Jökull - 01.12.1980, Page 15

by questions and problems that remain un-
answered or controversial. The local strati-
graphy is not very well known and a formal
lithostratigraphical scheme has only been
proposed for the youngest major unit on
Tjörnes, the Breiðavík Group (Eiríksson 1979).
Current work in progress at the University of
Iceland involves further mapping of that and
older units on Tjörnes. The sequence of lavas
between the Breiðavík Group and the Tjörnes
beds is fairly well known. The surface extent of
the two oldest lithozones, the Tjörnes beds
and the Kaldakvísl lavas is also fairly well
known, but there are many uncertainties
about their internal structure and strati-
graphy. Recent investigations at the Kalda-
kvísl locality (Eiríksson 1979) confirm T. Ein-
arsson’s (1958) and Strauch’s (1963) views about
the conformable relationship between the
Kaldakvísl lavas and the Tjörnes beds.
Preliminary radiometric ages from lava flows
at that locality (Albertsson 1976) indicate that
the Tjörnes beds may have accumulated fairly
rapidly in late Pliocene time as an age of
4.30 ±0.17 Ma was obtained from the young-
est flow beneath the Tapes Zone.
Although that part of the Tjörnes sequence
which contains direct lithological evidence
about glaciations (i. e. the Breiðavík Group)
has been bracketed in time by radiometric
ages (Albertsson 1976, 1978), there are still
uncertainties about the age of individual
glacial-interglacial cycles. More radiometric
ages are required to achieve such a goal.
The abundant palaeontological evidence of
the Tjörnes sequence has yet to be integrated
with sedimentological studies in a facies ana-
lysis and a reconstruction of palaeoenviron-
ments. Such a line of research is likely to
facilitate an interpretation of the fossil record
in terms of climatic changes, and the general
understanding of the geological history of the
area.
The geological structure of Tjörnes and its
relation to the tectonic history of North Ice-
land will probably continue to attract
researchers in the future. A meaningful inter-
pretation of the fault pattern on Tjörnes can
only be attempted when a detailed geological
map of the peninsula has been compiled and
the age of the faults determined.
One line of research which has been
neglected is a morphological analysis of Tjör-
nes. It seems possible that the present
landscape, which is characterized by step—
faults in the east, more gentle slopes in the
north and west, and young valleys in the cen-
tral and southern part, may hold clues to
recent tectonic developments.
The subsurface geology of Tjörnes is mostly
unknown and very little geophysical work has
been done in the area.
CONCLUSIONS
It transpires from the review of the long
research history of Tjörnes that the geological
sequence there has persistently played an im-
portant part in the study of stratigraphical,
climatological, and tectonic problems in Ice-
land and in the North Atlantic. Geologists
have visited Tjörnes with questions about the
age of the deposits and about the climatic
evidence of the fossils and rocks. Most of the
visits were brief, however, and careful analyses
of the Tjörnes sequence, based on objectively
planned field work, have been relatively few.
Bárðarson’s (1925) monograph on the Pliocene
deposits at Tjörnes was accompanied by the
statement that his investigations were still in
their initial stage. Strauch (1963) attempted to
reconstruct the palaeogeography of the Tjör-
nes area. Some of his conclusions, at least as
far as the Breiðavík Group rocks are con-
cerned, are let down by weaknesses in map-
ping (T. Einarsson 1963, Eiríksson 1979). Ans-
wers to questions about the origin and
depositional environments of the Tjörnes beds
can probably not be regarded as reliable until
they have been mapped thoroughly. Inter-
pretations, of which there are a legion, appear
to have suffered from a lack of detailed
knowledge of the stratigraphy of Tjörnes.
JÖKULL 30. ÁR 13