Jökull - 01.01.2013, Blaðsíða 35

Stratigraphy, 40Ar–39Ar dating and erosional history of Svínafell, SE–Iceland
Öræfajökull is the most active volcano within the
ÖVZ. The youngest rocks found in Snæfell have an
40Ar–39Ar age of about 240 ka (Helgason and Dun-
can, 2005). Rocks in Svínafell, from the Öræfajökull
volcano, typically belong to the tholeiitic fractiona-
tion series, i.e., tholeiites to rhyolites (e.g., Prestvik,
1979, 1982). More recently, the volcanic products in
the ÖVZ are defined as transitional alkalic (Jakobsson
et al., 2008).
Previous work
Nielsen and Noe-Nygaard (1936) and Noe-Nygaard
(1953) studied indurated moraines in the Svínafell
massif and identified several tillites in bedrock out-
crops, however, their exact location is unclear.
Þórarinsson’s (1963) pioneering work on the Svínafell
strata outlined the stratigraphic framework for the
plant-fossils found in the Svínafell lacustrine sedi-
ments, stating that the base of the Svínafell layers, just
north of Skjólgil lies about 160 m above the plain with
a thickness of about 15 m. The underlying basalts
are normally magnetized down to about 100 m and
reversed farther down. Einarsson (1977) concluded
that normally magnetized volcanic units overlying the
Svínafell sediments formed during the Brunhes po-
larity chron. Þórarinsson (1963) considered the Sví-
nafell sediments to have formed during the second
last inter-glacial whereas Einarsson (1968) regarded
them as being somewhat older, from the third last
inter-glacial, based on Albertsson’s (1976) K-Ar age
dating, which suggests an age of about 500 ka. Tor-
fason (1985) suggested a Tertiary age (>3.1 Ma) for
the lowest part of the Svínafell massif and Brunhes
age for the upper sequence. Prestvik (1979) cites Al-
bertsson (1976) on rocks from the upper part of the
Svínafell massif suggesting that the Svínafell lacus-
trine sediments may have an age of "between post-
Jaramillo (ca. 890 ka) and the beginning of the Elster
glaciations (ca. 600 ka)". Although his main contri-
bution was to Öræfi’s petrology, Prestvik (1979) re-
ported three glacial surfaces in Svínafell, one above
the Svínafell sediments and two below.
GEOLOGICAL MAPPING AND
STRATIGRAPHIC DATA
Stratigraphic classification in the Svínafell area re-
flects highly diversified lithology, frequent glacial-
interglacial transitions, subaerial as well as subglacial
volcanism, extensive transgressive erosion and mag-
netic reversals. During our geological mapping of the
Svínafell massif numerous stratigraphic profiles were
made through cliff sections from which we observed
rock lithology, magnetic polarity and general field re-
lations (Figures 2–4).
We compiled an 1832 m thick stratigraphic master
sequence. Initially, strata were mapped and divided
into 37 rock formations (Helgason, 2007), mainly em-
phasizing transitions between glacial and inter-glacial
conditions as well as magnetic reversals (Figure 2).
While mapping Quaternary strata in Svínafell we
focused on glacial to inter-glacial transitions indi-
cated by abundance of subglacially formed litholo-
gies, and erosion surfaces caused by the numerous
glaciations. Mapping in the Öræfi district has re-
vealed a general distinction of "upper" and "lower"
strata (e.g., Prestvik, 1979) where the upper strata
rest unconformably on the lower strata. The upper
strata are predominantly of Brunhes age (<0.781 Ma)
whereas the lower strata have an age range of about
5 to 0.781 Ma (Helgason and Duncan, 2001; Eiríks-
son, 2008). In the field, Walker’s rock classification
was used to divide lavas into specific lithologic types
(Walker, 1959); that is, plagioclase porphyritic lavas,
aphyric tholeiites and olivine tholeiites (the last type
not found in Svínafell). The aphyric, fine-grained
"tholeiites" of Walker tend to have alkali basalt chem-
istry in Svínafell.
Stratigraphic division: groups S1 to S7
Stratigraphic units, such as lavas or subglacially
erupted strata, are numbered for each profile and
divided into formations and groups which are cor-
related based on 40Ar–39Ar age determinations and
geomagnetic polarity measurements. Erosion sur-
faces, that predominantly coincide with subglacial
volcanic events, are commonly glacially striated.
Based on lithology the stratigraphic sequence is di-
vided into inter-glacial or glacial stages. Amount and
JÖKULL No. 63, 2013 35