Jökull


Jökull - 01.12.1982, Side 59

Jökull - 01.12.1982, Side 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
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