Jökull - 01.12.1989, Page 83

Fig. 4. (on facing page and above) Geological map of the Kækjuskörð rhyolitic dome.
Mynd 4. (til vinstri og að ofan) Jarðfrœðikort af Kœkjuskarða-eldstöðinni.
obsidion block pumiCeous block
surfoce
btecclQ ^dowfoot
obsidian—-T ' breccio
• - ^o*
stony_____
rhyollte
pumice fall deposit
Fig. 5a. Schematic vertical section through a rhyol-
ite lava flow (from Cas and Wright, 1987).
Mynd 5a. Einfaldað þversnið í gegnum rýólíthraun.
In the northem part of the mapped area only one
silicic lava flow is exposed, the Kollutungur rhyol-
he. It occurs in the basaltic lava pile about 200 m
below the silicic lava flows of the Kækjuskörð vol-
cano and was probably erupted from a now eroded
silicic eruption site to the north of the Kækjuskörð
volcano. The Herfell ignimbrite was deposited after
the Kækjuskörð volcano had become extinct and
was buried by a pile of basaltic lava flows. The
Pyroclastic rocks and silicic lava flows have been
named according to local place names. A schematic
profile through the volcano is presented in Fig. 9.
A more detailed description of the silicic rocks
follows, beginning with products of the Kækjuskörð
volcano.
SILICIC ROCKS OF THE KÆKJUSKÖRÐ
RHYOLITIC VOLCANO
Orustukambur ignimbrite I and II — In Skúmhattar-
dalur valley in the vicinity of the Orustukambur cliff
Fig. 5b. Cross section through the lenght of the
Rocche Rosse obsidian coulée, with generalizedflow
foliation patterns. (from Cas and Wright, 1987 [after
Hall 1978]).
Mynd 5b. Þversnið af rýólíthrauni með brotflötum.
(Fig. 10) two fonnations of breccia are exposed in a
stream gully. The breccias are composed of pumice
fragments and larger basaltic and silicic lithics
(0.1-1 m in diameter) and show incipient to dense
welding. They resemble a type of fragmental rock
termed ’co-ignimbrite breccia’, which was described
first by Wright and Walker (1977, 1981). These
breccias, now being recognised in many ignimbrite
flow units, represent the near-vent facies, with accu-
mulation of larger lithics and a relatively small
amount of ignimbrite matrix (Cas and Wright,
1987).
The Orustukambur ignimbrite I is only exposed in
a section of the gully south of the Orustukambur
cliff, 200-230 m above sea level. Basaltic and inter-
mediate rock components of a size up to 10 cm
appear as lithics in a glassy matrix. The strongly
proþylitised rocks are of pale green colour caused
by the formation of secondary chlorite and epidote.
The Orustukambur ignimbrite I represents the oldest
exposed ignimbrite of the Kækjuskörð volcano and
JÖKULL, No. 39, 1989 81