Jökull - 01.12.1972, Side 74

sheets and intrusions within the caldera are all
normally magnetized. On the other hand, the
great bulk o£ the rocks immediately surround-
ing and to the west o£ the caldera have revers-
ed magnetization; in all a pile over 1 km in
thickness includes only a 50—100 m thickness
of normally magnetized rocks, as was originally
shown by Einarsson (1957). Piper (1971) has
assigned the tliick reversely magnetized pile to
the Matuyama epoch. The thin normally
magnetized pile is most likely from the Olclu-
vai event (Fridleifsson 1973). The intra-caldera
filling and the intrusions within the caldera
are from the same normal event, which occurr-
ed about 2 million years ago.
The following geological features would be
expected to affect the shape and properties of
the broad magnetic anomaly in the Stardalur
area:
a) A circular caldera rim intensely brecciat-
ed ancl highly altered by hydrothermal ac-
tivity (i.e. having a low net magnetization).
b) Normally magnetized intra-caldera filling,
and a great bulk of normally magnetized
basic intrusives in the central and eastern
part of the caldera.
c) Surrounding country rocks with reversed
magnetization.
In order to elucidate further the cause of the
broad magnetic anomaly the remanence and
susceptibility values of 160 samples from sur-
face formations near Stardalur have been mea-
sured. The results of these measurements are
highly scattered, but logarithmic averages of
total magnetization (remanent plus induced, re-
gardless of sign) are given in Table 1 for the
main categories.
TABLE 1 - TAFLA 1
Total magnetization of samples from surface formations in the vicinty of Stardalur
Heildarsegulmögnun ýmis konar bergsýna úr nágrenni Stardals
No. of Samples Fjöldi sýna Type T egund Locality Staður Average magnetization in 10-3 g Meðal- segulmögnun
23 olivine tholeiite lavas Stardalur, Reykjavik 2-3
9 tholeiite lavas Stardalur 6-8
30 tholeiite lavas Stardalshnúkur, Skógarbringur, Skeggjastaðir, Úlfarsfell 1-3
20 basaltic andesite plugs Bláhnúkur, Tröllafoss, Múli 10-15
7 dolerite (olivine tho- leiite) intrusions Stardalshnúkur, Leiðhamrar, Kvensöðlar 1-2
16 dolerite (tholeiite) in- trusions Músarnes, Gráhnúkur 6-8
12 tholeiite dykes Stardalur, Skálafell, Skeggjastaðir 10-20
14 rhyolite Esja, Grímmansfell 0-1
72 JÖKULL 22. ÁR