Jökull - 01.12.1987, Page 72

margin where meltwater entered the sea from subglacial
tunnels. The change in texture and fabric, upwards with-
in the esker sediments, could reflect a change from a fast
flowing stream in a tunnel to a more deflecting stream in
front of the tunnel. On top of the esker sediments and
partly interfingering with them, a cap of diamicton was
deposited. The diamicton contains outsized clasts, in-
terpreted as dropstones, and I conclude that it was pro-
duced by undermelt and/or slumping of debris from the
glacier or from collapsing ice bergs. The silt and sand
laminae and thin beds within the diamicton could have
been produced by current reworking and/or turbidite
deposition from underflows.
As the ice margin retreated, the deposition gradually
changed from diamicton to interbedded, rhythmic silt
and sand with iceberg dropped debris. Sedimentary
structures in the rhythmic unit, such as “roll-up” struc-
tures and convolute bedding indicate strong current ac-
tivity and rapid sedimentation (Gustavson and Ashley
1975, Collinsson and Thompson 1982). The environment
was marine, but the small number of molluscs may in-
dicate rapid sedimentation or brackish water. If the
rhythmic control has been seasonal, a mean sedimenta-
tion rate of 0.17 m • y'1 for at least 160 years can be
inferred for the sediments. This is a sedimentation rate
at least fifty times faster than the one inferred for the
fossiliferous Asbakkar diamicton, but it is similar to
sedimentation rates in present day glaciomarine fjord
environments on Svalbard (Elverhöi et al. 1983) and on
Baffin Island (Gilbert 1982). The relative sea level dur-
ing the deposition of the glaciomarine facies was higher
than 23 m above the present level, which is the highest
occurrence of undisturbed rhythmic silt and sand in the
cliffs, probably considerably higher.
THE LANDHÓLMI SANDS, MELAR
DIAMICTON AND ÁSGIL GRAVELS: ICE
PROXIMAL, ICE CONTACT AND ICE-
MARGINAL FACIES ASSOCIATIONS
There are indications in the cliffs of a glacial advance
after the deposition of the Látrar beds glaciomarine
facies. Three major stratigraphic units resulted from this
advance: The Landhólmi sands, Melar diamicton and
the Ásgil gravels.
The Landhólmi sands: an ice proximal delta facies associ-
ation
Description:
Between 2000 m and 2400 m, landwards of the Landhólmi
skerry in the Látrar parts of the cliffs (Fig. 1), a unit of predom-
inantly sands lies on top of the Látrar beds (Fig. 3, log D).
There, the interbedded silt and sand facies of the Látrar beds
become increasingly sandy upwards, and are overlain by a 1.2-
1.6 m thick sequence of massive pebble gravel (Gu), stratified
silty sand (Ss) with discontinuous parallel laminations of sandy
silt, and poorly sorted sand (Su). The gravelly-sandy sequence is
conformably overlain by a 4-6 m thick single set of planar cross
stratified, pebbly sand (Sp). Its foresets are angular to tangential
based, 4-15 cm thick, with internal fining upwards from pebbly
sand to coarse sand. The foresets dip towards SE-SW at angles
between 10° and 27°. Interbedded in the cross stratified sand are
up to 20 cm thick units of clast supported pebble to cobble
gravels (Gu) and, in the upper part of the set, a number of
10-15 cm thick sandy-silty stratified diamicton lenses with peb-
ble-cobble clasts (Dms). The interbedded units have erosive
bases.
The upper parts of the Landhólmi sands are tectonically dis-
turbed, but the disturbance is complex and the exposure poor so
that structural analysis was not attempted. The deposits are at
places capped by ienses of boulder-rich diamicton and lag con-
centrations of boulders (Fig. 10C).
Interpretation of the Landhólmi sands
Thick single set cross stratified sands and gravels can
form where a glaciofluvial stream carries bedload into
quieter water and deposits it via grainflow avalances
down a foreset delta slope (e.g. Gustavson and Ashley
1975, Edwards 1978, Clemmensen andHoumark-Nielsen
1980). I suggest that the Landhólmi sands were depos-
ited as a fan delta in an ice- proximal glaciomarine
environment. The contact with the Látrar beds glacio-
marine facies is non-erosional, and possibly the upper-
most part of the Látrar beds are bottomsets of the delta,
deposited beyond its cross stratified seaward face. The
thin, graded foreset beds indicate deposition during a
period of reducing flow strength. The interbedded grav-
els and stratified diamictons could have been deposited
from periodic strong current induced avalances or grain-
flows and subaqueous slumps off an unstable delta front.
Some of the gravel trains could be lag deposits. Thomas
(1984a) described thin diamicton units, interbedded
with foresets in an ancient glaciofluvial fan delta, as
subaqueous slumps. The relative sea level during the
deposition of the delta sediments was at least 30 m above
the present level.
The Melar diamicton: a lodgement till facies
Description:
Beneath the farm of Melar (Fig. 1), a 1-2 m thick diamicton
sheet is exposed (Fig. 3, log C). It has an erosive base, and can
be followed from the base of the sequence at 1500 m, where it
disconformably overlies the Ásbakkar diamicton, concavely up-
wards across the glaciomarine unit boundaries to the top of the
sequence at 1700 m (Fig. 2). The diamicton is matrix supported
and massive (Dmu), with clasts of all sizes from pebbles to large
boulders embedded in a silty-sandy matrix (Fig. 10B), but clus-
ters of boulders also occur (Dcu). The clasts are subrounded to
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