Jökull - 01.12.2006, Page 53

A paleomagnetic study of stratigraphic relations in lava pile
Table 3. Polarity zones in a composite stratigraphic section in Norðurárdalur-Austurdalur, based on Figure 3. –
Samantekt segulstefna í staflanum.
No. of flows Thickness, m Polarity Note
TD 2-21, TB 4-59 80 550 R Sólheimar formation
TB 59A-81 23 220 N base of Merkidalur fm.
PG 33-39 7 65 R
PG 40 1 5 N
PG 41-44 4 30 R
Sedimentary rocks 50 Merkidalur sediment
PM 1-24 25 330 N w/some non-exposures
PN 1-6 6 100 R largely sedimentary rocks
PO 4-22 19 220 N
Sedimentary rocks 30 w/Skati acid tephra
PO 23-28 6 50 R
PO 29-35 7 70 N Thvera subchron
The entire profile PH is left out, as well as other profile parts which overlap with the above. As the thicknesses of polarity
zones may show considerable variation between profiles, they should not be taken too literally.
Table 4. Comparison of the composite lava sections of Skagafjörður and Eyjafjörður. – Samanburður á sam-
settum jarðlagasniðum.
Thickness Time span Max. age Min. age Reversals Accumulation
m Ma rate m/Ma
Skagafjörður 1700 4.5 9.5 5.0 9 380
Eyjafjörður 2900 4.4 9.2 4.8 17 660
2003), would support the above-mentioned date for
the Skati dome.
According to Ogg and Smith (2004) the geomag-
netic field had normal polarity during the Thvera sub-
chron 5.23–5.00 Ma ago whereas the Skati Dome is
reversely magnetized and overlain by several reverse
flows (Hjartarson 2005, Figure 7, cf. our PO profile).
If the Skati date is not misleading, the Skati Dome and
its rhyolite tuff were most likely emplaced during the
rather long interval of reverse polarity preceding the
Thvera subchron. This interval, i.e. the oldest part of
the Gilbert chron, began 6.03 Ma ago (Ogg and Smith
2004) and our suggested age for the Skati Dome is
therefore around 5.5 Ma. The uppermost 6-8 flows of
the PO profile and of our composite section may ac-
cordingly represent the Thvera subchron and have an
age slightly in excess of 5 Ma.
NUMBER OF REVERSALS AND
COMPARISON WITH EYJAFJÖRÐUR
PROFILES. POSSIBLE
UNCONFORMITIES
A total of 10 geomagnetic polarity zones are seen in
the Skagafjörður lava pile (Table 4) presumably em-
placed between approximately 9.3 and 5.1 Ma ago,
while Ogg and Smith (2004) list about 20 reversals
in that period. The composite section of Kristjánsson
et al. (2004) in Eyjafjörður which covers about the
same time interval is considerably thicker than that in
Skagafjörður and has recorded many more polarity re-
versals. The reasons for this difference are unknown:
the build-up in Skagafjörður may have been slower
or more episodic (especially above the Sólheimar for-
mation), or it may have suffered heavier erosion than
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