Jökull - 01.12.1970, Side 66

ar district, but actually filling the whole gap
between the two limbs of the active volcanic
zone in Southern Iceland. The only written
account of young, non tilted interglacial lavas
in this area is an unpublished report to the
State Electricity Authority by H. Tómasson
(1962), where he describes the geology around
Árbaejarfoss, a planned power plant site. He
noticed non tilted lava flows in the river bed
of Rangá, as well as an outcrop near Raudi-
laekur.
THE UNDERLYING
HREPPAR SERIES
The interglacial lavas (hereafter referred to
as the young lavas) appear everywhere to over-
lie plateau basalts of the Hreppar Series.
The Plreppar Series includes the characteristic
intercalations of hyaloclastites and tillites and
a varied suite of rocks in the northeastern
section of the map (Fig. 1) including rhyolites,
andesites and a large intrusive body (Fridleifs-
son 1970). South of Hlídarfjall an extensive
zone of propylitization is found. These are
characteristic phenomena only found in con-
nection with central volcanoes recently re-
cognized in great number within the Icelandic
plateau basalts.
As shown in Fig. 1 the Hreppar Series dips
northwestward in most of the area. Only in
the northeastern section of the map is the dip
directed towards northeast. The largest dip of
more than 20° is found north of the river
Stóra Laxá. Towards the south the dip is not
as steep, the values ranging between 4° to 12°.
The northeasterly dip is rather constant be-
tween 8° and 10°.
The change in direction of dip is related to
the Hreppar anticline (Th. Einarsson 1967,
Saemundsson 1967), that extends some 50 km
northeast of the map. The development of this
anticline most probably is a result of accumula-
tion of volcanic material in the two limbs of
the active volcanic zone to the west and east
(Saemundsson 1967). The occurrence of tillites
and thick hyaloclastites among the oldest strata
near the axis of the anticline in Stóri Núpur,
Mástungur and Thjórsárholt indicates that the
whole succession was formed after the first
glaciation set in.
64 JÖKULL 20. ÁR
Dykes are occasionally found in rocks of the
Hreppar Series. They usually stand perpendi-
cular to the enclosing rocks. The thickness of
the dykes ranges from less than 1 m to 10 m.
The dykes usually strike NE—SW but veins can
be found showing deviation from this direction.
SECONDARY MINERALS
OF THE HREPPAR SERIES
Rocks of the Hreppar Series often contain
secondary minerals such as zeolites, calcite and
silica minerals. A preliminary study of these
was made and a distribution pattern establish-
ed that allows conclusions to be drawn about
the amount of erosion of the Hreppar Series
prior to deposition of the interglacial lavas.
Fig. 2 shows the sampling localities of second-
ary minerals and the assemblage found in each.
The secondary mineral assemblage produced
is dependent of the rock type and is controlled
by isotherms within the lava pile (Walker
1960).
Detailed petrographic examination of the
basaltic rocks themselves was not made but
both olivine basalts, tholeiites and porphyritic
rocks occur. As expected the tholeiites are
characteristically poorer in secondary minerals.
In the olivine basalts only chabazite and some
thomsonite and opal are found in most places.
Scolecite was found in some localities in the
northern part of the area and was especially
abundant in hyaloclastites outcropping between
Minni Núpur and Mástungur. It occurs as a
devitrification product of basaltic glass. Amyg-
dales in the same rock contain chabazite and
thomsonite. Chalcedony, and quartz occur in a
few places.
Tlie intensity of zeolitization is very low
throughout the area and the secondary minerals
are small in size. Chabazite cubes usually do
not exceed 2 mm and radiating clusters of
thomsonite are usually only a few mm long.
Scolecite aggregates containing more than 1 cm
long needlelike crystals occur in the Stóri Núp-
ur—Mástungur hyaloclastite. Phillipsite occurs
in amygdales near Hruni.
Other seconclary minerals found include
levyne, apophyllite and calcite, which is parti-
cularly abundant in and arouncl the hydro-
thermal aureole of the Laxá-volcano.