Jökull - 01.12.1974, Blaðsíða 47

Fig. 3. Flow units form-
ing the uppermost part
of the Deildargil ignim-
brite. Each flow unit is
thoroughly welded in the
lower part but grades up-
wards into a less welcled
porous agglomerate. Gra-
nophyric xenoliths havc
been found only in this
uppermost flow unit fa-
cies of the ignimbrite.
Mynd 3. Beltun efst i
Deildargils-ignimbríti.
Hvert belti er sambrœtt
neðst en gengur yfir í
laust ignimbrit efst. —
Lausasteinar úr granófýri
hafa fundist aðeins i
þessum efsta beltaða hluta
ignimbrítlagsins.
the SW where it is about 20 m thick. We pro-
pose for it the name Deildargil ignimbrite as
being reminiscent of the good and easily acces-
sible exposure in this gorge.
A maximum thickness of about 100 m was
found in the slopes of Thorvaldsdalur. The
ignimbrite is rhyolitic in composition. The
main bulk consists of a single unit which is
thoroughly welded but horizontally laminated.
The base is chilled where the degree of weld-
ing is highest.. At the southwest limit of the
outcrop only the interior is welded. In the area
of greatest thickness the uppermost part of the
ignimbrite consists of several thin flow units
(Fig. 3) which are exceptionally rich in xeno-
liths and lumps of a porous rhyolite which
we interpret as having been fluicl inclusions of
the ignimbrite flow. At the time of eruption
of the ignimbrite the hyaloclastites of Merkja-
gil, Bæjarfell and Tunga formecl a relatively
high ground contiguous with the rliyolites of
Tunga and Fljótstunguháls. Both at Bæjargil
and Fljótstunguháls the ignimbrite can be seen
to thin out on the gentle slopes of this liigher
terrain. Thus like its preclecessor of the first
acid phase the ignimbrite did much to smooth
the relief which was in this case created by
subglacial volcanism and thick rhyolite flows.
In Tunga the Deildargil ignimbrite forms a
continuous sheet which is up to 15 meters thick
and overlies rhyolite flows and partly also
hyaloclastites against which the rhyolites are
banked. In this area the ignimbrite is a chaotic
breccia which is welded only in the very east
of Tunga. The chaotic facies was found also in
Selgil east of Húsafell. The distribution of it
might indicate a near source equivalent of the
flow facies of the ignimbrite originating per-
haps as spatter adjacent to the vents on higlier
ground not reached by the pyroclastic flow.
Tlie source of the ignimbrite is not known
but it may be supposed somewhere in the area
of Tunga or Fljótstunguháls. The thickness
variations of the ignimbrite indicate a very ex-
ceptional total volume of 20—30 km3. This
value is somewliat reduced if one assumes that
topographic relief channelled the flow into val-
leys and depressions for which tliere are indica-
tions nearest to the central volcano. An ignim-
brite eruption of tliis scale would normally
have been followed by a caldera collapse. We
are not able to locate such a structure with any
confidence but it might be concealed east of
the rhyolites of the core area. The only indica-
tion of a caldera ring fault is the fault contact
of rhyolites against acid breccias and tholeiite
JÖKULL 24. ÁR 45