Jökull - 01.01.2005, Page 20

Leó Kristjánsson and Ágúst Guðmundsson
Guðmundsson (1978) carried out comprehensive
geological mapping on both sides of Norðurdalur (26
profiles) and of the adjacent Suðurdalur valley (20
profiles). His work on the basement rocks of these val-
leys and the Múli ridge includes stratigraphy of lava
flows and sediments, tectonics, and alteration miner-
als. Using the available radiometric dates and rema-
nence directions along with his field polarity measure-
ments on lava flows, he correlated composite sections
from both valleys with the geomagnetic polarity time
scale between about 6.5 and 2 Ma ago. Guðmunds-
son (in unpublished reports prepared in connection
with the planning of hydroelectric power projects
in the Jökuldalur-Fljótsdalur area) subsequently ex-
tended the Norðurdalur section upwards to Brunhes
age rocks (<0.8Ma old) south and west of the topmost
profiles of Watkins andWalker (1977), and mapped in
detail the bedrock of the inner Fljótsdalur area.
Dagley et al. (1967), Walker (1974, 1983) and
Mussett et al. (1980) pointed out the presence of a
flexure zone in Eastern Iceland, which e.g. causes the
dip to increase fromwest to east across the southwest-
ern part of Lögurinn. Sæmundsson (1974) inferred
from the presence of this flexure zone and other ev-
idence a major hiatus in volcanic activity in Eastern
Iceland 8–4Ma ago; his suggestion was not altogether
confirmed by subsequent radiometric dating in Norð-
urdalur, but possibly the rate of buildup of the lava pile
was relatively low. The area studied here is west of the
flexure zone. No detailed stratigraphic or structural
mapping other than that of Guðmundsson (1978, and
unpublished) has been carried out in Múli, in Suður-
dalur, or on the eastern side of Fljótsdalur as far as we
are aware. No results of such mapping, radiometric
age determinations or new laboratory paleomagnetic
measurements have appeared in print from Fljótsdalur
and the surrounding region since 1980.
Geirsdóttir and Eiríksson (1994, 1996) who stud-
ied sediments in the Fljótsdalur-Jökuldalur area, state
that the first glacial deposits appear in the estimated
age range 4.3–3.0Ma in Fljótsdalur, with evidence for
full-scale glaciations of Iceland identified in strata of
2.7–2.6 Ma age.
Walker (1983) discussed the evolution of land-
scape in Eastern Iceland, as it has resulted from an
interplay of volcanic, tectonic and erosional phenom-
ena, and Ashwell (1985) described the geomorpho-
logy of the area around Lögurinn.
GEOLOGY OF THE FLJÓTSDALUR
AREA
In late Tertiary time, Eastern Iceland was a basaltic
plateau, with a number of central volcanoes proba-
bly rising several hundred meters above the plateau
to form a SW-NE running mountain chain. As a con-
sequence of gradual cooling of the climate, glaciers
formed on the central volcanoes and flowed eastward
down to the coast. The coastal areas today exhibit
typical alpine morphology. On the western side of the
mountain chain the glaciers may have gathered into
one major glacier stream, eroding the present Fljóts-
dalur valley. The valley is dissected into the 600–800
m high plateau of the central highlands.
Exposures
The landscape in the Fljótsdalur area (Figure 3 a) is a
typical erosional landscape, with largely outcropping
bedrock along the valley slopes where scattered till
remnants are encountered. Along the flanks of Norð-
urdalur and Suðurdalur, the basalt flows usually form
cliffs which can easily be followed for long distances.
Material from the weathered sediment interbeds and
from the less resistant scoria zones in the lava flows
forms an inclined berm which at the foot of the cliffs
is covered with blocky talus. The most complete ex-
posures are found along streams (Figure 3 b).
Stratigraphy
Based on the geological studies described in the pre-
vious section and unpublished observations by Á.G.
in various reports, the geological history of the Fljóts-
dalur area is as follows:
The bedrock in the Fljótsdalur area (including Norð-
urdalur and Suðurdalur) was formed during the last
6.5 or possibly 7 Ma. It consists of a 1500 m thick
sequence of basalt lava flows with interbedded sedi-
ments. The basalt in the area may be classified ac-
cording toWalker (1959) into three petrographic types
distinguishable in the field: tholeiite basalt, olivine
basalt and feldspar-porphyritic basalt. In addition to
20 JÖKULL No. 55