Jökull - 01.12.1987, Page 70

Fig. 11. The most common lithofacies
of the Ásgil gravels: (A) Massive grav-
els, (B) A multistorey sequence of mas-
sive and stratified gravels, (C) Planar
paralel stratified, poorly sorted sand,
overlain by massive gravels, (D) Angu-
lar based, planar cross stratified, peb-
bly sand, overlain by sandy, massive
gravel.
11. mynd. Malar- og sandeiningar í lag-
skiptum jaðargarði.
deposition from bedloads of meltwater streams at a
submarine fan apex, the gravelly and stratified sand
facies represent deposition in a meltwater channel, and
the silty sand and the massive to crudely bedded sand
facies represent proximal interchannel deposition. Rust
and Romanelli (1975), Cheel and Rust (1982) and Do-
mack (1983) described similar facies distribution from
ice-marginal subaqueous meltwater fans. The coarse
gravels and the tangential foresets of facies Gp indicate a
relatively fast flowing depositing stream. I interpret the
diamicton facies as glacigenic subaquatic debris flows,
representing a proximal facies to the intrabedded dia-
micton units found in the Ásbakkar diamicton.
Processes affecting the sandfacies deposition could
include bottom current or traction load deposition, re-
sulting in relatively well sorted sand, rippled bed surfac-
es and sand/gravel lags, and high concentration under-
flow turbidity currents, resulting in relatively poorly
sorted silty sand and graded bedding. Massive to crudely
bedded sands in the ice proximal glaciomarine envi-
ronment have been attributed to a deposition from high
concentration sediment flows during periods of high
meltwater discharge from a glacier (Cheel and Rust 1982,
Powell 1984, Eylesetal. 1985). The laminated silt andsilt
coatings on unit contacts could be due to fluctuations of
meltwater and its sedimentary load and/or shifts in the
concentration of glaciofluvial activity along the ice mar-
gin. Mackiewich etal. (1984) pointed out that interaction
of stream discharge with tidewater also causes fluctu-
ations in particle sizes, and that tidal currents can pro-
duce thin, discontinuous silt laminae. The loaded struc-
tures and the dish structures indicate rapid deposition
(Collinson and Thompson 1982), and by analogy to pre-
sent day sub-polar ice-proximal glaciomarine environ-
ments (e.g. Powell 1981, Gilbert 1982, Molnia 1983) a
very rapid deposition can be suggested for these sedi-
ments.
The abrupt lateral and vertical facies changes in an
overall coarsening upwards sequence, leads me to con-
clude that the facies association was deposited close to
the grounding line of the glacier. I interpret it as a strati-
fied ice proximal/ice marginal deposit (Fig. 5) built up as
a shoal or bank in front of a slowly advancing glacier.
Powell (1984) described glaciomarine processes related
to morainal bank construction in front of a warm based
tidewater outlet glacier, and modelled the inductive
lithofacies. Fle modelled a lithofacies association related
to a morainal bank deposition similar to the one de-
scribed above.
The contact between the Ás beds and the Ásbakkar
diamicton is transgressive, i.e. while the the morainal
bank built up at the glacier margin, glaciomarine sedi-
ments with interbedded diamictons and sands were be-
ing deposited beyond the bank. The relative sea level
during the deposition of the Ás beds was as high as
during the deposition of the Dmu facies of the Ásbakkar
diamicton.
THE LÁTRAR BEDS: AN ESKER FAN
FACIES AND GLACIOMARINE FACIES
ASSOCIATION
The Ás beds are discordantly overlain by the Látrar
beds, a major sequence of gravelly sands (Sp, St), and
diamictons (Dms), and laminated silts and sands (Fl, Sl)
(Fig. 3, logs C, D, F), exposed between about 1650 m
and 3650 m in the cliffs (Fig. 2). All lithofacies belonging
to the unit can be observed at Látrar, around 2100 m.
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