Jökull - 01.01.2005, Side 29

Stratigraphy and paleomagnetism of Suðurdalur, Eastern Iceland
The remanence intensity after 10 mT treatment
has an arithmetic average of 4.7 Am−1, whereas av-
erages of 3–4 Am−1 have been found in several pre-
vious studies of paleomagnetism in Tertiary Icelandic
lavas. The difference is partly due to unusually high
intensity values occurring in the two sites MK 41 and
MM 2.
Application of the magnetic directions in strati-
graphic correlation
A composite polarity column in Suðurdalur may be
constructed from the following flows: VV 1 to 16, VS
3 to 6, ST 1B to 6, MK 3 to 48, and KO from 9 up-
wards. This column corresponds satisfactorily with
that from Norðurdalur which is shown in Figure 7.
The figure also illustrates the stratigraphic succession
of the lava suites described above.
The flows VV 1 to 14 are reversely magnetized,
although the virtual pole is not far from the Equator
in VV 8 and 10. VV 15–20 consist of three thin po-
larity zones. The pole positions in VV 17 and VS 3
are similar and quite unusual (west of South America
in Figure 6). VS 6 has a remanence direction close to
that in ST 1 and 1A. The reverse flows Q 1–6A (Mc-
Dougall et al., 1976) may correspond to the upper part
of VV 1–14.
Flows MM 1 and MM 2 have similar reverse di-
rections as ST 1,1A and ST 2 respectively, and the
pole from MK 1 (in S-Australia in Figure 6) may be
from the same R to N transition as ST 2,3. This agrees
with the correlation of Guðmundsson (1978) that the
sediment overlyingMK 3 is the same as that overlying
ST 8.
In profile MK the polarity changes from reverse to
normal above the predominantly tholeiitic series MK
12 to 30. The normal-polarity feldspar-porphyritic se-
ries which begins at MK 31, reaches to well beyond
the last cored flowMK 41 according to field measure-
ments and it continues in profile KS up to KS 25 or
26. Guðmundsson (1978) has traced these series to
both sides of the Norðurdalur valley.
The reverse-polarity mostly tholeiitic lava group
Q 21 to 32 in Bessastaðaá (McDougall et al., 1976),
i.e. profile 3 of Guðmundsson (1978), is expected to
be the same as our MK 12 to 30. Near this polar-
ity boundary the petrography also changes from be-
ing mostly tholeiites to mostly feldsparphyricflows in
Bessastaðaá, as well as at a 10–20 m unexposed inter-
val between flows 24 and 25 in Watkins and Walker’s
(1977) profile R of Figure 1 (profile 5 of Guðmunds-
son, 1978). The normal-polarity porphyritic series in
profiles Q and R comprises at least 12–15 flows or
flow units, as in MK.
Although it is a tenuous attempt at correlation,
we note that sites MK 39, 40 have similar mid-
latitude VGPs as the compound flow R 33. Accord-
ing to Guðmundsson, the reverse-to-normal boundary
should also occur at the base of Watkins and Walker’s
(1977) profile S. The normal-polarity zone S1 to 17E
which is mostly composed of porphyritic lavas but in-
cludes some tholeiite ones, has the above-mentioned
mid-latitude poles in S 13D to 15.
The K-Ar dates of McDougall et al. (1976) in pro-
file Q and the Ar-Ar dates of Mussett et al. (1980) in
Q through V are somewhat scattered and stratigraph-
ically inconsistent. The three versions of the Ar-Ar
dates (isochron, plateau, and total fusion ages) are
also considerably different in some cases. McDougall
et al. (1976) and Watkins and Walker (1977) suggest
that the reverse polarity zone Q 21–32 ( = R 12–24 =
MK 12–30) corresponds to the lowermost part of the
Gilbert epoch, which is of 5.23–5.89 Ma age accord-
ing to Cande and Kent (1995). The normally magne-
tized flows Q 33–43 (or higher), R 25–33E (or higher)
and S 1–17 E in Norðurdalur as well as MK 31–50 and
the bottom part of KO in Suðurdalur would similarly
belong to the Thvera subchron C3n.4n of 4.98–5.23
Ma age. This correlation agrees fairly well with the
dates obtained by McDougall et al. (1976) on lavas in
the upper part of the extended Q profile, which may
be expected to be among the least altered material in
that age range in Fljótsdalur. The reverse flows KO
27–39 and associated sediments may correlate with
S 18–26, and KO 40–43 with S 27–31, emplaced in
the Nunivak or (cf. Watkins and Walker, 1977) in the
Cochiti subchron with an upper boundary at 4.2 Ma.
The Suðurdalur-Norðurdalur paleomagnetic correla-
tions are summarised in Table 3.
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