Jökull - 31.12.2001, Page 12

Jukka Kayhkö et al.
Figure 5. An example of the classes b3: Fine sediment and b4: Medium sediment, on the Dyngjujökull sandur
(8420300, 7200600; cf. Figure 3). In the foreground, the poorly sorted alluvium with boulder-size rocks repre-
sents class b4. Class b3 in the background shows fewer boulders, and a surface covered with aeolian sand. Note
the person for scale. - Horft til Herðubreiðar afsöndunum norðan Dyngjujökuls. Dæmi um yfirborð í flokkum
b3 og b4.
a lobe of sand ca. 12 km long and 2 km wide (Figure 3;
site N), and is gradually growing in length towards the
NE by filling depressions on the rough lava surface.
Dyngjufjalladalur is a N-S oriented gorge, be-
tween Dyngjufjöll ytri and Askja, with the charact-
eristics of a water-affected canyon. The bottom of
the steep-sided valley is covered with thick, poorly-
sorted sedimentary deposit that dips gently towards
the north. Large matrix-supported boulders, up to one
metre in diameter, crop out from the valley bed along
a river channel, indicating a very high-energy envi-
ronment during sediment deposition. Together, these
features suggest that a past flood event (perhaps even
a jökulhlaup) originated at Vatnajökull and drained to-
wards the north, passing Askja, this time on the west-
em side via Dyngjufjalladalur. An abrupt decrease in
transport momentum apparently occurred when these
flood waters exited the narrow gorge, inducing the
sandur deposition. To the north of the valley outlet, a
6xl0km sedimentary formation has developed, con-
sisting mainly of classes b3-b6 (Figure 3; site O).
Other evidence of past catastrophic floods can be
detected further east, along Jökulsá á Fjöllum. Down-
stream of several abrupt channel bends, large out-
wash plains extend several kilometres beyond the cur-
rent channel, in the direction of flood momentum.
The most prominent examples of flood deposits are
found near Möðrudalur and Grímsstaðir. The out-
wash plain surface (Grjót) in Möðrudalur measures
ca. 10 x lOkm on the eastern side of the present river
channel, and has a flat, winnowed lag surface with
ventifacted boulders (mostly classes b4 and b6) sev-
10 JÖKULLNo. 51