Jökull - 01.12.1970, Page 64

Interglacial Lava Flows in the Lowlands of Southern Iceland
and the Problem of Two-Tiered Columnar Jointing
KRISTJÁN SAEMUNDSSON,
NATIONAL ENERGY AUTHORITY, REYKJAVÍK, ICELAND
ABSTRACT
The present paper describes a major occurr-
nce of non tilted interglacial lava flows in
Southern Iceland overlying a tilted basement
of Quaternary plateau basalts. The geology of
the basement rocks, referred to as Hreppar
Series, is outlined. The secondary mineral cont-
ent of the Hreppar Series rochs is discussed in
order to help determine the degree of erosion
prior to the emplacement of the interglacial
lavas. From these data the probable thickness
of rocks removed was found to be some 500—
700 m. The interglacial lavas, which were
erupted after the main features of the present
landscape icere formed, cover an area of about
250 km2. The most coherent cover lies to the
southeast of the river Thjórsá in the lowlands
of Rangárvallasýsla, and remants of the lavas
are also found to the north of the river capping
some of the hills. The structure of the lavas
and the paleomorphology of their basement
make it obvious that to the north the lavas
ponded into valleys and depressions where they
developed columnar jointing, whereas to the
south they flowed into the open country. A
peculiar division of the flows to the north into
a lower colonnade and an upper chilled en-
tablature is explained by xuater flooding the
lavas luhile their interior was still molten.
Petrographically the lavas are tholeiites poor in
olivine to the north of Thjórsá but olivine
bearing basalts to the south of the river. The
greater erosion of the lavas north of Thjórsá
and the less advanced denudation of the
Hreppar Series prior to the eruption of the
interglacial lavas in that area make it probable
that the tholeiites are older. The interglacial
lava flows have normal magnetization limiting
their age to the present (Brunhes) normal pol-
arity epoch. The olivine basalts are considered
62 JÖKULL 20. ÁR
as possibly of last interglacial age but the tho-
leiites are thought to have been erupted during
an earlier interglacial period. The site of erupt-
ion is unknown in either case but certainly lies
outside the mapped area, most probably to the
east or northeast within the eastern limb of
the active volcanic zone.
INTRODUCTION
Non tilted interglacial lava flows in Iceland
occur mainly within or near the active volcanic
zone that extends across Iceland from SW to
NE. Outside this volcanic zone such lavas are
known in the Snæfellsnes region in the west
and in Skagafjördur and Húnavatnssýsla in the
north. This paper describes a recently discover-
ed occurrence in the lowlands of Southern Ice-
land, more precisely in the districts Hreppar
west of the river Thjórsá and in the western
part of Rangárvallasýsla east of the river (for
location see Fig. 1).
The interglacial lavas have been overlooked
by most geologists who have worked or travell-
ed in the area. This may be due to the un-
usually thick and continuous cover of sedi-
ments and soil leaving only steep scarps or
riverbeds exposed. The general geology of this
area was established by Kjartansson in 1943,
1958 and 1962. He regards all rocks underlying
late Pleistocene to recent sediments and post-
glacial lava flows as late Tertiary and/or Plei-
stocene in age and proposed the local term
Hreppamyndun (Hreppar Series), which he
later (1962) included in the “old grey basalts”
of his Geological Map of Iceland. Rocks of the
Hreppar Series are tilted plateau basalts with
abundant intercalatiorjs of hyaloclastites and
tillites exceptionally well exposed in the Hrepp-