Jökull - 01.12.1987, Page 73

angular, and serveral striated and facetted clasts with double
stoss-lee sides, characteristic for lodgement till (Kriiger 1984)
were recognized. Some of the elongated clasts show an apparent
imbricated fabric in the exposure, with a dip towards north. The
diamicton is lithified and very difficult to sample or check for
preferred orientation fabric. Where the diamicton sheet climbs
the cliffs, clasts can be seen thrust or lodged into the substratum
(Fig. 10A) and silt clasts from the substratum are incorporated
into the diamicton. Neither joints nor folds were observed with-
in the diamicton, but both occur in its substratum (see later). At
some places the upper contact is coated by a thin (1-5 cm)
sheared siltband. Concentrations of large boulders on top of the
strata between 1700 m and 2500 m and large, striated erratics on
the lowland surface above Látrar (Fig. 10D), could be lagdepos-
its from the Melar diamicton.
Interpretation of the Melar diamicton
The Melar diamicton has many of the properties com-
monly used for recognizing lodgement tills (for reviews
see Boulton 1976, Dreimanis and Schluter 1985): It is a
non sorted, massive sediment, containing large, striated
and facetted clasts. Elongated clasts show imbricated
fabric. Its lower contact is erosional and it has a sheet-
like depositional geometry. Its surface is sheared at
places, and glaciotectonic deformations occur in its sub-
stratum. On this basis I interpret the Melar diamicton as
lodgement till. Apparent fabric and direction of the
glaciotectonical deformations in its substratum (see lat-
er) indicate a deposition beneath a glacier coming from a
northerly direction, out of the Borgarfjördur main val-
ley.
The Asgil gravels: an ice-marginal outwash
facies association
Description:
A deposit of coarse gravels, trending NE-SW, is buried in the
sediments south of the Ásgil ravine, between 2725 m and 2875 m
(Fig. 2, Fig. 3, log E). It has a ridge-like morphology: It is about
150 m wide and 5-12 m high, and can be followed in exposures
some 300 m inland from the coast. The deposit is composed of
four major gravel facies (Gu, Gp, Gs, Ggn) and three major
sand facies (Ss, St, Sp). Most conspicuous are the facies of
massive to weakly stratified gravel (Gu, Fig. 11A), often with
well developed imbricate fabrics, arranged together with facies
Gs in poorly defined multistorey sets (Fig. 11B). Set thicknesses
are 1-2 m, set contacts are erosional or gradiational, and individ-
ual sets can be traced laterally for up to 45 m. Boulders are
abundant in the gravels, and the largest one encountered had a
diameter of 60 cm. The gravel and boulder clasts are sub-
rounded to well rounded. Some of the clasts derive from the
Ásbakkar diamicton and contain subfossil molluscs. Facies Gs
(sample 16, Fig. 4) is sandy, planar stratified, with imbricated
gravel trains and minor intrabeds of gravelly sand. The graded
facies (Ggn) grades upwards from clast supported gravel to
sandy, matrix-supported pebble gravel.
The Gp facies consists of large-scale isolated sets of planar
cross-stratified gravels, occurring within and below the multisto-
rey sequence of Gu and Gs facies. Set thicknessesobserved were
around 1 m, with foreset thicknesses of 20-30 cm, dipping
around 20° towards SE. The foresets are angular based to slight-
lyconcave.
Facies Ss is a roughly planar parallel stratified poorly sorted
sand, with occasional pebbles (Fig. 11C). It occurs both as about
1 m thick single units, and as thin and discontinuous lenses
within the multistorey sequence. The stratification is due to
normal grading from pebbly to poorly sorted sand within the
individual 4-10 cm thick sandbeds.
Facies St is gravelly sand (Fig 4, sample 17) in grouped trough
sets, with coset thickness of about 1.5 m. Set thicknesses are
20-30 cm and set widths about 1 m.
Planar cross stratified sand (Sp) occurs in large scale isolated
sets at the base of the sequence (Fig 11D). Set thicknesses are up
to 1.5 m, with foreset dip angles up to 26° towards SE and foreset
thicknesses of 10-20 cm. The foresets are angular based and
consist of alternating layers of coarse, pebbly sand and some-
what better sorted sand.
Interpretation of the Ásgil gravels
I interpret the Ásgil gravels to be ice-marginal glacioflu-
vial outwash deposits. Their sedimentology fits descrip-
tions of proximal glaciofluvial deposits by e.g. Booth-
royd and Ashley (1975), Frazer (1982) and Miall (1983).
Large boulders in glaciofluvial deposits have been used
by e.g. Thomas (1984a) to infer ice-contact depositional
environment, as the stream capacity to carry large boul-
ders downstream is limited. Haraldsson (1981), who
studied the recent Markarfljótssandur in S-Iceland,
rarely found boulders with diameters larger than 40 cm
outside its very proximal part.
The deposit is buried under glaciomarine sediments
(Fig. 2) belonging to the Melabakkar silts and sands
described below. Paleocurrent measurements from im-
bricated fabrics and foreset dips (Fig. 2) indicate a de-
positing stream direction from NW. The glaciofluvial
deposits interfinger as gravel trains with the Melabakkar
silts and sands glaciomarine facies. That, along with the
lack of faults or other tectonic features supports the
assumption that the broad ridge form is the syndeposi-
tional geometry of the deposits. I suggest that the Ásgil
gravels were deposited as an ice-marginal transverse
ridge where fast flowing subglacial streams entered the
marine environment. The waterdepth during the ridge
deposition was more than 25 m.
THE MELABAKKAR SILTS AND SANDS:
A GLACIOMARINE FACIES ASSOCI-
ATION AND SUBLITTORAL FACIES
A third major sequence of silty-sandy sediments, with a
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