Jökull - 01.12.1992, Page 42

esson 1989). The vector sum R for the poles is 115.5,
giving a c.s.d. of 30.6°which is also similar to results
from other lava sequences in Iceland.
The arithmetic average remanence intensity for all
the lavas of Table 1, after 100 Oe demagnetization, is
4.1 A/m, compared to values in the range 2.8-3.6 A/m
for various other Upper Tertiary and Lower Quatemary
areas of Iceland. Magnetic susceptibilities average
some 2.5 x 10-3 cgs volume units.
POTASSIUM-ARGON DATING
Hand samples for dating were collected in 1976
from profiles TL,TM and TN by one of us (I. McD.).
The only samples to pass all selection criteria were
from five tholeiite flows and one porphyritic flow in
profiles TL and TN, at the upper and lower ends of
the composite section of Fig. 8. Whole-rock K-Ar
dates were obtained in 1977 at the Australian National
University. Sampling procedures, sample preparation
and dating techniques as well as the decay constants
used are the same as described in McDougall et al.
(1984). The weight percent of K in the samples aver-
aged about 0.4% which is somewhat higherthan in, for
example, the lavas dated by McDougall et al. (1984)
in NW-Iceland. The alteration state of the lavas, de-
scribed above, is comparable to that in many of the
lavas dated by Sæmundsson et al. (1980) and Mc-
Dougall et al. (1984). Radiogenic Ar contents are in
the range 26—45%.
The results (Table 2) are very satisfactory, both as
regards duplicate K and Ar determinations, and in the
relation of the age data to the local stratigraphy. The
ages fall into two distinct groups, the three lavas from
Bólstaðarhlíð (TL) giving an average age of 8.19 ±
0.18 million years (s.d.), and those from mt. Holta-
staðafjall (TN) giving 7.26 ± 0.17 million years.
CORRELATION OF THE POLARITY
COLUMN AND AGES
A map of polarities in the Langidalur profiles is
shown in Fig. 8. A single-lava normal "event" occurs
in flow KM 2 and also in flow TM 11 at a similar strati-
graphic level. If these are assumed to represent one
real polarity subchron, then there is a total number
of seven reversals in the sampled Langidalur pile in
0.9-1 million years. This is in agreement with the re-
versal rate found elsewhere in the Upper Cenozoic lava
pile of Iceland (Kristjánssonand McDougall 1982). It
should be kept in mind that some reversals may have
gone unrecorded by lavas in these profiles, especially
at the fourmain sedimentary beds shown in Fig. 8.
The rate of buildup in the lava pile where sampled
and dated by us is close to 1.0 km/ Ma which is of a
similar order of magnitude as average values found in
other Upper Miocene to Pleistocene regions of Iceland
(e.g.,McDougalletal., 1984). However, thisvalue has
quite wide error limits and will in general also be de-
pendent on whether the mapping involved takes place
at high or low altitudes. Hence, it should only be taken
as a rough guide to conditions applying locally rather
than having wide-ranging significance. By extrapola-
tion, the surface rocks at the syncline axis of Fig. 1
may be 6-7 M.y. old, as inferred for the Snaefellsnes
syncline by Jóhannesson (1980). It is therefore likely
that these synclines represent the same tectonic event.
Correlation with geomagnetic polarity time scales
derived from ocean-ridge anomalies still remains
largely guesswork, but on the basis of the raw data
it is most likely that the normal polarity zones in the
uppermost part of the composite section of Fig. 8 cor-
respond to the period 4 n of mostly normal polarity
placed by Cande and Kent (1992, Table 6) between
7.25 and 7.9 million years ago (previously known as
Epoch 7). The main reverse part of the profile, includ-
ing the normal subzone, would then correlate with the
younger part (or all) of the mostly reverse period 7.9
to 8.5 M.y. ago. The reverse flows above TN 77 may
represent the oldest part of a predominantly reverse
polarity period ending 7.0 million years ago.
As for using these polarities in stratigraphic cor-
relation with other regions of Iceland, the main re-
verse zone of our section may correlate with that of
profile L in Hamarsfjörður of E-Iceland (Watkins and
Walker, 1977), and with the reverse zone in the profiles
BT-BU on the east coast of the Vestfirdir peninsula
(McDougall et al., 1984). In the latter profiles, strati-
graphic and dating control is not very good due to poor
exposures and to alteration caused by the Prestbakki
central volcano.
40 JÖKULL, No. 42, 1992