Jökull - 01.01.2005, Blaðsíða 31

Stratigraphy and paleomagnetism of Suðurdalur, Eastern Iceland
Table 3. Inferred correlations between paleomagnetic polarity zones in Suðurdalur and Norðurdalur. – Áætlaðar
tengingar milli Suðurdals og Norðurdals byggðar á segulstefnum.
Pol. Suðurdalur Thickness, m Corresponding zone in Norðurdalur
R VV 1 – 14 330 Q 1-6 (top part of the polarity zone)
N VV 15–16 25 Q 7–8
R VS 3–6, ST 1B–3 90 Q 9–12
N ST 4–6, MK 3–11 130 Q 13–20 (including thin R zone)
R MK 12–30 200 R 12–24
N KO 9–26, MK 31–48 310 S 1–17E (TE porphyritic series)
R KO 27–39 110 S 18–26
N KO 40–43 65 S 27–31
SUMMARY AND DISCUSSION
This paper presents an overview of the basement ge-
ology of the southern Fljótsdalur area. This strati-
graphic sequence which is estimated to cover the
period from 7 Ma ago, overlies the flexure zone
through northeastern and eastern Iceland pointed out
by Walker (1974) and other writers. We describe
briefly 11 distinct suites, mostly of basalt lava flows,
which constitute the major part (1.6 km, see Figure 7)
of the geological succession from Víðivellir ytri up to
Snæfell. The lavas have a westerly dip and we make
use of their petrographic characteristics for correla-
tions across Suðurdalur, Múli and Norðurdalur. The
correlations are aided by the the presence of clastic
sediment layers which reach tens of m or more in
thickness, and by reversals of magnetic polarity.
We sampled 76 lava sites for laboratory mea-
surements of characteristic remanence directions and
some other magnetic properties. The eight main po-
larity zones in the lower part of the sequence covering
the estimated interval 7 to 4 Ma ago, are as shown in
Table 3. They represent a pile of 1.2–1.3 km thickness
(including sediments) emplaced during a period of 3
million years. A higher rate of reversals is generally
found in studies covering comparable time intervals
in other areas of Iceland (e.g., Table 1b of Kristjans-
son and McDougall, 1982). It may be concluded that
the buildup of the lava pile in the Fljótsdalur area has
been rather slow, and that several geomagnetic polar-
ity chrons are not recorded due to hiatuses or erosion.
This makes any correlations of the polarity zones with
individual chrons of the geomagnetic polarity time
scale (Figure 7) very tentative.
Many aspects of the geological history of the area
remain to be studied in more detail, such as the ac-
tivity of volcanic centers contributing to its genesis,
the geochemistry of the volcanics, and the character
of the various sedimentary layers. Especially how-
ever, additional radiometric dates are needed; those
available from lavas older than 1 Ma (in Hengifossá,
Bessastaðaá and the west side of Norðurdalur) were
published a quarter-century or more ago.
ÁGRIP
Kortlagning jarðlaga í Suðurdal Fljótsdals og
nágrenni hans, ásamt segulstefnumælingum í
nokkrum sniðum
Um 1,8 km þykkur jarðlagastafli er í sunnanverðum
Fljótsdal, mælt frá Víðivöllum ytri austanmegin
og Bessastöðum vestanmegin upp að Snæfelli. Í
greininni er sagt frá fyrri rannsóknum á þessum stafla,
m.a. aldursgreiningum og öðrum gögnum sem má
nota til að áætla að hann hafi orðið til á tímabilinu frá
því fyrir um 6,5–7 milljón árum þar til fyrir innan við
1 milljón ára síðan. Aðgreina má í staflanum 14 syrp-
ur jarðlaga með mismunandi bergfræðileg sérkenni,
JÖKULL No. 55 31