Jökull - 01.01.2019, Page 65

Sgattoni et al.
Figure 9. Volcaniclastic deposits exposed along the NE side of Gæsavatn lake (a), with accretionary lapilli-rich
layers (b), low-angle cross-stratification (c) and bomb-sags (d). White arrows indicate the SW-to-NE direction
of provenance of these deposits based on the bedforms shape (c) and the ballistic trajectory of the bomb sags (d).
– Gjóskulög koma vel fram í sniði norðaustan Gæsavatns (a), með lögum af öskubaunum (b), skálögóttri gjósku
(c), og fallförum (d). Hvítar örvar sýna falláttina sem lesa má út úr lagskiptingunni (c) og lögun fallfarsins (d).
cal of crater rims. Anomalous high-angle dipping (up
to 70◦) was observed along a tectonically-controlled
ravine transverse to the sub-rounded geometry of the
lake.
Volcanologic and tectonic implications
Hereafter, we attempt to draw some conclusions about
the eruption types and environmental conditions of the
volcanic activity in the Gvendarfell area, bearing in
mind the difficulty in accessing most of the outcrops.
The most important feature observed is the occur-
rence of pervasively-jointed rhyolitic lava lobes and
domes embedded within fragmented material, inter-
preted as the products of effusive subglacial rhyolitic
eruptions similar to those described e.g. at Torfajökull
(Tuffen et al., 2001, 2008; Lescinsky and Fink, 2000),
Öræfajökull (Stevenson et al., 2006), Krafla (Tuffen
and Castro, 2009) and Prestahnúkur volcanoes (Mc-
Garvie et al., 2007). The rhyolitic lava flows at the top
of the succession have a distinctive near-horizontal
orientation and less pervasive jointing, similar to flat-
lying lava flows in the subaerial lava cap of rhyolitic
tuyas (e.g. Tuffen et al., 2002). Although we were un-
able to inspect the flow surface textures and contact
relationships in detail, we speculate that these were
generated in subaerial conditions. Some of the under-
lying rhyolitic lava lobes may have acted as feeding
points for these lava flows.
Notably, most of the feeder dykes of this lava suc-
cession are aligned approximately along a N-S direc-
tion, apparently as the continuation of the proposed
fault in the E side of the Stórihryggur ridge (Figure
6). We argue that there could have been a NNE-
SSW tectonically-controlled magma transport during
64 JÖKULL No. 69, 2019