Náttúrufræðingurinn - 1985, Side 19

SUMMARY
Geology of the Búrfell area
by
Elsa G. Vilmundardóttir,
Ágúsl Guðmundsson
and Snorri Páll Snorrason
National Energy Authority,
Grensásvegi 9,
108 Reykjavík
In the early 1950ies geological and hydrologi-
cal investigations began in the Búrfell area.
These investigations were a part of a project to
build an clcctric hydropowcr station at Mt. Búr-
fell, which started operating in 1970.
The enclosed geological map of the Búrfell
area shows geological formations which are
formcd in the last 2 m.y., or during Pleistocene
and Holocene times (Matuyama and Brunhes
magnetic epochs). In the stratigraphy evidence
for at least 10 glacial and 10 interstadial stagcs
are found. The bédrock is divided into several
distinct formations, identified witli letters on the
map.
Rock formations from the glacial stages are
mainly hyaloclastites and thick sediments, main-
ly tillites. Erosional features sucli as valleys
and hills are l'ound buried in the lava pile, latcr
l'illed or dammed up by lavas from the interglac-
ial stages. One of these infilled valleys is found
east of Mt. Búrfell (in drillholes) but the Þjórs-
árdalur valley is not filled yet, having one post-
glacial lava covering its U-shaped valley floor.
Extinct volcanoes are found buried in the lava
pile. A central volcano was situated in the Þjórs-
árdalur area giving birth to voluminous basaltie
eruptions, the hyaloclastites ol' the Reykholts-
formation (RE) as wcll as rocks of intermediate
to acid composition, number of cone sheets and
high alteration of thc bedrock.
Tectonic lineaments are conspicuous on aireal
photographs, but few faults could be identified
in the field. The main orientation of fractures is
SW-NE and N-S. Dykes are not common, due
to shallow depth of erosion, but nummerous
cone sheets are found at the bottom of the
Þjórsárdalur valley.
Hyaloclastites from the Brunhes magnetic ep-
och are shown with a light brown shade on the
geological map, older hyaloclastites are dark
brown.
Postglacial lava flows are from eithcr the
Hekla - or thc Veiðivötn volcanic systems.
Lava llows from either system are easily identifi-
ed. The Tungnárhraun lavas are tholcitic and
characterized by big phenocrysts ol' plagioclase
(up to 1 cm), but the lavas from the Hekla
system are transitional alkali basaltic and
phenocrysts are almost lacking. There is also a
great difference in volume and areal coverage of
these lavas. The volume of the Tungnárhraun
lavas ranges from 1-16 km’’ while the volume of
the Hekla lavast ranges from 11,1-0,3 km'. The
north easternmost part of the Hekla volcanic
system is within the mapped area. At least 2-3
volcanic eruptions have occurred in this part of
the Hekla system in Postglacial times. The crat-
er rows from which thc Tungnárhraun lava flows
wcre erupted are about 30 krn to the east of the
mapped area. The largest Tungnaárhraun lava
reaches the sea at the south coast and its total
length is at least 130 km.
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