Jökull - 01.12.1987, Page 68

grains in the matrix to be altered into palagonite, which
then can act as a cementing substance. The sediments
lack other diagnostic characteristics of palagonite, such
as a brownish color or precipitation of opal or zeolites.
The relative sea level was high during the deposition
of the Ásbakkar diamicton, possibly at the marine maxi-
mum limit at 80-90 m some time during its deposition. A
shell sample from the Skorradalur valley, dated to about
12.300 BP (Ashwell 1975), implies that at that time the
relative sea level was above 62 m a.s.L, which is the
elevation of the valley threshold.
THE ÁS BEDS: A MORAINAL BANK
FACIES ASSOCIATION
The Ásbakkar diamicton is overlain by a sequence of
gravels, sands and diamictons, the Ás beds. They are
exposed in the cliffs below the Ás farm (Fig. 1), between
3550 m and about 4700 m in the section (Fig. 2 and logs F
and G in Fig. 3). The Ás beds have four major character-
istics: (1) Their lower contact with the Ásbakkar diamic-
ton is Iocally gradational and conformable, indicating
continued submarine deposition, and locally a glaciotec-
tonic thrust plane. (2) Except for the lowest part of the
strata, beds are discontinuous and laterally variable in
thickness. (3) There is a clear coarsening upwards trend
in the unit, from silty sands to coarse gravels. (4) The
unit is bounded upwards by a surface of glacial erosion
and shear. Its uppermost part is at places so sheared and
mixed that it is difficult to recognize primary textures,
fabrics and geometries.
The Ás beds gravel facies
Description:
The gravel facies are most conspicuous in the Ás beds sequence,
especially in its upper part between 3550 m and 3900 m in the
cliffs, and constitute about half its total thickriess. Four major
lithofacies of gravel were recognized: normally graded gravel
(Ggn), reversely graded gravel (Ggr), massive to weakly strati-
fied gravel (Gu) and planar cross stratified gravel (Gp). The
gravel facies are interbedded with units of sands and diamictons
described below. The gravel facies are discontinuous and lat-
erally variable in thickness. Typical dimensions for individual
units are thicknesses of less than one m and a lateral extent of
less than 40 m. Unit contacts are sharp and sometimes erosional.
The gravel is unconsolidated and poorly sorted, and the graded
facies show variations from pebble-cobble gravels to sandy peb-
ble gravels or vice versa. Lenses of sorted grain-to-grain contact
pebbles, sometimes showing imbrication, occur in all gravel
facies. The imbricated fabrics indicate a depositing stream or
current from a northerly direction. Laminae and lenses of silt,
thin beds of crudely stratified to laminated sand and massive to
stratified diamicton occur in all gravel units, except in facies Gp.
Facies Gp is a 2-3 m thick isolated set of planar, tangential-
based cross stratified sandy gravel with foreset bed thicknesses
of 10-30cm. Individual beds extend over the full thickness of the
unit and dip at an angle of 8-17° towards SW.
The Ás beds sand and silt facies
Description:
The lowest part of the Ás beds, which can be seen at intervals
between 3650 m and 4700 m, ís up to about 8 m thick unit of
stratified sand (Ss). Its lower part is poorly sorted and pebbly
(samples 14 and 15, Fig. 4), with stratification due to normal
grading of individual 5-10 cm thick beds from gravelly sand to
poorly sorted sand. Upwards the unit grades to stratified silty
sand, with laminae and thin intrabeds of sandy silt (Fl) and
coarse to fine sand (S1 ,Su,Ss). Random rippled bedding surfaces
and isolated structures such as sand lags and rip-ups of sandy silt
laminae also occur. The unit is heavily sheared, and its lower
gravelly part is partly incorporated in a large fold (Fig. 12). The
silty upper part is lithified, at least partly due to palagonitization
(precipitates of opal present). The contact with the underlying
Ásbakkar diamicton is of two types: between about 3560 m and
4000 m it is obscure due to glaciotectonic shear, but beyond
4000 m the contact is gradational. Incorporated in the lowest
gravelly sand are pitted and worn shell fragments, probably
derived from the Ásbakkar diamicton.
The lowest stratified sand facies is conformably overiain by
the only major silt facies in the Ás beds sequence, a laminated
sandy silt (Fl), deformed due to loading. The thickness of facies
F1 ranges from a few cm to about 30 cm. Silt laminae and thin
siltbeds were also found coating bounding surfaces of many of
the overlying lithofacies in the sequence.
Fig. 9. The Látrar beds stratified diamicton: The con-
tact (broken line) between the cross stratified esker fan
sediments (Sp) and the stratified undermelt/ice-rafted
diamicton (Dms). Here, the diamicton is very rich in
outsized clasts.
9. mynd. Jakabornir hnullungar á mörkum áskeilu og
jökulsjávarsets.
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