Jökull - 01.01.2019, Side 64

The 2011 unrest at Katla volcano
Figure 8. N-S oriented, sub-vertical normal fault along the eastern side of Stórihryggur ridge. Pervasively-
jointed rhyolitic lavas (◦) are exposed on the opposite sides of the fault with a tens-of-meters vertical displace-
ment. The fault is sealed by a glacial moraine associated to the retreating Hafursárjökull glacier stream (*). –
Nærri lóðrétt siggengi með N-S strikstefnu í austurhlíð Stórahryggjar. Kubbaberg úr rýólíti sést báðum megin
misgengisins. Lóðfærslan um misgengið skiptir tugum metra. Misgengið er hulið jökulurð sem tengist hörfun
Hafursárjökuls.
On both sides of the Gvendarfell ridge, in its
middle-lower portion, we observed a very thick (up
to ∼200–250 m), sequence of dark to yellowish vol-
caniclastic deposits (Figure 7c), with no base exposed.
Their features resemble those of hyaloclastic deposits
with palagonitized portions, although it was not pos-
sible to closely investigate them, nor the contact rela-
tionships with the overlying rhyolitic lavas. The pres-
ence of basaltic hyaloclastic deposits in this area is
consistent with the petrochemical study of Lacasse et
al. (2007) and Jóhannesson and Saemundsson (2009).
Notably, a crude-layering of these deposits is visi-
ble, displaying a radial bedding attitude around the
Gvendarfell ridge (Figure 6).
Similar deposits crop out in the surroundings of
the Gæsavatn lake (Figures 6, 9a), to the south of
Gvendarfell. There, we observed a tens-of-meters
thick succession of massive to crudely-stratified yel-
lowish lapilli-tuffs composed of dense black (basaltic)
glass fragments, minor vesicular scoriae and rare
lithic clasts, with several fine ash and accretionary
lapilli-rich layers (Figure 9b). Low-angle cross-
stratification and sandwaves (cm- to dm-scale) are
visible (Figure 9c), together with a few SW-to-NE
directed bomb sags (diameter <10 cm; Figure 9d).
These deposits have an overall radial, gentle outward-
dip around the Gæsavatn lake, also showing in the
NE side of the lake an opposite dip-direction typi-
JÖKULL No. 69, 2019 63