Jökull - 01.12.1987, Page 76

zone of the fold, wing shaped, drawn-out boudins (Fig.
12-3) have developed on the boundary between the As
beds and the Ásbakkar diamicton. The boudins are
0.3-1.0 m long, 0.2-0.4 m thick, separated by a distance
of approximately half their width. The isoclinal fold is a
8-10 m high anticline. Its limbs consist of the lowest
lithofacies of the Ás beds, while the core is of Ásbakkar
diamicton. The fold axis trends 75°-255° and the axial
plane dips 28° towards 345°, indicating a glacial push
from NNW.
The Ás beds above the fold are also sheared and
folded. Immediately above the fold, in facies Ss, there
are boudins and small-scale overturned to recumbent
folds (Fig. 12-2). Higher up in the interbedded gravel-
and diamicton facies, larger overturned sheared folds
occur (Fig. 13). At 3850 m, a overturned folded structure
has developed in the mid-section of the Ás beds, at the
junction between the lowest sandy lithofacies and the
gravelly upper part (Fig. 14). Its fold axis trends 90°-
270°, and the axial plane dips 20° towards 360°, in-
dicating a deforming push from approximately north.
The uppermost part of the Ás beds is so heavily sheared
and mixed that it is often difficult to recognize primary
textures, fabrics and geometries. Boulton and Deynoux
(1981) suggested that such non-till glacially deformed
sediments should be called “deformation till”.
A large-scale (> 50 m long) recumbent structure of
concentric folded strata, with almost horizontal axial
plane, is exposed at around 4700 m in the cliffs, affecting
the Ásbakkar diamicton (Fig. 15). The inverted strata
and part of the anticlinal hinge zone are exposed in the
section. The upper part of the structure has been re-
moved by erosion along a horizon approximately coinci-
ding with the axial plane. The fold axis appears to trend
100°-280°. As the exposure trends 135°-315°, the SSW
facing hinge strata appear as a succession of concave
clastic dykes, stacked sub-parallel to the exposure. The
folded strata are transected by normal- and reverse
faults as well as overthrusts who converge towards
north. The dislocating force has operated from a north-
erly direction. The folded structure is discordantly over-
lain by heavily sheared gravel-diamicton admixture
(“deformation till").
At 1525 m, in the immediate substratum of the Melar
diamicton there is an example of a small scale, asym-
metrical, angular fold which has been transformed into a
thrust fault (Fig. 16A). The sharp trough hinge has frac-
tured and the inverted limb has been replaced by a slide
plane with an apparent dip of 26° towards NNE.
At 2400 m there is a folded structure affecting the
glaciomarine facies of the Látrar beds (Fig. 16B). It is a
5-6 m high structure of open (interlimb angle: 115°),
asymmetrical anticlinal strata. The fold axis trends 80°-
260° and the axial plane dips 50° towards 350°, indicating
a push from approximately NNW.
At 160 m, a synclinal part of a large overturned fold.
Fig. 15. Structural outline of heavily sheared and folded strata of the Ásbakkar diamicton at 4650-4700 m, drawn
from photographs. The deformed strata are truncated by a glacial discordance, approximately coinciding with the
axial plane of the folded structure. They are overlain by a heavily sheared gravel-diamicton admixture (“deforma-
tion till”, probably belonging to the Ás beds. The deforming shear has been from left to right. Height of section
about 20 m.
15. mynd. Óþyrmilega haggaður jarðlagabunki við 4650-4700 m.
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