SOME CHAPTERS OF THE TERTIARY HISTORY OF 



ICELAND 



TrAUSTI ElNARSSON 

 University of Iceland, Reykjavik, Iceland 



The oldest rocks in Iceland are Tertiary plateau basalts. These basalts, which 

 form the western, northern, and eastern parts of the country, are remnants of 

 a 5-7 km thick pile or plateau of lava flows. The plateau must originally have 

 had a much greater extension than the present island, as is evidenced by the 

 truncation of the plateau at the coasts. The lowest accessible parts are of 

 Early Tertiary, probably Eocene or possibly Uppermost Cretaceous age, 

 while the highest parts are of Upper, possibly Uppermost Tertiary age. The 

 lavas were formed entirely on land. Their base is unknown. 



The Icelandic basalts are usually grouped with similar rocks in north- 

 western Britain, the Faeroe Islands, Greenland, Spitsbergen, and Franz 

 Josef's Land under such names as the North Atlantic, Brito-Arctic, or 

 Thulean plateau basalts. The lowest parts of the Icelandic plateau and the 

 basalts in Britain, the Faeroe Islands, and Greenland seem to be of a similar 

 age, whereas the basalts of Spitsbergen are somewhat older. In Iceland no 

 marine fossils, nor fossil land fauna, are associated with the main part of the 

 plateau and the age is based on lignites which are found at a number of 

 horizons. This means that the age determinations are considerably uncertain. 



The assumption was made long ago that the plateau lavas were formed on 

 an extensive land mass occupying the area of the present North Atlantic or 

 at least large parts of it, and successive foundering of much of the land has 

 been postulated. The question is naturally important in the present context 

 and we shall therefore consider some aspects of it, although it must be said 

 at the outset that we can say very Httle with any certainty. 



Postulation of a large-scale foundering of a continental area cannot be 

 made without consideration of the isostatic equilibrium. This leads to specula- 

 tion about processes in deep-crustal or sub-crustal regions. Foundering could 

 have taken place if a relatively light deep layer grew thinner by spreading or if 

 such a layer became denser by crystallization. Thus, foundering is not defin- 

 itely excluded, but by postulating it we introduce inevitably deep- or sub- 

 crustal processes of a wholly hypothetical character. 



Paleontologically, there is no strong evidence that Iceland was connected 



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