Jökull - 01.01.2013, Side 51

Stratigraphy, 40Ar–39Ar dating and erosional history of Svínafell, SE–Iceland
chron (0.781–2.581 Ma). Within Matuyama
time there are four rather short normal magnetic
chrons/subchron, i.e., Jaramillo subchron (84 kyr),
Cobb subchron (12 kyr), Olduvai chron (167 kyr) and
Reunion subchron (20 kyr) (Gradstein et al., 2004).
Of these Cobb and Reunion have never been observed
in Iceland, probably due to their short duration. The
age of dyke D2 (1.67 Ma) is significantly older than
the Jaramillo subchron (C1r.1n: 0.988–1.072 Ma) but
similar in age to Olduvai (C2n: 1.778–1.945 Ma)
chron. Therefore, we correlate the N-lavas in Skjól-
gil with the Olduvai chron rather than Jaramillo sub-
chron. If our correlation of N-Skjólgil lavas with the
Olduvai chron is correct it follows that the R-lavas at
the top of the Skjólgil section have an age of 0.78–
1.78 Myr (C2n). Keeping in mind the mean age for
the dated samples above the sediments of 698 kyr, be-
low which are some undated older units within Brun-
hes, it follows that the Svínafell sediments have an
age between 0.70 and 1.78 Myr. This interval should
be even smaller considering the period of erosion af-
ter 1.78 Ma, i.e., when the depression into which the
Svínafell sediments accumulated was formed.
Regional comparison of the Svínafell stratigraphic
sequence
Each region in Iceland has its own character with
respect to accumulation of volcanic rocks and sedi-
ments that form the stratigraphic succession exposed
in dissected cliff sections. By this we mean that
the stratigraphic characteristics are determined by
the volcano–tectonic environment; i.e., dominantly
crustal accretion and subsidence, or erosion exceed-
ing accumulation. An example of this regionally vari-
able character, we compare the 1240 m thick sequence
in Svínafell to a stratigraphic sequence of the Öræfa-
jökull volcano at Kvíárjökull (Stevenson et al., 2006),
some 10 km to the east of Svínafell and Hafrafell
(Helgason, 2007).
Each region in Iceland has its own character with
respect to accumulation of volcanic rocks and sedi-
ments that form the stratigraphic succession exposed
in dissected cliff sections. By this we mean that
the stratigraphic characteristics are determined by
the volcano-tectonic environment; i.e., dominantly
crustal accretion and subsidence, or erosion exceeding
accumulation. As an example of this regionally vari-
able character, we compare the 1240 m thick sequence
in Svínafell to a stratigraphic sequence of the Öræfa-
jökull volcano at Kvíárjökull (Stevenson et al., 2006),
some 10 km to the east of Svínafell and Hafrafell
(Helgason, 2007).
The oldest strata at Kvíárjökull (Stevenson et al.,
2006) are the "laterally continuous basalt lavas and
hyaloclastites" (basal basalt succession), exposed be-
tween 200–900 m above sea level. They are nor-
mally magnetized and of Brunhes age (<0.781 Myr).
Prestvik’s (1979) "lower strata" are missing here, an
indication that prior to the deposition of Kvíárjökull
strata, erosion may have removed the "lower strata" to
at least the present 200 m level. At Kvíárjökull, the
basal basalt succession is intruded by rhyolitic dykes
and lavas of rhyolite and trachydacite composition
predominate the upper strata. Thus, this section seems
to be more central to the Öræfajökull volcano than is
the sequence at Svínafell. The distance, however, of
both Svínafell and Kvíárjökull areas from the present
Öræfajökull caldera is similar, about 8 km. This may
indicate that a channel was open from the caldera to-
ward the Kvíárjökull area during Brunhes or that the
Öræfajökull volcanic center had a more SE location
when the Kvíárjökull section erupted.
In Hafrafell, northwest of Svínafell, there oc-
curs a 350 to 600 m thick sequence of lavas from
lower Matuyama time (Helgason and Duncan, 2001)
whereas in Svínafell a sequence of this age is at most
120 m thick. In Hafrafell this sequence is exposed
at 300 to over 900 m elevation interval whereas in
Svínafell it is exposed at about 100 to 250 m eleva-
tion, i.e. lava formations SV6–SV9 in Skjólgil. Lava
flow strike and dip in the region show stratigraphic
age to increase toward south to southeast. Thus, the
older "lower strata" should thicken in that direction.
Why then do the "lower strata" almost disappear in
Svínafell when compared to Hafrafell, a distance of
some 2–3 km? The most probable explanation is that
they originally reached the Svínafell but have now
been eroded away. This erosion would have taken
place in the Svínafell area during erosion stage SR2.
Our conservative estimate is that erosion removed at
least 230 m of section but that it could have been
JÖKULL No. 63, 2013 51