Greinar (Vísindafélag Íslendinga) - 01.01.1977, Síða 26

Chapter 2
THE UPPER TERTIARY BEGINNING OF ZONAL VOLCANISM
AND TECTONISM IN ICELAND
THE AGE OF THE TJÖRNES DEPOSITS AND THE EARLY
TERTIARY HISTORY OF ICELAND
CONTENTS
The oldest untilted basalts in the Median and Eastern Active Zones.
The valley system eroded by non-glacial river into these basalts.
The formation of Table Mountains and other Tuff-Breccia masses; form-
ed under the influence of river and/or lake water in these valleys in Ter-
tiary and Interglacial times.
Search for marine sediments by drilling in Southern Iceland. The Upper
Pliocene Age of the Breiðavík deposits. Lower Pliocene age of the Cardium
division of Tjörnes. Beginning of deposition of the Skjálfandi-Tjörnes de-
posits in the Oligocene. Oligocene age of topography in the Basalt Areas.
Cold climate and mountain glaciation during the Eocene regression.
In a recent paper based on a detailed study of the Median
Active Zone of Iceland (15), the present author demonstrated
that no „spreading" has taken place within the Zone on the
south side of Lake Þingvallavatn during approximately the
last one million years. The dating is based, on one hand, on
the extensive morphological work done in the area, and on the
other, on the occurrence of a reversely polarized volcanic mem-
ber at one of the highest stratigraphic levels.
On this basis, the two Pleistocene shieldvolcanoes on each side
of the lake, the Mosfellsheiði and the Lyngdalsheiði, must
have an age of no less than one million years. In Chapter 3 we
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Fig. 5. The prototype of Icelandic Table Mountains, Skriða, is seen here to
lie astride a pre-existing valley, the Mjóidalur. The original mountain was
probably a flat shield, similar to Hvalfell (cf. Part I) formed under the
influence of river- and (soon) lake-water in the Mjóidalur durning inter-
glacial time. Then glaciers and glacial rivers eroded the flanks, to leave
the steepsided Table mountain (Air photograph, Icelandic Cartographic
Institute),