Náttúrufræðingurinn - 1997, Side 49

Walker, G. P. L. 1959. Geology of the
Reydarfjördur area, eastem Iceland. Q.
Journ. Geol. Soc. 114. 367-393.
Walker, G. P. L. 1960. Zeolite zone and dike
distribution in relation to the structure of the
basalts of eastern Iceland. Journ. Geol. 68.
515-528.
Walker, G. P. L. 1995. Plant molds in Hawai-
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Joum. Geol. 103. 85-93.
■ SUMMARY
Basalt tree casts in a tertiary lava in
MT. SKRIÐNAFELLSNÚrUR, NW-ICELAND
ln the summer of 1987 several basalt tree casts
were found in a Tertiary lava flow at
Barðaströnd, NW-Iceland. This part of the
country is built mainly of thick sequences of
basalt lavas interbedded with minor horizons
of sedimentary rocks. The bedrock of
Barðaströnd dates back to Miocene time, 10-
12 million years ago.
The tree casts were found in a cliff of a 9.5 m
thick tholeiite lava, forming the uppermost and
most prominent part of Skriðnafellsnúpur (Fig.
I and 2). At this locality altogether seven tree
casts were found, and additionally some five
lava tree molds. Near the casts a dyke has
pushed its way through the lava (Fig. 4 and 5).
Detailed fteld study revealed that one of the
casts can be traced directly to the dyke (Fig. 6).
All of the tree casts were found to lie within 15
m from the dyke, whereas Ihe molds are all lo-
cated farther away.
Petrographic and major elcment analyses
were made on thc dyke, the lava and the tree
casts. These examinations revealed that the pe-
trography of the tree casts are all similar, indi-
cating their common origin. Also, it emerged
that the petrography and chemical composition
of the tree casts are quite different from that of
the lava and clearly resemble more closely that
of the dyke (Table I and fig. 8). On the basis of
this it can be postulated that the tree casts in
Skriðnafellsnúpur originate from the dyke that
penetrates the lava. Although detailed descrip-
tion of how the tree casts formed can’t be
given, thc main sleps in their formation must
have been as follows: Predating the dyke, the
lava (containing the casts) was locally inter-
spersed with a lot of empty tree molds, most
propably intersecting each other to some extent
and tlius leaving some open channels between
them. Laler on, when the lava was penetrated
by a dyke, magma flowed laterally through the
cavities left by the tree molds. All the molds
located within approximately 15 m from the
dyke were filled by basalt magma but the more
distant ones were left empty.
Petrographical examination indicates that
(he Tertiary lavas of the upper part of
Skriðnafellsnúpur are mineralogically all simi-
lar, comparablc to many basalt lavas from the
active rift zones of Iceland. Chemical analyses
of lavas of the upper section of Skriðnafells-
núpur reveal that all of them group as tholeiites
(Table 1). Comparison was made of the major
element chemistry of the lavas of Skriðnafells-
núpur with publishcd analyses of basalt lavas
in lceland. On the basis of this it is implied that
geological conditions during the formation of
the Barðaströnd area in Miocene time, may
have been similar to that of the Reykjanes Pe-
ninsula in Recent and Late Quaternary times.
PÓSTEANC HÖFUNDA/
AUTHORS' ADDRESSES
Magntás Á. Sigurgeirsson
Orkustofnun Vatnamælingar Austurlandi/
National Energy Authority
Þverklettum 2-A
IS-700 EGILSST AÐIR
Sveinn P. Jakobsson
Náttúrufræðistofnun íslands/
Icelandic Institute of Natural History
Pósthólf/Box 5320
IS-125 REYKJAVÍK
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