Jökull - 01.12.1990, Page 9

ite composition. This section contains four diamictite
units (unit 5-8) which appear between the lava flows.
The third acid phase coincides with a magnetic rever-
sal from normal to reversed polarity. The entire group
ts of reversed polarity and comprises very distinct an-
desitic and rhyolitic flows besides acid hyaloclastite
and numerous acid and composite plugs and dykes.
The end of this acid phase is marked by diamictite
unit 9 where diamictite interfingers with hyaloclastite.
At the end of the third phase of acid volcanism, the
volcano stood above its surroundings for some time.
There are, however, indications that it became buried,
only to have its enveloping lavas totally removed dur-
>ng a period of erosion. The rocks that are now seen to
cover the volcano overlie an erosional unconformity
representing a hiatus of 300,000 to 600,000 years dura-
Oon (Sæmundsson and Noll, 1974; Albertsson, 1976).
Diamictite unit 10 is exposed above the unconformity.
Seven of the Borgarfjörður section diamictites
were deposited within the life-span of the central vol-
cano (ca. 2.9-2.4 Ma). Two are stratigraphically older
(> 2.9 Ma), and one diamictite horizon is younger
(< 1.8 Ma).
THE stratigraphy of the hvalfjörður
area
Hvalfjörður lies about 60 km southwest of the up-
Per Borgarfjörðurarea (Fig. 3). Regional stratigraphic
mapping in the area was initiatedby Einarsson (1957),
who employed field measurements of remanence po-
larity of the lavas to distinguish and correlate age
groups. The geological mapping north of the Hval-
fjörður was initiated by Franzson (1978), whereas the
urea south of the fjord was mapped by students at
the University of Iceland under Sæmundsson’s super-
vtsion (Gunnlaugsson et al., 1972; Jónasson et al.,
1973; Arason et al., 1974; Guðmundsson, 1976).
Detailed geologic mapping and magnetic measure-
ments were made in the Hvalfjörður region from the
mountain Esja on the south side of the Hvalfjörður
fjord to the mountain Akrafjall on the north side. This
enabled correlation of the lava succession on each side
ofthe fjord (Kristjánsson etal., 1980). Paleomagnetic
results confirmed the magnetic polarity stratigraphy
°f Einarsson (1957). The correlation implies that the
1000 m section spans the interval from the Mammoth
event (3.1 Ma) to strata younger than the Olduvai event
(1.8 Ma). Two K/Ar age determinations were made
for this study, on the lava flows enveloping the low-
est diamictite unit on top of the mountain Akrafjall.
The lava flow underlying the diamictite yielded an
age of 3.12± 0.23 Ma and the lava flow on top of the
diamictite 2.87 ± 0.23 Ma (Table I).
Stratigraphic interpretation suggests continuous
volcanic activity for over 1 m.y. with two central vol-
canoes active during the deposition of the strata. The
lowest part of the Hvalfjörður section corresponds to
the initial eruptions of the Hvalfjörðurcentral volcano
(Friðleifsson, 1985).
Above the basal 300 m of the section, the charac-
ter of the volcanic pile changes from a relatively uni-
form lava sequence to diamictite units and thick hyalo-
clastite sequences separated by tholeiitic and olivine
tholeiitic lava flows.
The Hvalfjörður section includes 10 major diamic-
tite and or hyaloclastite units. The lowest five are
interbedded within lavas deposited during the Gauss
magnetic polarity epoch (ca. 3.1-2.48 Ma) and the
upper five are all within lavas formed during the
Matuyama epoch (2.48-1.8 Ma). Unlike the Borg-
arfjörður section most of the interbeds are associated
with hyaloclastites.
PREVIOUS WORK ON THE DIAMICTITE UNITS
The sedimentology of the Borgarfjörður and Hval-
fjörður sections has recently been studied by Geirs-
dóttir (1988). Previously, little sedimentologic work
had been done on the interlava sediments. Although
the Borgarfjörður section has a much better defined
stratigraphy than the Hvalfjörður section, the interbed-
ded sediments of the latter have received much more
attention.
Sæmundsson and Noll (1974) described a total of 9
diamictite units from the upper Borgarfjörður section
and interpreted them as tillites. Each tillite horizon
was thought to mark a distinct episode of glaciation.
The oldest tillite unit was thought to be 3.1 Ma in
age, and the overlying seven glacial horizons suppos-
edly formed within the following 0.7 Ma (Sæmunds-
son and Noll, 1974; McDougall et al., 1977). Also
JÖKULL, No. 40, 1990 7