Jökull - 01.12.1982, Page 59

Fig. 6. A section in the channel bank near profile
IV. 1) Sand with gravel. 2) Lacustrine sediments.
3) Peat with 6100 years old tephra layer, H5.
Mynd 6. Snið í Itekjarbakkanum nálægt sniði IV. 1)
Sandur með möl. 2) Vatnaset. 3) Mór með hinu 6100 ára
gamla gjóskulagi H$.
INTERPRETATION AND DISCUSSION
In the Trjáviðarlækur basin, sediments have
been accumulating ever since the Weichselian main
ice melted away from the area. It is supposed to
have left the Búði end-moraines, which lie 35 km to
the SW of the basin, some 10,000 years ago (Kjart-
ansson 1943).
The sand with gravel, the oldest deposit of the
basin, is part of a much greater outwash plain,
which started to build up at the margin of the inland
high plateau during the deglaciation in Preboreal
time. The much greater and glacierfed forerunner of
the Fossá river carried a heavy sediment load into
and through the Þjórsárdalur valley and took part
in building the vast South-Iceland outwash plain.
Initially the sea level was 100-110 m higher than at
present, but regressed rapidly and had reached 20
m level below the present one 9,000 years ago
(Kjartansson et al. 1964). The sand thickness in the
basin is at least 25 m, as can be seen it the borehole
BH-16 (Einarsson and Tómasson 1962). The cross
bedding clearly shows the fluviatile character of the
sand. It reaches approximately 133 m elevation in
the basin and is found exposed in the banks of the
Fossá river, below the much younger (about 3,000
years old) Þjórsárdalshraun lava.
Quite abruptly the sandur formation came to an
end, followed by much more finegrained deposits of
a lacustrine character. These deposits are rather
poorly stratified and less than 1 m thick in the
channel banks. Their character points to shallow
ponds rather than a large lake. The 8,950 years old
peat layer in it gives a very interesting clue to the
age of the lower part of the basin deposits and also
shows that plants had colonized the area about
9,000 years ago. The plant remains show moss-like
character, but have not been studied so far.
The age of the peat deposit is well defined at
between 4,000 and 9,000 years BP l4C age.
Throughout the 5,000 years period, birch wood
grew in the Trjáviðarlækur basin. Below H5 (6,100
years BP) the peat is approximately 1 m thick,
giving an average 0.3 mm/year thickening. Between
H5 and H4 this is 0.55 mm/year, and the yearly rate
for the peat as a whole is 0.42 mm/year. The thick-
ening rate of the Icelandic peat deposits has not
been estimated so far (Einarsson 1975). Therefore it
is impossible to tell whether the above figures show
high, average or low thickening rate for this period
(4,000-9,000 years BP). According to Einarsson
(1975) the Icelandic peat bogs are usually 2 - 6 m
thick, but information on peat thicknesses in any
given period of the Holocene is not available.
The authors belive that the basin was continu-
ously covered with vegetation during this 5,000
years period. This opinion is supported by the fact
that the 1+C dated samples fromjust belowandjust
above the H5 tephra are of practically the same age.
Nowhere in the peat strata have unconformities or
weathering horizons been noticed. On the contrary,
the stratification in the peat seems to be regular and
continuous.
The peat accumulation came to a sudden halt
caused by the H4 tephra. The 2 m thick primary
tephra layer has spelt doom to the vegetation,
JÖKULL 32. ÁR 55