Jökull - 01.01.2014, Side 42

J. Helgason and R. Duncan
ies from different sites in Iceland have shown the first
glaciations of the present ice age to extend back at
least to 3–4 million years (Eiríksson and Geirsdóttir,
1996; Helgason and Duncan, 2001; Eiríksson, 2008;
Geirsdóttir et al., 2007). Refined correlation of these
glacial events across the country awaits detailed work
on high-resolution glacio-volcanic sequences such as
those preserved in the Öræfi district due to deep ero-
sion and volcanic accumulation in a rift flank zone.
RESEARCH AREA
The Hafrafell volcanic massif lies within the Öræfa-
jökull Volcanic Zone (ÖVZ) that is located southeast
of the accreting neovolcanic rift zone in NE Iceland
(Figure 1). Crustal accretion in this area is mini-
mal at present. Erosion, however, has exposed vast
intrusions, both sheet swarms and major intrusive
bodies (e.g. Walker, 1975). Hafrafell is located some
6 km west of the Öræfajökull stratovolcano and is
enveloped between two southward-flowing glaciers,
Skaftafellsjökull to the west and Svínafellsjökull to
the east. Immediately north of Hafrafell, are the Hrút-
fjallstindar volcanic massif with a highest peak of
about 1875 m. Hrútfjallstindar volcanics partly super-
impose on the Hafrafell north end. The bulk of Hrút-
fjallstindar are stratigraphically younger than Hafra-
fell and formed during the Brunhes magnetic chron
(< 0.781 Ma).
Previous work
Based on reconnaissance field work Prestvik (1979)
suggested a broad division of Hafrafell into 6 units,
the lowest being "basaltic lava flows with fine grained
layers of sediments in-between. This unit, where
alteration is most conspicuous, is frequently cut by
basaltic dikes". Stratigraphically above, he defined
two tillite beds and three units of "hyaloclastites and
basaltic lava flows" with the top-most unit separated
by an unconformity from "hyaloclastites and basaltic
lava flows" below. Prestvik regarded the strata gener-
ally as basaltic but mentions a "silicic massif" higher
up at Efri-Menn. Part of Prestvik’s geochemical work
included analyses of 5 dikes and lavas that he iden-
tified as tholeiite or dacite (Prestvik, 1985). On
a geological map of SE Iceland, scale 1:250.000,
Torfason (1985) presented a broad geological divi-
sion of the Öræfi district in which he regarded all
of Hafrafell’s lower strata to be older than 3.1 Ma.
For SE Iceland, Torfason’s map shows widespread
hyaloclastite formations of Quarternary age. Hel-
gason and Duncan (2001) divided the stratigraphy
of the Skaftafell area into glacial-interglacial stages
(0–5 Ma) on basis of paleomagnetic work and K-
Ar age dating. Helgason (2007) published a geo-
logical bedrock map of the Skaftafell area with de-
tailed division of strata into rock formations, namely
the Skaftafellsfjöll, Skaftafell, Hafrafell and Svínafell
mountains. The present study is based on this map-
ping effort.
METHODS
Field mapping and stratigraphy
The Hafrafell massif (Figure 2) rises steeply from the
sandur plain to the south, and is cut by a dike swarm
that has led to numerous gullies with good exposure
in the cliff section. Walker (1959) established a field
classification scheme for the Neogene lavas of Eastern
Iceland that we used during the mapping of Hafrafell
(Figures 3 and 4). The basaltic lavas are defined ei-
ther as (aphyric) tholeiite, (plagioclase) porphyritic
or olivine basalt. However, the stratigraphic record
includes frequent glacial-interglacial transitions with
great volumes of subglacially formed strata. Thus,
the geology of the area differs from typical Neogene
lava terrains, as found in Eastern and Western Ice-
land, in the greater occurrence of subglacially erupted
rocks, hyaloclastite horizons and evidence of ero-
sion. The highly diversified lithology in Hafrafell
required us to map numerous stratigraphic profiles.
Based on correlations between profiles we divided
the Hafrafell stratigraphy into 39 lithologic forma-
tions (HF1-HF39, Figure 4) that we then merged into
eight groups, H1 to H8 (Figure 5). To constrain the
time frame for stratigraphic evolution we sampled a
master section for magnetic polarities and dated key
units by the K-Ar method. Intercalated between lava
flows are up to 30-m-thick hyaloclastite sedimentary
units. We devide the brown hyaloclastite sediments
into two types, namely "primary" and "laminated".
The "primary" hyaloclastites typically massive, have
42 JÖKULL No. 64, 2014