Jökull - 01.01.2013, Page 40

J. Helgason and R. Duncan
jökull caldera, which presently produces highly acidic
compositions, is a relatively recent feature, which is in
agreement with work on the base of the Öræfajökull
volcanic sequence by Kvíarjökull (Stevenson et al.,
2006) as well as work by Prestvik (1979). Accumula-
tion of volcanic rocks within group S4, above surface
SR4, was rapid. S4 has a dip of 7◦ toward SE and
acts as a thick cap rock that prevented erosion of the
soft Svínafell sediments below. During the build-up
of group S4, accumulation of volcanic strata far ex-
ceeded removal by erosion.
Figure 7. Stratigraphic profile L in Skjólgil. – Jarð-
lagasnið L í Skjólgili.
Group S5 consists of miscellaneous rock formations
of mafic to intermediate composition which repre-
sents late stage volcanism in the Öræfajökull strato-
volcano. Formations SV24–SV34 within group S5,
have an estimated total thickness of 350 m and are
best exposed in the Svarthamragil gully (Figure 11).
They include tillites, subaerially erupted lavas, pillow
basalt and basalt andesite, that dip up to 40◦/292◦.
No attempt is made to correlate group S5 forma-
tions within a specific glacial or inter-glacial period.
The S5 formations, which vary 10–50 m in thick-
ness, were deposited on steeply sloping surfaces in
a valley on the western side of the Svínafell mas-
sif. A major erosional unconformity is present below
S5. Although glacial erosion removed several hun-
dred meters of strata prior to deposition of group S5,
the hiatus below S6 and S5 may, however, represent
only a short time interval, about 0.1 to 0.2 Myr. The
erosion period and subsequent formation of S5 lavas
occurred well within the Brunhes magnetic chron.
Group S6 contains volcanic strata, formations SV35–
SV36 with a total thickness of 134 m, on the eastern
side of Svínafell. These strata accumulated on an ir-
regular surface and flowed down to the eastern side
of Svínafell, banking against the Svínafell sediments,
perhaps in a depression between glacier ice and the
mountain cliff. Instability of the underlying ice may
have contributed to the uneven accumulation in this
group. The boundary between groups S3 and S6 is
nearly vertical, as S6 lavas are banked up against
eroded S3 lavas. Group S6 is distinctly younger than
S3, whereas Þórarinsson (1963) believed S6 to lie
below S3. Measurements with a handheld fluxgate
magnetometer show S6 to be normally magnetized
and, thus, of upper Brunhes age.
Group S7 consists of highly porphyritic, 153 m thick,
pristine section of basalt lava flows, formation SV37,
which is correlated with formation HF43 in Hafrafell
where it has been dated at 215 (±12) ka (Helgason
and Duncan, 2001). S6 has thus been mapped on both
sides of the Svínafellsjökull, where it has accumu-
lated against the steep cliff side, 250–353 m above
sea level. Based on K-Ar age determination this inter-
glacial formation was most probably deposited dur-
ing the third last inter-glacial (Mindel-Riss for the
Alps; Holsteinian for N-Europe). The S7 lavas flowed
40 JÖKULL No. 63, 2013