Jökull - 01.12.1980, Síða 11

suggested a maximum sea water temperature
of 5 °C higher than now. He considered the
analyses of trees to show that air temperature
was also higher. Schwarzbach concluded that
the Breiðavík deposits must be considered of
Pleistocene age as they rested on a moraine. In
his work “Climates of the Past”, Schwarzbach
(1963) discussed Tertiary and Quaternary
climatic evidence from Tjörnes. The pollen
flora of the Pliocene deposits (Tjörnes beds)
and the Pleistocene deposits (Breiðavík beds)
on Tjörnes was described for the first time by
Schwarzbach and Pflug (1957), and was found to
point to a climate similar to that of the
Holocene Atlanticum of Central Europe. A
climatic cycle with an optimum in the Mactra
Zone was inferred. Pflug (1959) studied spores
from the Tjörnes and Breiðavík beds. He con-
cluded that the Tapes and Mactra Zones were
of Pliocene character and age, but that the
Breiðavík beds were of Quaternary age.
Schönfeld (1956) studied tree remnants from
the Tapes Zone and suggested similarities bet-
ween the Icelandic Tertiary flora and the
present North American one.
Reinterpretation of the position of Tjörnes with
regard to the geology of Iceland
Apart from palynological studies, the fifties
brought a new effort by Dutch scientists
towards an understanding of the origin of the
Palagonite Formation of Iceland and the
structure of the Icelandic Central Graben.
The first contribution from this direction
came from Hospers (1953, 1954), who studied
palaeomagnetism in eastern North Iceland.
Bemmelen and Rutten (1955) studied amongst
other things questions of tillites and breccias,
morainic and fluvioglacial sediments, and
related phenomena. Tjörnes was among the
areas visited by Bemmelen and Rutten. They
agreed with the generally held view (Thorodd-
sen 1906, Bárðarson 1927) that Iceland was
symmetrical about a central depression, and
that the country represented a horst structure.
According to Bemmelen and Rutten the oldest
rocks, the Tertiary Plateau Basalts, were cov-
ered by a series of Older Pleistocene Basalts
with intercalations of tillites (Laxárdalur
Series to the west of the graben in Northeast
Iceland, corresponding to the Jökuldalur
Series on the eastern side, both being identical
with Pjetursson’s Grey Stage unit). In the ex-
treme north, the marine Pliocene deposits of
Tjörnes were intercalated between the Ter-
tiary Plateau Basalts and the Laxárdalur
Series. The Tjörnes sediments had, according
to Bemmelen and Rutten, accumulated in a top
depression of the Icelandic horst. Rutten and
Wensink (1960a, 1960b) discussed the structure
of the Central Icelandic Graben as well as
palaeomagnetic dating, glaciations, and the
chronology of the Plio-Pleistocene in Iceland.
They correlated the Grey Stage unit with the
Nl, R1 and N2 palaeomagnetic groups of
Plio-Pleistocene age (Astian — Villa-
franchian), and suggested that the Ter-
tiary/Quaternary boundary should be
defined at the reversal between the N2 and R1
groups in Iceland, providing a much better
datum line than any index fossils or climatic
indices. The palaeomagnetic studies were
continued by Wensink (1964), who correlated
the Tjörnes sequence with the palaeomagnetic
groups. His correlation was based on a
geological and palaeomagnetic study of the
sequence by Broekman (1961). Wensink corre-
lated the Tjörnes beds with the N2 geomag-
netic period and placed the Breiðavík deposits
at the geomagnetic N2/R1 boundary, thus
being of a possible early Pleistocene age. The
correlation of the Jökuldalur Series with the
geomagnetic groups suggested by Rutten and
Wensink (1960a, 1960b) was later confirmed by
radiometric dating of the Jökuldalur Series,
and the R1 and N2 polarity groups were
found to correspond to the Matuyama rever-
sed and the Gauss normal polarity epochs
respectively (McDougall and Wensink 1966).
McDougall and Wensink found a tillite im-
mediately above a basalt dated at 3.10 ± 0.10
Ma and suggested that the base of the
Pleistocene might be as old as 3 Ma.
The origin of breccias and conglomerates in
the Icelandic Palagonite Formation was dis-
cussed in a treatise on the geomorphology of
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