Jökull


Jökull - 01.12.1971, Side 56

Jökull - 01.12.1971, Side 56
1968). He has defined, as lst tectonic phase, the tilting of the basalts. Then followecl a period of very widespread, if not quite general, peneplanation of Iceland. During the second main tectonic phase, the country was cut into large blocks (along shear faults in a stress field which can now be considered as known by the cutting off and shear movement of dykes) which were uplifted to different elevations. In the Borgarfjördur area the peneplain is very clear at a level of 400 m, and here covered by flatlying younger rocks, of reverse and normal polarity (Einarsson, 1960, Fig. 3a, 1962). The time span represented by the period of peneplanation is difficult to estimate. The author has discussed this (Einarsson, 1968, p. 7567) and pointed out that there is a discrep- ancy between the time which geology seems to demand for a peneplanation (hardly less tlian 5—10 My) and K/Ar-datings of the topmost dipping groups which are certainly older than the peneplanation, and are found to be 2— 3 My. “If this age is accepted, planation and over 90% of the valley erosion would have taken place during 2 My, and during the re- maining 1 My relatively insignificant erosion would have taken place. This seems unlikely, and it would appear that radioactive dating of the basalts must still be used with caution, and that the age of the second tectonic phase is uncertain.” I still consider it an unsolved geological problem how a peneplanation of the hard basalts of Iceland could have been per- formed in about 1 My, to leave another 1 My for the valley erosion, in three generations with threefold grading of the main valleys to the respective base levels. Is K/Ar-dating of basalts so certain that such geological enigmas can be thrust upon us? The uppermost magnetic groups of the pene- plained plateau basalts contain vestiges of cold climate. This has mostly been taken to mean Pleistocene or Late Pliocene age, always by in- vestigators who have not known the later pene- planation, nor realized the three-generation development of the present valley landscape of the basalt areas. But taking that erosional work into consideration, one should realize that there is really no reason why Iceland could not have had a spell of cold climate long before Plei- stocene time. Cold climate flora would natur- ally accompany this cold climate, and by the content of the flora the age could not be deter- mined within the Upper Tertiary. Yet, it should be mentioned that the cold climate flora of the upper plateau groups has been considered to be of late Pliocene age (Jux, 1960). This would naturally be in keeping with the K/Ar-datings, but leave us with our erosional enigma. In the Hvalfjördur area, six polarity groups lie above the deepest horizon of cold climate (and local glacial) vestiges, but by extending the survey into middle southern Iceland, I con- cluded (Einarsson, 1962) that about 10 polarity groups do lie above this horizon. In the present East Iceland area, the number of polarity groups above the lowest cold climate vestiges is commensurate or a little larger. The Gilsá event was i. a. suggested by an ob- servation by one member, Hupjé, of a students training group led by Wensink (1964). A single normally polarized lava was found “ in tlie Gilsá river” within what was taken to be an otherwise uninterruptecl reversely polarized group of bas- alts. The elevation of this normal flow is given as 560—570 m above sea-level. The locality is not indicated closer, but “in the Gilsá river” at this altitude, indicates a locality where the river flows in flat terrain at about 65° 20.5' N, 15° 17' W, or outside the map showing the localities of samples, Fig. 1 in Wensink’s paper. Nor is the locality indicated in Wensink (1966), in the polarity map of Fig. 1. The Gilsá event was, however, basecl on the dating of other lavas, as indicated later. On the south side of the lower Gilsá river gorge, there is the high valley side of Jökul- dalur, reaching 636 m in the north (Arnórs- stadahnjúkur), high above the Gilsá gorge. Conditions for the study of polarity groups are in various respects much better here than up along the Gilsá river. Four sections of the northerly part of this Jökuldalur valley slope — besides further stu- dies farther south — were studied by me, jointly with Th. Sigurgeirsson and G. P. L. Walker. These sections were along the brooks Hnappá, Flnjúksá, Kringilsá, and Búdará. In all sections we founcl an about 60 m thick normal lava group overlying, with a clear contact, a sedi- ment that separates it from the underlying re- verse group which is found along the lower 54 JÖKULL 21. ÁR

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