Jökull - 01.12.1987, Side 66

water could explain the disappearance of molluscs from
the sediments. I interpret the sandy matrix of facies
Dmu to have accumulated through a combination of
settling of suspended material from turbid owerflows
(the silt fraction), icerafting (the sand and outsized clast
fraction) and possible density flow input of sand. The
interbedded units will be treated separately below.
There is some controversy in the scientific literature
as to the proximity of ice-rafted glaciomarine facies to a
glacier margin (Boulton and Dreynoux 1981, Molnia
1983, Powell 1983b, 1984, Mackiewich et al. 1984).
Though iceberg dropped coarse detritus enters the prox-
imal glaciomarine environment, it can be masked when
incorporated in the fjord floor sediments due to the
dilution effect of the much greater volume of finer grain
sizes. The relative magnitude of iceberg dropped mate-
rial, compared to other genetic components in the ice-
proximal environment, is a function of calving activity,
meltwater activity and debris content of the ice, i.e. it
varies in time and space for individual depositional ba-
sins, and given favourable conditions should be recog-
nizable very close to the ice front. In the case of Borgar-
fjördur, the high and steep mountains on the fjord/
valley eastern flank, and their easily weathered volcanic
rocks, could have introduced large quantities of debris
onto the surface of a passing valley glacier. In a few
instances in the cliffs, isolated mounds, pods or lenses of
material, on the whole much coarser than the surround-
ing sediments, were observed. These fit well to descrip-
tions given by Thomas and Connell (1985) and Domack
and Lawson (1985) of dump structures resulting from
release of debris aggregates from overturning icebergs.
Powell (1983b, 1984), Domack (1983) and Domack and
Lawson (1985) have modelled ice-rafted diamicton dep-
osition in a proglacial marine environment. I suggest
that facies Dmu, with a peak in dropped detritus, was
deposited in an environment similar to Powell's (1983b)
glaciomarine environment in front of a slowly advanc-
ing, actively calving tidewater glacier, and corresponds
to the dispersed meltwater and ice-rafted facies of Do-
mack (1983). I interpret this environment as an ice-
intermediate (molluscs disappear) to ice-proximal depo-
sitional environment (Fig. 5).
The intrabedded units
Description:
Intrabedded in the Asbakkar diamicton are units of interbedded
sand (Ss, Sl) and diamictons (Dms, Dmu). These are most
conspicuous in the southernmost part of the section (4900-
5250 m in Fig. 2). They usually occur in poorly defined lenses or
tongues, randomly stacked or superimposed on each other,
often with a lamina or thin bed of silt on the bounding surfaces
(Fig. 8). Occasionally, the intrabedded units have a iobate-like
appearance, with sharp to erosive base and slightly convex
upper contacts (Fig. 6B). There is a tendency towards concent-
ration of coarser clasts at the base of some of the units.
The diamicton lenses (Dms, Dmu) differ from the surround-
ing Asbakkar diamicton in having a coarser texture, with sub-
angular to rounded gravel clasts, floating in a sandy-silty matrix.
The stratified diamicton lenses have weakly stratified silt, sand
and gravels in the outer parts, and a core of either silty diamicton
or massive silty sand. The stratification is an expression of
intraformational discontinuous sand and silt bandings and lami-
nae, which sometimes appear to be sheared. I could not decide
whether this deformation is syndepositional or due to later
glaciotectonic deformation. Some of the interbedded facies also
show loaded structures, interpreted as convolute structures
(Fig. 7B) and small scale folds. The stratified sand (Ss) is poorly
sorted and sometimes pebbly. The stratification is due to normal
grading of the 5-10 cm thick sand beds, intralaminae of silt and
occasional rippled surfaces. Isolated stratified sand units also
occur, both as interbedded, 10-40 cm thick units, ranging lat-
erally up to 50 m, and as isolated bodies, resembling large scale
ball and pillow structures. The upper contact of interbedded
sand and diamicton units with facies Dmu is usually sharp, but
appears to be conformable.
Interpretation and discussion:
The Asbakkar diamicton is glaciotectonically deformed
(see later) and the intrabedded units are often displaced
so that it is difficult to reconstruct their primary sedi-
mentary geometry. A glaciotectonic origin of some of
the structures observed cannot be ruled out. I propose
that the interbedded sand and diamicton facies are
formed due to resedimentation processes and meltwater
influx, where semi-plastic to liquefied glaciogenic sedi-
Fig. 8. A close up of an interbedded diamicton and sand
body in the Ásbakkar diamicton at 5100 m: (1) silty-
sandy, stratified diamicton, (2) pebbly, massive diam-
icton. Black arrows point at the sharp but conformable
upper contact with the Ásbakkar diamicton. White ar-
row points at an intraformational silt lamina.
8. mynd. Seteining í Asbökkum.
64