Jökull - 01.01.2013, Blaðsíða 37

Stratigraphy, 40Ar–39Ar dating and erosional history of Svínafell, SE–Iceland
thick and reaches an elevation of about 800 m (Helga-
son, 2007) whereas in Svínafell comparable lavas ex-
tend only to about 250 m above sea level. While lava
tilt and/or tectonics could partly explain this differ-
ence in elevation, erosion (surface SR2) must have re-
moved at least several hundred meters of the lava sec-
tion.
Figure 3. Geological map of Svínafell showing
groups S1–S7 with erosion surfaces SR1–SR12. –
Jarðfræðikort af Svínafelli sem sýnir yfirmyndanir S1
til S7 ásamt rofflötum SR1 til SR12.
Group S2 consists of four lava formations (SV6–
SV9), the basal Skjólgil lavas with a total thickness of
104 m (Figures 4, 6 and 7). S2 lies on top of a poorly
exposed, thin, dark brown, reworked hyaloclastite
sedimentary unit. The two lowest lavas (SV6) are thin
reversely magnetized tholeiites which are overlain by
three normally magnetized lavas, of which the first
(LK, Figure 8) is 28 m thick with a 15 m high cen-
tral colonnade, indicating depositional ponding early
within an inter-glacial. The first (unit LK) and third
(unit LI) N-lavas of formation SV7 in Skjólgil were
drilled for paleomagnetic measurements (Figure 7).
The second lava (LJ) was measured as normal with
a handheld magnetometer. Above are four reversely
magnetized tholeiite lavas (SV8) and four basaltic an-
desite lavas (SV9), also reversely magnetized. Lava
flows of group S2 become gradually thinner upwards.
Following deposition of the Skjólgil lavas glaciers
eroded this formation, almost to obliteration and
shaped S2 into a small hill with tillites on its west-
ern side and a large valley on its eastern side, into
which the Svínafell sediments (S3) were deposited
during an inter-glacial interval.
Group S3 includes formation SV10, the 80 m thick
Svínafell sediments. The sediments have distinct ver-
tical and lateral variations, with basal silt-sized beds
grading upwards to homogenous massive hyaloclastic
units, 10–15 m in thickness at the top. With increas-
ing hyaloclastite content massive "walls" with zeo-
lite coatings become more frequent (Figure 9, left).
The lower part of the Svínafell sediments consists
mostly of fine-grained greyish silt and some light
brown palagonite-rich beds, 1 to 3 cm thick, dipping
1–2◦ southward. Lateral variations are seen in grain
size, degree of lamination and dip. From north to
south, the sediments can be divided into central-and
distal-facies strata. The central facies are seen in
Goðagil and farther north but the distal facies strata
are found to the south in Sniðagil and surroundings.
The lower part of Sniðagil is mostly fine-grained silt
in good agreement with a southward deepening of
the sediment basin. There, plant-bearing fossils (leaf
imprints and pollen) are found. Based on sediment
layer thickness, apparent small regional extent and
fine grain size we regard these sediments of lacustrine
origin. The northern boundary of the sedimentary
basin is not exposed but the base of group S2 extends
southward (Figure 4). The Svínafell sediments are
exposed over the same interval as group S2 indicating
that they piled up against the Skjólgil lavas.
Group S4 consists of formations SV11–SV23 that have
a total thickness of 862 m. All units above SV10 in
Svínafell are normally magnetized, as measured with
JÖKULL No. 63, 2013 37