Jökull - 01.01.2012, Page 160

S. Brynjólfsson et al.
The middle layer (2) is a 40–50 cm thick, com-
pact, massive, matrix supported diamicton, with a
grey colour. It has fine-grained matrix, of clay to sand,
and carries numerous subangular to angular pebbles
and cobbles. It is very similar to layer 1, except for be-
ing somewhat less compact. Its lower contact is sharp
and erosional. We interpret layer 2 as subglacial till
formed during the 1912 surge.
The top layer (3) is about 40 cm thick, consist-
ing of a massive, dark grey, matrix supported diamic-
ton. The clasts are subangular to very angular cobbles,
pebbles and boulders. Layer 3 rests unconformably on
a compact diamicton with a sharp unit contact. We in-
terpret layer 3 as being supraglacial and/or englacial
debris that has melted out of the stagnant glacier ice
after the 1912 surge. The angular and very angu-
lar rocks indicate supraglacial and/or englacial trans-
portation of rockfall material. The sand grained ma-
trix is interpreted to originate from reworked basal
diamicton brought to the ice surface in the marginal
zone. We conclude that during a surge some sub-
glacial sediments are produced by erosion of the sub-
stratum and reworking of rock fall debris.
Landforms at the Búrfellsjökull and Teigarjökull
glaciers
The moraines formed by the surging cirque glaciers
Búrfellsjökull and Teigarjökull have similar charac-
teristics. Generally, the moraines are relatively small
and irregular; their crests are 1–3 m high and typ-
ically they are not symmetric (Figure 6A and B).
However, two of the moraines formed by the Teig-
arjökull glacier are different and much larger than the
other moraines. These moraines consist of diamic-
ton, rich in subangular to angular cobbles and boul-
ders. Occasionally, huge boulders occur on top of the
moraine crests. In some cases, these moraines appear
as steps in the landscape, proximally bordering to a
hummocky debris sheet or tongue (Figure 6C). The
process of formation for the moraine ridges is not en-
tirely clear, as there were no sections where their in-
ternal architecture could be studied, but the process of
dumping is indicated by the huge boulders deposited
on the moraine crests. The very coarse grained di-
amicton in the moraines reflects sediment supply from
the steep mountain slopes, supraglacial and englacial
transport of debris as well as short transport distances.
Hummocky moraine is prevalent in the forefields
of the Búrfellsjökull and Teigarjökull glaciers. Its sur-
face consists mainly of coarse debris of angular cob-
bles and boulders, and in the stratigraphic exposures
a coarse grained diamicton was observed. The hum-
mocky moraines are spotwise ice-cored, especially in
the younger formations where buried dead-ice is rela-
tively extensive, like in the drift from the 1971 surge
of Teigarjökull. Signatures of active dead-ice melting,
like extension cracks, backslumping, sinkholes and
collapse are widespread in the hummocky areas. The
hummocky terrain is the product of supraglacial and
englacial debris cover of the stagnant glacier snout as
it melts after surge.
Crevasse-fill ridges are relatively common in front
of Teigarjökull, but only a few occur in front of Búr-
fellsjökull. The crevasse-fill ridges (Figure 6D) reach
less than one metre in height and extend up to a few
tens of metres. They consist of coarse-grained diamic-
ton. Few poorly-preserved flutes, up to 10 metres in
length and not higher than 0.5 m, consisting of matrix-
supported diamicton, occur close to present margin of
Teigarjökull. They probably formed during the 1971
surge of Teigarjökull, and suggest basal sliding during
the surge. The flutes are situated on drift covering 10–
20 m thick body of stagnant-ice close to the present
glacier margin, indicating that a stagnant glacier ice
was overridden by the 1971 surge.
Prominent, ice-flow parallel low-amplitude ridges
occur from the ablation zone down into the glacier
forefields of Búrfellsjökull and Teigarjökull (Figure
7). The ridges consist mainly of angular to very angu-
lar cobbles and boulders. We interpret them as medial
moraines. Formation of such landforms has been de-
scribed by folding in response to lateral compression
as the ice moves from a wide cirque head into a nar-
row tongue further down the valley (Hambrey et al.,
1999). At the margins and boundaries of flow units,
the ice and the englacial debris is strongly folded. In
the ablation zone the debris is exposed at the surface
of the hinges and limbs of the folds, producing medial
moraines which combine towards the snout (Hambrey
et al., 1999).
158 JÖKULL No. 62, 2012