Jökull - 01.12.1983, Side 65

Both the lava and associated breccia are coarsely
porphyritic with numerous, 2-4 mm sized, olivine
phenocrysts and even larger (up to 1.5 cm) but
somewhat scarcer augite phenocrysts. This distinc-
ttve phenocryst content and the lack of plagioclase
phenocrysts contrasts with the basaltic lava type in
all the younger units.
Although most of the lavas in the area are at least
partly vesicular, the Unit A lava is distinctive both
tn the large size of some of the vesicles, some being
UP to 2 cm across, and the fact that they are invari-
ably lined with zeolites or more rarely lined, or even
totally filled, with pale yellow brown banded pala-
gonite. Chabazite of pseudocubic form is the most
abundant zeolite, and is associated with thompson-
1 te, phillipsite and commonly also calcite. Scarce
development of zeolites including analcite was also
seen in the overlying Unit B, in particular near
where Skógafjallsgil emerges onto the glacier flood
plain. Zeolites are however, absent from all the
succeeding volcanic units.
Unit B: This volcanic unit is separated from the
underlying coarsely porphyritic unit by a well de-
fined erosion surface with clear glacial striae where
the erosion level is on relatively massive basalt.
There is no evidence ofsub-aerial weathering or tuff
deposits, or indeed of any regular tillite. However,
the lowest few feet of the overlying palagonite tuíf-
breccia formation contain scattered boulders, some
With ice striated surfaces, of various lava types and
some large augite and olivine crystals which have
clearly been derived from the underlying unit. It
therefore seems likely that ice conditions and glacial
erosion prevailed after the formation of Unit A and
that glacial till boulders lying on the glaciated pave-
uicnt were incorporated in the base of this next
eruptive unit.
rhis Unit as a whole consists largely of yellow-
brown palagonite tuíf-breccias again with irregul-
arly shaped, highly fractured, basalt lava masses,
which become somewhat more regular and promin-
ent towards the top of the unit. The basalt type is
typical of that found in all but the lowest Unit, at
some levels being essentially aphyric, although oft-
en in fact microporphyritic, elsewhere being more
uoticeably porphyritic with labradorite plagioclase,
augite and olivine phenocrysts, commonly roughly
5-10 vol.% of each of 2 mm maximum grain size.
This unit as a whole appears to thin drastically both
to the west and south ofjökulhaus North, but this is
almost certainly largely a post-eruption erosional
feature rather than a primary feature.
Unit C\ The underlying Unit B appears to have
suffered extensive subaerial erosion prior to the
eruption of the volcanic rocks of unit C. The evid-
ence suggests that the earliest Unit C rocks were
erupted subaerially rather than sub-glacially over
an eroded surface. Thus the junction between Units
B and C is an irregular and rather complex erosion
surface. It is marked in place by a thick fluvioglacial
conglomerate horizon, in particular in the stream
around the south side of Jökulhaus South and on
the flanks ofjökulhaus North.
On the south side of Sólheimajökull local pockets
of black volcanic tuffs are preserved above the con-
glomerate horizon, including a striking lapilli tuff
on the eastern flank ofjökulhaus South. In addition
the presence of a really thick coherent basalt flow
forming much of the north-east corner ofJökulhaus
South, the local evidence of haematite development
indicative of sub-aerial weathering, and the general
presence of more regular basalt flow units than in
the other volcanic units together suggest that most
of the volcanic rocks of Unit C, were erupted sub-
aerially during an Interglacial period. However,
palagonite tuff-breccias become more prominent
again towards the top of this unit indicating that
glacial conditions were probably beginning to pre-
vail once again.
Thus the volcanic rocks of this unit may have
been erupted over an extended period of time, un-
like those of Unit B which may conceivably have
formed as the result of a single sub-glacial eruptive
episode. However, apart from some slightly more
plagioclase-rich (about 15-20 vol. % plagioclase
phenocrysts) thin basalts flows in the “valley-fill”
sequence, there is little significant variation appar-
ent in the basalt magma type throughout Unit C.
The basalt erupted was either porphyritic with
plagioclase and augite phenocrysts, and olivine
generally either scarce or absent, or essentially
aphyric.
Unit D: The impressively thick sequences of pala-
gonite tuff-breccias exposed in the deeply dissected
canyons to the west of the glacier flood plain large-
ly belong to this unit. The erosion surface which
separates Units C and D is best observed in the
virtually dry first canyon west of the Jökulsa river. It
is a well defined glaciated surface with good glacial
striae apparent in places. Although it has a roughly
horizontal disposition in the canyon it is still far
from being ideally planar. It becomes increasingly
irregular as it is traced north-eastwards and on the
west side of Skógafjallsgil, where the river emerges
JÖKULL 33. ÁR 63