Náttúrufræðingurinn - 1993, Page 95

Sveinn Jakobsson 1979. Outline ofthe pe-
trology of Iceland. Jökull 29. 57-73.
Þorleifur Einarsson 1962. Askja og Öskju-
gosið. Náttúrufrœðingurinn 32. 1-18.
S U M M A R Y
At the North Border of
Vatnajökull
II. The Strata and
the Geological Map
by
Guttormur Sigbjarnarson
Skuftahlíð 8
IS-105 REYKJAVÍK
Iceland
In the last volume of Náttúrufræð-
ingurinn, there appeared the first article
of a series under the collective name “At
the North Border of Vatnajökull”, where
I described the preparation and the fíeld
survey for geological sudies and mapping
of the uppermost part of the watershed of
Jökulsá á Fjöllum, performed by the Na-
tional Energy Authority in connection with
the development of the hydro-power in
that area. In the present paper, the geolog-
ical map and the main features of the stra-
ta sequence is represented, with the ex-
ception of the postglacial volcanism and
recent lavas, which will be introduced in
the next article of this series in the fol-
lowing issue of Náttúrufræðingurinn.
The western part of the rnapped area
extends into the Neovolcanic Zone, which
is charcterized by fissure swarms from
three central volcanoes, the mountain ridg-
es of Bárðarbunga, Kverkfjöll and
Dyngjufjöll. Eruptions of the Hawaiian
type have also been common in connec-
tion with the fissure swarms. Shield vol-
canoes and table mountains formed in late
Quaternary time and upto Holocene are
therefore numerous.
The tablemountains represent the gla-
cial epochs but the shield volcanoes the
interglacials. Herdubreid and Kistufell are
typical table mountains dating from the
last glacial epoch, but Urðarháls,
Hrímalda, Vaðalda, Álftadalsdyngja og
Arnardalsalda are shield volcanoes erupt-
ed during different interglacials of the late
Quarternary epoch. Other mountains of the
mapped area are mostly built up by one or
more subglacially erupted hyaloclastite
formations. Most of them consist of co-
lumnar jointed basalt, pillow lava or brec-
cia, but palagonite tuff is rather rare. Co-
lumnar jointed basalts is fairly common
in comparison with other palagonite for-
mations. Upptyppingar, Lindatjöll, Fagra-
dalsfjall, Alftadalshnjúkur og Hvannstóðs-
fjöll are the mest prominent palagonite
mountains. Interglacial lava flows are not
common but are easily buried under young-
er formations. They form distinctive stra-
ta in Sauðárdalur and Vesturdalur and on
the west bank of Kreppa and in Kverk-
árnes, as well as some other places.
The active neovolcanic zone extends
from SW to NE through the western part
of the mapped area, i.e. on the western
bank of Jökulsá á Fjöllum and in Kreppu-
tunga. There, the formations are Holocene
in age, or with very few exceptions
orginate from the last glaciation. Along
river Kreppa and on its east bank, older
formations appear in the strata and tillites
become very common, both between thc
volcanic formations and on the top of them.
The strata grow older toward east, with
rnany exceptions though. The magnetic po-
larity of all the formation is norrnal and
their age is therefore less than 700.000
years, except in the bottom of Sauðárdalur
valley where interglacial lavas and
palagonite formations of reversed polarity
can be found. The lava layers at
Sauðárfoss, farthest to the east on the map,
are one million years old and thus are the
oldest formations within the mapped area.
The next article will deal with the post-
glacial volcanism and Holocene lava flows
as well as the active tectonics of the
Neovolcanic Zone.
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