Náttúrufræðingurinn - 1964, Blaðsíða 48

142
NÁTTÚ RU F RÆÐINGURINN
e. g. dating ol purely minerogenic deposits. Already before the application of
the C14 method a tolerably exact tephrochronology for the last 5000 years or
so had been established as shown e. g. by the good agreement between the
tephrochronological and the C14 dating of the layers H3 and H4.
The combination of C14 datings aml tephrochronology offers great possibili-
ties for the establishing of a detailed and exact chronology of postglacial de-
posits and postglacial development in Iceland and is also important from a
volcanological point og view.
Charcoal beneath the lava-tlow Laxárhraun yngra (8)
1940 ± 270 years (Y-87)
2110 ± 140 years (K-139)
1990 ± 50 years (Winnipeg)
The charcoal was formed wiien tlie lava overflowed vegetation-covered ground
near the mouth of the Laxá canyon. The section is shown on Fig. 7. Esti-
mated age of the flow was 2050 ± 250 years.
Of the same age as lliis flow are the famous pseudocrater groups in the
Mývatn area and the diatomite deposits 011 tlie bottom of the southern part
of lake Mývatn, which rests on this iava flow.
Peat beneath the tephra layer II, in EyjafjörSur, N.-lceland (7)
2720 ± 130 years (Y-85)
Charcoal formed when a birch tree was buried by layer H:i in Rangárbotnar (14)
2820 ± 70 years (St. 813)
The rhyolitic Hekla layer I I:i is the biggest tephra layer produced in Iceland
in Postglacial Time and covers about 80% of the country’s area. The estimated
age was “not less than 2500 — probably nearer 3000 — year old”. The position of
this layer in the stratigraphy of the Icelandic peatbogs (cf. Fig. 9) shows that
the severe subatlantic climatic deterioration began about 600 B. C.
Peat beneath the tephra layer H: in Eyjafjörður, North Iceland (7)
4200 ± 150 years (K-120)
3850 ± 150 years (K-120)
Aver. 4030 ± 150 years
This Hekla teplira layer is the second Uiggest postglacial tephra layer in
Iceland and covers about the same area as H:i. Both are found all over N and
NE Iceland (cf. Figs. 8 and 10) as well as in the central part of the country.
Estimated age was “about 4000 years”.
Peat bcncath tlie tephra layer H.-, in Eyjafjörður, North Iceland (5)
6520 ± 190 years (K-141)
6710 ± 250 years (K-141)
Aver. 6610 ± 170 years
This layer is the oldest rhyolitic Hekla layer found in postglacial soils in
Iceland. Spread over central and northern Iceland (cf. Figs. 9 and 10). Esti-
mated age “7000—8000 years, probably somewhat less”.