Horda Formation
Hordaland Group
Name
English / Norwegian
Horda Formation / Hordaformasjonen
Derivatio nominis
From the Horda Platform. The formation is formally named by Knox & Holloway
(1992), but the term has been widely used informally since 1987 where it first appeared
in an unpublished report by RRI (now Simon Robertson Ltd.).
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 Horda Formation consists predominantly of grayish mudstone with subordinate
sandstone.
Thickness
The formation is 387.5 m in the UK type well 22/1-1A, and 504 m in the reference well
16/18-1, while the thickness is 808.5 m in reference well 9/4-1 and 158 m in reference
well 21/22-1.
Geographical distribution
The Horda Formation is present over much of the Central and Northern North Sea, but
passes laterally into the Mousa Formation along the eastern margin of the East Shetland
Platform and in the west of the Outer Moray Firth (Knox & Holloway, 1992).
Type well
Well name
UK well 22/1-1A
Location
WGS84 coordinates:
UTM coordinates:
UTM zone:
Drilling operator name
Completion date
Interval of type section (m)
1992-2379.5 m
Reference wells
Well names
UK wells 9/4-1, 16/18-1 and 21/22-1.
Locations
9/4-1:
WGS84 coordinates:
UTM coordinates:
UTM zone:
16/18-1:
WGS84 coordinates:
UTM coordinates:
UTM zone:
21/22-1:
WGS84 coordinates:
UTM coordinates:
UTM zone:
Drilling operator name
Completion date
Interval of reference section (m)
UK 9/4-1: 1026.5-1835 m
UK 16/18-1: 1852.5-2356.5 m
UK 21/22-1: 1076-1234.5 m
Boundaries
Lower boundary (basal stratotype)
Over most of its areas, the Horda Formation rests on the
Balder Formation, and the
boundary is marked a upward change from dark grey, carbonaceous claystone to
overlying variegated grayish mudstone and claystone. The lower boundary shows an
increase in gamma-ray intensity and a decrease in velocity from the laminated tuffs of
the Balder Formation into the claystones of the Hordaland Group (Fig. …..). Where
the Frigg Member is present at the base of the Hordaland Group the lower boundary
normally shows a decrease in gamma-ray response and an increase in velocity from
the Balder Formation into the Frigg Member (Fig. …….)
Upper boundary (characteristics)
The upper boundary is placed at the contact with grey mudstones and claystones of the
Horda Formation and the overlying brown mudstone of the
Lark Formation. On wireline
logs, the boundary is marked by a upward increase in gamma-ray readings.
"Reference" seismic sections
Location of section
[figure]
Seismic section
[Colour figure]
Fossil events/zones dating the formation
Age
Early Eocene to earliest Oligocene.
Correlations
[Graph]
Depositional environment
The mudstones of the Horda Formation were deposited in an open marine environment,
with pelagic sedimentation initially in water depths of several hundred metres (Knox &
Holloway, 1992). The associated sandstones were deposited as submarine fans (Frigg
Member), by gravity flows, or as open marine sands during a regressive period (Grid
Member) (Isaksen & Tonstad, 1989).
Remarks
The Horda Formation is here redefined to also include the
Frigg and Grid members on
the Norwegian sector.
