Jökull - 01.12.1971, Page 55

Magnetic Polarity Groups in the Fljótsdalsheidi Area,
including Gilsá.
TRAUSTI E I NARSSON
DEPARTMENT OF ENGINEERING AND SCIENCE, AND SCIENCE INSTITUTE, UNIVERSITY OF ICELAND
The accompanying map of magnetic polarity
groups is based on field measurements with the
compass method (cf. Einarsson, 1957 a and b),
done in 1956. The westernmost part of the
region covered by the map was studied jointly
by the author, Th. Sigurgeirsson, and G. P. L.
Walker, while the Gilsá area, and the ESE-part
(Fljótsdalur) was done by the author.
This area is particularly interesting, because
here we have a clear picture of the youngest
polarity groups found in this part of Eastern
Iceland, and we can show that there is a great
erosional uncpnformity (peneplanation) between
the two or three topmost polarity groups, on
one hand, and the uninterrupted series of the
underlying plateau basalts groups, on the other.
Further, Wensink (1964, 1966) worked later in
this area, and the dating of collected material
by McDougall (McDougall and Wensink, 1966)
led to the introduction of the Gilsá event.
Finally, Dagley et al. (1967), making a com-
plete polarity section of the East Iceland basalts,
used Fljótsdalur sections as the top of their
column. Their sections Q, R, S, T, U, V are
shown in the present map, and a direct com-
parison with my measurements is made.
The sections T, S, and R are in full agree-
ment with the map. In Q there is a difference
of thickness of groups. The paleomagnetic map
given by Wensink (1966) disagrees with both
studies.
The Fljótsdalsheidi area, between the rivers
Lagarfljót and Jökulsá, consists of plateau
basalts with a dip of 6—11°W in the east, but
the dip decreases markedly towards west and
is 2° at the foot of Snæfell. In the east, indi-
vidual lavas and lavagroups can easily be trac-
ed over considerable distances, as shown in the
map. With the aid of these, and the dip and
strike of the lavas in the area of the sections
Q—U, it is possible to extend approximately
the magnetic groups in these sections into the
Jökulsá-Gilsá area. From this connection it
appears that the reverse group in the middle
of T (Kleif) corresponds to the reverse group
at the lower Gilsá-river, or very nearly so.
The present surface of Fljótsdalsheidi is a
600—700 m high, nearly horizontal erosion
plain. On this plain (in the west) is a number
of separate heaps and mountains of more or
lcss glassy volcanic breccias (“palagonite“ brec-
cias). Some of these heaps consist of reversely
magnetized roc.ks, others of normally magnetiz-
ed ones. At the base of two of the reverse
heaps, Eiríksstadahneflar and Fjallkollur, there
is a thin normal group, about which it is un-
certain whether it rests on the erosion plain
or whether it belongs to a pre-erosion plain
group. These outcrops are too small to decide
upon this by the faint dip of the plateau
groups in this area.
One of the main mountains is Snæfell, of
normal polarity, at the base of which is a fresh
lava of reverse polarity. The plateau basalts
dive with a dip of 2° under Snæfell and they
are seen with this dip under the horizontal
erosion plain on which the scattered heaps of
the younger “irregular” rocks rest. From this it
is clear that between the last two or three
magnetic groups, on one h.and, and the plateau
basalts of the Fljótsdalsheidi area, on the other,
there is atn unconformity — a time interval,
during which the plateau basalts were tilted
and peneplained. The Fljótsdalsheidi is only
one of such basalt areas in Iceland, where
peneplanation can clearly be demonstrated.
The present author has pointed this out in a
number of papers (Einarsson, 1960, 1962, 1967,
JÖKULL 21. ÁR 53