Jökull - 01.12.1987, Page 74

maximum thickness of about 25 m, is exposed between 0
and 1670 m and2720m and3400 m (Fig. 2, Fig. 3: logs B,
C and E). It differs from the underlying glaciomarine
sequences in containing almost no fossils or dropstones,
being neither fractured nor sheared, and being lithified
to soft.
Description:
The Melabakkar silts and sands consist of four major lithofacies
(Fig. 6D): (1) the lowest, a massive to weakly stratifiedsandy silt
(Fu), which grades upwards over (2) sandy laminated silt (Fl) to
(3) interbedded laminated silt (Fl) and stratified sand (Ss).
Stratified sand (Ss) (4), containing numerous burrows, tops the
sequence.
The lowest facies, Fu, is compact to lithified due to palagonit-
ization. A grain size analysis (sample 12, Fig. 4) and a thin
section study showed it to be poorly sorted, silt (75%), sand
(15-20%), and granules (5-10%). The weak stratification is due
to occasional discontinuous interlaminae of fine to medium
sand, spaced at 20-30 cm intervals. The Fu facies is dark gray in
color, sometimes with a slight brownish shade due to palagon-
itization. Its thickness varies between 0.5 m and 5 m.
Facies Fu grades into facies Fl, a fine, well sorted (sample 13,
Fig. 4) and very tight, laminated silt. The palagonitization de-
creases upwards, and facies F1 is soft when wet. A thin section
study showed the fine lamination to be due to bandings of
grain-to-grain contact coarse silt and fine sand sized fragments
in a finer matrix. The laminae are spaced at one to a few mm
intervals and can be difficult to see in the field without the help
of a loupe. Facies F1 is so impervious that groundwater can not
percolate through it, but is conducted on its upper surface to the
cliffs. The thickness of facies F1 is 2-5 m. It is gray to bluish in
color when dry.
Upwards in the sequence, thin interbeds of well sorted fine to
silty sand (Ss, sample 11, Fig. 4) appear. The sandbeds have
sharp lower contacts, but grade upwards into silt with thin
intralaminae of fine sand. The total thickness of the planar
parallel interbedded sand and silt is about 4 m, and two counts of
a number of couplets gave 33 and 41 couplets. Upwards the
facies grades into a 3-7 m thick unit of planar parallel stratified,
well sorted fine to medium sand (Ss, sample 10, Fig. 4). I
counted 60 beds of sand, 8-18 cm thick (mean: 12 cm).Bed
contacts are sharp, and going down from the upper bounding
surfaces are numerous burrows (Arenicolites). The sand is trun-
cated by sharp to erosional contact with the Melagil gravels and
sands described below.
Interpretation of the Melabakkar silts and sands:
I interpret the Melabakkar silts and sands to be a distal
glaciomarine facies association and sublittoral facies,
deposited after the second glacial advance registered in
the cliffs. I interpret the facies changes from facies Fu to
facies F1 to reflect an increasing distance of the basin to
the sediment input, i.e. glacial meltwater streams enter-
ing the marine environment during a rapid retreat of the
glacial front to a position within the present coast. The
massive appearance of the lowest facies could be due to
rapid sediment accumulation due to flocculation, only
occasionally disturbed by underflows during periods of
high meltwater discharge. The better sorting upwards
could also reflect increasing waterdepths due to a ma-
rine transgression in connection with the deglaciation. I
interpret the well sorted facies F1 to indicate a change
from dispersed meltwater and underflow sedimentation
to sedimentation mainly from overflows. At this stage,
pherhaps, the glaciers had retreated to a position within
the present coast, so that the meltwater streams reached
the depositional basin via coastal deltas. The very fine
Fig. 13. An overturned anticlinal, fold-
ed and sheared structure, developed in
interbedded gravels and diamictons of
the Ás beds at ca. 3750 m. The deform-
ing push has been from left to right.
Photographed with a 400 mm telelense
from the beach.
13. mynd. Andhverfa við 3750 m.
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