Jökull - 01.12.1989, Síða 78

should have attracted more interest. From the
regional geological setting it is clear that the coun-
try rock is of Tertiary age, most likely Miocene (Sæ-
mundsson, 1979). Thoroddsen (1906) visited the
area before the tum of the century and mapped a
large area of silicic rocks in the mountains around
Borgarfjörður and Loðmundarfjörður. Deamley
(1954) mapped a small area on the northem side of
Loðmundarfjörður and carried out some petrograph-
ical research. He described both alkalic and
tholeiitic rocks occurring there. This led him to
believe that the rocks of this part of Eastem Iceland
had strong affinities to the Tertiary volcanic rocks of
Great Britain, which was in accordance with the
generally accepted view that the volcanic rocks of
Britain and Iceland were part of the Thulean Vol-
canic Province with its main rock suites, the alkalic
and tholeiitic ones. Walker (1963, 1966) and his col-
leagues revealed the structure of the Tertiary rock
pile of Eastern Iceland comprising central volcanoes
and a large volume of intermittent flood basalts.
Most of the central volcanoes in Eastem Iceland
have now been described (Carmichael, 1962; Gib-
son, 1963; Walker, 1963; Blake, 1970; Torfason,
1979; Fridleifsson, 1983) (Fig. 1). Using the map-
ping of Thoroddsen (1906), Walker (1964) sug-
gested a volcanic centre at Loðmundarfjörður,
which he named "Loðmundarfjörður central vol-
cano". According to the stratigraphic sequence that
Walker (1964) developed subsequently, this Loð-
mundarfjörður volcano should be the oldest in the
Tertiary rock pile of Eastem Iceland. This has, how-
ever, not yet been confirmed, since no radiometric or
other age determinations have been carried out in
the Borgarfjörður/Loðmundarfjörður area.
GEOLOGICAL SITUATION OF THE
BORGARFJÖRÐUR/-
LOÐMUNDARFJÖRÐUR AREA
The widespread occurence of light-coloured sili-
cic rocks in this area suggests the existence of at
least one, and probably several central volcanoes,
from which considerable amounts have been
removed by erosion, making it difficult to reveal
their former shape. It is, however, likely that they
15°W 14°W
Fig. 1. Tertiary central volcanoes in eastem Iceland
(stippled, with associated dyke swarms where they
have been mapped). They are, from north to south:
V = Vopnafjörður, D = Dyrfjöll, BRV = Breiðavík,
K = Kækjuskörð, H = Herfell, ÁS =Álftavík/Seyðis-
fjörður, B = Barðsnes, Þ = Þingmúli, R = Reyðar-
fjörður, BRD = Breiðdalur, KO = Kollumúli-Lóns-
öræfi, F = Flugustaðatindar, L = Lón, Á = Álfta-
fjörður, G = Geitafell. Large gabbro intrusions are
the Austurhom and Vesturhorn intrusions, major sil-
icic intrusions are the Sandfell Laccolith and Ketil-
laugarfjall intrusions.
Mynd 1. Tertíer megineldstöðvar á Austurlandi.
stood out as elevations in an otherwise flat environ-
ment. On the basis of large amounts of silicic rocks
and voluminous basaltic eruptives in this area the
following central volcanoes or eruption sites have
been defined. They are, from north to south (Fig. 2):
76 JÖKULL, No. 39, 1989