Jökull - 01.12.1991, Page 50

Fig. 1. Geological setting of the Mosfellssveit region.
Circles are extinct central volcanoes and the dotted
zone represents the active Krísuvík fissure swarm. -
Megindrœttir í jarðfrœði Mosfellssveitar.
extended, in an attempt to construct a composite sec-
tion from the Gufunes peninsula to the Hafrahlíð hill
and to correlate the area with the above-mentioned
study in Mt. Esja. Uncertainties in the mapping dur-
ing its initial stages have resulted in considerable
stratigraphic overlap between some of the profile
segments we sampled.
GEOLOGICAL SETTING
The Mosfellssveit area (approx. position 64.15°N,
21.7°W) south of Mt. Esja is adjacent to the
Reykjanes-Langjökull active rift zone. The geology
of the area is dominated by three extinct central vol-
canoes of Gauss to Lower Matuyama (i.e. about
3.0-1.8 M.y.) age. They are named Kjalarnes, Viðey
and Stardalur volcanoes (Fig.l). The Kjalarnes and
Stardalur volcanoes have been described by Friðleif-
sson (1973, 1985) and the Viðey volcano by
Jóhannesson (1985). Each of these was associated
with a NE-SW trending swarm of dykes, faults and
fissures.
The succession consists predominantly of series of
interglacial lava flows and subglacial hyaloclastites,
intercalated with minor tillites and detrital beds. The
lavas are generally thin, of the order of 5 m as com-
pared with an average of 12 m in older parts of
Iceland (Walker 1959). This may reflect the proxim-
ity of the Mosfellssveit area to the source volcanoes.
The lava pile has been tilted towards the southeast,
i.e. towards the present volcanic zone. The amount of
dip is quite variable, from a few degrees up to about
30. Superimposed on this old tectonics is a more
recent tectonic activity associated with the Krísuvík
fissure swarm (Fig. 1) which stretches towards NE
from the active Krísuvík volcanic system. The trend
of the recent faults and fissures is more or less the
same as that of the older faults.
The most common secondary mineral in the suc-
cession is chabazite, but analcime also occurs along
with chalcedony, opal, calcite and clay minerals.
Scolecite was not found during our field work.
The original pile has been heavily eroded by later
glaciations, which have left a landscape of wide val-
leys and low (< 300 m) hills. Parts of the area are
largely covered by interglacial lava flows of Late
Brunhes age, which were not studied here, and by
postglacial detritus. Continuous profiles are accessi-
ble only in sections along the coast, in streams and in
cliffs. The poor exposures and the large number of
normal faults make detailed correlations difficult.
THE MAPPED SUCCESSION AND ITS
CORRELATION WITH MT. ESJA
Twelve profiles were mapped and sampled for this
study (Figs. 2, 3). Their cumulative thickness
between the Gufunes promontory and the top of the
Hafrahlíð hill is about 800 m.
The lava flows were classified according to
Walker's (1959) classification system into four types,
i.e. olivine basalt, porphyric basalt, tholeiite and
compound flows. It should be noted that this system
is based only on field observations of lava lithology :
petrographic or geochemical analyses were not made
in the present study. No units of acidic or intermedi-
ate composition were noted in our profiles, but boul-
ders of rhyolite in unit UA 19 in Úlfarsfell are
thought to be derived from the subglacial rhyolites
erupted on both the northern and southern rim of the
48 JÖKULL,No. 41, 1991