Jökull - 01.12.2006, Síða 52

Kristjánsson et al.
Table 2. Ar/Ar and K/Ar dates on Miocene lava flows from Skagafjörður valleys. – Aldursgreiningar úr Skaga-
fjarðardölum.
Locality m a.s.l. Rock type Polarity No. Age, Ma Ref.*
Skati Dome Rhyolite R 17181 5.212±0.016 2)
Sólheimafjall PF 48 1050 Ol. tholeiite N 78-1005 8.88±0.12 1)
Sólheimafjall PF 43 1000 Tholeiite N 78-1002 8.72±0.10 1)
Sólheimafjall PF 39 1000 Tholeiite N ? 75-144 9.04±0.18 1)
Sólheimafjall PF 36 960 Tholeiite N ? 75-141 9.12±0.12 1)
Sólheimafjall PF 34 940 Porph. basalt R 75-140 9.16±0.20 1)
1) Sæmundsson et al. 1980. 2) Hjartarson et al. 2003b; in Hjartarson 2003
not been recognized in the PH-profile but are found
between flows PG 43 and 46. Hjartarson et al. (1997,
2003a) have traced these sediments which they name
“Merkidalur sedimentary member (me3)” all the way
to the base of our profile PM where they are overlain
by at least 25 normal-polarity flows. The sedimen-
tary horizon has also been identified west of Austari
Jökulsá but not in Vesturdalur valley where exposures
are limited. No stratigraphic correlation seems to be
available between the profiles PM and PN. However,
the base of PN might be a direct continuation of PM.
THE AGE OF THE LAVA PILE
As mentioned above, Saemundsson et al. (1980) pub-
lished a series of K-Ar dates along with the results
of their geological and paleomagnetic study in Trölla-
skagi. These indicate an intensive and continuous
building up of the lava pile between 9.7–8.7 Ma.
Their uppermost dated samples (Table 2) were col-
lected around a reverse-to-normal polarity reversal in
Mt. Sólheimafjall (profile PF) that has been corre-
lated to the boundary between the Sólheimar forma-
tion and the Merkidalur formation (Table 3) in Aust-
urdalur (Hjartarson 2003). This is the lowest reversal
in our composite section (recognized in TB, PH and
PG). Kristjánsson et al. (2004) have correlated this
reversal to the lowest one in their Eyjafjörður pro-
files and suggest that the overlying normal-polarity
zone (flows GL 17-29 in Gloppufjall, Öxnadalur) cor-
responds to the geomagnetic chron C4An. In the re-
cent compilation of Ogg and Smith (2004) this chron
is estimated to span the time interval 9.1–8.8 Ma ago,
and the preceding normal-polarity period spanned the
interval 9.3–9.4 Ma ago. We therefore suggest that the
base of our composite section does not reach 9.3 Ma
age, which is reasonable if the 550 m thick sequence
of reversely magnetized lavas below the reversal at
TB 59 (Table 3) was emplaced at the rate of almost
4 km/Ma inferred by Saemundsson et al. (1980).
Higher in the lava pile, a thick zone of mostly re-
verse polarities occurs in Eyjafjörður (in profiles SG
and KG of Kristjánsson et al. 2004) but does not have
a comparable counterpart in Skagafjörður. The corre-
lation of individual polarity zones between the cen-
tral and upper parts of these two areas, or further
afield, therefore becomes ambiguous and cannot be
attempted here.
Near the top of our composite section, the Skati
Dome and the Tinná Central Volcano were suggested
to be 6–8 million years old by Jóhannesson (1991) and
7–8 million years by Hjartarson et al. (1997) based
on stratigraphic and paleomagnetic correlations. No
hiatuses, or considerable gaps, were supposed to be
hidden in the strata pile below it and only a few mag-
netic subchrons were believed to be missing between
the well-dated magnetic reversal at the top of the Sól-
heimar formation (Table 3) and the Tinná Volcano.
The recently acquired Ar-Ar date of 5.2 million years
for the Skati Dome, given in Table 2, was therefore
much younger than expected. However, several un-
published dating results from the Upper Skagafjörður
valleys which point to a low age (Hjartarson et al.
50 JÖKULL No. 56