Magne Formation (Magneformasjonen)

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

 

Introduction

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in the Central Graben is after Fritsen & Riis (2000: ‘A revised chalk lithostratigraphic nomenclature’; NPD Report, unpublished) (see Table)

Name

After the Norse god Magne, who was the son of Tor, and who supported him after his great fight with the giant Hrungnir (Sturluson, 1954).

Lithology

Lithofacies associated with the crestal biofacies on the Valhall structure are typically (textural) mudstones and wackestones. Those associated with the open marine platform, high productivity upper slope and basinal biofacies are typically bioturbated argillaceous wackestones and chalks or interbedded chalks and claystones (sometimes referred to as periodites), indicative of slow sedimentation or, in the case of the basinal biofacies, incipient hardgrounds, indicative of extremely slow sedimentation (sediment starvation). Those associated with the eutrophic sub-biofacies are bioturbated, those associated with the dysoxic sub-biofacies are laminated (i.e., non-bioturbated) pyritic chalks.

Thickness

The thickness of this formation ranges from zero to a few hundred metres in the study wells.

Geographical distribution

It is absent in the 31/26A-10 well in the United Kingdom Sector, thin (typically less than 100 m thick) on structural highs such as Valhall-Hod, Eldfisk and Tor in the Norwegian Sector, and thickest in wells in basinal lows such as those in the 1/3 and, locally, 2/5 areas in the Norwegian Sector.

Type well

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

4125-3952 m MD

 

Reference Well

Well name: 2/11-A-2 T2

3486-3427.4 m MD

Upper and lower boundaries

Upper Boundary

Picked on seismic, biostratigraphic and log criteria. The seismic criterion is a reflector (which may also locally be an onlap surface) separating a lower amplitude interval below from a higher amplitude interval above, at least in structurally higher areas. The biostratigraphic criterion is penetration of microzone FCS21. The log criterion is a downhole gamma increase (the Magne is more clay-rich than the overlying Tor).

Lower Boundary

Picked primarily on a seismic criterion (onlap surface). The formation onlaps the Thud Formation and oversteps on to the Narve (?) Formation on flanks of structures.

 

Biostratigraphy

Microzones intraFCS20pp-intraFCS22; nannozones intraUC14pp-intraUC16pp. The base is taken near 81 Ma, using the criterion of 40% up in UC14.

Age

Late early to early late Campanian (for practical purposes, Campanian).

Depositional environment

The formation is variably associated with the open marine platform, high productivity upper slope and basinal environments, locally eutrophic.

References

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

 



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