Jökull - 01.01.2013, Blaðsíða 53

Stratigraphy, 40Ar–39Ar dating and erosional history of Svínafell, SE–Iceland
flows. The oldest erosion surface in Svínafell is be-
tween R-lava sequences of Upper-Gilbert and Lower-
Matuyama age and coincides with a considerable hia-
tus, based on the absence of normally magnetized
rocks of Gauss age in Svínafell.
A major erosion phase dominated the area dur-
ing Lower-Matuyama time (ca. 1.95 Ma) that resulted
in complete removal of at least 230 m of strata. At
this time a ridge-valley system had already devel-
oped that was amplified from then until the present.
A small hill-shaped remnant of some 13 lavas, ex-
posed in Skjólgil, includes rocks correlated with the
Olduvai normal chron (1.78–1.95 Ma). The upper-
most lavas in Skjólgil are inferred to be of Upper-
Matuyama age. Glaciers now carved a depression into
which the Svínafell sediments were deposited dur-
ing an interglacial period. Stratigraphic dip then in-
creased, especially within sedimentary units and on
erosion surfaces, up to 30◦ toward SW and SE (forma-
tion SV11). At this stage deposition of sediments was
intensive and followed by volcanism in the Öræfa-
jökull volcano where onset of volcanism occurred af-
ter 0.78 Ma. This timing is based on the fact that all
units above the Svínafell sediments are normally mag-
netized from the Brunhes magnetic chron, consistent
with three 40Ar–39Ar total fusion age determinations
from Svínafell’s upper strata.
During Upper Brunhes time the thick volcanic se-
quence from Öræfajökull, here defined as group S4,
was deeply eroded and now stands out as an isolated
mountain only to be further carved down and have,
from time to time, additions in the form of volcanic
strata (groups S5 to S7) that reach to the lowland, as
low as 100 m above the present sea level. Group S4
forms the upper half of Svínafell and was deposited
during Upper-Brunhes time. After its deposition se-
vere erosion set in that led to the carving out of the
Svínafell mountain. Subsequently erupted volcanic
groups, i.e., S5 to S7, were therefore deposited at
lower levels and added to the mountain sides, as low
as 100 m above the present sea level. At this stage the
present day "valley network" was fully developed.
Acknowledgements
We extend special thanks to Dr. Leó Kristjánsson,
University of Iceland, for use of his paleomagnetic
laboratory and assistance in the field with core sam-
pling. We thank the people at Skaftafell, Freysnes
and Svínafell for their invaluable help during field-
work in Öræfi. JH expresses gratitude to Friends of
Vatnajökull for financial aid. An early phase of this
research was funded by the Science Fund in Iceland
through a grant to JH. We thank Dr. John Stevenson
and an anonymous reviewer for their constructive crit-
icism.
ÁGRIP
Jarðlagafræði, 40Ar–39Ar aldursgreiningar og rof-
saga Svínafells, Suð-Austurlandi.
Þrátt fyrir öra upphleðslu hraunlaga í Öræfasveit hef-
ur rof þar af völdum jökla verið gríðarlegt og orsak-
að mislægi og mótað mjög landslag. Rakið er sam-
spil eldvirkni og rofs við mótun Svínafells í Öræf-
um með kortlagningu jarðlaga, seguljarðlagafræði og
40Ar–39Ar aldursgreiningum á bergi. Aðallega er lýst
skiptingu gosbergs frá Kvarter, þ.e. frá segulmund-
um á neðri-Matuyama og efri-Brunhes. Um 1832 m
þykkur stafli í Svínafelli er gerður af 37 myndunum
jarðlaga (Helgason, 2007), sem við höfum deilt í 7
yfirmyndanir (e. groups). Í Svínafelli sést að upp-
haf eldvirkni frá megineldstöðinni í Öræfajökli varð
á neðri-Brunhes (C1n < 781 k ár), en skömmu áð-
ur höfðu Svínafellssetlögin safnast fyrir í lægð á hlý-
skeiði. Á myndunartíma jarðlaga í Svínafelli urðu að
minnsta kosti 8 jökulskeið og hlýskeið. Tólf roffletir
marka rofsögu Svínafells og landslagsþróun. Niður-
staða okkar er að aldur Svínafellssetlaganna sé á bil-
inu 0.70 til 1.78 M ár. Við rof á Brunhes segultíma
hefur rof sífellt aukist með áberandi dýpkun dala. Af-
leiðing eldgosa undir jökli og rofs af völdum jökla er
veruleg landlyfting í Öræfum umfram aðra landshluta.
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Einarsson, Þ. 1977. Um gróður á ísöld á Íslandi (On veg-
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JÖKULL No. 63, 2013 53