Jökull - 01.12.1982, Side 41

Stratigraphy and Structure of a coastal Sediment Wedge
of glacial Origin inferred from Sparker Measurements
in glacial Lake Jökulsárlón in southeastern Iceland
G.S. BOULTON, P.W.V. HARRIS andj. JARVIS
School of Environmental Sciences,
University of East Anglia, Norwich, Norfolk NR4 7TJ, U.K.
ABSTRACT
Bathymetric and sparker surveys of Jökulsárlón, a large
lake at the margin of Breidamerkurjökull were undertaken
in 1975 and 1976 respectively. A detailed bathymetric map is
presented which shows the lake basin to have a depth of 150
m near to the ice margin and a total volume of500 x l(f m3.
The lake does not occupy a rock basin as had previously
been thought but has been entirely excavatedfrom an un-
lithified sediment sequence at least 120 m thick during the
Little Ice Age (between 1794 and 1932- 1965); a minimum
erosion rate of 0.37m/yr averaged over the whole lake area.
The area of Breidamerkursandur may not therefore be a
rock-floored coastal plain but a deep valley in which sedi-
ments have accumulated to above sea level. It ispossible that
the same is true for the whole coastal plain south of Vatna-
jökull.
Two principal glacial suites are recognised around the
Jökulsárlón basin; an upper one dating from the Little Ice
Age, and up to 35 m or more in thickness, and a lower one up
to at least 15 m in thickness deposited prior to 5,500 B.P.
andprobably of Late Weichselian age.
It is suggested that Breidamerkursandur is a thick coastal
sediment wedge which infills the deep glaáally-eroded valley
of Breidamerkurjökull (which extends out into the contin-
ental shelf) and that it developed during the Holocene.
INTRODUCTION
The fluctuations of the margins of outlet glaciers
of Iceland’s largest ice-cap, Vatnajökull, are relati-
vely well-known over recent centuries from histori-
cal records. These tell of glacier growth after about
1400 A.D. to a maximum during theso-called Little
Ice Age, with peaks in 1710, 1750 — 60, 1810(?),
1840-50and 1890 A.D. (Thorarinsson 1939; 1943).
Tomasson and Vilmundardottir (1967) believe that out-
let glaciersof Vatnajökull may have advanced by 20
km to their Little Ice Age maximum since the early
Mediaeval period, whilst Thorarinsson suggests
that Breidamerkurjökull advanced by up to 9 km
between 1740 and its Little Ice Age maximum in
1894. The rate and pattern of retreat ofglaciers from
their Little Ice Age maximum is generally well
established.
Unfortunately, the earlier history of Vatnajökull
is diflicult to establish. Much of the sedimentary
evidence of that earlier history which one would
expect to íind in the valleys radiating from Vatna-
jökull has been removed by fluviatile erosion, cover-
ed by recent glacial outwash or hidden by the still
relatively extensive ice cap.
A window through the sedimentary cover is pro-
vided by proglacial lakes which allow high resolu-
tion seismic surveys to be undertaken, and reveal a
stratigraphic record which is otherwise hidden.
A seismic survey was undertaken of Jökulsárlón
(Fig. 1) at the frontal margin of Breidamerkurjökull
during summer 1975 as part of a long-term Uni-
versity of East Anglia project on the glaciology and
glacial geology of this glacier and its forefield. The
object of the survey was not only to throw light on
the history of glacier lluctuation but also to enquire
into the origin of Jökulsárlón, the processes ofsedi-
mentation within the lake and the structure of the
sandur plain.
Howarth and Price (1969) surveyed the other lakes
on Breidamerkursandur apart from Jökulsárlón.
Derbyshire (1974) produced a rough bathymetric
map ol' Jökulsárlón and suggested that the lake
occupied a rock basin produced by prolonged glac-
ier erosion.
JÖKULL 32. ÁR 37