Jökull - 01.12.1972, Page 73

are intruded by basaltic dykes, and large sheets
represent localised intrusive centres.
In Stardalur a central volcanic complex is
exposed. The main features of the complex are
a caldera, a cone sheet swarm, several intrusive
sheets and sills, and a large laccolithic intru-
sion (Fig. 1).
The calclera is almost circular and about 6.5
km in diameter. The northern margin of the
caldera is clearly marked by a zone (a few tens
to 200—300 m in width) of intensive faulting
and brecciation of the rocks. There is a marked
change in dip, the rocks within the caldera
rim dipping steeply towards the centre of the
caldera in a markedly regular fashion, and the
rocks outside the rim dipping away from the
caldera or showing the regional southeasterly
dip. The southern half of the caldera is cover-
ed by younger formations. Hydrothermal activi-
ty in the area has been most intense along the
fractured zone of the caldera rim, and has
altered the rocks severely.
At an early stage of the caldera a lake has
formed within it. Then, due to intense vol-
canic activity within the caldera and on its
rim, the caldera became filled with volcanic
Fig. 1. Geological map of the Stardalur cald-
era, showing palaeomagnetic polarities. The
polarity units are numbered as in EÁnarsson
(1957). The normal N-1 unit is from the pre-
sent Brunhes epoch, the reversed units R-l,
R-2 and R-3 are from the Matuyama epoch.
The normal N-2 and N-3 units are most likely
from the Gilsá and Olduvai events respectively
(Friðleifsson 1973). The term basic hyaloclas-
tites includes here subaquatic volcanic rocks
and detrital beds. — The magnetic intensity pro-
file is drawn lrom the results of an unpublish-
ed survey at 360 m altitude, kindly provided
by Prof. Th. Sigurgeirsson.
Mynd 1. Jarðfrœði- og segulskeiðakort af Star-
dalsöskjunni. Tölasetnijigu Trausta Einarsson-
ar (1957) d segulskeiðunum er haldið, en N-1
er Brunhes-skeiðið, R-l, R-2 og R-3 eru hlut.ar
Matuyama-skeiðsins, og N-2 og N-3 eru að öll-
um líkindum Gilsár- og Olduvai-skeiðin (Frið-
leifsson 1973). Segulsviðssnið er fengið frá Þor-
birni Sigurgeirssyni og sýnir heildarsviðsstyrk
i 360 m hœð.
Fig. 2. Index map, showing the location of
samples collected outside the Stardalur caldera
and listed in Table 1. Hatched: area of Fig. 1.
Mynd 2. Afstöðumynd af Stardal og ýmsum
öðrum stöðum, þar sem sýnum var safnað til
bergsegulmcelinga.
products, mainly basaltic pillow lavas and tuffs.
The intra-caldera filling was subsequently in-
truded by an intense swarm of basaltic cone
slieets (each I—10 m thick), larger sheets and
sills. Several parasitic plugs can be seen on and
outside the caldera rim. The last intrusive phase
gave rise to a laccolithic body, 2.3 km long
and 1 krn wide.
The cone sheets have a range in dip from
20—50°, and an average dip of about 35°. A
downward projection of accurate dip measure-
ments of the cone sheets indicates the upper
margin of a more or less continuous layer of
intrusions at a depth of 600—700 m in the
central and eastern part of the caldera. Seismic
measurements (Pálmason 1971) indicate crustal
layer 3 at a depth of 500—600 m in this area.
This is the shallowest depth to layer 3 record-
ed so far in Iceland. The cone sheets and the
larger intrusions in the area strongly suggest
that layer 3 is formed rnostly of basic intru-
sions. A positive gravity anomaly (Einarsso?i
1954) is associated with the intrusions within
the caldera.
THE BROAD MAGNETIC ANOMALY
Palaeomagnetic polarity mapping of the Star-
dalur area (Fridleifsson 1973) has shown that
the intra-caldera filling as well as the cone
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