Jökull - 01.12.1980, Blaðsíða 12

Iceland by Bout (1953), who also presented an
outline of the stratigraphy of Iceland and in-
cluded the Tjörnes sequence in his scheme.
Reexamination of the Tjörnes sequence
In the early sixties Strauch (1961, 1963) car-
ried out a detailed study of the stratigraphy
and palaeontology of the Tjörnes sequence.
He agreed with earlier authors about an early
Quaternary age of the Breiðavík beds (Table
1), and suggested that the “West Tjörnes
Beds” might correspond to the Jökuldalur
Series of Bemmelen and Rutten (1955). Strauch
also studied climatic evidence, tectonic
aspects, and the palaeogeographical evolution
of Tjörnes. He agreed with earlier views on the
Pliocene topography of Iceland, which was
considered a relatively flat country. A slight
depression, Strauch suggested, was formed in
the vicinity of the present Central Icelandic
Graben, where the 550 m thick “West Tjörnes
Beds” accumulated during slow and uninter-
rupted subsidence. Strauch concluded that
sedimentation in the Pliocene Kaldakvísl area
began with bog formation and deposition of
fluviatile sands coupled with an initial sub-
sidence of the area and a lull in volcanic
activity. Only occasional lava flows reached
the area during an early phase of relative sea
level changes. Sedimentation then continued
in a fiord which according to Strauch was open
towards north with sediment supply from the
south. From time to time the sediment supply
exceeded the rate of subsidence and the shal-
low fiord was rapidly filled with sediments.
The relief of the area was gentle, but when it
came to the deposition of the upper “West
Tjörnes Beds” an increased rate of subsidence
and local uplifts in border areas enhanced the
topographical relief. Soon after that volcanic
activity set in and the intercalated basalts
were piled up in an environment charac-
terized by tectonism and volcanism. Coastal
and fluvioglacial sedimentation began in the
present Breiðavík area, but continued sub-
sidence to the west and uplift of the Tjörnes
horst led to relative sea level changes and un-
conformities. Strauch suggested that the upper
Breiðavík beds had accumulated in a fiord of
similar proportions as the earlier “West Tjör-
nes Beds” fiord. Sedimentary dykes in the
Tjörnes sequence were the subject of separate
publications (Strauch 1966, 1968a), and
sedimentological and palaeontological
evidence from Tjörnes was used in a later
paper on a North Atlantic land bridge (Strauch
1970). Palaeontological results from Tjörnes
were to a various extent the subject of further
publications (Strauch 1968b, 1968c, 1972a,
1972b, 1972c).
The Tjörnes sequence was reexamined and
sampled for palaeontological analysis and
palaeomagnetic measurements by Doell, Th.
Einarsson, and Hopkins (Hopkins et al. 1965, Th.
Einarsson et al. 1967, Th. Einarsson 1966, 1967,
1969, Doell 1972). They found evidence of ten
regional glaciations within the sequence and
attempted a correlation with the geomagnetic
polarity time scale. According to their prefer-
red alternative the Mactra/Serripes Zone
boundary lay close to the Gilbert/Gauss
reversal, and the Breiðavík beds corresponded
to the Matuyama epoch. The fossil material
was analyzed by MacNeil (1965, 1967) and
Zullo (1968), and discussed by Durham and
MacNeil (1967) and by MacNeil (1973) in terms
of faunal migrations between the Pacific and
Atlantic Oceans. Durham and MacNeil iden-
tified 32 mollusc species of Pacific origin in the
Tjörnes sequence, 22 out of which appeared
for the first time in the Serripes Zone. Th. Ein-
arsson et al. (1967) correlated the Serripes Zone
with the Red Crag of England, and suggested
that the Zone was largely and perhaps entirely
of early Pleistocene age. They pointed out that
the mollusc fauna of the two lower Zones was
similar to that of the Coralline Crag of East
Anglia (Astian or late Pliocene). In his text-
book on general geology and the geology of
Iceland, Th. Einarsson (1968) correlated the
Mactra/Serripes Zone boundary with the Gil-
bert /Gauss reversal and interpreted the
faunal and floral changes at this boundary as
evidence of a sudden dramatic deterioration
of climate. The Pliocene/Pleistocene boun-
10 JÖKULL 30. ÁR