From the Fladen Ground Spur, a structural feature.
The group is typically developed in the UK sector of the Central North Sea particularly in the area between the Piper and Forties oilfields.
Within the volcanic province the thickest section penetrated so far is 1100 m and in this well the base of the volcanic was not reached. A considerable amount of thickness information was presented by Howitt and others (1975).
The group embraces both volcanic and non-volcanic formations. Therefore the group contains a wide range of lithologies including basalts, tuff agglomerates, and normal continental to shallow water sediments.
The group normally rests on pre-Jurassic rocks, frequently Triassic sediments, and the lower boundary is marked either by the incoming of igneous rocks or the change from Triassic continental sediments to paralic sediments containing tuff horizons. The upper boundary is the contact with the marine sediments of the Humber Group. The Piper or Kimmeridge Clay formations may overlie the Fladen Group and in some sections the
may be present but as noted earlier the complex interdigitations of the component formations of the Humber Group are not precisely known for this part of the Central North Sea.
The group is largely restricted to the area of UK quadrants 14, 15, 16, and the more northerly parts of 21 and 22.
As noted above the rocks of the Fladen Group are divided into two formations which are essentially end members of an interdigitating sequence. These are named the
and Pentland formations. For present purpose if the encounters section contains more than half volcanic it is assigned to the
while if it contains more than half sedimentary rocks then it is placed in the Pentland Formation. As more data become available it will almost certainly be possible to subdivide the two formations further, and a proper understanding of the relationship between these end members may necessitate revision of the nomenclature in the future.
Deegan, C. E. and Scull, B. J. (compilers) 1977: A standard lithostratigraphic nomenclature for the Central and Northern North Sea. UK Institute of Geological Sciences, Report 77/25; the Norwegian Petroleum Directorate, NPD-Bulletin No. 1, 36 pp.