Jökull - 01.12.1987, Page 75

laminae could reflect tidewater current activity or fluc-
tuations in the meltwater input.
I interpret the transition upwards in the sequence,
over interbedded silt and sand to stratified sand with
burrows, to reflect the growing proximity of the basin to
a beach environment due to isostatic uplift and marine
regression, after the glaciers had retreated from the
coastal areas. The relative sea level during the deposi-
tion of the lower-middle part of the Melabakkar silts and
sands was at the regional marine limit at 60-70 m above
the present sea level. Only a few broken and pitted shell
fragments were found in the Melabakkar silts, and three
field seasons yielded 23 gr. of dateable material. Radio-
carbon dating of this sample gave the age of 11.985±120
BP (Table II, sample 10). There are indications that this
dating value is too high for the sediments, and that the
shell fragments derive from older sediments: Firstly, it
predates the Melabakkar silts and sands to the glacioma-
rine facies of the Látrar beds, which is inconsistent with
the stratigraphical and other dating evidence. Secondly,
radiocarbon dated fossiliferous marine and sublittoral
sediments in a similar stratigraphic position as the Mela-
bakkar silts and sands, found in Leirárvogar, some 4-5
km east of the Melabakkar-Ásbakkar cliffs, date around
10.000 BP (Ingólfsson 1985).
THE MELAGIL GRAVELS AND SANDS:
EMERGENCE FACIES ASSOCIATION OF
BEACH GRAVELS AND EOLIAN SANDS
The Melabakkar-Ásbakkar section is truncated by a
gravel lag horizon, G(l). The unconfirmity is very dis-
tinct and can be mapped in sections throughout the
region. Above the lag horizon there are four major
facies of gravels and sands, up to 15 m thick, below
recent soils (Fig. 3, all logs), which can be observed in
the vicinity of the Melagil ravine: (1) a planar parallel
stratified sandy pebble-gravel (Gs, sample 20, Fig. 4),
(2) planar cross stratified gravel (Gp), (3) planar parallel
stratified pebbly sand (Ss, sample 19, Fig. 4) and (4)
planar crossstratified silty sand (Sp, sample 18, Fig. 4). I
interpret facies Gs, Gp and Ss as littoral deposits, and
facies Sp as eolian dune deposits. The littoral and eolian
sediments are found regionally at and below 60 m a.s.l.,
and have been deposited during and after the marine
regression that resulted from the isostatic rebound of the
depositional basin after the glaciers had retreated from
the Iowlands.
GLACIOTECTONIC STRUCTURES
A number of tectonic structures, ranging from small
scale fracturing and shearing to large scale overfolding
and thrusting, are found in the cliffs. If considered as
isolated phenomena, many of the structures could be
interpreted as disturbances caused by local sliding or
slumping of sediments down a slope, or as load-cast
structures. But the consistent dislocation direction (Fig.
2) as well as the pattern of overthrusts and overfolding
makes glaciotectonism the most plausible explaination
for the deformations. The deformations occur at two
stratigraphic levels: a lower level with extensive defor-
mations affecting the Ásbakkar diamicton and the Ás
beds, and a upper level with less intense deformations,
mainly affecting the glaciomarine facies of the Látrar
beds and the Landhólmi sands. The following is a brief
description of deformation features, with emphasis on
good structural indicators of glacial push and its direc-
tion, such as large-scale structures (>10 m), overfolds
and Iow angled thrusts.
Folds
Descriptions of folds were done in two steps in the field:
(1) fold appearance in profile was determined, and (2)
orientations of its axial plane and fold axis were estima-
ted from dip and strike measurements.
At around 3700 m (Figs. 2 and 12) a large scale isocli-
nal fold, affecting the Ásbakkar diamicton and the low-
est lithofacies of the Ás beds, is truncated by a rising
thrust plane that carries the bulk of the Ás beds across
the axis of the fold. The thrust plane can be followed
about 450 m to the SE. The lowest Iithofacies of the Ás
beds is thickest foreward of the fold. Beyond the hinge
Fig. 14. An overturned, isoclinal folded structure, de-
veloped in the mid-section of the Ás beds. A core of silty
sand penetrates the overlying sandy-gravelly strata
which constitute the limbs. Deforming push obliquely
upwards and out of the photograph from left to right.
Photographed with a 400 mm telelense from the beach.
14. mynd. Andhverfa við 3850 m.
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