Jökull - 01.12.1983, Side 109

Fig. 8. Grain-size curve and
histogram of rhyolite tephra
from section C at Óþoli.
Mynd 8. Kornastœrðardreijing og
histogram fyrir líþarítgjósku úr
sniði C við Ófiola.
ions sometimes occur in the clay (winter) layers.
Such thin clay and silt layers were always taken as a
part of a varve in counting and measuring them.
Average thickness of380 measured varves is about 4
cm. The thickness of the five rhythmic beds is at
least 46 m and it can be assumed that the Skagafjall
deposits are composed of about 1200 varves at least.
It is possible that the rhythmic beds at Skagi belong
to younger sedimentation than the five measured
rhythmic beds and the lake has therefbre possibly
been in existence for some 2000 years.
The delta front must have been close to Óþoli
Table 1. Chemical composition of the tephra in
section C.
Tafla 1. Efnainnihald lípantgjóskunnar úr sniði C.
1 2 3 Mean Meðaltal
Si02: 71.92 70.91 71.43 71.42
Ti02: 0.14 0.19 0.18 0.18
Al203: 11.20 11.26 11.32 11.60
FeO: 2.59 2.75 2.63 2.36
MnO: 0.48 0.36 0.25 0.36
MgO: 0.02 0.00 0.00 0.00
CaO: 1.32 1.28 1.37 1.30
NasO: 4.30 4.51 4.39 4.40
K20: 2.56 2.46 2.50 2.51
P205: 0.00 0.02 0.00 0.00
H20: not measured — ekki mælt.
Sum: 94.55 93.74 94.07 94.13
most of the time when the sedimentation took place
in the Skagafjall lake. Ashley (1975) distinguishes
three types of rhythmites (varves) in glacial lake
deposits depending upon the location ofdeptosition:
Group I-clay thickness greater than silt thickness.
Group II-clay thickness approximately equal to silt
thickness. Group III-clay thickness less than silt
thickness. Groups I and II were formed in still
water away from river mouths, where little sedi-
ments was received directly from density currents,
Group III were formed relatively close to delta
fronts, associated with high sedimentation rates.
The glacial lake deposits on Skagafjall are so well
preserved that it is hardly older than the last
glaciation. Two stages ofreadvance are known dur-
ing the recession of the main ice sheet at the end of
the Pleistocene. First, the Alftanes-stage 12,500-
12,000 years B. P. (Older Dryas) when the ice had
retreated onto the present western and northern
coast. Second, the Búdi-stage 11,000-10,000 years
B. P. (Younger Dryas) after the ice had retreated
rapidly upto the highland during the Alleröd-inter-
stadial (Einarsson 197\). Norddahl (1981) hasdivided
the VVeichselian glaciation in North Iceland into
three main stages: (1) The maximum stage, when
North Iceland was ice-covered northwards to the
island of Grímsey, (2) the ice-lake stage, when a
series of ice-dammed lakes were formed in Fnjóska-
dalur, and (3) the Langhóll stadial, 14C age about
10,000 B. P., and advance restricted to the valleys
on both sides of Eyjafjördur, after the final emptying
of the Younger lake in Fnjóskadalur. His estimated
age for the oldest ice-dammed lake is about 20,700
B.P. and postdated the maximum glaciation in
North Iceland.
The Skagafjall lake must have formed after the
JÖKULL 33. ÁR 107