Náttúrufræðingurinn

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Náttúrufræðingurinn - 1993, Side 27

Náttúrufræðingurinn - 1993, Side 27
constitute less than one percent by volume. The tephra is ntoderately sorted and consists of fine and mediurn ash. On the basis of texture analyses, it can be con- cluded that these tephra layers formed in hydromagmatic eruptions, when hol magma comes into contact with suriace water or ice. This conclusion is in agreement with the locations oi the most potential sources, i.e. Grfmsvötn or Kverkfjöll volcanic systems, which are mostly covered by ice of the Vatnajökull ice cap. No direct dating method has been applied on the tephra layers or the peat deposits in Sogamýri, but several criteria can be used to evaluate the age of the tephra layers. Firstly, the close strati- graphic proximity of the tephra layers to the underlying clay indicates that they must be from early postglacial time. The ice of the last glaciation (Weichselian) retreated from the Reykjavík area, approxi- mately 10000 years ago. Secondly, with the aid of pollen analyses and radiocarbon dating of the peats in the Reykjavtk area it can be concluded that these two tephra layers are about 9000 years old. Thirdly, the upper tephra layer in Sogamýri can possibly be correlated with a tephra layer found in a Late Weichselian-Early Holo- cene lake sediments in North Iceland. The age of this tephra layer is about 8900 years B.P., according to radiocarbon dating. The geochemical composition of this layer and the upper Sogamýri tephra are sufficiently congruous to support this correlation. Recently, a basaltic tephra layer has been identified in the Saksunarvatn lake sediments in the Faroe Islands (Mangerud et al. 1986). Radiocarbon dating indicates an age of 9000-9100 years BP. A com- parison of textural and geochemical data reveals that the upper tephra layer in Sogamýri, rather than the lower one, closely resembles the Saksunarvatn lephra layer (Table 1). On the basis of this comparison, it is reasonable to conclude that these two tephra layers were probably formed by the same eruption event in either the Grímsvötn or Kverkfjöll volcanic systems about 9000 years ago. The thickness and areal distribution of the tephra layers found in Sogamýri is not known yet, but present data indicates that il must be substantial if thcir sources are subglacial in the Vatnajökull ice cap, as postulated here. Plausible equivalents to these tephra layers have recently been identified in several scattered locations in Iceland (Fig. 3). 137
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