Jökull - 01.12.1982, Blaðsíða 53

Holocene sedimentary Sequence at Tráviðarlækur Basin,
Pjórsárdalur, southern Iceland
ELSA G. VILMUNDARDÓTTIR and INGIBJÖRG KALDAL
National Energy Authority, Grensásvegur 9, 108 Reykjavík, Iceland
ABSTRACT
The Trjáviðarlakur basin is situated NE of the Búrfell
Hydropower Plant which started operating in 1970. Spill-
water Jrom the intake lake has cut a channel into the basin
floor and exposed a sedimentary sequence of Holocene age in
the channel banks. The deposits show that drastic environ-
mental changes have taken place during the last 10,000
years. The radiocarbon (l4C) method and tephrochronology
have been used Jor dating the sedimentary sequence. The
main points oj the Holocene hislory oj the basin are as
Jollows:
~~ 10,000-ca. 9,000years BP. Outwashplain was built up
mjront oj the Weichselian ice sheet.
~~ 9,000 years BP. Lacustrine sediments accumulated in
shatlow lakes or ponds. Colonization by plants took place.
~~ 9,000 - ca. 4,000years BP. The basin was covered with
vegetation as evidenced by a 2.5 m thick peat layer. Birch
(Betulaj was the most conspicuousplant. Within thepeat is
the Jinegrained acid tephra layer H5Jrom Hekla volcano
(approximately 6,100years BP, 10-15 an thick).
~ 4,000 ~ l,200years BP. Abruptly the basin was cover-
ed with more than 2 m thick H4 tephra (approximately 4,000
years BP), which sujfocated all vegetation. Winddrijted
tephra andsand accumulated (ca. 4 m), unlil the H3 tephra
Jell 2,800years BP. 1,200years BP loessial soil began to
Jorm and the basin was revegetated.
1104 AD untilpresent. In 1104 the Ht eruplion in Hekla
took place. The tephra caused desolation ojjarms in the area.
The vegetation recovered relatively quickly and ancient re-
cords give evidence oj wood utilization which culminated in
the 16th and 17th centuries. In the 18th and 19th centuries
the vegetation began dedining due to overgrazing and deter-
toration oj climatic conditions. Soil erosion set in and the
Trjáviðarltekur basin became denuded once again in the
Holocene. Atpresent the basin is barren with the exception oj
crooked birch bushes on adjacent slopes and some cultivated
grass pasture.
INTRODUCTION
The Trjáviðarlækur basin is situated at aboutone
kilometer’s distance to the southeast of the Búrfell
Hydropower Plant, which was started up in 1970.
The summit ofMt. Hekla is 14 km to the southeast
(Fig.l). To the south, Mt. Búrfell rises 669 m a.s.l.,
and the somewhat lower Sámsstaðamúli south of
Skeljafell forms the northern border. These moun-
tains form the south edge ofthe high central plateau
in Iceland. The Trjáviðarlækur basin, which is
about 1x2 km, slopes gently towards the Fossá
river, at 125-150 m elevation. It is almost barren
and covered with thick deposits, which are mostly
tephra. Before the construction of the Búrfell
Hydropower Plant a stream named Trjáviðarlækur
had an irregular course through the basin without
cutting into the underlying sediments. Since the
construction of the power plant the spillwater from
the intake lake has cut a channel, exposing the
sedimentary sequence of the basin.
GEOLOGICAL SETTING
The geological formations in the area shown on
Fig. 1 fall into two main classes:
1. Pleistocene formations (10,000-2,000,000
years).
2. Holocene formations (< 10,000 years).
1. Pleistocene Jormations, of which there are two
main types:
1.1 Lavas and hyaloclastites belonging to the
Matuyama reverse geomagnetic epoch, and clastic
interbeds, especially tillites, are also common.
Rocks of this type and age are found in the western
JÖKULL 32. ÁR 49