Jökull


Jökull - 01.12.1980, Síða 11

Jökull - 01.12.1980, Síða 11
suggested a maximum sea water temperature of 5 °C higher than now. He considered the analyses of trees to show that air temperature was also higher. Schwarzbach concluded that the Breiðavík deposits must be considered of Pleistocene age as they rested on a moraine. In his work “Climates of the Past”, Schwarzbach (1963) discussed Tertiary and Quaternary climatic evidence from Tjörnes. The pollen flora of the Pliocene deposits (Tjörnes beds) and the Pleistocene deposits (Breiðavík beds) on Tjörnes was described for the first time by Schwarzbach and Pflug (1957), and was found to point to a climate similar to that of the Holocene Atlanticum of Central Europe. A climatic cycle with an optimum in the Mactra Zone was inferred. Pflug (1959) studied spores from the Tjörnes and Breiðavík beds. He con- cluded that the Tapes and Mactra Zones were of Pliocene character and age, but that the Breiðavík beds were of Quaternary age. Schönfeld (1956) studied tree remnants from the Tapes Zone and suggested similarities bet- ween the Icelandic Tertiary flora and the present North American one. Reinterpretation of the position of Tjörnes with regard to the geology of Iceland Apart from palynological studies, the fifties brought a new effort by Dutch scientists towards an understanding of the origin of the Palagonite Formation of Iceland and the structure of the Icelandic Central Graben. The first contribution from this direction came from Hospers (1953, 1954), who studied palaeomagnetism in eastern North Iceland. Bemmelen and Rutten (1955) studied amongst other things questions of tillites and breccias, morainic and fluvioglacial sediments, and related phenomena. Tjörnes was among the areas visited by Bemmelen and Rutten. They agreed with the generally held view (Thorodd- sen 1906, Bárðarson 1927) that Iceland was symmetrical about a central depression, and that the country represented a horst structure. According to Bemmelen and Rutten the oldest rocks, the Tertiary Plateau Basalts, were cov- ered by a series of Older Pleistocene Basalts with intercalations of tillites (Laxárdalur Series to the west of the graben in Northeast Iceland, corresponding to the Jökuldalur Series on the eastern side, both being identical with Pjetursson’s Grey Stage unit). In the ex- treme north, the marine Pliocene deposits of Tjörnes were intercalated between the Ter- tiary Plateau Basalts and the Laxárdalur Series. The Tjörnes sediments had, according to Bemmelen and Rutten, accumulated in a top depression of the Icelandic horst. Rutten and Wensink (1960a, 1960b) discussed the structure of the Central Icelandic Graben as well as palaeomagnetic dating, glaciations, and the chronology of the Plio-Pleistocene in Iceland. They correlated the Grey Stage unit with the Nl, R1 and N2 palaeomagnetic groups of Plio-Pleistocene age (Astian — Villa- franchian), and suggested that the Ter- tiary/Quaternary boundary should be defined at the reversal between the N2 and R1 groups in Iceland, providing a much better datum line than any index fossils or climatic indices. The palaeomagnetic studies were continued by Wensink (1964), who correlated the Tjörnes sequence with the palaeomagnetic groups. His correlation was based on a geological and palaeomagnetic study of the sequence by Broekman (1961). Wensink corre- lated the Tjörnes beds with the N2 geomag- netic period and placed the Breiðavík deposits at the geomagnetic N2/R1 boundary, thus being of a possible early Pleistocene age. The correlation of the Jökuldalur Series with the geomagnetic groups suggested by Rutten and Wensink (1960a, 1960b) was later confirmed by radiometric dating of the Jökuldalur Series, and the R1 and N2 polarity groups were found to correspond to the Matuyama rever- sed and the Gauss normal polarity epochs respectively (McDougall and Wensink 1966). McDougall and Wensink found a tillite im- mediately above a basalt dated at 3.10 ± 0.10 Ma and suggested that the base of the Pleistocene might be as old as 3 Ma. The origin of breccias and conglomerates in the Icelandic Palagonite Formation was dis- cussed in a treatise on the geomorphology of JÖKULL 30. ÁR 9
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