Náttúrufræðingurinn - 1985, Page 53

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and magnetic surveys in Iceland. —
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SUMMARY
The composite dyke on
Streitishvarf, Breiðdalsvík,
Eastern Iceland.
by
Agúst Guðmundsson,
Menntaskólinn við Sund,
104 Reykjavík
The composite dyke on the point Streitishvarf
is described and its origin is discussed. The dyke
is at least 14 km long (Fig. 1), but both its ends
are uncertain: the south end is under the sea in
Berufjörður (Fig. 2), the north end is in a
mountain north of Stöðvarfjörður (fjörður =
fjord). The petrology of the dyke is described by
Guppy and Hawkes (1925) and by Gunn and
Watkins (1969).
At the south end of Streitishvarf the dyke is
about 30 m thick, the rhyolite part being 7.5 m
thick (Fig. 2). The thickness decreases some-
what along Streitishvarf, but the strike, N20°E,
remains the same. The dip of the dyke is 74°E.
The rhyolite is light grey and contains a number
of basaltic xenoliths (Fig. 3), all of which have
roundish edges. Near the northern end of
Streitishvarf the dyke is very similar, but the
rhyolite part is proportionally thicker, being 13
m thick of a total of 26 m (Fig. 4).
On the north side of thc cove Breiðdalsvík the
dyke follows a small gully up to the top of the
mountain Lambafell (Fig. 5). The exposure in
the gully is almost continuous right up to the top
of the mountain (Fig. 6). At the entrance to the
gully the dyke is about 11 m thick and is com-
posed of two basalt parts, 1.5 m and 2 m thick,
and two rhyolite parts, 6 m and 1.5 m thick.
Little to the north in the gully, the dyke consists
of seven parts, four basalt parts and three
rhyolite parts, the total thickness being 13 m
(Fig. 8). Basaltic xenoliths are common but
more angular than those in the exposures on
Streitishvarf. According to Guppy and Hawkes
(1925), the basaltic xenoliths form 10-20 per-
cent of the rhyolite part of the dyke. The dyke is
vertical in this gully, and it does not occupy a
dip-slip fault; nor was any evidence found for
strike-slip movement along the dyke.
At the altitude of about 440 m the dyke meets
a 120 m thick basaltic sill (Guppy and Hawkes
1925). The sill has a very beautiful columnar
structure (Fig. 9). The sill dissects the dyke and
is therefore younger than the dyke. Above the
sill the rhyolite forms the whole of the dyke, so
that the basalt parts must end where the sill and
the dyke intersect. At the top of the mountain the
dyke is about 35 m thick and forms a narrow
ridge (Fig. 10). The rhyolite at the top contains
basaltic xenoliths, but they are not as conspi-
cuous as in the lower sections in the gully. The
rhyolite is mostly without vesicles.
On the south coast of Stöðvarfjörður the dyke
is found in a poor exposure near a small rill. The
total thickness is about 18 m, the rhyolite part
being 15 m thick. The basaltic xenoliths are
much smaller than in the exposures further to
the south, and many of them have been weath-
ered away. The direction from this exposure to
the exposure on the north coast of Stöðvar-
fjörður is N20°E, so that the dyke curves some-
what to the north in Stöðvarfjörður.
On the north coast of Stöðvarfjörður the dyke
is composed of two parts. The eastern part is
rhyolite, very similar to the rhyolite in the other
exposures. It contains basaltic xenoliths, but
many of them have been weathered away. This
part is 7 m thick. The western part is brown to
gray, it has xenoliths, most of which have been
weathered away, and is in places similar to
scoria. This part is 15 m thick, so that together
the parts are 22 m thick. No basalt dykes were
found in this exposure.
The northernmost outcrop of the dyke is in a
hill north of the main road in the area. The dyke
is seen in a rill and its visible thickness is only 2
m. However, rhyolite is found in the nearby
scree, thus the real thickness of the dyke is at least
147