Jökull - 01.12.2007, Blaðsíða 5

Early Pleistocene molluscan migration to Iceland
strated between air temperatures and sea surface tem-
peratures around Iceland (Stefánsson, 1991). The
North Atlantic Current carries warm, saline water past
Iceland on its way to the Norwegian-Greenland Seas,
where it sinks and forms the dense North Atlantic
Deep Water (NADW) which flows back south across
the North Atlantic ocean ridges towards the Equa-
tor. The front between the surface water masses of
the Labrador and Greenland-Norwegian Seas on the
one hand and the warm Atlantic water on the other
shifted southward during the last glaciation and north-
ward across Iceland during the Weichselian deglacia-
tion (Ruddiman and McIntyre, 1973). Similar events
might have repeatedly happened during earlier Pleis-
tocene glaciations and deglaciations.
The purpose of this paper is to report Early Pleis-
tocene shallow water palaeoceanographic data from
western and northern Iceland, based on studies of ma-
rine molluscan assemblages in coeval sediments on
the northern side of the Snæfellsnes Peninsula in West
Iceland and in the eastern part of Breiðavík on the
Tjörnes Peninsula in North Iceland. The Tjörnes data
are from the Máná Formation of the Breiðavík Group
(Eiríksson, 1981; Vilhjálmsson, 1985).
MATERIAL AND METHODS
Several geological sections have been measured by
the authors on the northern side of the Snæfellsnes
Peninsula, western Iceland, to gain insight into the
form and infilling of a Pleistocene sedimentary basin
encountered during extensivemapping in 1998 (Leifs-
dóttir, 1999). Sections from the deepest parts of the
basin in Búlandshöfði and Stöð are presented here
(Figure 2), both consisting of sediments capped with
basaltic lavas. Most of the sediments are marine and
samples have been collected during several years from
seven different layers for analyses of marine macro-
fossils. They were both bulk samples and shells
picked from the sediments in the field. Microfossils
are not dealt with in this paper. We could not process
the bulk samples by sieving because of the consolida-
tion by silica and palagonitization. Therefore the sam-
ples were crushed in a vice and the shells picked from
the fragmented sediment. The sedimentological data
from the Máná Formation of the Breiðavík Group,
northern Iceland, are from Eiríksson (1981) and the
palaeontological data from Vilhjálmsson (1985) and
the authors.
In Tables 1-2 the numbers of the species were
worked out as follows: whole gastropod shells and
fragments with the spire preserved were counted;
valves and umbonal fragments of bivalves were
counted and their number divided by two; parietal
plates of barnacles were counted and their number di-
vided by six.
These investigations were concentrated on the
Early Pleistocene migrational history of three arc-
tic molluscan species, Tachyrhynchus erosus, Port-
landia arctica, and Tridonta placenta, and two boreal-
lusitanian species, Littorina littorea andNucella lapil-
lus. Their presence or absence in contemporaneous
sediments in western and northern Iceland is thought
to provide insight into the migration history of ma-
rine molluscs in Iceland during the Early Pleistocene
as well as changes in the pattern of ocean currents.
THE BÚLANDSHÖFÐI FORMATION,
SNÆFELLSNES, WESTERN ICELAND
On the northern side of the Snæfellsnes Peninsula in
western Iceland, from Kirkjufell in the east to Skarðs-
lækur in the west (Figure 2), it is possible to trace
fossiliferous sediments which are up to 50 metres in
thickness and rest unconformably on glacially striated
Tertiary basalts of reversed polarity (Pjetursson, 1904;
Hospers, 1953). Here the Búlandshöfði Formation in
Búlandshöfði and Stöð is the best known.
The stratotype for the Búlandshöfði Formation is
composite and consists of three members. In the lower
Búland Member, consisting of pebbly diamicton with
dropstone boulders, flat-bedded conglomerates, and
sandstones, arctic marine fossils occur mainly in the
diamictite which is interpreted as a glaciomarine de-
posit (Figure 3). The upper Höfði Member in the Bú-
landshöfði sequence consists mainly of shallow wa-
ter, flat-bedded marine sandstones and conglomerates
with thermophilic faunal assemblages. The shallow
water marine sediments are not present in Stöð (Fig-
ure 3). They are replaced by the StöðMember, deltaic
sandstones and conglomerate overlain by finegrained
JÖKULL No. 57 3