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An informal three-fold subdivision has been identified, and is best recognizable along the basin rim areas where a middle high gamma ray zone separates two zones of lower gamma ray response (e.g. 210/30-1).
In basinal areas the formation generally has a diachronous contact with the Heather Formation. On marginal highs the formation onlaps pre-Upper Jurassic rocks. On the northern Horda Platform, Upper Jurassic sandstones of the Sognefjord Formation mark the base of the Draupne Formation.
The upper boundary is often an unconformity or discontinuity, usually overlain by Cretaceous sediments which have a higher velocity and lower gamma ray response than the Draupne Formation. The importance and regional validity of this unconformity is currently under debate; see for example Rawson and Riley (1982).
The type area for the Kimmeridge Clay Formation is Kimmeridge Bay in Dorset, southern England (e.g. Arkell 1947). There the unit consists of dark, organic-rich claystones of Kimme-ridgian to Volgian age (using Boreal stage terminology). The term was extended by Rhys (1974 and 1975) into the Southern North Sea to describe a unit of comparable age overlying Oxfordian limestones (Corallian Formation) in type well 47/15-1. Deegan and Scull (1977) further extended the Kimmeridge Clay into the Northern North Sea to denote a generally highly radioactive, generally low velocity claystone sequence. The age range of the formation was broadened to include part of the Oxfordian and Ryazanian. It was remarked that the sediments in the Southern North Sea were less radioactive (less than 60 A.P.I, units compared with 100-200 A.P.I, units in the north). However, the low seismic velocities shown by both units were considered sufficient grounds to justify a correlation.
Deegan and Scull did not specifically discuss the application of the term "Kimmeridge Clay" to the Central Graben region of the Central North Sea. Nevertheless, the name is in common usage there (e.g. Ofstad, 1983). It has, among other meanings, been taken to denote: (i) all claystones of Upper Jurassic age regardless of log character, or more usually, (ii) only the upper, most highly radioactive claystone unit of Volgian-Ryazanian age. To the northwest, in the Norwegian-Danish Basin, a similar deposit is present (the Tau Formation, described in this publication). This unit has never been referred to the Kimmeridge Clay Formation but is nevertheless a typical Upper Jurassic organic shale. It is of Kimmeridgian-Volgian age and occupies a lower relative position in the overall Upper Jurassic shale sequence than the Central Graben "hot shales". (Table 4). A connection between the two deposits across the Southern Vestland Arch is most unlikely.
In extending the Kimmeridge Clay Formation from the type area to the Northern North Sea (a distance of
approximately 1300 km) the stratigraphic meaning of the name has varied considerably (Table 5). It is recognised
that deposits of this type were laid down over a very wide area in intervals of the late Jurassic, the
widespread deoxygenated bottom conditions reflecting the combined effect of paleogeography with general
sea level rise (e.g. Tyson et al., 1979; Ziegler 1982). However, acceptance of this overall regime should
not alone be taken as a basis for lithostratigraphic classification. The nature and timing of the organic
shales appears to have varied according to the evolution of individual basins. Use of the generic term
Kimmeridge Clay Formation is a potential cause of mistaken correlation and does not further the understanding
of inter-basin relationships. Replacement of the name is a more palatable alternative to its continued
extension northwards to the Arctic and Russia, where alternative nomenclature already exists.
It therefore seems appropriate to limit the "Kimmeridge Clay Formation"
of the North Sea to to the area south of the Mid-North Sea High, as described
by Rhys (1974). The Norwegian Lithostratigraphic Nomenclature Committee
proposes substitution of the name "Draupne Formation" for the "Kimmeridge
Clay Formation" of the Northern North Sea as defined by Deegan and Schull
(1977). It is suggested that the name "Mandal Formation" should be applied
specifically to the "hot" shales of Volgian-Ryazanian age in the Central
Graben region (Table 4), following the precedent set by Hamar et al. (1982).
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