Jökull - 01.01.2013, Side 52

J. Helgason and R. Duncan
much greater. This seems reasonable keeping in mind
that eastward from Hafrafell the "lower strata" dis-
appear and are not exposed again until perhaps in
Breiðamerkurfjall, some 20 km farther east. The "lat-
erally continuous basalt lavas and hyaloclastites" in
Vatnafjöll (Stevenson et al., 2006) are all normally
magnetized and of Brunhes age and thus not part of
the "lower strata". This emphasizes the need for de-
tailed fieldwork and mapping of erosion surfaces that
may eventually reveal how relief was modified to its
present state.
Age, nature and stratigraphic relationship be-
tween groups S4–S7
The youngest groups, S5 to S7, rest unconformably on
the sides of Svínafell and their age relationship to the
dated group S4 is of interest. The youngest group in
Svínafell, S7, was not dated but can be correlated with
an identical dated group in Hafrafell that has an age of
215 ka. In the present study group S4 was dated at 698
(±54) ka. From a stratigraphic point of view group
S5 is considerably younger than group S4. Since we
have evidence for several glacial–interglacial transi-
tions occurring after its deposition, we estimate group
S5 to have an age of about 300–400 ka. We consider
group S6, deposited at only 100 m above sea level, to
be of upper Brunhes age (ca. 200–300 ka).
Landscape evolution in Iceland during the
Matuyama chron
The effect of glacial ice in sculpting landscape in
Iceland during the Matuyama (0.781–2.581 Ma) may
have been highly variable and dependent on many fac-
tors, including the distribution and thickness of ice
sheets. The Matuyama lower boundary is close to
the onset of the main northern hemisphere glaciations.
Recently, the Subcommission on Quaternary Strati-
graphy agreed to define the onset of the Quaternary
period at 2.58 Ma, a time that almost coincides with
the boundary between the Matuyama and Gauss mag-
netic intervals (Gibbard et al., 2010). Global Strato-
type Section and Point (GSSP) for the Quaternary is
just 1 m above the Gauss-Matuyama paleomagnetic
reversal (Lourens, 2008). For work on glaciations and
erosion history in Iceland, this new definition signifi-
cantly eases the mapping of the main ice age boundary
as it is contemporaneous with a magnetic reversal be-
tween two relatively long chrons. Clearly, however,
Iceland was covered with ice prior to 2.58 Ma (Eiríks-
son and Geirsdóttir, 1996).
Tertiary landscape in Iceland
We assume that prior to the onset of northern hemi-
sphere glaciations, at roughly 2.7 Ma, Iceland’s sur-
face was relatively flat as a result of prolonged lava
accumulation under subaerial conditions during the
Neogene (roughly 20 to 2.7 Ma). For the last 2.7 Ma
Iceland has been shaped by glacial erosion during
some 19 glacial intervals (Geirsdóttir et al., 2006)
and, at least during Brunhes, these glacials lasted
longer than the intervening ice-free inter-glacials. Not
only has erosion been much greater during glacials but
the amount of volcanic material lost, with either melt-
water or explosive activity under sub-ice volcanism,
must have been substantial. On the other hand vol-
canic material lost to sea under ice-free volcanic con-
ditions must have been negligible. Valleys and fjords
continued to deepen and grow during this period caus-
ing the land eventually to rise isostatically. Therefore,
compared to present day conditions, we suppose that
Iceland had, say at 10 Ma, a much greater subaerial
extent as well as having a highland that was mostly
below the snow line, as indicated by rare hyaloclastite
sediments of Tertiary age. The erosion stages in
Svínafell bear witness to this developing landscape
and clearly hiatuses have become greater with time.
We observe erosion surfaces in Svínafell’s stratigra-
phy. We expect many of these surfaces to be coeval
with similar surfaces found elsewhere in the region
(Helgason and Duncan, 2001; Helgason, 2007) and
that with additional work these will prove to be re-
gional markers in documenting the glaciation history
of Iceland.
SUMMARY AND CONCLUSIONS
The lowest strata in Svínafell are probably some 4
million years old, i.e., reversely magnetized rocks
from the Upper-Gilbert magnetic chron (3.60–4.19
Ma). These Neogene rocks belong to the lower strata
in Öræfi that formed prior to the beginning of glacial
conditions (>2.7 Ma) when the landscape was most
likely relatively flat accumulations of basaltic lava
52 JÖKULL No. 63, 2013