VIKING GRABEN
1.11.2 Formatlon/Sequences: Definitions
Seismic Sequence 2: This has been subdivided into three Subsequences (a-c) by Stewart, Sequences 2a-2b being equatable with the Maureen Formation, which is of mixed Üthology and contains firequent conglomeratic pebbles and clasts of reworked limestone and shales of Daman age (Deegan & Scull 1977). In areas removed from fan deposition the hemipelagic claystones are referrable to the lista Formation (Vale Formation, Norwegian North Sea). The highstand mudstone of Subsequence 2c is referable to the lista Formation.
Seismic Sequence 3: In the Central North Sea the sand input associated with the basinward shift in coastal onlap is referable to the Andrew Formation. Ih the South Viking Graben any sands developed would be attributed to the lower parts of the Heimdal Formation. Ih areas without sand deposition the hemipelagic claystones fades is attributed to the lista Formation.
Seismic Sequence 4: This sequence is recognised by the presence of Phase 1 volcanics (Knox & Morton 1983). From Stewart (1987) it would appear that these volcanics define the top of the Andrew Formation. Sands developed above this sequence in the Central North Sea are probably attributable to the
"Undifferentiated Montrose sands" of Deegan and Sciill (1977) or the Heimdal Formation (pars) in the South Viking Graben. Hemipelagic claystones are termed the lista Formation.
Seismic Sequence 5: Ih the Central North Sea sands deposited during this relative lowstand are termed the "Undifferentiated Montrose sands” (Deegan & ScuH
1977), whilst in the South Viking Graben the dastics define the upper limit of the Heimdal Formation. The lista Formation is assigned to the hemipelagic claystones.
Seismic Sequence
6
: This represents a highstand system in which there are only rare, as yet un-named, sand deposits (Fig. 19). The claystones of this sequence represent the upper limit of the lista Formation. Ih the wells studied fiom theBalder Field (Norwegian 25 quadrant) this sequence is represented by dive green claystones. Associated with this is the last period when the basin is oxygenated prior to the onset of dysoxia/anoxia during Sele/Balder times. The latter is represented by a change fix>m hioturbated claystones to the laminated shales of
Sequence 7 (Stewart 1987, Deegan & ScuH 1977). The wireHne log can be seen to change across tins boundary with a increase in the g a m m a response in the Sele Formation (e.g. in wells 25/10-1, 25/10-2 and 25/8-1; Appendix Al.l). This boundary has probably been incorrectly identified in 25/10-1, being recorded at
5794ft, according to NPD Report 28. The boundary is here considered to occur at 5912ft, reflecting the change firom oxic to dysoxic conditions at this depth, as indicated by preservation state of amorphous organic matter (see section
1
.12
,and Chapter Four).
Seismic Sequence 7: The base of this sequence as discussed earlier represents the regional discontinuity of Rochow (1981), and also the initiation of dysoxic
conditions (Stewart 1987). Ih the Central North Sea, sands are referred to as the Forties Formation (see earlier discussion, Stewart 1987), while in the South
Viking Graben any sand depostion has been named the Hermod Formation in the Norwegian sector. However, as discussed, there seems to be a highstand claystone which terminates Sequence 7 sedimentation (Forties Formation) i.e. Sequence
8
, therefore any sands younger than Sequence8
in the Sele Formation are stfll un named, and cannot, as inferred by isaksen and Tonstad (1989), be called theHermod Formation. The sand deposition of Sequence 7 may be related to the relative basdnward shift in coastal onlap. The absence of coarse clastic
sedimentation leaves only the depostion of hemipelagic shales termed the Sele Formation (sensu Deegan & ScuH 1977).
Seismic Sequence
8
: This sequence represents a highstand system; the resulting hemipelagic shales are termed the Sele Formation. Stewart (1987) relates this highstand system with the stacking of elastics in the Moray Firth area giving rise to the sands and coal deposUs of the deltaic Dornoch and Beauly Formations. This sequence represents an intra-Sele highstand shale, and may be recognised on g a m m a log responses by a ’hotter' Ëiale overlying the Forties/Her mod Sands. Examples of this 'hotter' diale in the literature include 16/7a-2 (1720m), 25/8-1 (1748m) and 21/10-1 (2125m), all from Morton and Knox (1990). This hotter shale is accounted for by reduced sedimentation rates during this relative highstand, resulting in condensing in AOM-rich shales. However, it must be stressed that this shale is generally difficult to recognise.Seismic Sequence 9: The base of this sequence, as discussed earlier (Fig. 19), represents a rapid lowering in sea level and a resultant basinward shift in coastal onlap. In the Central North Sea and in the South Viking Graben any sands are un-named and are therefore here termed the Sele Sands/Balder Sands. As
disciossed by Deegan and Sciill (1977) and Stewart (1987), the increase in
volcanic ash layers represents the Balder Formation, with less volcanic-a^-nch shales seen in lower Sequence 9 times, being termed the Sele Formation. The characteristic wireline "dumbell” shape of the Balder Formation is shown in Pig. 11
.
Seismic Sequence 10: This sequence is represented at its base by red/brown claystones, perhaps attributable to altered volcanic ash of volcanic phase
2
d (Knox & Morton 1983). Ih the South Viking Graben, sands in the Frigg area are termed the Frigg Formation, outside this area clastic input is un-named. The tdoturbated claystones reflecting hemipelagic sedimentation are termed theHordaland Group (Deegan & Scull 1977).