Jökull - 01.01.2013, Side 36

J. Helgason and R. Duncan
Figure 2. Detailed geological division of Svínafell with formations (SV1–SV37 and groups (S1 to S7). – Ítarleg
skipting jarðlaga í Svínafelli í myndanir SV1–SV37 og yfirmyndanir S1–S7.
timing of erosion can be estimated based on miss-
ing stratigraphic units. Based on lithology, magnetic
polarity and lava field classification (International
Stratigraphic Guide, 1976) we have simplified and
combined 37 rock formations into a broader subdivi-
sion of 7 groups, S1 to S7, from oldest to youngest,
and 12 erosion surfaces, SR1–12 (Figures 3 and 4).
Group S1 includes formations SV1–SV5, comprising
149 m of basalt lavas. The lowest strata in Svína-
fell, i.e., Hrútagil, consist mainly of rather thin basalt
lavas with thin red sedimentary interbeds of sand to
silt size. Massive, subglacially erupted volcanic rocks
are not preserved from this time, only subaerially
erupted lava flows. The lavas are hydrothermally al-
tered within the mesolite-scolecite zeolite zone and
intruded by numerous dykes trending N66◦E. Field
measurements indicate either reverse or anomalous
magnetization for these lavas. The lack of hyalo-
clastite beds within this group and parallel lava flow
surfaces suggests that it predates major glaciations
and development of irregular relief in the region.
Within this group is, however, a sedimentary horizon
with two units, i.e., a lower breccias/tillite?, 3 m thick
with boulder size fragments up to 1 m (Figure 5) and
an upper unit of sandy dark brown hyaloclastite. In
between these two units is erosion surface SR1 that
dips 14◦ toward 75◦.
Below the unconformity the lava flows dip 7◦ to-
ward N60◦W (SV1), but 4◦ toward N120◦E above.
Erosion at surface SR1 formed minor landscape and
created a hiatus. After accumulation of SV5 lavas sub-
stantial glacial erosion began, as indicated by compar-
ison of strata in Svínafell and Hafrafell, 2–3 km north
of Svínafell. The Hafrafell lava sequence is 600 m
36 JÖKULL No. 63, 2013