Jökull - 01.01.2005, Side 35

The Late Miocene Tinná Central Volcano, North Iceland
coal deposits hidden deep inside the mountains. Hall-
grímsson (1989) visited Skagafjörður in 1839 to study
and take samples from the supposed “coal seams” for
the Danish government. In his report they are stated to
be thin layers of lignite of poor quality and too small
in quantity to be of any practical value. After this no
geological investigations were made in the area for
more than a hundred years.
The acid rocks in the Skagafjörður Valleys were
first shown on Kjartansson’s (1965) Geological Map
of Iceland, but there they cover far too limited an area.
The first detailed map indicating the real distribution
of acid rocks in the valleys was the one of Hjartarson
et al. (1998).
Various studies of the stratigraphy, local tecton-
ics and geothermal activity of the Skagafjörður Val-
leys were carried out between 1970 and 2000, mainly
in association with geothermal research and utiliza-
tion and hydropower planning (Jónsson et al. 1977,
Kaldal and Víkingsson 1978, 1979, Harðarson and
Guðmundsson 1986, Karlsdóttir et al. 1991, Guð-
mundsson 1991, Jóhannesson 1991, Hjartarson et al.
1998).
The stratigraphy
The strata pile belonging to the Tinná Central Volcano
has been defined as one stratigraphic group, the Tinná
Group. It is divided further into 11 formations. These
will be described in ascending order. Table 1 gives an
overview of the stratigraphic names, the thickness of
the units and their polarity. The classification has not
been given a formal stratigraphic status.
The Fjóslækur formation is the lowest formation
of the Tinná Group. In Austurdalur it is 350 m thick.
In Vesturdalur it is 125m thick. The Fjóslækur forma-
tion is dominated by normal paleomagnetic polarity
(N) with two short reverse subchrons (R). The lower
one, which contains only two lavas, wedges into the
pile in the outer Goðdaladalur canyon. The other one
is found near the top of the formation. No acid or
intermediate layers have been recognized inside this
formation.
The Ágúll rhyolite dome. The first clear signs of
the onset of acid magmatism and the evolution of a
central volcano in the Skagafjörður Valleys are found
near the abandoned vicarage Ábær. A tributary river
flows in rapids and cascades in a deep and rocky gorge
from the hanging Ábær valley to form a broad alluvial
fan joining the Jökulsá river in Austurdalur. On the
north side the bedrock is covered by a rugged scree,
while in the south wall of the gorge the river has ex-
cavated the lowest part of a large rhyolite extrusion
that can be classified as a typical lava dome (Fink and
Anderson 2000). This rhyolite is the lowest of several
acidic layers in the Skagafjörður Valleys, and it indi-
cates the early formation of a magma chamber under-
neath the area (Figure 2).
Table 1: The stratigraphic units of the Tinná Group –
Helstu myndanir Tinnárhóps
Average Polar-
Group Formation thickness ity*
m
Tinná Nýibær rhyolite 80 N
Nýibær andesite 100 N
Nýibær tholeiite 150 N
Tinná tholeiite 150 R
Tinná andesite 50 R
Skati rhyolite 100 R
Tinná olivine tholeiite 25 R
Tinná lignite sediments 18
Ábær tholeiite 280 N
Ágúll rhyolite dome 100 N
Fjóslækur olivine tholeiite 300 N/R/
N/R/N
Tholeiite N
*N = normal polarity, R= reverse polarity
The acid layers of the dome in Ábær gorge can
be divided into three units. The lowest is a light-
coloured, fine-bedded, sandy tephra, 15–20 m thick
with coarser pyroclastic lenses containing angular
fragments; most often these are 2–4 cm in diameter
but can reach up to 50 cm. The upper part of the tephra
layer is coarser than the lower one.
Brecciated rhyolite and acid scoria are found
above the tephra, at 280 m a.s.l. Angular rhyolite
blocks, up to 1 m in diameter, are found in more
finely-grained material. This unit attains a thickness
of 10 m and forms the basal breccia of a massive lava
JÖKULL No. 55 35