Jökull


Jökull - 01.12.1975, Blaðsíða 29

Jökull - 01.12.1975, Blaðsíða 29
shape, while the smaller valleys of the system were not so affected, and either still retain their V-shape cross section or their smooth floor at 250—300 m elevation. Thus, most of the erosional work was done before the beginning of the Pleistocene Ice Ages. After a further general uplift of the Tertiary country bv about 200 m, the large valleys were graded anew to a lower base level (3rd generation). At the same time a widespread strandflat was eroded. When this was practically finished, it was covered by lavas of reverse polarity, the Iv/Ar-date of which is close to 1 My. Using this — probably too low — age for the late strandflat stage as a basis, and comparing the quantity of material eroded and carried away during the older erosional stages, the present author came many years ago to the minimum figure of 15—20 My for the whole erosional time. But instead of a comparable figure as to order of magnitude, K/Ar-datings in Iceland leave even less than 1 My for all this erosional work. We think that every scient- ist, acquainted with geomorphology, will agree that these K/Ar-datings are too low by more than an order of magnitude. This includes the paleomagnetic Gilsá event. In this connection we shall return to the question of reconstructing groundwater history. Near-horizontal zeolite zones in the Tertiary basalts of the Eastern Fjords were first described by Walker (1960) in a long section co-direc- tional with the dip (6—8°) of the basalts. Corre- sponding zones have later been studied in other Tertiary areas in the country, and can probably be considered as general. The near-horizontality of the zones implies, as can be demonstrated by onyxes, that the process of tilting was nearly finished and had stopped during the development of the zones, for the onyxes and the zones dip 1—2°, to in- crease the earlier tilting. The formation of the zeolite zones suggests a relatively flat horizontal land surface at the time, emphasized further by the indications of near-stagnancy of the groundwater (Walker, 1. c., found that the material of zeolites is derived from the local rock), ancl the zones may thus be linked with horizontal isotherms under the peneplained, previously tilted blocks. The zeo- lites imply such temperatures, and such solution of feldspars by groundwater, as to make it highly questionable whether argon could have been retained in feldspars — disregarding glass — to be still found in the exposed rocks. But after the blockfaulting and uplift of the peneplained land during the Second Tectonic Phase, as defined by the author in earlier papers, ancl especially after the dissection of the raised land by the lst, and especially the 2nd valley generation, the rocks which are now exposed, and were above or a little below sea- level, would have been permeated and cooled by a relatively cold downward flowing ground- water because of the then existing hydrostatic head. From now on, the feldspars in such rocks might better retain the produced argon, if they were deep enough under the surface. The highest K/Ar-ages (12—16 My) found in rocks rapidly uncovered during the Pleistocene, might then give a lower age limit for this dissection stage of the basalts. This interpretation of the measured ages is in a remarkably good harmony with the author’s earlier estimate, on morpho- logical grounds, of the age of the earliest val- lays. On the basis of palynological work (Pflug, 1959), an Eocene age was inferred for the base of the basalts of Eastern Iceland (12 My by K/Ar-dating), and by the lack of any signific- ant erosional or depositional intervals, the higher lava groups are not much younger than the lowest ones. Also the Brjánslækur flora in western Iceland is considered Eocene by Pflug (1959). A more general study of that flora (Friedrich, 1966) led to this result: "nothing speaks against a Lower Tertiary age but, on the other hand, nothing against a Miocene age”. We suggest that the safest way of dating Tertiary rocks in Iceland, and the tectono- morphological stages of these rocks, may in the end be the coupling of climatological, tectonic, magnetic polarity, and marine-paleontological data, as indicated in a separate paper soon to be printed. Our interpretation of the maximum ages in Iceland leads to the question why higher ages (50—60 My) are found in the basalts of NW- Britain, because these are all altered, to even a higher degree than the Icelandic ones. A temperature of up to 200°C or even higher, under an original cover of the present expo- JÖKULL 25. ÁR 27
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