Jökull - 01.12.1990, Page 18

the diamictite. This diamictite unit is correlated with
tuffaceous and scoriaceous sediment one km farther
east (Fig. 3).
Diamictite unit 3 which is separated from diamic-
tite unit 2 by approximately 160 m of olivine tholeiite
and tholeiite lavas, has very similar attributes in com-
parison to the underlying diamictite unit 2. Toward
west the diamictite is capped with a tuffaceous sand
and granule bed. The diamictite itself does not con-
tain any features that can be related to direct glacial
deposition. It is composed of a massive and graded
diamictite with medium to fine grained sandstone ma-
trix. Stratified and bedded tuffaceous sandstone and
granule facies lie on top of the diamictite. Interpreta-
tion of the characteristics of these sedimentary facies
do not require direct deposition by a glacier, but sub-
aqueous or subglacial activity in the source area may
have promoted flowage of tuffaceous sand, granule
and brecciated basalt into an aqueous environment as
indicated by the stratified and bedded tuffaceous sed-
iments on top of the mass flow deposit.
Diamictite unit 4 can be traced along the slope of
the mountain Eyrarfjall as a laterally extensive hyalo-
clastite, approximately 120 m higher in the stratigra-
phy. The base of the diamictite is mostly fluviatile
conglomerate which is capped by a thick sequence of
tuffaceous sandstone, breccia and pillow lava. The
unit thickens towards the Kjalarnes central volcano
according to Friðleifsson (1973) and Kristjánsson et
al. (1980). The underlying tholeiite lavas are thought
to represent the first volcanic phase of the Kjalarnes
central volcano (Kristjánsson et al., 1980).
A glacial deposit (diamictite unit 5) is recorded at a
locality approximately 60 m up section and 10 km east
along the coastline from the underlying one. It appears
as a basal tillite at the coastline, but is associated with
a laterally extensive hyaloclastite that can be traced
parallel with unit 4 along the uppermost slope of the
Eyrarfjall mountain (Fig. 3). The basal tillite at the
coastline is characterized by shear features, a variety
of clast lithologies, and the underlying striated bedrock
surface showing orientations from 330° to 340°. The
striations are consistent with the orientation of clast
long axes in the diamictite (Fig. 7a).
Another glacial deposit (diamictite unit 6) crops
out approximately 5 km east from diamictite unit 5 in
Brynjudalsvogur. This glacial deposit was probably
deposited as melt-out till from a debris rich glacier
with subsequent flowage, as shown in both clast fabric
data as well as the rock magnetic data. The clast fabric
shows a preferred orientation NNW-SSE similar to the
fabric of diamictite unit 4, with a minor axis oriented
perpendicular to the preferred orientation. The contact
between this glacial deposit and an underlying massive
tuffaceous sandstone is erosional (Fig. 7b). Diamictite
unit 6 is correlated with hyaloclastite that can be traced
along the slope of Reynivallaháls (Fig. 3).
Sedimentary units 7 and 8 in Hvalfjörður which
are interbedded within lava flows do not contain any
diamictites in the Þrándarstaðarfjall outcrops (Fig. 3).
The bottom layer of both units is a thin (up to 1 m
thick), tuffaceous, medium-grained sandstone which
in unit 7 grades up into a 30 cm thick tuffaceous con-
glomerate. The uppermost part of both sedimentary
units is a brecciated hyaloclastite underlying pillow
lavas. The surface of the underlying lavas does not
show any sign of erosion or polishing effect.
Two intercalated diamictites (diamictite units 9
and 10) are recognized as glacial deposits in Botns-
dalur, approximately 150 m up section from diamic-
tite unit 5 (Fig. 3). The first of the two diamictites
was deposited as a basal till followed by glaciofluvial
activity (Fig. 7c). This interpretation is supported by
both lithologic and lithofacies analysis, pebble fabric
and magnetic remanence pattem. The basal one metre
is a massive diamictite, overlain by a clast-rich diamic-
tite with fine sand lenses, which grades again into a
current-reworked diamictite. This basal tillite shows
a strong unimodal pebble fabric with a preferred ori-
entation of 330°. The overlying sorted unit shows a
more dispersed fabric.
The other diamictite in the Botnsdalur section (di-
amictite unit 10) is separated from the first by a sin-
gle lava-flow, which suggests ice free condition at
that locality. This deposit underlies lacustrine sedi-
ments with some load cast structures and other de-
formational structures indicative of rapid deposition
within a glacial lake. Both diamictite units 9 and 10
are recognized in the form of tillites and hyaloclastite
sheets on both sides of the fjord.
16 JÖKULL, No. 40, 1990