Jökull - 01.12.1985, Page 74

Fig. 2. Lichen growth curves for Rhiz-
ocarpon geographicum after Jaksch
(1975), and Dugmore and Williams,
in relation to maximum thallus dia-
meter on the proglacial sandur depos-
its, Sólheimajökull. 2. mynd. Þvermál
stærstu flétta sem fall af aldri á Sól-
heimasandi.
unvegetated scoured sandur remnants, numerous scat-
tered boulders and large-scale ripple and dune struc-
tures. The terrace surfaces are mantled by frost-shat-
tered and wind-blasted basalt and yellow-brown palago-
nite breccia fragments, embedded in a black gravelly
pumice matrix. These high terrace and channel features
clearly relate to former jökulhlaup events associated
with a period of ice expansion when marginal melt-
waters drained through a deeply dissected gorge
(Thurragil, Fig. 1). The northern margin of this high
sandur is truncated by a wide, incised channel meander
that sweeps first towards the south and then northwards
towards the valley wall. This channel finally emerges
about 4 km to the west at Skóga, and then turns
southwards to the sea. Area B represents the Sólheima-
sandur terraces lying to the east of the Jökulsá River.
The upper terrace surfaces lie at an elevation of c. 58—
80 m above sea level, and clearly postdate the formation
of the Skógasandur terraces. The lowermost terrace
(Zone X), however, is mantled by deeply weathered
cobble deposits, and appears to represent an exhumed
surface of much greater age. This Zone X deposit
indeed appears to extend westwards beneath the Skóga-
sandur, and southwards as far as the coast. Three relict
soil patches are present on the otherwise unvegetated,
soil-free terraces (Fig. 1), with the deepest soil section
reaching c. 2 m in depth, and comprising up to ten
tephra layers overlying the cobbly terrace surface. (See
later).
Areas C and D are located in the central Jökulsá
valley, incised between the Skóga- and Sólheimasandur
terraces. The Jökulsá sandur deposits include gravel,
cobble and boulder terraces, often with a substantial
mat of Rhacomitrium lanuginosum moss, Empetrum
nigrum, Vaccinium uliginosum, Salix lanata and S.
herbacea. The terrace deposits are clearly related to
terminal moraine ridges and lateral spillway channels in
the adjacent upland (e.g. Jökulsárgil, Fig. 1), associated
with a progressively retreating ice mass. The boundary
between areas C and D is marked by a relatively high
(up to approx. 100 m). steeply graded bouldery outwash
deposit (Zone XI on Fig. 1) known as the Holar deposit
(Landmaelingar Islands, 1973). This Holar deposit is
mantled with large washed moraine deposits („Proximal
outwash“ of Carswell at al., op. cit.) which appear to
represent the oldest deposit within the Jökulsá valley
itself (i.e. within zones C and D).
The distal zones of the Holar deposit are buried by a
lower, more gently graded valley sandur deposit (Zone
XIII in Fig. 1) in area C, associated with meltwaters
draining through Gorge 3 (Jökulsárgil). Area D extends
northwards from the Holar deposit to the present ice-
margin, and includes the series of moraines studied in
detail by Jaksch (1970, 1975) and by Dugmore and
Williams in 1983. In addition to the Holar deposit, 4
other main terraces, T2 to T5, have been identified,
with the higher T2 deposit occurring only in area C, and
the lower T3 to T5 terraces extending progressively
northwards through areas C and D (details given in
Maizels, in prep.).
Hence, according to morphologic and topographic
evidence the likely sequence of outwash formation com-
menced with Zone X, which formerly extended across
most of the glacier foreland, followed by areas A, B
(excluding Zone X), C and D. Within each of these
areas, the relative age of a number of individual
deposits has also been determined according to these
geomorphological criteria, and are listed in Table 1.
72 JÖKULL 35. ÁR