Jökull - 01.12.1970, Page 68

Several boreholes have been drilled in recent
years throughout this area, the deepest of which
are in Midfell (347 m) and lilesastadir (270 m).
Cuttings from these boreholes and also cores
from a 100 m deep borehole near Kaldárholt
were examined for secondary minerals. Chaba-
zite and tliomsonite are still by far the most
abundant secondary minerals in all the bore-
holes followed by some scolecite, stilbite and
some calcite.
The restricted assemblage of secondary min-
erals found in the Hreppar Series and the
low intensity of zeolitization corresponcls well
to the lower half of the chabazite-lhomsonite
zone of Walker (1960) established for Eastern
Iceland. In rocks of the Hreppar Series anal-
cite, the index mineral of the zeolite zone next
below the chabazite-thomsonite zone, was neith-
er found in the exposed rocks nor in the bore-
hole cuttings. The highest level at which zeo-
lites were found is in Berghylsfjall at 250 m
altitude on the northern border of the map
(Fig. 2). Here a group o£ olivine basalts con-
tains abundant chabazite, besides some thom-
sonite and opal. The top of the chabazite-
thomsonite zone thus must lie higher than this
and the top of the lava pile may originally
have been several 100 m higher. Walker and
Charmichael (1962) assume a combined thick-
ness of 800—1000 m for the chabazite-thom-
sonite zone and a pile of rocks void of second-
ary minerals on top of it. According to this in
Hreppar a considerable thickness of rocks pos-
sibly some 500—700 m has been eroded before
the outpouring of the interglacial lavas.
As compared to Eastern Iceland, however,
several factors complicate the reconstruction of
secondary mineral zoning and the determinat-
ion of the top level of the succession.
Most significant are probably abundant
hyaloclastites within the Hreppar Series, since
basaltic glass is particularly liable to devitri-
fication and zeolitization. The Hreppar Series
is of relatively young age, but it has neverthe-
less been strongly tilted and eroded. It is pos-
sible that the geoisotherms reached their high-
est level only subsequent to the deepest burial
of the Hreppar Series. In other words the
process of zeolitization may have been inter-
rupted by rapid erosion. The present day high
geothermal gradient and widespread hydro-
66 JÖKULL 20. ÁR
thermal activity of the lowlands of Southern
Iceland probably is the surface expression of
the process of zeolitization still taking place at
depth in this region.
FAULTING
The area described here has been much af-
fected by faulting that lias continued into post-
glacial time. The most common trend of faults
is N 15°—30° E. These are normal faults some
of which have a downthrow of several tens of
meters ancl they are arranged in a step fault
pattern with downthrow to the east towards
the axis of the Hreppar anticline. Strike slip
faults exist in Vördufell trending N 10—25° E
and probably also in Midfell ancl south of
Hruni, trending N 50—60° E. In Vördufell
strike slip movement of about 6 m with right
lateral movement was measured in one case,
where several dykes had been moved apart
along a fault trending N 20° E. This fault
forms the continuation of a fissure swarm
mentioned by Tr. Einarsson (1967) and inter-
preted by him as indicating strike slip move-
ment of the same sense as the fault in Vördu-
fell. Faults are sometimes seen to cut through
the young lavas. In Skardsfjall a fault trending
N 60° E evidently is present and another with
the same trend occurs in Háholtsfjall. To the
south of Thjórsá, the only faults were found
along the course of Steinslaekur trending N
30° E, and north of Holtsmúli, trending N
15° E. To the west of Thjórsá open fissures
occur in the districts Skeid and Hraungerdis-
hreppur in postglacial lava where conditions
are favourable for their preservation. These are
the only recent faults observed.
DISTRIBUTION OF THE
INTERGLACIAL LAVA FLOWS
With a sharp unconformity (Fig. 3) the tilted
basement of the Hreppar Series is overlain by
extremely fresh looking but strongly eroded
liorizontal lava flows. These young lavas extend
over an area of nearly 250 km2 mainly between
the rivers Thjórsá and Rangá (Fig. 1). The
easternmost occurrence is along the river bed of
Rangá from Geldingalaekur down to Aegisídu-
foss. East of Rangá extensive river deposits