Tarbert Formation
(From NPD Bulletin no. 3)
Brent Group
Name
Named by Deegan and Scull (1977) who gave it "sub-unit" status.
Well type section
UK well 211/29-3 (Shell), from 2602.5 m to 2633.5 m, coord N 61°08'06", E 01'36.5", (Fig. 10).
Well reference sections
Norwegian wells 33/9-1 (Mobil), from 2464 m to 2509m, coord N61°15'07.05" E01°50'25.8" (Fig. 11), 31/4-4 (Norsk Hydro) from 2680 m to 2695 m, coord N60°40'01.12" E03°06'54.12" (Fig. 17) and 30/6-7 (Norsk Hydro) from 2632 m to 2646 m, coord N 60°38' 39.49'', E02°45'21.74", (Fig. 16).
Thickness
31 m in the type well, 45 m in 33/9-1, 15 m in 31/4-4 and 14 m in 30/6-7.
Lithology
In the type well section it consists of grey to brown relatively massive fine to medium grained sandstone with subordinate thin siltstone, shale and coal beds and some calcareous bands. On the Horda Platform the formation usually constitutes one or more "coarsening upward" sequences of fine to medium, occasionally coarse, micaceous and carbonaceous sandstones, which become increasingly silty and argillaceous downward. Stringers of calcite cemented sandstone and coal beds occur in this area.
Boundaries
The base of the formation is taken at the top of the last "fining upward unit of the Ness Formation, i.e. at the top of an argillaceous bed or coal-bed. The upper boundary coincides with the top of the Brent Group (see above).
Distribution
The distribution of the formation is not known in detail, but is usually well developed within the western part of the East Shetland Basin, including the central and western part of the Viking Graben. The formation has a sporadic distribution on the Horda Platform.
Age
Bajocian to Bathonian
Depositional environment
The environment of deposition of the formation was marginal marine. In places it may rest with minor disconformity on the coaly Ness Formation (Hodson, 1975), and possibly in part represents reworked delta plain deposits at the onset of the marine transgression which deposited the overlying Upper Jurassic argillaceous sediments, sediments.
