The Neogene stratigraphy of the glaciated European margin from Lofoten to Porcupine

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The Kai Formation can be traced from the mid-Norwegian margin westwards into the Norwegian Basin. Although most mapping north of 66°N has focused on the mid-Norwegian Slope, wells, such as 6607/5-1, have proved that it extends landwards onto the Trøndelag Platform, below the Vøring Shelf. Farther south, the Kai Formation locally becomes very thin or is absent due to large-scale sliding associated with the Storegga Slide Complex (Evans et al. 2002); however, it is well preserved in the Møre Basin, below the North Sea Fan.

External form and thickness

The Kai Formation displays a predominantly sheetform geometry of variable thickness across much of the area. In the north of the region, the formation is commonly thin or absent on the Trøndelag Platform and eastern flank of the Vøring Basin, but thickens generally in the western Vøring Basin where discrete depositional build-ups are preserved, largely beyond the distal edge of the prograding wedge of the overlying Naust Formation. Seismic data reveal thicknesses of up to 1000ms TWTT associated with these build-ups. Elsewhere, significant thinning of the succession, locally to less than 100ms TWTT, occurs in areas affected by mid-Cenozoic doming, e.g. the Helland-Hansen Arch, as well as other positive structural features such as the Vøring Marginal High that underlies the Vøring Plateau, and west of the Vøring Escarpment. Farther west, this unit has not been mapped in detail. However, from the available seismic data the Kai Formation has been shown to increase in thickness on the outer Vøring Marginal High (Brekke 2000). Farther south, the Kai Formation has a maximum thickness of up to 1000ms TWTT in the Møre Basin, thinning to the west. On the SE flank of the Møre Basin, the geometry of the Kai Formation is interpreted by Martinsen et al. (1999) to depict a prograding lowstand wedge. 

The thickest section penetrated in a commercial well is 428m in well 6607/5-1, which proved middle–upper Miocene sediments on the eastern flank of the Vøring Basin. This contrasts with the nearby well of 6607/5-2 and wells 6610/7-1 and 7-2 on the northern Trøndelag Platform where the Kai Formation is absent (Eidvin et al. 1998). Farther west, ODP site 642 on the Vøring Marginal High proved 212.3m of lower Miocene–lower/upper Pliocene sediments. On the western flank of the marginal high, ODP site 643 proved ~330m of Miocene–‘middle’ Pliocene sediments (Eldholm et al. 1987). From the southernmost part of the Møre Basin the Kai Formation was found to be very thin. Wells such as 34/8-3A have proved lower and upper Miocene sediments separated by an intra-Miocene unconformity (Eidvin et al. 2000).

Seismic facies

On the Trøndelag Platform, the Kai Formation is characterised by a parallel-laminated internal seismic signature. In the Vøring Basin, the continuity of this acoustic configuration is commonly disrupted by faulting. Locally, the internal reflections of the Kai Formation display an onlapping pattern onto structural highs, such as the Vøring Marginal High and the Helland-Hansen Arch (Brekke 2000). In the Møre Basin, a progradational internal configuration is implied by Martinsen et al. 1999.


Upper boundary

The upper boundary of the Kai Formation is the Base Naust Unconformity (BNU). The BNU is an angular unconformity downlapped by clinoforms of the prograding Naust Formation along the Vøring and Møre shelf margins. Further west the unconformity gradually becomes a conformable surface, and on the Vøring Plateau no obvious hiatus has been identified at ODP Sites 642 and 643 (Bruns et al. 1998).

Lower boundary

The lower boundary is the Base Kai Unconformity (BKU). The BKU is an erosional, possibly diachronous, hiatus over much of the area. In well 6607/5-1, in the eastern Vøring Basin, the Kai Formation overlies lower to middle Eocene sediments of the Brygge Formation (Eidvin et al. 1998). On the Vøring Marginal High, the Kai Formation overlies lower Eocene volcaniclastic sediments in ODP site 642. On the outer Vøring Marginal High, a major break separates upper Oligocene and upper Eocene strata at ODP site 643, with no apparent hiatus at the Oligo-Miocene boundary (Eldholm et al. 1987). On the southern Trøndelag Platform, a hiatus is reported between the Kai Formation and the underlying Brygge Formation in well 6506/12-4 (Eidvin et al. 1998). Commercial wells from the southernmost part of the Møre Basin reveal lower Miocene sediments unconformable on upper Oligocene sediments (Eidvin et al. 2000). 


At ODP site 642 on the Vøring Marginal High, the upper Miocene–lower/upper Pliocene section comprises siliceous/nannofossil mud and siliceous and calcareous ooze whilst the middle and lower Miocene section is dominated by siliceous mud and ooze (Eldholm et al. 1987). At ODP site 643, to the west, the Kai Formation was found to comprise mud, sandy mud, siliceous mud, diatom and nannofossil ooze of late Miocene–mid-Pliocene age, diatom ooze of mid- and late early Miocene age, and laminated mudstone of early Miocene age (Eldholm et al. 1987). These data suggest that the Miocene–middle Pliocene succession on the Vøring Marginal High is characterised by shifts between intervals of high biogenic opal and carbonate deposition (Bohrmann et al. 1990). In the eastern Vøring Basin and on the Trøndelag Platform wells 6607/5-1and 6506/12-4, respectively, proved claystone with small proportions of sand and silt (Eidvin et al. 1998, 2000). In the northern North Sea, well 34/8-3A proved lower Miocene basinal mudstones unconformably overlain by upper Miocene–lower Pliocene glauconitic sandstones (Eidvin & Rundberg 2001).


On the Vøring and Møre margins, the Kai Formation has been subdivided into a number of seismic subunits, A (incorporating Aa and Ab), B and C (oldest) (McNeill et al. 1998). Kai Unit A is only found north of the Storegga Slide.

Regional correlation

The Kai Formation can be correlated with the lower Nordland Group and upper Hordaland Group in the central and northern North Sea (Eidvin et al. 2000), megasequence FSN-2 in the Faroe–Shetland region, and megasequence RPb and the upper part of RPc in the Rockall–Porcupine region.


Genetic interpretation

The influence of ocean currents on the style of sedimentation has been important during the Miocene, and we propose a contouritic origin for much of the Miocene sediments on the Mid-Norwegian continental margin. Our interpretation is consistent with that of Bruns et al. (1998) who, based on ODP site 642, favour deposition from contour currents for the middle–upper Miocene succession on the Vøring Plateau. The alternating intervals of high carbonate and opal accumulation have been linked to increased water mass exchange with the central North Atlantic and decreased surface and deep-water exchange, respectively (Bohrmann et al. 1990). Contourite drift development has also been reported from the Lofoten margin (Laberg et al. 1999), the northern North Sea (Galloway et al. 1993) and the northern Atlantic Ocean from the Miocene period onwards (Wold 1994). In contrast, the Møre Basin prograding complex of Martinsen et al. (1999) is interpreted to reflect the building of a slope system, and a turbiditic input to the basin is envisaged. The influence of contour-currents on this part of the margin remains uncertain but cannot be discounted.

Biostratigraphic characterisation

In well 6607/5-1 the uppermost Kai Formation corresponds to the Cibicides telegdieponides pygmeus–Neogloboquadrina atlantica (dex.) biozone where the top of the zone is taken at the highest consistent occurrence of N. atlantica (dex.) (faunal zone M–H). Faunal zone M–I is the Ehrenbergina variabilis–Globocassidulina subglobosa–N. atlantica (dextral) zone while zone M–J is the Bolboforma Metzmacheri zone. The lowermost part of the Kai Formation belongs to the Bolboforma subfragori–Bolboforma fragori Zone (faunal zone M–K). The base of the zone is marked by the highest consistent occurrence of Cenosphaera sp. (Eidvin et al. 1998). For details on the Miocene succession of ODP sites 642 and 643, see Eldholm et al. (1987).


The age of the Kai Formation has been proposed to be early Miocene to late Pliocene by Dalland et al. (1988) and c. 25 to c. 7 Ma by Swiecicki et al. (1998). Regional biostratigraphic data presented in this study (Fig. 11) support an early Miocene to ‘mid’-Pliocene age range.


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[ WP1 ] Figure 18 ] WP2 ] Figure 19 ] WP3 ] Figure 20 ]

Preface ] Introduction ] Unified Stratigraphy ] Pre-Neogene Framework ] Miocene to Lower Pliocene ] Lower Pliocene To Holocene ] High-Resolution Stratigraphy ] Key Geoseismic Sections ]

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