Lark Formation
Hordaland Group
Name
English / Norwegian
Lark Formation / Larkformasjonen
Derivatio nominis
Named after the bird Lark.
Original definition
Knox, R.W.O.B. & Halloway, S. 1992. 1. Paleogene of the central and northern North
Sea. In Knox, R.W.O.B. & Cordey, W.G. (eds.), Lithostratigraphic nomernclature of the
UK North Sea. British Geological Survey, Nottingham.
Lithology
The formation consists of brown-grey to grey-brown mudstone, silty in part, and locally
grading upwards into sandstones of the Skade Member.
Thickness
The Lark formation is 650 m in the type section well
21/10-4
. The formation reaches a
maximum thickness of about 1000 m in the Central Graben (Knox & Holloway, 1992).
Geographical distribution
The Lark Formation is present over much of the Central and Northern North Sea.
Occurrences of formation tops in wells
Type well
Well name
UK 21/10-4.
Location
WGS84 coordinates:
UTM coordinates:
UTM zone:
Drilling operator name
Completion date
Interval of type section (m)
From 1217-1867 m in well 21/10-4.
Reference well
No reference well is presently defined.
Boundaries
Lower boundary (basal stratotype)
The lower boundary is marked by an upward change from a variable association of green-
grey mudstone, grey silty mudstone, or sandstone of the
Horda Formation, to grey-brown
or brown-grey mudstone of the Lark Formation.
Upper boundary (characteristics)
The upper boundary of the Lark Formation is marked by an upward change from grey-
brown to brown-grey mudstones of the Lark Formation to overlying green-grey,
commonly glauconitic, silty mudstones or basal sandstones of the
Nordland Group.
In the Central Trough, a zone occurs which has high gamma-ray readings and usually a
slightly lower velocity than the underlying and overlying claystones. The upper boundary
of the Hordaland Group is placed at the base of this zone (Fig. ...).
"Reference" seismic sections
Location of section
[figure]
Seismic section
[Colour figure]
Fossil events/zones dating the formation
Age
Early Oligocene to early mid Miocene (Knox & Holloway, 1992).
Correlations
[Graph]
Depositional environment
The mudstones of the Lark Formation were deposited in a distal shelf setting (Andrews et
al., 1990; Knox & Holloway, 1992).
Remarks
In blocks 2/2 and 2/3 the Lark Formation contains a unit of Upper Oligocene shallow
marine sandstones assigned to the Vade Member.
