Jökull - 01.12.1972, Page 71

The Stardalur Magnetic Anomaly, SW-Iceland
INGVAR B. FRI DL.EI FSSON,
DEPARTMENT OF GEOLOGY AND MINERALOGY UNIVERSITY OF OXFORD,
OXFORD, ENGLAND
LEÓ KR I STJÁNSSON , *
DEPARTMENT OF PHYSICS, MEMORIAL UNIVERSITY OF NEWFOUNDLAND,
ST. JOHN’S, NEWFOUNDLAND, CANADA
A15STRACT
Geological mapping at the site of major
magnetic, gravity and seismic travel-time ano-
malies near Stardalur, SW-Iceland, has demon-
strated the existence of a Qiiaternary central
volca.nic complex. The complex includes a
caldera of 6.5 hm size, a cone sheet swarm,
sills, plugs and a laccolithic body, all of which
are of Olduvai event age. A positive magnetic
anomaly of 4500 y at 700 m above ground,
essentially coincident with the caldera, is most
probably due to the norma.lly magnetized rocks,
in particular the basic intrusives witliin the
caldera.
A series of highly magnetic tholeiite lava
flows was penelrated by drilling at the center
of the highest maximum in the anomaly (79000
y) as delineated by a ground survey. Investiga-
tions on samples from the drill core shoiu that
their high remanence intensity (averaging 0.06
Gauss) is partly causecl by their unusually high
magnetite content and partly by an unusually
high ambient field strength at the time of th.eir
formation.
INTRODUCTION
The aeromagnetic survey oí’ Iceland by Sig-
urgeirsson (1970; pers. comra. 1972) has revealed
several distinctly localized magnetic anomalies.
The strongest of these is a positive one located
at Stardalur, about 20 km NE of Reykjavík,
and a few km NW of the Langjökull-Reykja-
nes volcanic zone. The Stardalur magnetic ano-
maly is limited to a slightly oval shaped area
8—10 km across and amplitucle 4500 y at 700
m altitude above ground. Subsequent ground
surveys (Kristjánsson 1970; Búason 1971) indi-
cated that the magnetic anomaly is composed of
two parts. First, there is an anomaly of dimen-
sions 7 krn by 5000 y; second, there are a few
sharp local maxima superimposed on it, the
highest one reaching 79000 y at ground level
(regional: 51700 y).
From preliminary ground survey data Krist-
jánsson (1970) computed the dimensions of a
possible single body causing the highest maxim-
um as 200 by 600 m, striking NE, with an
estimated upper surface at the depth of 50—
70 m, ancl a total magnetic intensity (remanent
plus induced) of 0.05—0.06 Gauss. A detailed
survey and analytical treatment by Búason
(1971) yielded a total magnetization of 0.08
Gauss and slightly different dimensions.
GEOLOGICAL SETTING
The rocks immediately surrounding and to
the west of Stardalur are of Quaternary age,
with thick successions of basaltic lava flows
intercalated, at intervals, with hyaloclastite
ridges, morainic horizons and glacial tillites cor-
responding to several glacial periods (Frid-
leifsson 1973). 'The regional dip of the rocks
is southeasterly with the oldest rocks outcropp-
ing on the coast furthest to the west and the
rocks becoming progressively younger east-
wards. The regular piles of basaltic lava flows
and the less regular hyaloclastite intercalations
*) Present address: Science Institute, Uni-
versity of Iceland, Dunhaga 3, Reykjavík, Ice-
land.
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