Jökull - 01.12.1980, Page 4

Fig. 1. Geological map of Iceland showing
location of Tjörnes. Legend: 1. Bedrock older
than ca. 3 Ma. 2. Bedrock younger than ca. 3
Ma. 3. Active volcanic zones. (After Th. Ein-
arsson (1974), K. Sœmundsson (1978) and H. Jó-
hannesson (pers. comm.)). — Mynd 1. Jarðfrœði-
kort af Islandi með ramma um Tjörnes. Skýringar: I.
Berggrunnur eldri en 3ja milljón ára. 2. Berggrunnur
yngn en 3ja milljón ára. 3. Virk gosbelti. (Teiknað
eftir Þorleifi Einarssyni (1974), Kristjáni
Sæmundssym (1978) og Hauki Jóhannessyni
(munnlegar upplýsingar) ).
The Tjörnes sequence is mentioned in a few
compilatory works. Woldstedt (1965) related
evidence on climate from Tjörnes, and refer-
ring to Strauch’s (1963) work, concluded that
glaciations were becoming imminent at the
time of deposition of the Breiðavík beds
(Table 1). Woldstedt viewed reports of earlier
glaciations with scepticism and suggested that
the “tillites” might be explained in some other
way. Flint (1971) discussed the evidence about
ten glaciations in Iceland dating back to 3 Ma
and also reviewed the evidence about the ex-
tent and nature of a separate Pleistocene ice
cap in Iceland. Berggren and Van Couvering
(1974) discussed the chronology of glaciations
in the Northern Hemisphere, and considered
the ice rafting chronology to be corroborated
by the evidence from Tjörnes. Their review of
Zagwijn’s (1974) climatic levels during the
Pleistocene and at the Pleistocene/Holocene
boundary also involves the Tjörnes sequence.
Confusion in the use of local place names on
Tjörnes in the geological literature has
recently been discussed and rectified by
Björnsson (1977).
This survey of the geological literature
about Tjörnes is an offspring of a recent phase
of sedimentological work on Tjörnes which
was originally inspired by Professor Th. Ein-
arsson and organized concurrently with a dat-
ingeffort by Dr. K.J. Albertsson. The objective
of the sedimentological work was to study the
origin of the sedimentary rocks and to analyze
the vertical cyclicity of the upper part of the
sequence. Several uncertainties about the
stratigraphy of the Breiðavík area were inves-
tigated, and a formal lithostratigraphical
scheme was erected. Areal extent of the four
major lithological units on Tjörnes is shown in
Fig. 2 and a composite vertical section in Fig.
3. The results included a new interpretation of
the origin of the sedimentary rocks in terms of
glacial-interglacial cycles and have recently
been presented in a thesis (Eiríksson 1979).
The new results about the age and origin of
the upper Tjörnes rocks confirm some of the
conclusions of earlier workers, while other
results now seem controversial. It is hoped that
the literature survey will be useful in the
assessment of the present state of knowledge
and understanding, show up weaknesses or
gaps in the knowledge, and stimulate further
work.
RESEARCH HISTORY
Early discovery
The research history of Tjörnes as reflected
by the geological literature dates back to 1749,
when the Icelandic naturalist Úlafsson (1749)
first mentioned the Hallbjarnarstaðir locality
in his work on natural conditions in Iceland.
The locality was later described more fully by
Ólafsson (1772), who noted some extinct mol-
lusc species in the marine beds there, and also
discussed the lignite beds briefly. The next
contribution came nearly a century later,
when Winkler (1863) mentioned 24 mollusc
species from Tjörnes. He ascribed the marine
2 JÖKULL 30. ÁR