Jökull - 01.01.2010, Blaðsíða 160

L. Kristjánsson
Figure 9. Angular standard devia-
tion (a.s.d.) values for collections
of virtual magnetic poles from
Icelandic lavas, approximated by
the parameter δ of Wilson (1959,
Appendix). Each horizontal line
indicates a single survey (or a few
smaller lava collections of simi-
lar ages combined, including the
present one) on 300–500 lavas
of the basalt lava pile, carried
out since 1972 (5139 in total).
In more than half of the lavas,
at least four samples were col-
lected from each. The 95% con-
fidence limits for the a.s.d. val-
ues do not exceed ±2◦. Each age
range is uncertain by at least 0.5
m.y. at either end. – Á lóðrétta
ás teikningarinnar er mælikvarði
á flökt segulskautsins frá meðal-
staðsetningu þess. Þetta er reikn-
að fyrir syrpur hraunlaga af svip-
uðum aldri í mismunandi lands-
hlutum, 300–500 í hóp. Aldurs-
óvissa á endum hverrar línu er
a.m.k. 0,5 milljón ár.
be traced for over 2 km from the Stóra-Sauðafell area
(Figures 4 and 6) across the Laxá river to Stóröxl.
Here, the reverse zone contains rhyolitic/andesitic ma-
terial seen in profiles KP and KO. It is probably older
than the normal-polarity Móskarðshnjúkar rhyolite 7
km west of Stóra-Sauðafell which has been K-Ar
dated at about 1.9 Ma (Kristjánsson et al., 1980).
The results presented above demonstrate the oc-
currence of three normal-polarity zones FO 20–21,
25–32, 36B–37 and the base of a fourth zone FO 39–
40 in an otherwise reversely magnetized sequence ex-
pected to date from the Matuyama chron. Further
radiometric (40Ar–39Ar) age determinations of these
geomagnetic events and some of the main sediment
sequences of Figure 8 would greatly aid in under-
standing the buildup of the Hreppar formation. While
it is shown here that the paleomagnetic method is use-
ful in detecting repetition of strata due to faults cross-
ing the sampled profiles, additional mapping in the
area is needed in order to exclude the possibility of
stratigraphic confusion caused by landscape effects
during the period of active volcanism and by subse-
quent tectonic movements.
The present results confirm previous observations
by the author regarding a long-term decrease in the
scatter of virtual geomagnetic poles. One possible
cause of this decrease is that reversal transitions may
each have taken less time (compared to chron lengths)
in the Pleistocene than in the period 3–15 Ma ago.
Detailed transitional pole paths as found at the R3–
N3 boundary (Sigurgeirsson, 1957; Kristjánsson and
Sigurgeirsson, 1993) are therefore a rare occurrence
in the lava pile of Iceland, and attempts to locate them
should concentrate on rapidly emplaced pahoehoe se-
ries of "flow units".
160 JÖKULL No. 60