Jökull - 01.12.1987, Page 71

The gravelly sands and diamictons
Description:
At the base of the sequence, between 2175 m and 2400 m and
between 2575 m and 2710 m, there are two occurrences of low
(up to 5 m), broad, mound-like deposits of sands and gravels.
Their major lithofacies are planar cross stratified pebbly sand
(Sp) and trough cross stratified pebbly sand (St), but minor
lithofacies of pebble-cobble gravels (Gu, Ggn) and poorly sort-
ed sand (Ss) also occur, The trough sets are large scale, with a
maximum thickness of roughly one m and a maximum width of
four m, though thicknesses of 30-50 cm and widths of 1.5 to 2 m
are most common. The angles of plunge in both planar- and
trough cross stratified sets vary between 20° and 37°, and com-
mon foreset bed thickness is around 20 cm. The lower sets strike
105°-285° and dip towards SSW, but upwards in the unit the sets
strike 160°-340° and dip towards WSW. There is a rough grading
upwards in texture: The lower sets tend to be more pebbly with
more frequent pebble-cobble trains while the upper sets tend to
be more silty. Some of the pebble-cobble trains show imbricated
fabrics. A paleocurrent estimation from the imbricated structur-
es gave flow direction towards.SSE.
The cross bedded pebbly sand unit is capped by 15-100 cm
thick lenses of silty-sandy diamicton (Dms) containing angular
gravel- and boulder clasts (Fig. 9). The stratification of the
diamicton is due to Iaminae of silt and sand. Sometimes the
diamicton interfingers with the uppermost sets of cross bedded
pebbly sand, suggesting simultaneous deposition. The clasts do
not show any preferred orientation. Downbending of silt and
sand laminae was observed below some of the clasts.
The laminated silts and sands
Description:
The stratified diamicton grades upwards into a unit of laminated
to thinly bedded sandy silt (Fl) and silty sand (Sl) containing
subfossil molluscs and numerous gravel to boulder clasts (Fig.
6C). The unit is a rhythmic deposit, where 5-40 cm thick beds of
laminated sandy silt (samples 7 and 8, Fig. 4) alternate with
1-4 cm thick beds of laminated silty sand (sample 9, Fig. 4). The
unit is up to 20 m thick, and two counts of a number of silt-sand
couplets gave 143 and 160 sets. The average couplet thickness for
303 sets was 17 cm, where the silt- and sand beds were on the
average 15 cm and 2 cm thick, respectively. Each set begins with
a silty sand bed with sharp lower contact, which then grades into
laminated silt.
The unit smoothes out all undulations in the underlying strata
until it reaches planar parallel stratification. Structures observ-
ed within the unit include small scale folds near the base where
deposition has taken place on a sloping surface, convolute bed-
ding structures and “roll-up” structures (Fig. 7C).
The fossil molluscs in the interbedded silt and sand unit be-
long to the same faunal assemblage as the fossils in the Asbakkar
diamicton (Table II) but there are fewer species and individuals.
No fossils were found in situ nor as paired bivalves, the finds
being mostly fragments bearing evidence of transport. Some of
the fossils sampled may be derived from older sediments, and
therefore the radiocarbon dates (samples R(, - R9, Table III)
should be considered as maximum ages for the sediments. All
radiocarbon dated samples were collected from the lowest part
of the interbedded silt and sand unit (location in Fig. 2). Their
radiocarbon ages range from 11.715 + 120 BP to 11.465 + 100 BP.
Interpretation of the Látrar beds
I suggest that the Látrar beds were deposited during a
transition from an ice-marginal environment to an ice-
proximal glaciomarine environment (Fig. 5) with the
following sequence of events: The glacier retreated after
having overrun the Ás beds and the Ásbakkar diamicton
in the southern part of the cliffs. It left an erosional
hollow which could function as a sediment trap for glaci-
genic sediments carried into the basin by subglacial
meltwater streams. I interpret the cross stratified pebbly
sand facies as esker fan sediments, deposited at the ice
Fig. 12. Stratigraphy and structural fea-
tures at 3700 m, drawn from photo-
graphs. In the center, a large isoclinal
fold affecting the lowest lithofacies of
the Ás beds and the Ásbakkar diam-
icton. 12.1: (A) Ásbakkar diamicton,
(B) lowest sandy part of the Ás beds.
Zone of maximum shear shaded in
gray, (C) upper gravelly part of the Ás
beds, (D) stratified glaciomarine facies
of the Látrar beds, (E) Melagil gravels
and sands. 12.2: Sheared and folded
structures within the lowest silty-sandy
Ás beds lithofacies. 12.3 Boudins devel-
oped in facies Ss at the contact with the
Ásbakkar diamicton.
12. mynd. Jarðlagaskipan og dœmi um
höggun og hnik við 3700 m. Fyrir miðju
er stór felling.
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