Jökull - 01.12.1974, Síða 53

time. There are indications, however, that it
became buried even though tlie enveloping
lavas were totally removed during a period of
denudation. The rocks which are now seeri to
cover the volcano overlie a plane of uncon-
formity and are thus separated from the central
volcano by a wide liiatus. The significance of
this unconformity will be discussed in the last
section of this paper. Here we give a short
description of the rock types and stratigraphic
units which make up this heterogeneous succes-
sion.
Above the unconformity an extensive sedi-
mentary horizon (no. 9 of Plate 1) is present
with tillites, thick boulder beds and conglo-
merates besides a number of hyaloclastite units
which thicken towards the east. The lower con-
tact of this layer is particularly instructive in
the area south of Húsafell where the tillite
overlies the eroded third phase rhyolites all the
way from Selgil to Asgil in the west. The thick-
ness of this unit is greatest in the east wliere
up to 50 m thick liyaloclastite piles occur, but
west of Asgil it is represented only by a detrital
bed of 10—20 m thickness.
In the westernmost branches of Ásgil a 50 m
thick series of alternating tholeiite and olivine
tholeiite lavas overlies the detrital bed. In the
lower part the lavas show clear signs of influ-
ence by water, being interbedded with gravel,
and showing pillow structures and the develop-
ment of entablature. Tliis series continues to-
wards the west beyond Valagil. It looks as
though it formed marginally to the hyaloclastites
which represent the lowest member of the post-
unconformity series farther east.
The hyaloclastites are elsewhere overlain by a
series of tholeiite flows which have been traced
from the top of Bæjarfell southeast of Húsafell
farm to Kaldidalur in the east (Tr. Einarsson,
1962). The aggregate thickness of the tholeiite
flows may exceed 200 m in the Bæjarfell—Kaldi-
dalur area. They are thinner in the north, some
40 m being exposed in the soutlieast slope of
Strútur. Sedimentary horizons are insignificant
so that the lavas appear to have been emitted
during a short interval of time. A conglomerate
bed was seen at Lambá, however, and an acid
tuff horizon which is welded secondarily due
to the overlying tholeiite flow occurs in the
upper part of the succession west of Sláttulág.
The Bæjarfell—Ivaldidalur group is of normal
polarity and it dips about 2—3° SE, whereas the
rocks belonging to the central volcano have
steeper and more variable dips.
At the base of Hafrafell 3 km SE of Strútur
three lavas of reverse polarity and an inter-
calated tillite are exposed wliich are strati-
graphically higher than tlie Bæjarfell—Ivaldidal-
ur tholeiites but like tliem they are gently tilted
towards the SE.
Of reverse polarity is also a remnant of a
basalt flow capping the summit of Búrfell up-
wards of 340 m (Tr. Einarsson, 1962). From
there a narrow ridge extends towards the SE
which is probably also made up of the same
flow or flow sequence. This lava cap sliows no
tilt and it overlies a fluvial conglomerate. It
shows an entablature structure typical of sur-
ficial cooling by water (Sœmundsson, 1970). This
indicates that it solidified on a valley bottom.
A K/Ar age determination (see Table 1) sug-
gests post Jaramillo age for this valley filling.
Other rocks of the Húsafell area are much
younger, being formed after the main features
of the present landscape had developed. All
these latest units are of normal polarity. They
comprise interglacial and postglacial lavas which
poured into valleys dissecting the area, and also
some hyaloclastite — pillow lava units erupted
subglacially within the confines of the mappecl
area.
The oldest member of these rocks are liyalo-
clastites which occur in three separate areas.
The largest occurrence is found in the northern
slope of Ok at 450—800 m altitude culminating
in the Vinnumannahnúkar hill. Tr. Einarsson
(1962) has described this occurrence which con-
sists mainly of light brown hyaloclastite tuffs
and breccias with intercalated entablature type
flows near the base, possibly extruded sub-
glacially. The two other occurrences are found
on the south side of Strútur near its base and
on the western slope of Hafrafell above the
reverse polarity lavas. In both localities the
hyaloclastites are of the same yellowish variety
as the one north of Ok and they form piles of
more than 100 m thickness. Both liave also as-
sociated with them large stratiform bodies of
entablature type rock.
An interglacial sedimentary valley filling
covered by a lava flow is exposed south of
JÖKULL 24. ÁR 5 1