Skade Member
Hordaland Group,
Lark Formation
Name
Named after the female giant in Norse mythology, the wife of the god Njord.
Well type section
Norwegian well
24/12-1
from 1007 to 851 m, coordinates N 59°02'29.80", E
01°52'57.93" (Figure xx). No cores.
Well reference section
Norwegian well
15/9-13
from 1224 to 1143 m, coordinates N 58°22'25.96", E
01°56'02.86" (Figure xx). No cores.
Thickness
Up to 300 m in the southern Viking Graben (e.g. in well
16/1-4
). The thickness is 156 m
in the type well and 81 m in the reference well. The formation shows a general eastward
thinning.
Lithology
The Skade Member consists of alternating sandstones and thinner claystones. The
sandstones are clear to light grey, usually fine to medium-grained and occasionally
coarse-grained. The sand grains are subrounded to rounded, and moderately to well
sorted. Marine fossil fragments, including shell debris, and mica and abundant glauconite
are present. In some wells the claystones are silty. Towards the southern pinchout of the
Skade sands (Figure xx) the formation comprises a stacked succession of coarsening-
upward units, each up to 50m thick.
While in the Viking Graben the typical Skade sands are present, toward Tampen the
lithology becomes more muddy, and grades laterally in the xxxxx formation.
Basal stratotype
The lower boundary is marked by a general coarsening upwards trend together with a
general decrease in gamma-ray response. The underlying upper Oligocene strata
comprise a predominantly mudrock succession, with intercalated thin sands present in the
northern Viking Graben. In the north there is a marked coarsening upwards trend from
mudstones to siltstones and silty-sandstones, while in the south this trend is less
pronounced. The basal sands and siltstones may be rich in glauconite.
Characteristics of the upper boundary
The upper boundary of the member corresponds approximately to the top levels assigned
an Early Miocene age (according to the biostratigraphy and Sr-isotope stratigraphy of
Rundberg & Eidvin, 2005). The Skade sands are overlain by mudstones which onlap the
middle Miocene erosive surface to the east. In the southern Viking Graben these authors
identified the upper boundary of the Skade Member, synonymous with the
Hordaland/Nordland transition, as coinciding with a very distinctive high radioactive
marker, a high-amplitude seismic reflector, and a high gamma-ray log marker. (Figure
xx). This unconformable surface is interpreted as a base middle Miocene sequence
boundary.
Logs from well 15/3-3
(pdf)
Logs from well 15/9-13
(pdf)
Logs from well 16/1-1
(pdf)
Logs from well 24/12-1
(pdf)
Distribution
See figure xx - Lateral extent of the sands of the Skade Member
Biostratigraphy
A characteristic assemblage of mostly pyritised diatoms, together with sponge spicules,
radiolaria, agglutinated benthic and planktonic foraminifera (index taxa to be named)
here is typical of the lowermost horizons of this formation. The Skade sands differ from
the overlying middle Miocene mudstone unit by lacking the presence of a Bolboforma
badenensis and Bolboforma reticulata assemblage (Rundberg & Eidvin, in prep.).
Age
Early Miocene (Rundberg & Eidvin, in prep.), and previously assigned to Oligocene by
Isaksen & Tonstad (1989). It has been noted by several workers that the
Oligocene/Miocene boundary is often difficult to determine in well sections, using either
biostratigraphy or seismic. This is particularly pertinent in the north Viking Graben.
Based on both biostratigraphy and Sr-isotope stratigraphy, the top of the Skade sands
corresponds closely to the top of the Lower Miocene. See note on age of
Utsira Member.
Depositional environment
Isaksen & Tonstad (1989) interpret the depositional environment to be an open marine
setting with deposition occurring in response to a global sea-level regression.
