Jökull - 01.01.2005, Page 22

Leó Kristjánsson and Ágúst Guðmundsson
series of basalt and intermediate volcanic rock was
mapped. The intermediate rock is classified as an-
desite and is possibly related to a central volcano,
buried in the plateau south of Fljótsdalur. The sedi-
ments mainly consist of brown tuffaceous sandstone
and reddish siltstone. This suite is estimated to have
formed about 7–6.5 Ma ago, on the west side of the
flexure zone crossing Lögurinn. Only its uppermost
part was included in the mapping by Guðmundsson
(1978).
HF-suite (Hengifossársyrpa), named after the
Hengifossá river in Fljótsdalur. This is a more than
300 m suite of various basaltic lavas at the south-
western end of Lögurinn, with very thick sedimen-
tary interbeds which may be observed in Hengifossá
and Bessastaðaá. The sediments include thin lenses
of acidic tuffs and lignite. In the inner part of Suð-
urdalur (east of Kiðufell, see Figure 1 and the lowest
part of Figure 2) the HF-suite contains an acidic lava
and ignimbrites, suggesting a buried central volcano
somewhere towards south (probably the same one as
produced intermediate lavas deeper in the strata). In
the upper half of the suite, a prominent olivinic com-
pound lava group (up to 30–50 m thick) forms steep
cliffs in the outer part of the Suðurdalur valley. The
HF-suite was formed 6.5–6 Ma ago according to ra-
diometric dates in Hengifossá and in the lower part of
Bessastaðaá (McDougall et al., 1976).
ML-suite (Marklækjarsyrpa), named after the
stream Marklækur (MK in Figure 1, Figure 3b) in
Suðurdalur, formed 6–5 Ma ago and is 400 m thick.
It consists of rather thin basaltic lavas with sediments
of various types. Most of the basalts are tholeiitic but
among them is a prominent group of two or three por-
phyritic lava flows which can be traced over relatively
long distances. The sediments are tuffs of basaltic
and acidic composition, sandstones and conglomer-
ates. Some of the sediments in this suite are consid-
ered to be of fluvioglacial origin, suggesting cold cli-
mate towards the end of Tertiary time (Geirsdóttir and
Eiríksson, 1994).
TB-suite (Teigsbjargssyrpa) is named after a steep
cliff southwest of profile R in Figure 1. The TB-
suite which was formed 5–4.5 Ma ago is about 100 m
thick, and consists almost entirely of thick porphyritic
lavas with thin red sandstone interbeds. The upper
and lower boundaries of this suite are well marked
by thick sediments. At the boundary between the TB
suite and the overlying FA suite, there is a thick sedi-
ment of sandy conglomerate and diamictite, probably
the first definite tillite in the Fljótsdalur lava pile.
FA-suite (Fossársyrpa) is named after a creek on
the southeastern side of Norðurdalur valley. The FA-
suite which formed about 4.5–3.5Ma ago, is 200–400
m in thickness. It consists of basaltic lavas of various
types, mostly tholeiite basalt with occasional layers of
olivine basalt and rather frequent sediment interbeds.
The sedimentary layers are up to 25 m thick, made of
sandstones, reddish tuffs, conglomerates and tillites.
ÞF-suite (Þverfells-Fossöldusyrpa) was formed
3.5–3 Ma ago. It is found in the middle part of Múli
and the hills in the north-eastern part of the Fljótsdals-
heiði plateau of Figure 1. The ÞF suite is 100–200 m
thick, consisting predominantly of porphyritic basalt
(most often of tholeiitic character) with interbeds of
siltstone, sandstone and conglomerates.
SA-suite (Sníkilsársyrpa) was formed 3–2.5 Ma
ago. This suite is unique in many ways. On a re-
gional scale, it is a 50–200 m thick suite of tholei-
ite basalt and porphyritic basalt, with sedimentary in-
terbeds whose thickness varies from about 50 m in the
northern part of Fljótsdalsheiði to more than 200 m in
the southern part. From field observations and drill
holes cored for the Kárahnjúkar hydroelectric project,
it appears that the thick sediment fill was deposited
in an eroded valley or glacier trough: relics of tholei-
itic and porphyritic lava sequences were found at the
lower levels of the SA-suite. The sediments of the
suite mainly consist of tillites with boulders, fluvio-
glacial coarse-grained conglomerates, sandstones and
siltstones. These sediments show graded bedding
(Geirsdóttir and Eiríksson, 1994). The grain-size dis-
tribution of the fill changes both in the vertical and
the horizontal direction. The strata show evidence of
some 100–200 m deep glacially eroded valleys head-
ing slightly east of north, gradually filled with sedi-
ments. Intermittent volcanic activity during that time
is suggested by the presence of interbedded tuffa-
ceous sandstones and occasional lava flows which de-
veloped rather narrow tongues following depressions,
22 JÖKULL No. 55