Jökull - 01.01.2013, Side 49

Stratigraphy, 40Ar–39Ar dating and erosional history of Svínafell, SE–Iceland
Ma. Likewise the Skjólgil lavas were deposited into
a depression formed during SR2 and they resemble
the Olduvai valley-filling sequence in Hafrafell that
grades upwards from R to N to R lavas, where one
of the upper R lavas has been dated at 1.69±0.29 Ma
(Helgason and Duncan, 2001). East of Hafrafell the
"lower strata" are last seen in Svínafell and disap-
pear farther east, at least as far as Vatnafjöll, 18 km
to the east where the lowest strata are of Brunhes age
(Stevenson et al., 2006). The small outcrop of lower
strata in Svínafell and its disappearance to the east
suggests a time-transgressive erosion interval.
SR3. The basin into which the Svínafell sediments
were deposited formed during this erosion stage. The
top of group S2 (formation SV9) in Skjólgil has
glacial striations trending 213◦. Erosion surface SR3
is slightly younger than the Olduvai chron that has
an upper age of about 1.78 Ma. Presumably, ero-
sion first carved out the depressions on both sides of
the Skjólgil hill of group S2 lava flows. We observe
that erosion during SR3 was extensive, amounting to
at least the thickness of the Skjólgil lavas, or in excess
of 120 m.
Lacustrine sediments are rarely found in the SE-
Iceland bedrock and it is thus likely that rather unique
landscape conditions prevailed in the region after for-
mation of surface SR3. Then, an interglacial stage be-
gan with the formation of a lake into which the Svína-
fell sediments were deposited. Upward, lithology of
the sediment changed from fine-grained dark grayish
silt with thin lamination to coarse-grained brown vol-
canic sands and more massive hyaloclastite. This up-
ward change in lithology indicates that climate was
relatively warm and stable during deposition of the
lower part of the sedimentary sequence. Then, in-
creasing influx of hyaloclastite indicates a change to
volcanic eruptions under ice, culminating in intense
subglacial volcanism that filled the lake. Lithification
of the hyaloclastite presumably formed a cap rock that
preserved the soft lower part of the sediments.
SR4. A >60 m thick tillite in Bæjargil (profile K),
formation SV12, was deposited during erosion stage
SR4. Considerable relief was generated during SR4
based on steep dipping of the tillite upper surface, i.e.,
by 24◦ toward SW (232◦) in Skjólgil and substantial
ponding of basal cooling units within formation SV13.
The formation of this tillite preceded the build up of a
thick Brunhes age volcanic sequence that presumably
erupted from Öræfajökull volcano.
SR5. This is an erosion stage defined by a 40 m thick
pebble and boulder conglomerate below the 120 m
thick subglacially erupted volcanic formation SV14.
SR5 dates from the Brunhes polarity chron (Figure
13b).
SR6. Erosion surface SR6 in group S4 is represented
by a 15–20 m thick tillite, i.e., formation SV16 that is
intercalated between formations SV15 and SV17.
SR7. Erosion surface SR7, within group S4, is below
formation SV21. In Öskuhnúta, at about 880 m el-
evation, it is located at a sharp boundary between a
subglacially erupted volcanic formation (SV21) and
subaerially erupted lavas of intermediate composition
(SV20).
SR8. This erosion surface, also within group S4, fol-
lows formation SV23 lower surface in Öskuhnúta at
about 920 m elevation. Erosion during this stage has
clearly been much greater than during stage SR7. In
Öskuhnúta surface SR8 is on formation SV22. How-
ever, farther down in Svínafell, at about 420 m, it is on
top of formation SV17. There, surfaces SR7 and SR8
coincide.
SR9. Erosional stage SR9 was marked by the contact
between groups S6 and the much older Svínafell sed-
iments, S3.
SR10. A 25 m thick tillite formed during this erosion
stage. It is intercalated between pillow basalt lavas
and a 12 to 14 m thick tholeiite lava flow at the top
of section SV-AU, both within group S6. This sur-
face, within one of the youngest groups, is of upper
Brunhes age. Group S6 must have formed at the val-
ley floor east of Svínafell, very late in its erosion his-
tory. The lowest part of formation SV35 is unusual, in
that it is neither a typical subaerial lava flow sequence
nor a pillow basalt sequence. It may have formed in
the depression between the glacier and the adjoining
Svínafell ridge. The lava flows on top of SR10 (SV36),
however, presumably formed when the valley of the
present Virkisjökull glacier was in part or wholly free
of glacial ice.
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