Geoscience Reference
In-Depth Information
represents a single, deep, permanent lake of con-
siderable size and stability. It contains quite a differ-
ent vertebrate assemblage that lacks the otherwise
ubiquitous placoderms and contains only the acti-
nopterygian fish Cuneognathus (Friedman & Blom
2006). This lake is coincident with the Devonian-
Carboniferous boundary with the interval immedi-
ately beneath the organic-rich sediments containing
a LN* spore assemblage, whereas the upper part of
the Obrutschew Bjerg Formation contains VI spores
(Streel & Marshall 2006).
the individual cycles can then be plotted against
cycle number through the section. This enables
a correlation with the Nathorst Bjerg section
(Fig. 8). The datum for the correlation is the top of
the Wimans Bjerg Formation. This places the peak
in cycle thickness at the top of megacycle 1 in
Gauss Halvø exactly coincident with a peak in
Paralleldal. The peak with the bundle of sandstones
grouped around 1174 m is coincident with the first
occurrence of a single sandstone bed and associated
sand stringers in Gauss Halvø. This shows that the
position of the 1174 m sandstone is about half way
through the second megacycle.
The poor condition of the remaining outcrop of
the 1174 m sandstone prevents any direct study of
its immediate environment. However, along strike
it is represented by a 1.6 m thick sandstone which
has a flat basal contact with the underlying siltstone.
The sandstone is largely parallel bedded but does
contain inclined scour surfaces. Also present are
scattered bone fragments together with abundant
plant debris at its base. Some of these plant frag-
ments are in good condition and are fern-like,
perhaps suggestive of a local short-lived vegetation
that grew adjacent to the active fluvial systems
following floods.
Interestingly, a similar thick sandstone occurs at
about this stratigraphical level on northern Nathorst
Bjerg (916 m altimeter; 904 m GPS, 0422863
8156887) where it is the second sandstone above
the base of the Britta Dal Formation. This sandstone
contains both abundant basal shale clasts (dark-
coloured silts but barren of palynomorphs) and
numerous vertebrate fragments which include
Holoptychius scales. It would appear to be a more
proximal equivalent of the sandstone in Paralleldal.
Harder Bjerg Formation
This interval is truncated by Permian sandstones on
Gauss Halvø but is more fully developed on the
mountain of Celsius Bjerg on Ymer Ø. It marks a
return to a sequence of thick fluvial sandstones
with minor calcretic mudstone intervals.
The 1174 m Ichthyostega locality on
Sederholm Bjerg, Paralleldal
The 1174 m Ichthyostega locality in Paralleldal
(Fig. 5a) was relocated (2006) in Profile Ravine
(Jarvik 1996, fig. 1B; Blom et al. 2005, figs 3, 4 &
10, section 11) at 0427913 8159913 and at an esti-
mated altitude of 1198 m (GPS) or 1182 m (alti-
meter). The first sandstone in the section occurred
at 1180 m (altimeter). The bed with the in situ
Ichthyostega is the thickest sandstone seen within
the section and contains bone fragments and abun-
dant plant debris. It occurs in the lower part of the
Britta Dal Formation within a sequence of red,
green and purple mudstones. The 1174 m bed is
still present in the ravine (Fig. 5b) but the exposure
is now very reduced, presumably by extensive fossil
collecting (Jarvik 1996). This 1174 m bed is at a
high altitude close to the flat-topped summit of
Sederholm Berg, such that the exposure is generally
reduced to the block crop typical of Arctic mountain
tops apart from in the ravine itself.
However, a correlative section (Fig. 5e) has been
measured on a prominent spur of red, green and
purple-coloured siltstone on the eastern side of
Profile Ravine. The base of the section is placed at
the last thin grey dolostone unit, that is, the top of
the Wimans Bjerg Formation, (0427933 8159551,
height GPS 1079 m, altimeter 1044 m) and termi-
nated along strike from the 1174 m locality at
0428102 8159778. The different-coloured siltstone
cycles in the section were measured in an identical
manner to those defined from the section on
Nathorst Bjerg. The thicknesses of the individual
cycles were measured from the base of a green
siltstone through red/purple siltstones to the base
of the overlying green siltstone. The thicknesses of
The Acanthostega locality on the Stensi¨
Bjerg Ridge
The well-documented (Bendix-Almgreen et al.
1988, 1990; Blom et al. 2005) in situ occurrence
of many articulated individuals of Acanthostega
is from the SE ridge of Stensi¨ Bjerg. It occurs at
an altitude of 759 m (GPS) or 785 m (altimeter)
with GPS co-ordinates 0429827 8146803. These
occur within a sandstone bed towards the top of
megacycle 6. The sandstone was described in
Bendix-Almgreen et al. (1990) as typical of the
Stensi¨ Bjerg Formation. The sandstone was des-
cribed as two-storied and dominantly parallel lami-
nated, although with low angle bedding in the lower
storey indicative of lateral accretion on a point bar.
The upper storey was again stated to have low-angle
bedding of inferred point bar origin overlaid by a
fine-grained channel fill. The two storeys were
regarded as probably superimposed by downstream
