Jökull


Jökull - 01.12.1982, Page 53

Jökull - 01.12.1982, Page 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
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