Jökull - 01.12.1982, Page 47

reflects a disorderly arrangement of physical
properties at the resolution scale of the system,
which could reflect materials deformed in a
complex fashion, or till. In either case, a glacial
unit is implied.
b) A coherent parallel-layered reflector which
coring reveals as a well-bedded silt with minor
sand and clay components and occasional
dropstones. We presume that internal reflect-
ors represent the gross-disposition of bedding
within these sediments.
c) A texture composed of locally coherent parall-
ed-layered reflectors which are similar to those
of b) but which lack its continuity. This texture
occurs between glacial units. We presume it to
represent disturbed equivalents of b).
d) A strong reflector located oíf the fronts of obvi-
ous deltas and fans, in which there is overall
parallelism of intemal reflectors, though in-
dividually they may intersect. Grab samples
show these to be sand or gravelly-sand bodies.
Application of these interpretations to the profil-
es allows us to suggest a seismic para-stratigraphy
(Krumbein and Sloss 1951). This is not a litho-strati-
graphy in the conventional sense, but it allows us to
identify the geometry of the principal sediment
masses. From this geometry and the generalised
interpretation of their lithological character summ-
arised above we reach generalised conclusions
about the origin of the individual masses, although
m detail difierent lithologies ofdiflerent genesis may
occur within them. Our interpretations of the profil-
es are shown in Fig. 7.
On the basis of these profiles we propose the
following parastratigraphy: -
6) Upper Lake Sediments - Unit F. These are sedi-
ments which have accumulated in the lake after
it was revealed by the retreating glacier. They
are marginal beach sediments, deltas and fan
deltas, slumped beds and the beds accumulat-
ing on the lake floor. They will be described in
detail in a subsequent paper. In the central
part of the lake they are up to 40 m in thickness.
5) Glacial Unit E. This is almost exclusively re-
stricted to the southern margin of the lake,
where it forms a wedge up to 35 m in thickness.
It is the equivalent of the till and glacially-
deformed sediments exposed at surface at the
southern margin of the lake. The major mor-
aine ridges which formed at the glacier termin-
us during its late 19th century maximum
beyond the southern end of the lake mark the
southern extremity of this wedge of glacial mat-
erial.
4) Inter-Bed Unit D. Up to 15 m in thickness.
3) Glaáal Unit C. Up to at least 15 m in thickness.
2) Inter-Bed Unit B. Up to at least 12 m in thick-
ness.
1) Glacial Unit A. Thickness unknown.
Several important conclusions may be
drawn about the Jökulsárlón basin: -
a) The basin has been cut out of unlithified sedi-
ment. No bedrock has been positively identifi-
ed.
b) The flat floor of the basin is a result of sedi-
mentation after ice retreat. The concentration
of the Upper Lake Sediments in the lowest part
of the basin presumably reflects strong slump-
ing down its flanks.
c) Glacial Unit E can be traced directly into the
tills which comprise the moraines deposited
during the maximum extent of Breidamerkur-
jökull in the Little Ice Age, and into the tills
revealed beyond the glacier margin as it re-
treated from this maximum (Fig. 6, 7). Thus,
we conclude that Glacial Unit E belongs to the
Little Ice Age.
d) The gross disposition of reflectors in the sequ-
ence deposited prior to Glacial Unit E does not
reflect the form ofthe lake basin (Fig. 6 and 7).
There appears to be a major unconformity at
the base of Units E and F which does reflect the
lake basin form. We thus conclude that Units
A-D were deposited prior to the formation of
the lake basin, which occured beforeor during
the deposition ofUnit E in the Little Ice Age.
e) The net relief of the lake basin is the combined
results of erosion of Units A-D, the varying
thickness of Glacial Unit E, and subsequent
infilling of the basin by the Upper Lake Sedi-
ments (F).
Some limits can beset to theageof the pre-Unit E
sequence. On the western bankofjökulsá, some 150
m below its exit from the lake, a series of silts and
sands are interfolded within till. These sediments
contain shells, comprising some whole as well as
fragmental valves, predominantly of marine moll-
uscs. A carbon-14 assay on several whole valves
yielded a date of 5,464 ± 60 B.P. (SRR-948). These
mollusc-bearing sediments are clearly not in their
original position of deposition but have suffered
glaciotectonic transport. We argue that they are
most likely to have been derived from Inter-Bed
Unit D, as at no place has Unit E been found to lie
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