Jökull - 01.12.1990, Page 22

phy suggest that there were at least two glaciations
in each area within the Gauss epoch. Paleomagnetic
measurements made on 10 samples from three differ-
ent locations within diamictite unit 7 (glacial deposit
3) in Borgarfjörður, however, gave reversed polarity in
contrast to normal polarities of both under- and over-
lying lavaflows in all sections (Figs. 5 and 8) (Geirs-
dóttir, 1988). These paleomagnetic results from the
diamictites in Borgarfjörður are scattered, but the po-
larity signal is consistent indicating a reversed event
during their formation. However, before question-
ing the previously assumed stratigraphic position of
the diamictites within the Gauss magnetic epoch, fur-
ther investigation of the reliability of the paleomag-
netic signals within diamictites is necessary. The two
stratigraphically youngest glacial deposits in Borgar-
fjörður and the four stratigraphically youngest glacial
deposits in Hvalfjörður fall within the upper part of
the Matuyama polarity epoch (Fig. 8) (McDougall et
al., 1977; Kristjánssoneí al., 1980; Geirsdóttir, 1988).
Indications of changes in the style of glacial ac-
tivity from the first identified glacial unit to the last,
in both the upper Borgarfjörður area and the Hval-
fjörður area, appear mostly in the regional extent of
hyaloclastites. In Borgarfjörður, hyaloclastites are ini-
tially restricted to the volcanic center and only later
spread out over most of the southem part of the up-
per Borgarfjörður area. This may indicate that the
oldest glacial deposit was formed by a valley glacier
that extended from a small ice cap covering the active
and growing volcano. There is, however, no evidence
of valley erosion at this stratigraphic level in Borgar-
fjörður and the stratigraphy points to a rather low relief
apart from the growing volcano. But the clast fabric
measurement from this first glacial deposit shows a
very consistent orientation of the particle long axes,
a NNE-SSW orientation, whereas all the other strati-
graphically younger glacial deposits show an orienta-
tion directed N-S or NNW-SSE. The second glacial
deposit (diamictite 6) in Borgarfjörður section was
deposited subglacially and shows features related to
both lodgement and melt-out of debris from ice. This
glacial unit shows a strong unimodal fabric pattern
oriented NNW-SSE at all site locations. This is also
valid for the third glacial deposit (diamictite 7), apart
from the westemmost site location at Valagil, where
particle long axes are oriented more toward N-S. This
third glacial deposit resembles basal melt-out tillite. It
is, however, in places less well preserved than the un-
derlying glacial units, but in other places it is followed
by glaciofluvial and thick lake deposit (Asgil). This
may be attributed to more effective fluvial erosion
associated with the retreat of the glacier or possibly
lessened volcanic activity within the central volcano
which promoted erosion rather than burial by contin-
ued eruptions. Neither the second or the third glacial
unit are directly associated with hyaloclastites except
within the volcanic center. The next diamictite unit
(diamictite 8) which is separated from the underlying
sedimentary deposit by olivine tholeiite lava flows, is
only found in one gully as a thin debris bed. A 100
metre thick hyaloclastite pile located within the central
volcano is correlated with this sediment (Sæmundsson
andNoll, 1974). Hence, itmay indicatetheeruptionof
an ice-capped volcano and thus reflect local deposition
rather than a major glacial activity. Hyaloclastite as-
sociated with the oldest tillite of the area is restricted
to the volcanic center, whereas the youngest tillites
in the area are directly overlain by tuffaceous hyalo-
clastites. Thick glaciolacustrine deposits or tuffaceous
hyaloclastites are preserved with the stratigraphically
younger glacial deposits but these are minor or absent
from the two oldest tillites.
Fragmentation of extruded lavas as overlying ice
melted and the formation of avalanches of hyalo-
clastite sheets may explain how some diamictite units
became intercalated with basalts and hyaloclastites in
the Hvalfjörður stratigraphy. Clast fabric measure-
ments made on glacial deposits in the Hvalfjörður
area all show southeasterly to northwesterly orienta-
tion. These glacial deposits become thicker towards
east, whereas diamictite units 2 and 3, become thicker
towards west and show an orientation perpendicular
to the prevalent orientation. These deposits, units 2
and 3 have been attributed to mass flow and fluvial
deposition. The Hvalfjörður section resembles the
Borgarfjörður section in containing thick glaciolacus-
trine deposits or tuffaceous hyaloclastites in associa-
tion with the stratigraphically younger deposits, but
these are not as extensive facies associations with the
20 JÖKULL, No. 40, 1990