Jökull - 01.12.1983, Side 104

The term rhythmite was applied by Bruno Sander
(Flint 1971) to individual units or couplets of
rhythmic strata, without implication as to thickness
of pair or period of rhythm. The term diamicton is
used to describe nonsorted terrigenous sediments
containing a wide range of particle size, regardless
of genesis (Flint et al. 1960).
Skörd, section A
I. On an erosional surface of the Tertiary basalts
rests diamicton at least 3. 5 m thick. Subangular
and subrounded pebbles and boulders in a silty-
sandy matrix. Sharp nonerosional top.
II. At least 6 m thick rhythmic bed. Alternating
units of laminated silt layers up to 15 cm thick, and
clay layers upto 1 cm thick. The silt layers are
composed of íine grained units, and fine to medium
grained sand beds are common at the base of each
silt layer. The uppermost laminae are convoluted
and some are in overturned folds in WNW direction
(fold planes dip ESE). Erosional, interfingering top.
III. Up to 3 m thick diamicton as I.
IV. At least 9.5 m thick rhythmic bed. Erosional
top but locally gradational. The lowermost 1 m is
composed of a thinly laminated bed ofsilt and clay
with occasional sand partings up to 5 cm thick. The
next 2 m are composed of about 40 couplets of silt
and clay, 5 cm thick on average. The silt layers are
composed of thin laminations that are commonly
graded; occasionally sand layers occur in the basal
units. The silt layers become thinner upwards in
this part but the clay layers are thickest (up to 3.5
cm) at the top. Although the rhythmites exhibit a
thinning upward trend in this part there is consider-
able fluctuation about this trend (Fig. 3). The next 3
m are composed of couplets up to 35 cm thick.
Multiple graded units are common within the silt
layer and very thin bands of clay occur. The
couplets show upward thinning. The uppermost
laminae of this rhythmic bed are convoluted and
overturned folds in WNW direction are common.
One sedimentary dyke runs through the convoluted
part, dip 40° WNW.
V. At least 2 m thick diamicton. Erosional base,
gradational nonerosional top. Subangular and sub-
rounded boulders, up to 60 cm in diameter, in a
silty-sandy matrix.
VI. a. At least 2.7 m thick bedded sand and
gravel. Consolidated silty-sand layer at the top.
Gradational top.
VI. b. At least 16 m thickrhythmicbed consisdng
of alternating thick laminated, fine graded silt lay-
ers and up to 3.5 cm thick clay layers. The basal
laminae are convoluted. The next 70 couplets, 6.2
cm thick on average, are undist'urbed. The basal silt
layers are thickest an become thinner upwards (Fig.
4) This part grades into distorted laminae ofsilt and
clay. At a higher level the laminae are in overtumed
folds in WNW direction. Shear planes are observed
at the basal part of the overturned folds. The next
part consists of 1.5 m of thick undisturbed laminae.
The topmost silt layer is 1 m thick grading into a 1
cm thick clay layer. The uppermost 9 m are com-
posed of convoluted laminae. The silt layers show
upward thinning but the thickness of the clay layers
remains the same. At the base of this part runs
sedimentary dike (Fig. 5), dip 5° WNW. About 300
couplets were counted in the whole rhythmic bed.
VII. At least 10 cm thick diamicton. Becomes
more stony upwards. Largest boulder 1 m in dia-
meter. Becomes thinner in WNW direction. Sharp
nonerosional top.
VIII. At least 4 m thick rhythmic bed with 10 cm
thick gravel at the base. Gradational and erosional
top. The silt layers have a sharp and sometimes
erosional sole but gradational transition at the top.
Clay fragments are common in the basal silt layers.
Occasionally bands of pebbles occur in the basal silt
layers. Numerous dropstones up to 10 cm in dia-
meter occur in the lowermost couplets (Fig. 6). At
higher level slump structures and small scale fault-
ing are common. The uppermost couplets are
slightly deformed and grade into thin bedded sand
and gravel. 124 couplets were counted in this bed
and are 2.8 cm thick on average. Within a few
meters vertically, the silt layers vary in thickness
while the clay layers remain about the same. Thin
sections were made offive silt-clay couplets from the
middle part of this bed VIII. The silt layers consist
of micrograded units. Cross-lamination was seen in
one 2 cm thick siltlayer with an erosional base. The
clay layers consist of two parts. The lower part is
silty and grades to the upper part with rather sharp
contact. The clay/silt contact is always sharp and
abrupt. Micro-ripples, 0.1 mm in length and 0.02
mm in height, were found on the surface of one 1.5
mm thick clay layer. The ripple crests are rounded
and symmetrical.
IX. At least 7 m thick diamicton. Increasing
stone size upwards. Largest boulder 1 m in dia-
meter. Sharp nonerosional top.
X. Rhythmic bed. No complete exposure was
found in this bed but it is at least 8 m thick. In this
section only the lowermost 4 m were seen. There it is
102 JÖKULL 33. ÁR