Jökull - 01.12.1990, Page 15

pebbles of a very uniform size. All pebbles are very
vesicular and are evenly distributed throughout the
sediment. Except for a polished pavement beneath the
diamictite, these are features that argue against a direct
glacial deposition for the sediment. The boulder bed
at the bottom within the layered lowermost one metre
suggests traction at the base and the nearly uniform
dispersion of clasts through the thickness of the oth-
erwise structureless fine matrix is probably the result
of dispersive pressure between the colliding particles.
These are conditions indicative of rapidly deposited
debris flow or surge type deposit. The principal vec-
tor direction from both pebble and magnetic fabric
measurements is transverse to the inferred orientation
of lineations of the polished surface of the underly-
ing basalt, and parallel to the strike direction of the
diamictite unit. This fabric pattem is comparable to
a fabric pattem found for Mt.St. Helens debris flows
(Mills, 1984; Major and Voight, 1986). Based on
similar stratigraphic position, Sæmundsson and Noll
(1974) correlate this diamictite with ill sorted and in
places clast supported sediment that crops out at Hvít-
ársíða. However, the Giljafoss section has very differ-
ent attributes in comparison to the correlative section
m Hvítársíða which suggests two different, but local
source areas. Fabric pattem derived from the Hvít-
ársíða section is dispersed and shows low significant
values (s \) similar to the debris flow samples from the
volcano Hekla in Iceland (Geirsdóttir, 1988).
The second diamictite unit is separated from the
first one by a 120 m thick lava sequence. It con-
tains thin massive tuffaceous beds intercalated with
faintly bedded conglomerate capped with trough cross
bedded tuffaceous sandstone. In thin section, this
sediment is almost entirely made up of glass frag-
ments (fresh sideromelane glass) with very few rock
or mineral fragments in the matrix. It shows less lat-
eral extent than most other sedimentary horizons in
the area and considerably less variation in composi-
tion. Its existence is attributedto flood-flow processes
mainly because of the lack of bedforms, followed by
fluvial activity which promoted the bedding and cross-
stratification in the upper part of the unit.
Between the second unit and diamictite unit 3
is a 110 m thick succession of basalts, dacite, ign-
imbrite and andesite (Sæmundsson and Noll, 1974).
The acidic affinity of the eruptional phase is clearly
imprinted on the third diamictite unit. It is in places
made up of light coloured lenticular pumice fragments.
In some locations this deposit has many similarities in
field appearance to the floodflow deposit of the sec-
ond diamictite unit except it is much more volumi-
nous, in places coarser grained and has greater lateral
facies variability. Furthermore, it is intercalated with
lava flows which suggests a continuing and concurrent
volcanic activity and sedimentary deposition.
The five glacial deposits found in the upper Borg-
arfjörður section have a distinctly different structure,
texture, and more heterogeneous lithology than the
other sedimentary units within the area. The oldest
identified glacigenic sediment in the upper Borgar-
fjörður (diamictite unit 4) preserves fissility and shear
features and boulder pavement indicative of lodge-
ment and basal melt-out of debris from ice. The
eastemmost section of the same diamictite unit (in
Deildargil) is also associated with channelized fluvial
deposits (Figs. 2 & 6a). In most cases both pebble
fabric and remanent magnetic measurements show a
unimodal pattem which is thought to be diagnostic
of both melt-out and flow mechanisms, but in a few
a bimodal pattem occurs, probably indicating glacial
marginal deposition and shearing of basal till. This
fourth diamictite unit is also the oldest unit in the area
that is associated with hyaloclastite formation within
the volcanic center (Fig. 2).
Approximately 50 m of basaltic and acidic rocks
separate the oldest glacial deposit from the next di-
amictite unit (Fig. 5). A huge ignimbrite sheet un-
derlies two diamictite units (diamictite units 5 and 6)
which are separated by a very thin lava flow (0.1-1 m
thick). The lower bed (unit 5) was probably formed by
localized mass flow processes. It rests on a thin sco-
riaceous lava-top which has been broken up in places
and incorporated within the diamictite unit. The scori-
aceous top of the lava is filled with fine laminated silt
and clay, and the base of the diamictite is almost clast
supported. It becomes more matrix rich towards the
top but still includes boulders up to 3 m in diameter.
Its matrix contains a considerable amount of glass and
angular rock fragments similar to the stratigraphically
JÖKULL, No. 40, 1990 13