocene – Lower Miocene boundary is marked by major shifts in sedimentation, with onset of sand deposition in the Viking Graben and Jutland (Map 1).
Northern North Sea
The Lower Miocene (unit UH-4 of Rundberg & Eidvin 2005) comprises the topmost part of the Hordaland Group. In the southern Viking Graben, it conformably overlies Oligocene strata. It is overlain by Middle Miocene sediments in the centre of the basin and Pliocene sediments at the margins (Profile 4). Just south of 61ºN and northwards, the Lower Miocene unit is only present in the central basin and absent at the margins to the west and east (Fig. 10). It may be present locally in depressions between diapirs. On seismic sections, the top of the unit can be defined by erosional truncation, as illustrated schematically in Profile 4. In the northernmost North Sea, between 61º30' and 62ºN, the unit has been completely eroded (Rundberg & Eidvin 2005).
In the Tampen area the Lower Miocene strata comprise mud-prone lithologies. In large parts of the Viking Graben, a sandy section, sourced from the East Shetland Platform, makes up a great proportion of the Lower Miocene unit. These sands are referred to as the Skade Formation and reach a gross thickness of up to 300 m (well 16/1-4). The areal extent and thickness of the sands are shown in Fig. 10. They comprise a succession of amalgamated sands alternating with thinner mudstones.
The deposition of the Skade Formation represents a southern shift in coarse clastic influx to the basin from the East Shetland Platform, relative to Oligocene time (Map 1). In most parts, the deposits are turbiditic in origin and were probably deposited in quite deep parts of the shelf. The Skade sections in wells 25/2-10 S and 25/1-8 S 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 (Fig. 10). Hutton sand is an informal term used in the UK sector by several oil companies to describe all sands above the Lower Eocene Balder Formation in the Northern North Sea (British Geological Survey 2000). As seen in Fig. 10, the Hutton sand as defined by Gregersen & Johannessen (2007) extends into the Norwegian sector and continues into the Skade Formation, However, we recommend using the established Norwegian stratigraphy in Norwegian waters.
The Skade sands pinch out to the east. According to our investigations, the sands were deposited between approximately 24 and 15.5 Ma and they represent a huge sand volume comparable to the Utsira Formation. It has been suggested that they are a result of a new tectonic uplift event affecting the East Shetland Platform, possibly associated with a renewed compressional tectonic phase along the northwest European margin (Lundin & Dore 2002, Boldreel & Andersen 1994).
Northern Central Graben
In well 2/4-C-11 the Lower Miocene consists mainly of clay, but small components of silt and fine-grained sand are also recorded (Eidvin et al. 1999 and 2000). The sand-rich units can be correlated to the Miocene delta of the Norwegian-Danish Basin, and the best developed sand unit correlates with sequence D of Rasmussen (2004).
In the Norwegian-Danish Basin, Lower Miocene sediments rest unconformably on Oligocene deposits. In Jutland, they are characterised by coarse-grained, dominantly sand-rich, deltaic deposits (Profile P3, Larsen & Dinesen 1959, Friis et al. 1998, Hansen & Rasmussen 2008, Rasmussen & Dybkjær 2005, Rasmussen et al. 2010, Map 1 and Link to Danish Miocene Sr isotope ages). The delta was sourced from the southern Scandes in Norway and central Sweden and prograded towards the south and southwest. The deltaic succession of the Ribe Group is composed of three discrete units referred to sequences B, C and D of Rasmussen (2004, Profile P1). The delta succession is about 200 m thick with a gross thickness of sand up to 150 m. The abrupt incursion of sand in the southern part of the Norwegian-Danish Basin is interpreted to be the result of an Early Miocene inversion of the basin and coincident uplift of the source area. Towards the Central Graben area the deltaic unit is wedging out and is dominated there by clay-rich deposits (Profile P3).