Jökull - 01.01.2009, Síða 42

Leó Kristjánsson
A list of all paleomagnetic directions appears in
Table 2 in a similar format as in previous publications
(e.g. Kristjánsson et al., 2003). A noteworthy aspect is
the consistently low values of directional uncertainty
(α95). This is mostly due to the excellent magnetic
stability of the primary remanence in the present col-
lection, but the number of samples collected per flow
and the number of demagnetization steps have also
been increased from some of the major previous col-
lections (e.g. McDougall et al., 1984).
NOTES ON THE STRATIGRAPHY OF
THE INDIVIDUAL PROFILES, WITH
PARTICULAR REFERENCE TO THEIR
REMANENCE DIRECTIONS
It has been attempted to fit the sampled profiles into
the overall column of J. Preston which is seen at the
right-hand side of Figure 3. For this, use has been
made of the stratigraphic position of his distinctive
units where the profiles have crossed these. The strati-
graphic heights of other profiles in the column have
been estimated from the dip and strike, aided by pre-
sumed paleomagnetic correlations.
TE: Exposures of the scoria-cone products or the
cumulate plagioclase basalts indicated in J. Preston’s
stratigraphic scheme of Figure 3 are not noted in this
profile, but it contains an outcrop of fossil-bearing
sediments between flows 36 and 37. The sediments
seem to have limited extent, but the ankaramite flow
TE 41 was traced by Preston across to Tálknafjörður
and along that fjord several kilometers to the south-
east. The normal-to-reverse polarity change taking
place just above the ankaramite will hopefully serve
as a useful marker in future mapping on the north
shore of Tálknafjörður.
KV, FD 0–20, HV: These profiles are assumed to
lie within the reverse-polarity zone beginning at the
top of profile TE. According to field mapping by stu-
dents in 2002, the top of the normal-polarity zone in
TE may occur in their two profiles on the coast be-
tween KV and HV. They also noted that flows HV
11–19 (not cored) may contain a geomagnetic excur-
sion similar to that in FD 16.
AC on the coast of Arnarfjörður consists of flow
units in the series called “cone basalts” by J. Preston.
These are also reversely magnetized but the bottom
six units sampled yield very similar mid-latitude ge-
omagnetic poles, indicating rapid buildup. The top
flow unit has a transitional direction.
FD 21–35, FH, HM, lower part of VR, TR/TS: It
seems very likely that the reverse-to-normal polarity
transition between FD 20 and 21 in Tálknafjörður is
the same as that between TR 3 and TS 1 farther east in
that fjord. The transition is also assumed to occur on
the north coast of Patreksfjörður between flows VR
1 and 2, and on its south coast between FH 4 and 5
as well as under HM 1. In all these locations, an un-
usual series of low- to mid-latitude geomagnetic poles
is seen above the reversal, namely in FD 22–27, TS 1–
4 or 7 (Figure 4a), VR 2–5, FH 6–14, and HM 2–5.
Upper part of VR, VS: A major apparent excur-
sion of the field occurs in flows VR 9–10 and VS 3–8
(Figure 4b), all yielding nearly identical geomagnetic
poles near the Equator. Again, this grouping indicates
rapid buildup of the lava pile at the time. Near the
top of the otherwise normally magnetized profile VS,
we find another geomagnetic excursion in VS 28–30,
the middle flow being very porphyritic (Figure 2). It
is probably what J. Preston’s maps (Kristjánsson et
al., 1975) refer to as the “Lower cumulate plagio-
clase basalt”, traceable both to the southeast and to the
northwest along north shore of Patreksfjörður across
into Tálknafjörður. It would clearly be interesting to
look for the upper excursion in the vicinity of other
outcrops of this cumulate basalt.
AI, AJ, AB 1–41, AK 1–16 and RF 1–2 in Arn-
arfjörður are assumed to belong to the same normal-
polarity zone as VS. However, they do not contain any
major excursions or very porphyritic basalts.
TW, TV and HP also are thought to belong to this
normal-polarity zone. Very similar low-latitude re-
verse directions occur in flows TW 14–15 and in HP
16, and judging from the strike of the lava pile it is
quite possible that these represent the same excursion.
Almost all the lava flows in both these profiles are of
a similar sparsely feldspar-porphyritic type. The ex-
cursion was not seen in TV. Flows PS 1–6 may lie at
the top of this zone.
42 JÖKULL No. 59