Jökull - 01.12.1987, Page 69

Fig. 10. (A) The Melar diamicton at
1640 m. Its lower contact with facies
Dms of the Ásbakkar diamicton is very
sharp, and clasts are thrust or lodged
into the substratum. The arrow points
at a double facetted stone. The diam-
icton is overlain by the glaciomarine
facies of the Melabakkar silts and
sands, (B) The Melar diamicton at 1550
m, where it rests on facies Dmu of the
Ásbakkar diamicton. Notice the large
number of angular boulders, (C) A lag
concentration of boulders on top the
Landhólmi sands delta deposit, (D)
Large, striated boulder on the surface
immediately above Látrar.
10. mynd. A og B: Jökulruðningur í
bökkunum. C og D: Stórgrýtisdreif frá
ruðningnum í sniði og á yfirborði.
There are three major facies of sand in the sequence overlying
the silt facies (logs F and G, Fig. 3), divided into a number of
beds: Massive to crudely bedded, moderately sorted to silty
sand (Su) stratified sand (Ss) and laminated silty sand (Sl). The
sand units range in thickness from about 0.2 m to about 1.2 m,
with conformable and loaded or erosional lower contacts, and
stratification due to normal to reversed grading of the individual
beds. Random intraformational lags of coarse sand to pebbly
sand occur in most units. In facies Su, intralaminae of silt,
resembling dish structures, were observed. Facies S1 is a succes-
sion of thin, silt coated sandbeds.
The Ás beds diamictons
Description:
Interbedded with the sand- and gravel facies in the sequence are
discontinuous sheets or lenses of massive and stratified diam-
ictons (Dmu, Dms, logs F and G, Fig. 3). Their matrix is
sandy-silty, but they display considerable varibility in both tex-
ture and clast content. Individual units range in thickness from a
few centimetres to about one m, and, where occurring isolated,
have sharp or erosive bases and a tendency towards convex
upper contacts. On one occasion a 3 m thick diamicton succes-
sion was observed (Log F). Facies Dms has intraformational
bands of silt and thin sandbeds. Both facies Dms and Dmu show
a tendency towards concentration of gravels at the base. The
diamicton units appear to be randomly interbedded with the
lithofácies of sand and gravel, suggesting penecontemporane-
ous deposition1.
Interpretation of the Ás beds
The transition from the Ásbakkar diamicton to the gen-
erally coarser overlying sequence indicates an increased
energy input into the depositional basin. In the sub-
polar glaciomarine environment, proximity to both the
ice margin and to the source of meltwater plays a key
role in determining the lithofacies distribution (e.g.
Molnia 1983, Powell 1984). Powell (1983b) pointed out
that supraglacial streams are absent and englacial
streams rarely flow from tidewater glaciers, due to their
highly crevassed nature. The meltwater discharge from
such glaciers is almost entirely subglacial, and results in
a proglacial submarine outwash deposit, composed of
several lithofacies reflecting discharge, sediment load
and the proximity of the deposit to the glacier margin.
The lithologies and structures of the Ás beds are best
explained as being a result of a combination of sedi-
mentation from subaquatic ice-marginal outwash
streams and glacigenic subaquatic debris flows. I suggest
that the sand and gravel facies are ice-proximal melt-
water fan sediments, where the gravel facies represent
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