Tofte Formation (Tofteformasjonen)
(From NPD Bulletin no. 4)
Båt Group
Name
Norwegian word for thwart. The unit has informally been called the Leka
sandstone (within Hl-3).
Well type section
6506/12-1
(Statoil), coordinates 65°10'07.58"N, 06°43'44.07"E, from 4229 m to 4164 m
(Fig. 11). Three cores (41 m recovery), including the base and middle part
of the unit.
Well reference section
6407/4-1
(Statoil), coordinates 64°35'45.36"N, 07°08'42.17 "E, from 4208,5 m to
4150 m (Fig. 12). Approximately 40 m of the section belongs to the Tofte
Formation, the rest comprises fine-grained deposits of the interfingering
Ror Formation.
Thickness
65 m in the type well, approximately 40 m in the reference well. The
formation thins rapidly eastwards across the Halten Terrace.
Lithology
The Tofte Formation consists of moderately to poorly sorted coarse-grained
sandstones which often show large-scale cross bedding. In the type section
the quartz content is generally higher than 90%, although the sediment is
texturally immature. Bioturbation occurs throughout the cored intervals,
especially in zones of very poor sorting and high clay content.
Basal Stratotype
In the type well the lower boundary of the Tofte Formation occurs at the
base of an upwards coarsening sequence. Fine-grained heavily bioturbated
sandstones rest on medium-grained sandstones of the Tilje Formation and
the gamma ray log shows a marked increase. In wells further to the south
and east the Tofte Formation overlies mudstones and shales of the
Ror
Formation. The transition is there associated with a marked decrease in
gamma ray response.
Lateral extent and variation
The Tofte Formation is only recognized on the western part of Halten
Terrace, where it consists of a continuous succession of coarsegrained
sandstones. The sandstones wedge out eastwards and interfinger with the
Ror Formation. Reference well
6407/4-1
(Fig. 12) shows a relatively distal
development of the formation where lateral interfingering occurs.
Age
Pliensbachian to Toarcian.
Depositional environment
The sandstones were deposited by eastwards prograding fan deltas which
reflect tectonic uplift to the west.
Correlation
No known time-equivalent lithostratigraphic units in surrounding areas are
similar to the Tofte Formation.
