Jökull - 01.12.1982, Page 49

on earlier sediments, and it is the shallowest non-till
unit from which they could have been derived.
1 hus we suggest the following temporal correlat-
ions: -
Unit F Upper Lake Sediments. These represent sedi-
mentation in the lakesince about 1930.
Unit E Glacial sediments laid down during the Little
Ice Age advance of the glacier since 1794.
Unit D Non-glacial sediments, probably partly or
wholly marine, in part of mid-Holocene
age.
Unit C Glacial sediments, possibly produced by ex-
tensive late-Weichselian glaciation. The
principal reason for this tentative conclus-
ion is that a marine limit of about 40 m
a.s.l. is present locally. This could corre-
late with the Late Weichselian marine lim-
it (Einarsson 1964), and if the Unit C glacia-
tion had post-dated this, and unless it were
by coincidence to terminate just beyond
Jökulsárlón, we would expect it to have
destroyed the evidence for the marine
limit.
Units B & A Non glacial and glacial sediments of in-
determinate age.
STRUCTURE OF THE COASTAL PLAIN
Seismic soundings of Breidarmerkjurjökull along
a ílow line directly up-glacier of Jökulsárlón (Ey-
thorsson 1953) reveal a subglacial bed which begins
to rise significantly above sea level some 10 km from
the glacier terminus (Fig. 8). It is suggested that
this point coincides with the emergence of bedrock
from beneath a cover ofglacial drift (e.g. Sigbjamar-
son 1970). Figure 8 also shows Jökulsárlón and the
unlithified sediments around it, and the sea-bed
profile directly south-south-east ofjökulsá for some
20 km ofi'shore. We suggest as a plausible hypothes-
is that the bedrock surface runs deep beneath
Breidamerkursandur and the immediately offshore
area, and that the coastal plain of Breidamerkur-
sandur is composed of a wedge of Late-Weichselian
and Holocene sediments rather than being a bed-
rock platform (Fig. 8). It is possible that this is also
true of other parts of the coastal pfain south of
Vatnajökull, especially east of Breidamerkurjökull,
where Pleistocene and Holocene volcanism has not
been a complicating factor. The postulated wedge
ofQuatemary sediment whichmakes up the coastal
plain, locally fdls to above sea level the deep bed-
rock valley in which Breidamerkurjökull fiows and
which extends across the continental shelf as
Breidamerkurdjúp (Fig. 8). Similar troughs on tfie
continental shelf lie offshore from the other major
southern valley glacier outletsof Vatnajökull.
It seems likely that the Late-Weichselian ancest-
or of Vatnajökull flowed onto the continental shelf,
and that the troughs on the shelf such as Breida-
merkurdjúp refiect glacial erosion during this and
earlier periods of major glacier expansion. We pre-
sume that the marine limit in this area at about 40 m
a.s.l. (Einarsson 1964) dates from the final recession
of Weichselian ice from the coastline, and reflects
strong isostatic depression of the crust below a low-
ered eustatic level. VVe suggest that the top of our
Glacial Unit C was exposed during this recession at
a time when sea level was 40 m above the present
sea level. At this time, the coastal plain at Jökulsár-
lón was 60 m below present sea level or -—100 m
below contemporary sea level. Our coastal sedi-
ment wedge hardly existed at that time. Sub-
sequently, relative sea level fell, leading to coastline
regression, which was aided by substantial prograd-
ation of the coastline during production ofa coastal
sediment wedge whose materials were primarily
derived from the glacier.
This hypothesis is schematically illustrated in
Fig. 8. 8a shows a Late Weichselian glacier flowing
into the sea and eroding a bedrock channel. Fig. 8b
shows retreat of such a glacier leaving a suite of
glacial sediments behind. Fig. 8c shows a largely
deglaciated coastal area in mid-Holocene times and
progressive progradation of the shoreline with sedi-
ments from a glacial source. VVe presume that this
continued until the Little IceAge glacial readvance.
Fig. 8d shows the effect of this readvance in eroding
away a large part of the previously deposited coastal
wedge in the Jökulsárlón basin, the deposition ofa
major glaciogenic suite and associated moraine
system, and the deposition of an outwash suite
beyond the moraine. The accumulation of sedi-
ments in the basin after this century’s deglaciation
is shown in Fig. 7.
ACKNOWLEDGEMENTS
We gratefully acknowledge the generous advice and ass-
istance of Flosi and Sigurdur Bjömsson of Kvísker and oj
Thorleijur Einarsson oj the University oj Iceland, all oj
whose practical and scientijic advice has been ojgreat help to
University ojEast Anglia parties in OraejiJor manyyears.
We also acknowledge generous grants Jrom the Natural
Environment Research Council and the Royal Society.
JÖKULL 32. ÁR 45