Jökull - 01.12.1982, Page 56

Fig. 3. Schematic section through the sedimentary sequence in the Trjávidarlækur basin. Legend: 1)
Primary tephra. 2) Reworked tephra. 3) Loessial soil with tephra layers. 4) Peat. 5) Lacustrine
sediments. 6) Sand with gravel. 7) Tillite.
Mynd 3. Einfólduð mynd af setl'ógum við Trjáviðarltek. Skyringar: 1) Gjóskul'óg, 2) Uml'óguð gjóska, 3) Fokjarðveg-
ur með gjóskul'ógum. 4) Mór. 5) Vatnaset. 6) Malarríkur sandur. 7) J'ókulberg.
Olsson at Uppsala University. The plant remains in
the peat are moss-like, but have not been studied.
Peat. The íirst signs of plant remains were found
in the cited silty layer, upon which rests a 2—2.5 m
thick peat deposit, which can be seen in proíiles II,
III and IV in Fig. 4and in Figs. 6, 8and 9. Regrett-
ably no botanic investigation has been carried out
yet. Birch (Betula) remains are most common in the
peat. Often logs and trunks are found in the peat as
indicated in the proflles. The biggest one observed
is about 10 cm in diameter (Fig. 8).
VVithin the peat deposit, there are sandy beds up
to 25 cm thick. The most conspicuous one was
identified as the acid tephra layer H5. It is light grey
in color and finegrained (sandy/silty) and up to 15
cnt thick in the channel banks. According to Thorar-
insson (1976), it is the earliest sign of Holocene volc-
anism in Hekla so far idendfied. Two samples were
taken for I4C dating, just above and below H5 (Pro-
flle III in Fig. 4). They gave the approximate age of
6,100 years BP (see also Table 1). The results are in
good accordance with previous datings of H5 which
were carried out on samples from the powerhouse
site of the Búrfell Plant (Thorarinsson 1971). The
peat deposit can be followed in the channel for
about 900 m.
The H4 tephra both prímary and reworked. The peat is
overlain by the second oldest acid tephra layer from
Hekla, designated H4, which is 4,000 years old
(Thorarinsson 1958, Larsen and Thorarinsson 1977). It
is found upstream in the channel and in profile II
(Fig. 4) the primary part is 2.2 m thick. It consists of
pumice fragments up to 3—4 cm in diameter, especi-
ally in the upper half. The lower part is greyish
white in color, but the uppermost 0.5 m become
gradually darker upwards. The primary tephra is
52 JÖKULL 32. ÁR