Jökull - 01.01.2019, Page 63

Sgattoni et al.
Figure 7. Rhyolitic lavas in the Gvendarfell area. a-b) Pervasively jointed rhyolitic lava lobes (*) and overlying
lava flows (rh) along the eastern side of the Stórihryggur ridge. c) Panoramic view of the Gvendarfell ridge from
the West with lava flows (rh) lying above yellowish hyaloclastic deposits (hy). – Rýólít-hraun við Gvendarfell.
a-b) rýólít-kubbaberg (*) og hraunlag sem liggur ofan á því (rh), í austurhlíð Stórahryggjar. c) Yfirlitsmynd af
Gvendarfelli séð úr vestri, sýnir hraunlag (rh) ofan á gulleitu, súru móbergi (hy).
tens of cm (Figure 7a,b). Some of these lava bod-
ies display the platy jointing structure typical of lava
body interiors (Forbes et al., 2014).
The top of the Stórihryggur ridge consists of a
40–50 meters thick sequence of three viscous lava
flow units that gently dip southwards, overlying the
lava lobes/domes pile. The lava flows comprise 2–
3 metre-thick zones of vertical columnar jointing and
glassier texture at tops and bases, which surround a
paler-coloured and more crystalline flow interior that
is cut by platy fractures (Figure 7b). We collected two
samples of the lava flows that were analyzed by David
Budd (Budd, 2015). The chemical analysis revealed
a rhyolitic composition ranging in SiO2 from 69 to
72 wt%. Similar lithostratigraphic features are visi-
ble also in the upper portion of the Gvendarfell ridge
(Figure 7c), both on its western and eastern flanks,
although we could not reach the outcrops and investi-
gate directly in the field the rock lithotypes. However,
the occurrence of rhyolitic lava deposits in this area is
consistent with the petrochemical study of Lacasse et
al. (2007) and Jóhannesson and Saemundsson (2009)
along the southern Katla’s caldera rim.
A N-S oriented, sub-vertical cliff along the east-
ern cliff of the Stórihryggur ridge was interpreted as a
normal fault cutting the exposed lava succession with
a tens-of-meters displacement (Figures 6, 8). This is
confirmed by the evidence for vertical displacement
of the lava flow units on the opposite sides of the
fault (Figure 8) and by its almost rectilinear and sub-
vertical shape. Alternative interpretations as a glacial
erosive feature or as a constructional margin of an ice-
marginal lava flow (Tuffen et al., 2002) are therefore
considered less likely.
62 JÖKULL No. 69, 2019