Tuxen Formation (Tuxenformasjonen)
(From NPD Bulletin no. 5)
Cromer Knoll Group
Name
Named by Jensen et al. (1986) from a bathymetric feature west of Blåvandshuk,
the westernmost point of Jylland.
Well type section
Danish well 1-1 from 2986 to 2898 m, coordinates N 56°03'10", E 04°(14160" (Fig. 14). No cores.
Well reference sections
Norwegian well 2/11-1
from 3063 to 2988 m, coordinates N 56°14'16.98", E 03°27'07.05" (Fig. 12). No cores.
Norwegian well 2/6-2
from 3935 to 3864 m, coordinates N 56°30'48.90", E 03°42'39.66" (Fig. 15). No cores.
Thickness
The thickness of the formation varies from 1 m along
structural highs to about 100 m in basinal areas. In the
reference wells the thicknesses are 75 m (2/11-1) and
71 m (2/6-2). In the type well (1-1) the thickness is 88 m.
Lithology
The formation is dominated by white to greyish-pink, calcareous claystones
and marlstones. Along some of the structural highs the marlstones grade into
purer limestones. Generally, the formation terminates vertically upwards
with a chalk sequence containing subordinate marlstone layers. This chalk is white to pale orange or yellowish-grey, occasionally greenish and reddish. The marlstones are generally light grey to greenish-grey or olive-grey, but may be reddish-brown in some wells.
A 0.3-1 m thick, radioactive, marlstone bed is frequently encountered within the Tuxen Formation in the Danish sector where it is defined as the Munk Marl Bed (Jensen et al. 1986). This characteristic unit has also been recognised in some wells in the central Norwegian sector (e.g. 2/1-2, 2/1-3, 2/1-8, 2/6-2, 2/11-7,6/3-1, 16/8-1 and 16/10-1), (see also Fig. 15). In the Norwegian sector, the Tuxen Formation above the Munk Marl Bed is often more calcareous than the rest of the sequence.
Basal stratotype
The lower boundary is defined as the base of an upward decrease in gamma-ray
readings and an increase in velocity, reflecting the passage from the
slightly calcareous claystones of the underlying Åsgard
Formation up into the more calcareous claystones and marlstones of the
Tuxen Formation (Figs. 12, 14 and 15). The transition is generally gradual
in basinal areas. Purer limestones were deposited along some structural highs,
causing more distinct log breaks.
Characteristics of the upper boundary
Upwards, the Tuxen Formation is generally in contact with the micaceous
claystones and organic-rich shales of the Sola
Formation (Figs. 12 and 14). This boundary is marked by an upward increase
in gamma-ray readings and a decrease in velocity. Where the Sola Formation
is missing, the Tuxen Formation is in contact with the marlstones of the
overlying Rødby Formation (Fig. 15). The boundary is
usually defined by an upward increase in gamma-ray readings.
Distribution
The Tuxen Formation is widely distributed in the Norwegian and Danish
sectors (Jensen et al. 1986). In the Norwegian sector it is developed in
the Central Trough, along the Jæren High and in parts of the Norwegian-Danish
Basin.
In basinal areas in the Norwegian sector it inter-fingers laterally with
claystones and marlstones of the Åsgard Formation
(Figs. 4 and 7).
Occurrences of formation tops in wells
Age
Late Hauterivian to Late Barremian (Heilmann-
Clausen 1987,Thomsen 1987).
Depositional environment
Deposition was dominated by pelagic marl and chalk oozes, which covered
large areas of the North Sea. The bottom waters were mainly well oxygenated
(Jensen et al. 1986).
