Jökull - 01.12.1991, Blaðsíða 55

22 to correlate with the lower and niiddle parts of our
Mosfellssveit section. There are indications of three
glaciations (no. 9,10 and 11) in this part of Esja. Five
glacial horizons are found in the Mosfellssveit sec-
tion, two within the parts which we correlate to
Kristjánsson et al.'s (1980) Esja section and three in
the topmost part which is younger than the Esja suc-
cession.
The glacial horizons may either occur as hyalo-
clastite series of subglacial volcanic origin and/or as
tillite beds. We correlate the Gufunes hyaloclastite
with the glacial horizon 9 of Kristjánsson et al.
(1980) and Esja Unit 18 which forms the base of the
Stardalur central volcano (Friðleifsson 1973). The
base of the series is not exposed and thus no tillite
bed is observed. The tillite bed between lavas GB 8
and 9 corresponds presumably to glacial horizon 10
in Esja. The tillite bed resting on flow UE 8B and the
Lágafell hyaloclastite seems to have no counterpart
in Esja other than two conglomerate beds (SB 46/49
and SB 61/62) which Kristjánsson et al. (1980) sug-
gested may represent separate glaciations. The tillite
bed seen in profiles UL and UA at the base of the
upper normal magnetic event corresponds to glacial
horizon 11 in Esja. Rhyolite boulders in the tillite are
thought to correspond to rhyolite hyaloclastites erupt-
ed subglacially in Móskarðshnúkar and Grím-
mannsfell during glaciation 11 of Esja (Friðleifsson
1973, Kristjánsson et al. 1980, Friðleifsson 1985).
Two apparently separate glacial horizons occur with-
in the Stórihnúkur series, one at the base of the lay-
ered silt beds and the other as a hyaloclastite unit at
its top. These two horizons may represent one glacial
advance with a shorter readvance. The uppermost
glacial horizon is the Hafrahlíð hyaloclastite and
underlying tillite.
PALEOMAGNETIC METHODS
2.5-cm core samples were collected using portable
drills and oriented as described by Kristjánsson et al.
(1980). All profiles are numbered upwards from the
base; the suffixes A, B etc. indicate outcrops which
were located after the initial mapping and which may
represent separate units. Some thin, inaccessible,
poorly exposed, or crumbly flows were not sampled.
In the first profile (UL), two cores were generally
taken from each unit and AF-demagnetized at a sin-
gle peak field of 150 Oe. In subsequent profiles, three
cores were taken from most units and demagnetized
either at 100 and 200 Oe (profiles UA,UB,UC,UD,
UH 6-15) or at 100,150 and 200 Oe, in a two-axis
tumbler system. Laboratory measurements, mostly
done at the University of Iceland, employed a static
four-probe fluxgate arrangement. A brief description
of the sampling locations (Figs. 2, 3) and procedures
is given in the Appendix.
PALEOMAGNETIC RESULTS
In most of the samples it was easy to isolate a prima-
ry direction of remanence; the mean intensity of this
remanence is similar to the averages for other collec-
tions published by Kristjánsson (1984). In cases of
within-unit directional inconsistencies, samples were
occasionally demagnetized at 250 or 300 Oe, or the
units were resampled. All lava-mean paleomagnetic
results, listed in Table 1, have been corrected for tec-
tonic tilt. The amount of tilt which is assumed to be
due southeast, is also given in Table 1.
Stratigraphy. For stratigraphic work, the most
useful paleomagnetic parameter in this case is the
polarity, shown for each flow in Fig. 3. We have
attempted to set up a composite stratigraphic column
for the area (Figs. 4, 5). An N to R reversal takes
place near the bottom of the composite section, at GB
5/6 and UE 1/2. The outcrop of UE 1 east of Koipa
gave ambiguous paleomagnetic results, so we later
resampled the flow on the beach west of the stream,
obtaining consistent directions. The bearing between
the two outcrops of the N to R transition is ENE (cf.
Fig. 4) rather than NE, suggesting the presence of
complications such as an unconformity or a set of
large faults.
We propose that the normal-polarity lavas corre-
late with Einarsson's (1957) N3 zone which is of
variable thickness in the Hvalfjörður area, but
Einarsson gives a mean thickness of 50 m. In the
Kistufell profile SB of Kristjánsson et al. (1980)
flows no. 24-36 belong to this zone. The flow UK 7,
which has a transitional magnetic direction (Table 1),
JÖKULL, No. 41, 1991 53