Thud Formation (Thudformasjonen)

updated to follow: Stratigraphic Guide to the Cromer Knoll, Shetland and Chalk Groups of the North Sea and Norwegian Sea. Felix M. Gradstein & Colin C. Waters (editors), Mike Charnock, Dirk Munsterman,  Michelle Hollerbach, Harald Brunstad, Øyvind Hammer & Luis Vergara (contributors). Newsletter on Stratigraphy, vol 49/1 pp71-280, 2016

Chalk Group


The definition of the Thud Formation in the Central Graben is after Fritsen & Riis (2000: ‘A revised chalk lithostratigraphic nomenclature’; NPD Report, unpublished) (see Table)


After an alternative name for the Norse god Odin (Sturluson, 1954). Thud means “thin one”.


Poorly constrained owing to the lack of core coverage. Relatively argillaceous chalks appear to alternate with clean chalks.


The thickness of this formation ranges from zero to a few hundred metres in the study wells. It is thickest in wells in basinal lows, such as those in the 1/3 area in the Norwegian Sector and the Roar area in the Danish Sector.

Geographical distribution

The formation is confined to the Central Graben of the Norwegian and Danish Sectors. It is absent in the 2/2 wells and in wells on structural highs, such as Valhall-Hod and Eldfisk in the Norwegian Sector and in the Lulu-1 well in the Danish Sector.


Type well

Well name: 1/3-8 (Norwegian Sector)

4337-4125 m MD


Reference Well

Not assigned.

Upper and lower boundaries

Upper Boundary

Picked primarily on a seismic criterion (onlap surface), corresponding to the original informal middle Hod sequence boundary.

Lower Boundary

Picked primarily on seismic and biostratigraphic criteria. The seismic criterion is an onlap surface. The biostratigraphic criterion is penetration of nannozone UC11. The formation laps on to the Narve Formation on the flanks of structures.

Well log characteristics

A complete Thud Formation has been penetrated in few wells, because of its basinal setting. In the northern part of the region, the gamma values in 1/3-1 and 1/3-8 indicate a clay content that is higher than in the Magne Formation. The relatively argillaceous chalks seem to alternate with clean chalks, giving rise to a ”box-shaped” pattern of the gamma log. The time equivalent to the Thud Formation in the Roar-2 well in the Danish Sector has developed a different log and seismic pattern.


Microzones FCS18pp-FCS20pp; nannozones UC12-UC14 pp.


Late early Santonian to earliest Campanian (for practical purposes, Santonian).

Depositional environment

Biofacies in cuttings samples from some wells are characterised by moderately abundant and diverse planktonic foraminifera and radiolaria and generally rarer carbonate agglutinated and calcareous benthonic foraminifera. This indicates affinity with the open marine platform or “shallow water” pelagic biofacies of the Valhall structure.


After Fritsen, A. & Riis, F. 2000. A revised chalk lithostratigraphic nomenclature; NPD Report, unpublished)

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