Jökull - 01.12.1983, Blaðsíða 101

Weichselian glacial Lake Deposits in the Highlands
of North-Western Iceland
JÓN R. SIGURVINSSON,*
Department of Geology, Faculty of Engineering and Science,
University of Iceland, 101 Reykjavík, lceland
ABSTRACT
At the maximum extent of the Weichselian glaáation in
North-Western Iceland the ice thickness at the mouth of
Dýrafjördur was at least 750 m and at least 600 m in
Önundarfjördur. An ice dammed lake wasformed between
the northem margin of the Dýrajjördur glaáer and the
bedrock slope of Skagafjall. The remains of the lacustrine
deposits is a 7 km long terrace along the south-westem edge
of Skagafjall. Maximum thickness of the deposils in one
locality is about 100 m and lotal thickness is at least 160 m.
The lacustrine sediments are composed of at least five
rhythmic beds separated by diamicton. The aggregate thick-
ness of the rhythmites is at least 46m. It is suggested that the
rhythmites are varves and that the lake existedfor about 2000
years. A rhyolite tephra layer, al least 10 m thick, rests on the
topmost rhythmic bed. Chemical composition indicales that
the tephra belongs to the tholeiiticseri.es. Suggested age of the
lake deposits is 17000-20000years B. P.
INTRODUCTION
This paper deals with observations made on the
peninsula between Dýrafjördur and Önundarfjörd-
ur, North-West Iceland (Fig. 1). On the northem
side of the mouth of Dýrafjördur is Skagafjall, a
highland plateau of 500-720 m altitude, which is a
part of Vestfirdir highlands. The plateau is 9 km
long and varies in width from about 600 m at Skagi
to 2 km between Fjallaskagi and Nesdalsskard.
Along the south-western edge of Skagafjall there
are extensive deposits resting on an erosional un-
conformity. The terrace is about 7 km long and its
surface forms an up to 400 m broad rim on the
* Present adress: National Power Company
Háaleitisbraut 68, Reykjavík, Iceland.
plateau. The highest level is about 680 m close to
Óþoli.
The gradient of the terrace surface is 1:50, but the
gradient of the contact between the deposits and the
bedrock is 1:80. The bedrock surface on the south-
western edge of Skagafjall lies up to 100 m lower
than on the north-eastern edge. In the crest at Oþoli
there is a little hollow covered by rounded boulders
up to 1 m in diameter. Terracettes reaching up to
695 m altitude are situated on the uppermost part of
Arnarneshvilft. The surface is covered by sub-
rounded boulders up to 1.5 m in diameter. Numer-
ous meltwater channels, cut in bedrock run along
the northeastern side of Skagafjall and the highest
channels reach to 605 m a. s.l.
DESCRIPTION OF LITHOLOGIES
The sedimentary successions for three exposures
are presented in Fig. 2. The surface of the terrace
slope is covered by talus so exposures are very few.
The steepness of the terrace slope often makes
access to exposure diflicult. The section A, at Skörd
was measured at meltwater channels. The channels
do not reach up to the crest slope so sections are
therefore measured in discontinuous exposures.
Section B at Skörd was measured about 100 m
southeastwards from section A. Because of thick
talus cover at the base only the uppermost 30 m
were measured. Section C at Oþoli was measured
about 1700 m southeastwards from section B. The
sections are not complete and it is possible that
other beds exist but were not recorded. Geographi-
cal location of the sections is shown on fig. 1.
Ingram’s (1954) classification is used in the des-
cription of bedding thickness. The term rhythmic
bed is used to describe laminae of two or three
different kinds of composition, texture and color.
JÖKULL 33. ÁR 99