Geoscience Reference
In-Depth Information
interval where the system flooded sufficiently to
allow the deposition of thin mudstone layers in
shallow temporary lakes that were then preserved
under a decreasing sedimentation rate.
sequence (Fig. 4c) of red, green and purple silt-
stones. The cyclicity when fully developed is
defined by the rock colour with the alternation
(Fig. 4d) from green through purple to red and
then back to purple and green. These differently
coloured beds define the characteristic 'banks' of
S¨ve-S¨derbergh (unpublished m/s discussed in
Blom et al. 2005, p.17) that were used to internally
sub-divide and correlate the tetrapod-bearing inter-
vals within the formation. In Paralleldal, some of
these green siltstone beds (Fig. 5d) contain thin
sandstone stringers which show that it was these
drab units that were the wetter parts of the cycle.
The cyclicity is systematically variable as the green
siltstones can be missing from parts of the sequence
where the cycles alternate between red and purple
colours. All the different coloured siltstones
contain conspicuous arcuate and sub-horizontal
crack systems (Figs 4d-f & 5c) expressed as a
very pervasive brecciation (Fig. 4f ) that represents
continuous in situ shrinkage and expansion. No
calcrete nodules were found in these siltstones.
These siltstones are entirely characteristic of
vertisols (Retallack 1997; Ahmad & Mermut
2006) and represent an extensive mud-rich flood-
plain dominated by soil-forming processes. These
are arid soils where there was successive seasonal
wetting and drying. There are some 196 vertisol
cycles present within the Wimans Berg to Britta
Dal interval on Gauss Halvø and, as such, these
formations represent a major sustained episode of
aridity. These cycles are grouped into 6 megacycles
of c. 20 vertisol cycles defined by systematic
changes in the relative thickness of the green and
red silts. The Britta Dal Formation on the mountain
of Celsius Bjerg (Ymer Ø, Fig. 1; see also Clack &
Neininger 2000) is much more proximal in character
and contains significant sandstone beds (Fig. 4h)
that replace the green intervals within a sequence
of red-coloured palaeosols.
On Gauss Halvø the Britta Dal Formation con-
tains fine-grained sandstones that are laterally very
extensive (Fig. 6a-c) and have flat bases (i.e. they
were weakly to non-erosive). This demonstrates
their origin as unconfined (i.e. poorly channelized)
major flood events that spread across an extensive
alluvial plain. Many of
Aina Dal Formation
The overlying 45 m thick Aina Dal Formation
(Figs 2 & 3) marks a transition to meandering
rivers (Olsen & Larsen 1993) with a significant pro-
portion of more mud-dominated overbank deposits.
The evidence for the higher sinuosity is lateral
accretion within channel sandstones including
several point bar sequences. There is pervasive brec-
ciation throughout the formation which together
with the more typical desiccation cracks indicates
that the climate was becoming increasingly arid.
Many Ichthyostega specimens have been found
from Aina Dal Formation scree together with a
single Acanthostega specimen (Bendix-Almgreen
et al. 1990; Blom et al. 2005) from a loose block
that is also entirely characteristic of Aina Dal
Formation lithology.
Wimans Bjerg Formation
The Aina Dal to Wimans Bjerg contact is an upward
transition through a drab-coloured (but non-
palyniferous) interval of siltstone. The Wimans
Bjerg Formation (c. 160 m, Figs 2 & 3) is character-
ized (Olsen & Larsen 1993) by parallel laminated
siltstones and thin sandstones with abundant wave
ripples. These are interbedded with dolomitic silt-
stones that are also wave rippled and with abundant
brecciation (Fig. 4a), desiccation cracks and
gypsum pseudomorphs (Fig. 4b). The environment
(Fig. 3) represents an alternating lacustrine and
inland playa with a fluctuating but generally high
water table giving rise to the dark sediment colour.
Despite this dark colour, however, the formation is
devoid of preserved organic matter (102 barren
palynological samples in the Wimans Bjerg and
Britta Dal Formations).
Britta Dal Formation
The Britta Dal Formation is a c. 460 m interval
(Figs 2 & 3) that comprises a spectacular cyclic
these sand bodies are
Fig. 4. (Continued ) the red (darker colour) vertisol cycles can be seen to thicken in the middle part of the exposure.
(d) Single vertisol cycle, Britta Dal Formation, Nathorst Bjerg. The green colour is paler middle section, purple is
the darkest colour beneath this, red the upper darker layer. (e) Close-up of green-purple-red colour boundary (green is
the lower two thirds, red the uppermost darker colour, purple the intermediate band between red and green) showing
arcuate cracks and in situ brecciation, Britta Dal Formation, Nathorst Bjerg. (f ) Bedding plane view of green vertisol
cycle showing in situ brecciation, Britta Dal Formation, Nathorst Bjerg. (g) Fluvial sandstone in the Britta Dal
Formation showing lateral change to green colours (right hand side of the sandstone) as soil turbation mixes the sand
with floodplain mudstones, Nathorst Bjerg. (h) Britta Dal Formation from Celsius Bjerg, Ymer Ø. This shows the
replacement proximally of the green, wetter vertisol cycles by fluvial sandstones.
