Jökull - 01.12.1989, Blaðsíða 24

Fig. 1. The Fossvogur sedimentary layers. Note the
coarse glacial till layers of the Younger-Dryas rest-
ing on the Alleröd fine marine fossiliferous silt. This
site is now hidden by a land-fill. Photo Á.Hj.
Mynd 1. Fossvogslögin við utanverða Nauthólsvík.
Grófur jökulruðningur frá yngra-dryas hvílir á fín-
gerðu lagskiptu seti frá Alleröd skeiði. Þessi staður
hefur nú verið huiinn með uppfyllingu. Ljósm. A.Hj.
but a period of altemating glacials and interglacials.
He inferred an old age of the Fossvogur layers from
the hardness of the sediments, from calcium car-
bonate in cracks in the layers, and from calcite nee-
dles that have sometimes formed inside the shells.
He also thought that he could identify glacial stria-
tions on top of the uppermost till (Péturss, 1905).
Péturss (1909) wrote about "die Fossvog-Inter-
glazialzeit" without placing the layers in any inter-
national geochronological system. In Einarsson
(1968), Péturss’ ideas have been developed into a
statement that the layers were formed during Eem,
the last interglacial. This age has been accepted by
most textbooks on the stratigraphy of Iceland
(Einarsson, 1971, 1978; Einarsson, 1972; Guð-
mundsson, 1982)
Recent observations have shown that some Ice-
landic sediments can become a solid rock in a short
period. The consolidation is especially rapid at high
temperatures in tephra (Jakobsson, 1972 and 1986),
but even at low temperatures sediments mixed with
tephra can consolidate in a relatively short period of
time (Hjartarson and Gísladóttir, 1982). Dating of
shells from the Búði end-moraine in Southem Ice-
land, has shown that calcite fillings are present in
Preboreal shells (Hjartarson and Ingólfsson, 1988).
This was why a radiocarbon dating was performed
on a shell specimen from the Fossvogur layers. The
dating, that was made at the University of Lund,
Sweden, revealed an age 100,000 years younger
than expected. The layers were found to be from the
Alleröd Interstadial and not from the Eem Intergla-
cial (Hákansson, 1987; Hjartarson, 1987). This con-
clusion has been disputed (see Hjartarson, 1988),
but subsequent radiocarbon datings performed at the
Aarhus Tandem Accelerator show a similar age
(Andersen and others, in press). They were the first
datings of Icelandic material made by accelerator
mass spectrometry.
THE SEDIMENT SUCCESSION
AT NAUTHÓLSVÍK
The first dated shell sample from Fossvogur, the
much disputed sample from 1985 (Lu-2599, Table
I), was taken from a small outcrop above the sand
beach by the boat harbour at Nauthólsvík (Fig. 2),
about 3 m above sea level. The sampled Mya trun-
cata (Icel. smyrslingur) were in situ, i.e. the bivalves
are roughly in the position that they were in when
the creatures lived. Macoma calcaria (Icel.
hallloka) and Balanus (Icel. hrúðurkarl) are also
found at this exposure.
The sediment forms a steep cliff named Kýrhamar
just west of this outcrop. A coarse wedge of till is
found at the foot of the cliff. The till was deposited
by a glacier that covered the Fossvogur area prior to
the formation of the Fossvogur layers. A 5-6 m thick
layer of sediment lies on top of the till. It can be
divided into several units (Fig. 3):
1. A shell-free varved silt at the base.
2. A dark coloured poorly stratified layer of fine
sand, fairly rich in shells.
3. A lighter coloured and layered unit of coarser
sand with gravel clasts. This unit is not as rich in
shells as the underlying one.
22 JÖKULL, No. 39, 1989