Jökull - 01.12.1987, Síða 65

cum and ligament are often preserved, even though the shells
may be broken. Whole individuals of Balanus balanus were also
found. The fossil fauna assemblage (Table II) is a mixture of
epifaunal and infaunal species.
Interpretation:
The sedimentological and paleontological data suggest a
shallow water, low salinity, boreal to mid-Arctic fjord
environment. The indistinct rhythmic character of the
sediments suggests that sedimentation occurred primar-
ily from turbid overflows and suspension (Fig. 5) but
stratification may have been destroyed by bioturbation.
The laminae and thin beds of sand cannot be related to
seasonal or other periodic control. More likely they
represent random density underflows from delta slopes
or other unstable surfaces. The random intrabeds of
gravelly sand are stratified to graded, occasionally with
erosive lower contact. I interpret them as lag deposits
caused by bottom currents. The tidal amplitude in Borg-
arfjördur today is about 4 m, generating strong tidal
currents.
Palaeoecological interpretation of the fossil fauna as-
semblage puts it with the Macoma calcarea community,
a boreal to arctic mollusc community described in East
Greenland by Thorson (1933) and Ockelmann (1958)
and from Vestspitsbergen by Feyling-Hanssen (1955).
Most individuals of Mya truncata belong to forma udde-
vallensis, which is a thick-shelled, panarctic, circum-
polar forma, e.g. described from Late Weichselian sedi-
ments on Vestspitsbergen (Feyling-Hanssen 1955) and
Upper Pleistocene sediments on West Greenland (Sí-
monarson 1981). The Macoma calcarea community pre-
fers silty and sandy-silty fjord bottoms with low salinity,
water temperatures below +5 °C and waterdepths less
than 45-50 m (Thorson 1933, 1957). Spjaeldnes (1978)
pointed out that the Macoma community prefers low
salinity “fjord water” conditions, and thus can be found
at greater waterdepths than 50 m if the local fjord topog-
raphy and sufficient input of freshwater allow it.
There is no conclusive evidence of a nearby glacier
during the deposition of facies Dms, and I interpret this
as a ice-distal to ice-intermediate facies (Fig. 5). The
occasional granule and pebble clasts are probably ice
rafted, either as dropstones from icebergs or rafted from
winter ice.
Powell (1981,1983b) and Molnia (1983) have described
distal glaciomarine sediments off the Alaskan coast as
bimodal due to a combination of sedimentation from
suspension, underflows and ice rafting. Domack (1984)
and Stevens (1985) described rhythmically bedded gla-
ciomarine sediments of late Pleistocene age, where fluc-
tuations of meltwater discharge and sediment load into a
density stratified nearshore marine environment are
called upon to explain the stratification. Stevens (1985)
pointed out that more distally within the glaciomarine
environment the rhythmic character probably becomes
less distinct due to weaker and more irregular density
stratification, lower and more homogeneous sediment
supply and increased bioturbation.
Facies Dmu
Description:
Facies Dms grades into facies Dmu, a sandy-silty, massive dia-
micton, with relatively fewer fossils but with an increased num-
ber of gravel and boulder clasts (Fig. 6A). The increased ratio of
sand and gravel in facies Dmu compared to facies Dms is reflect-
ed in samples 3,4 and 5 in Fig. 4. Upwards in the facies the fossil
molluscs gradually disappear, and intrabedded units of stratified
and massive diamictons (Dms, Dmu) and stratified sand (Ss, Sl)
occur frequently. The gravel and boulder clasts are angular to
subrounded, and on one occasion a facetted and striated boul-
der was recognized. The clast distribution varies both laterally
and vertically, from isolated clasts to clusters with grain-to-grain
contacts. No preferred clast orientation was observed, but on a
few occasions the stratum below a clast was bent downwards
(Fig. 7A).
Interpretation:
I interpret the transition from facies Dms to Dmu, to
reflect increasing proximity of the depositional basin to
the sediment source, an advancing ice front. Increase
both in the rate of sedimentation and brackishness of the
Fig. 7. Structures from the Melabakkar-Ásbakkar sedi-
ments, drawn after field sketches and photographs: (a)
downbended strata below outsized clasts in the Ásbakk-
ar diamicton, (b) convolute structures of stratified diam-
ictons and sands in the Ásbakkar diamicton at 4750 m,
(c) “roll-up” structures in the Látrar glaciomarine fa-
cies. Paleocurrent from right.
7. mynd. Dcemi um byggingarlag sets í bökkunum.
63