Náttúrufræðingurinn

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Náttúrufræðingurinn - 1978, Síða 53

Náttúrufræðingurinn - 1978, Síða 53
HEIMILDIR Eliasson, Sigunjin, 1974: Eldsumbrot í Jökulsárgljúfrum. Náttúrul'ræðingur- inn, 44: 52—70, Reykjavík. Sigurðsson, Oddur et al., 1975: Dettifoss- virkjun, jarðfræðiskýrsla. Orkustofn- un, Reykjavík. Sœmundsson, Kristján, 1973: Straumrák- aðar klappir kringum Asbyrgi. Nátt- úrufr. 43: 52—60. Tómasson, Haukur, 1973: Hamfarahlaup í Tökulsá á Fiöllum. Náttúrufr. 43: 12-34. Þórarinsson, Sigurður, 1956: On the Varia- tions of Svínafellsjökull, Skaftafells- jökull and Kvíárjökull in Öræfi. Jök- ull, 6: 1-15. — 1958: The Öræfajökull Eruption of 1362. Náttúrugripasafn Islands, Reykjavík. — 1959: Some Geological Problems in- volved in the Hydroelectric Develop- ment of Jökulsá á Fjöllum. Raforku- málaskrifstofan, Reykjavík. — 1960: On the Geology and Geophysics of Iceland by J. Askelsson et al. VIII. Jökulsárgljúfur and Asbyrgi, 69—74. — 1964: On the Age of the Terminal Moraines of Brúarjökull and Hálsa- jökull. Jökull, 14: 67—75. — 1971: Aldur ljósu gjóskulaganna úr Heklu samkvæmt leiðrcttu geislakols- tímatali. Náttúrufr. 41: 99—105. S U M M A R Y Some Remarks on Prehistoric Floods in the Jökulsá á Fjöllum Glacial River, Northeast-Iceland by Siguruin Eliasson, Skinnastadur, 671 Kópasker, Iceland. The author traced old flood deposits in several traverses across the Jökulsá River Canyons, NE-Iceland, in relation to C14-dated ashlayers (S. Thorarinsson, 1958, 1971) cf. Fig. 4. The main results are as follows: Striated lavas of probable Younger Dryas Period age were found in the Ásheidi uplands, pointing towards a more northerly advance of the ice-margin in that period than argued by K. Sæ- mundsson (1973). A thin layer of boulders and gravel in the western part of Ásheidi, dispersed on pahoehoe lava, is thouglit to be of glacial and glaciofluvial origin (Fig. 1). Striations arouncl the dry Ás- byrgi Canyon, attributed by Sæmundsson to transport of rock debris by catastrophic floods in earliest postglacial times, are identified as genuine glacial striae (Fig. 2). Deposits of three tremendous glacial floods were found in soil profiles around the Jökulsá River Canyons, dated re- spectively at 4600 y B.P., 3000 y B.P. and 2000 y B.P. (C14-age, half-value period 5730 years). High, coarse gravel terraces in the Forvöd Valley and the Svínadalur Valley were deposited in the flood of 3000 y B.P. All three floods split in two main forks a short distance north of the Hljódaklettar Crater Row, one flowing down to the Ásbyrgi Canyon, the other down the more easterly Landsgljúfur Canyon. The flood of 2000 y B.P. eroded many of the present wide canyons and united the two older gorges of the Ás- byrgi Canyon. On the Eyjan, a high cliffbound island-remnant of the wall be- tween the older gorges, is a boidderlayer, situated on top of the H3-ashlayer (C14- age 2900 y B.P.), Fig. 7 and 8. The prob- nble peak flow of this flood was at least 200 thousand m3/discharge/s, and was, perhaps, the biggest of the three floods. The Jökulsá River, however, heads in the Vatnajökull ice-cap. The Ásbyrgi Canyon is mostly eroded in three successive cata- strophic floods during the past 5000 years, but has probably never been the actual channel of the Jökulsá River. 179

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