Jökull - 01.01.2014, Blaðsíða 55

Magnetostratigraphy, K-AR dating and erosion history of Hafrafell, SE–Iceland
been characterized by nunataks with heights well
above the pre-glacial level. The time-averaged effects
of these processes may have governed elevation and
relief of SE Iceland. As these factors are very different
from the situation in the accreting volcanic rift zone
the question arises: "how high was the land in the ac-
creting rift zone compared to SE Iceland when an ice
sheet first formed in Iceland?" This may be difficult to
estimate. It may be argued, however, that during the
Neogene, prior to major glaciations, the SE Iceland
region was more elevated than the rest of Iceland as a
result of the off-axis volcanic activity. We agree with
Geirsdóttir (2011) who argues that an ice sheet first
formed in SE Iceland and spread outward from there
to the north and west.
A flexure zone was already present in SE Iceland
in the late Neogene (e.g. Torfason, 1979; Klausen,
1999). This large-scale feature may possibly have
contributed to the regional topography but we believe
more detailed work is needed to conclusively evaluate
its impact on Neogene landscape evolution.
The oldest tillite in Hafrafell
Based on the abundance of hyaloclastite and boulder
conglomerates associated with the lowest erosion sur-
face HR1 in Hafrafell, we conclude that it formed in
response to glacial erosion. The age of HR1, about 4
million years, is almost the same as that of the tillite
located at the base of Jökulfell (Thome, 1968; Helga-
son and Duncan, 2001; Helgason, 2007). The derived
stratigraphic sequence there and radiometric age dat-
ing for the tillite, some 10 km west of Hafrafell (Hel-
gason and Duncan, work in progress), suggests very
similar age for both horizons. The similar age and
stratigraphic location of these two outcrops suggests
that the same glacial event formed the tillite at both
sites. The importance of this tillite lies in its location
and age that are in support of the idea that the earliest
glaciations were confined to SE Iceland (Geirsdóttir,
2011). Further mapping of this horizon in SE and cen-
tral E Iceland would therefore be valuable.
Gauss strata (Neogene-Quaternary transition) at
Hafrafell
Near the end of the Gauss chron (∼2.6 Ma), a thick
sequence of hyaloclastite is found intercalated with
lavas in Hafrafell. Erosion surfaces, HR2 and HR3
occur at the base and top of the Gauss strata, respec-
tively. It is clear from Figure 9 that the strata from
the Gauss normal polarity interval show a distinct
north-south variation in lithology. On the north side
sequence begins with highly plagioclase porphyritic
lava flows in section U that grade up dip into sim-
ilar composition pillow basalts some 400 m further
south in section HM. On top of porphyritic formation
HF7 are hyaloclastite sediments of formation HF8.
Here tholeiite N-lavas (Gauss) inter-finger with the
sediment in sections HM and HL (e.g. unit HL26),
suggesting interglacial conditions in Hafrafell at this
time. However, we conclude that the abundant hyalo-
clastite sediments associated with lava flows at this
time means that glaciers were present further inland.
Finally, a 70-m-thick sequence of tholeiite N-lavas
overlies the sediments, being thickest at the southern
tip of Hafrafell. There, steep but local depositional
dips toward south are noted. Overall, the build-up
of volcanic strata during Gauss time occurred through
lenses of material added progressively at the southern
end of Hafrafell.
Gauss-age strata in Hafrafell compared with
Borgarfjörður and Fljótsdalur
Most magnetostratigraphic studies on Gauss-age
rocks (2.581–3.596 Ma) in Iceland have been con-
ducted on sequences that accumulated within accret-
ing rift zones. Models of crustal accretion, such as
that of Pálmason (1980), assume symmetrical build-
up across the rift zones, with major subsidence and
burial of erupted material at the rift axis. Following
burial and spreading away from the rift zone, strata
may eventually resurface due to erosion and uplift. In
the Hafrafell rift flank region, however, the more vo-
luminous lavas, or those that flowed greater distances
from the rift zone axis, may preferentially be repre-
sented. Consequently, the exposed sections in Hafra-
fell are dominantly distal accumulations of the erup-
tive sequence. It is therefore interesting to compare
results of the present study, from a rift flank region
such as Hafrafell, with results from sequences orig-
inating in an accreting rift zone, namely from Borg-
arfjörður, W Iceland and Fljótsdalur, E Iceland (Fig-
ure 1).
JÖKULL No. 64, 2014 55