Jökull - 01.12.1974, Page 49

the upper liorizon is also the youngest strati-
graphic unit in Tunga. This is a layer of re-
worked rhyolite debris, containing also tillites
(Hrafnagil, Hestagil), and a basaltic hyaloclas-
tite (Hrafnagil) which is overlain by the sup-
posedly subglacially extruded rhyolite on the
summit of Tunga. At the same stratigraphic
level on the southern side of Tunga a 10 m
thick relic of compound type olivine tholeiites
is exposed.
The next glacial horizon (no. 7 of Plate 1)
whicli forms the topmost layer of the group
tliroughout rnost of the area is on the other
itand associated with more than 100 m tliick
piles of hyaloclastites and pillow lavas which
are exposed in the area between Selfjall and
Reydarfell. In Strútur this liorizon is made up
of a 30—50 m thick conglomerate riclt in hyalo-
clastite debris, which can be followed from the
east of Kalmanstunga to the north side of
Strútur. On the northern slope of Selfjall, just
west of Lambá, the same horizon is represent-
ed by layered mudstone, probably a lake de-
posit, over 20 m thick. The hyaloclastites are
seen to taper out west of Reydarfell and they
are not present at Asgil. There, liowever, a
clastic bed only 2 m thick, wliich is probably
a tillite, could be correlated with them strati-
graphically. West of Asgil tliis clastic bed is
overlain by 4 lava flows of about 40 m total
tliickness which were probably banked up
against the slopes of the hyaloclastite liills
further east.
The olivine tholeiite compound flows and
hyaloclastites wliich were erupted between the
seconcl and third acid phases largely buried
tlie uneven topography which existed in the
area of Bæjarfell and Tunga. This is evident
from the lavas which are found banked up
against the preexisting slopes ancl occasionally
as remnants on top of the rhyolites and hyalo-
clastites of the core area in Tunga and Bæjar-
fell. At the end of tlris group, however, a new
higli grouncl had come into existence in Tunga
and west of Bæjarfell mainly as a result of sub-
glacial volcanism.
Compound type olivine tholeiites in the pre-
sently active volcanic zone of Iceland forrn lava
flows of a very distinct shape, i. e. the lava
shields. Their shape and internal structure are
determined by low viscosity and low rate of
extrusion of the lava ('Walker, 1972). Probably
the compound lava flows, which followed upon
the second acid phase formed lava shields also.
Their tectonic setting is very similar to the
situation found today at some of the central
volcanoes of the active volcanic zone, such as
Theistareykir or Fremrinámur. Both are almost
completely buried by the olivine tholeiites of
lava shields which were the dominant type of
extrusion in Postglacial time. Occasional rhyo-
lite peaks projecting through the lavas and
active steam fields indicate the presence of
buried central volcanoes.
2.2.5 The tliird acicl phase,
The beginning of the third acid phase coin-
cides with a magnetic reversal from normal to
reversed. The entire group is of reversed po-
larity and comprises very distinct flows of ande-
site and rhyolite (Fig. 4), besides acid hyalo-
clastites and numerous acid and composite
plugs ancl dykes. Tlie acid rocks are found in
Fljótstunguliáls, Strútur and on the hills soutli
of Húsafell from Selfjall to Ásgil in the west
in a zone which runs generally NE—SW. The
average thickness of tliis group is about 100 m
but it grows thicker where hyaloclastites are
present. The group starts with one to four
tholeiite flows which include two thick ande-
site flows in the upper branclies of Hringsgil
and Deildargil. Tliey are followed by a rhyo-
litic tuff horizon which can be traced from
Ivleppatagl and Strútur, where it is very thin!
across Selgil (southern wall) to Hringsgil and
Deildargil, where it is thickest. At the last men-
tioned locality it is composed of a bedded green-
ish airfall tuff overlain by a welded pink ignim-
brite cliaracterized by flattened fiamme (Gilja-
tungur ignimbrite). The ignimbrite, which is
10 m thick at Deildargil, thins rapidly towards
Ásgil and Skeidvallagil where it is only 2 m
thick. In the area of greatest tlrickness (Gilja-
tungur, west of Reydarfell) the ignimbrite con-
sists of many flow units each sliowing the
highest degree of welding at tlie base. The
ignimbrite is overlain by a thick rhyolite flow
which shows no sharp contact with tlie ignim-
brite (a pitchstone base is lacking), indicating
that both were erupted in a rapid succession
from tlie same source whicli could be located
in the nearby Reydarfell (Fig. 5).
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