Greinar (Vísindafélag Íslendinga) - 01.01.1976, Blaðsíða 156

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partly below water and partly above it (Fig. 10, Ei). But also the
state of the upper part of this layer (about an 80 m thickness of it)
was largely due to the influence of water — by causing explosivity
in the funnel(s).
A series of subaerial unporphyritic tuffs and lavas soon followed
in the Hengill area, while farther away to the north was formed
the extensive unporphyritic Stangarháls-Lómatjarnarháls sub-aqua-
tic tuff-breccia. It was formed at about 200 m stage of the lake level.
In Hengill the unporphyritic series was cut into tilted blocks and
then denuded to a plain surface, perhaps near to or at lake level,
for erosional valleys seem not to have been formed. This was at
the same time a denudation to the level of the nearby parts of the
westem dolerite, for likely outliers from these or older Hengill
rocks on the dolerite are not found. On the other hand, an ap-
parently much younger layer of stratified coarse tuff with clear
signs of transport rests on the dolerite at the mouth of Marardalur,
on the erosion plain of Sleggja, and on the denuded basalts of
Hengill, where it forms the topmost layer of the westem part of
Hengill, after the great later unlift of the latter. (The flowage of
this layer during the uplift, as described earlier, is perhaps the
best indication of the relative youth, for such flowage is not seen
in the older Hengill layers).
The unporphyritic Stangarháls-Lómatjamarháls sub-aquatic tuff-
breccia reached a denuded abrasion bench of Mælifell and Sandfell,
and covered sediments formed by the run-off from the pass be-
tween these mountains. Soon, the breccia was covered here by an
extruded mass of olivine-porphyritic basalt of reverse magnetiza-
tion. By present knowledge of the geomagnetic time scaie, this
rock cannot by younger than 0.69 My. Some such age would also
have been expected by consideration of the dissection of the under-
lying extensive unporphyritic tuff-breccia by broad valleys and
other indications. But the occurrence of the reverse magnetization
is a most important support for the relatively high age of the main
mass of the rocks covering the gap between the westem and eastern
dolerites, and overlying these dolerites. All considered, we are lead
to the conclusion that the dolerites can hardly by younger than
about one milhon years.
There are no indications of such a process as “spreading” since