Tímarit Verkfræðingafélags Íslands - 01.12.1966, Page 50

74
TÍMARIT VFl 1966
Summary.
A geological investigation at the Búrfell Hydro-
power site on the Thjorsá River has been going
on since approximately 1960, guided each time
by the project under consideration. First it was
proposed to build a large dam between Búrfell
and Sauðafell but in 1961 it was abandoned be-
cause of difficult geological conditions on the dam-
site and a new project was proposed with a dam
farther upstream near a site already chosen
1915 by a Norwegian engineer, Sætersmoen, in
a study on the hydro-power potentials in the
rivers Thjorsá and Hvítá. This new site was
investigated thoroughly in 1962 and onward. To
begin with a tailrace development was proposed
but in 1964 a headrace development was planned
and that project is now under construction.
The rocktypes in the project area are a variety
of volcanic rocks with basaltic lava flows as the
most common rock. Other types are tuff, brecci&s
and pillowlavas mostly formed through sub-
glacial and subaqueous eruptions, dolerite (gray
basalt (diabase) lava flows, andesitic lava
flows and rhyolitic lava flows and intrusions.
Some sedimentary rock does also exist but has
similar properties as tuffs or breccias.
The geological history of the Búrfell area
starts some 4 million years ago and the oldest
formation is a basalt formation at the south toe
of Búrfell. Next in age is the older Burfell forma-
tion which was built up by an active volcanism
at some distance west of the Búrfell area. The
erupting products range in composition from
basalt through andesite to rhyolite. In Sáms-
staðamúli that formation is named Older Bur-
fell Formation. In Sámsstaðamúli a valley was
eroded in the Older Burfell Formation which
afterwards was filled with móberg, a subaqueous
formation. Sandstone also deposited in the water
and then along the hillsides a talus breccia was
formed and finally the valley was filled with
basalt lava flows. These formations are all named
Sámsstaðamúli Group SM, During that time the
eruption fissure was just west of Sámsstaðamúli
or even in the westemmost part of it.
The first unmistakable glacial occured after
the time of the SM group and during that time
a subglacial eruption occured in Burfell and the
thick pillow lava top of Búrfell was formed. It
was almost one million years ago. In this eruption
a very thick intmsive layer which is undemeath
most of Sámsstaðamúli was formed.
After this there was a valley or a canyon
erosion in Sámsstaðaklif which shortly after-
wards was filled with basaltic lava flows. This
formation is named Sámsstaðaklif Basalt Forma-
tion SB. When this was formed the volcanic zone
was north or northeast of Búrfell.
Doleritic rock was formed extensively during
the last interglacial in Iceland. The dolerite (gray
basalt) at Skeljafell might be of that age. The
layers are almost horizontal and there is every
indication of low age. The formation is called
Skeljafell Dolerite SD.
During the last glacial the land was sculptured
almost to the present form. The móberg ridges
east of Ytri Rangá were also formed in sub-
glacial eruptions. In Postglacial time í.e. during
the last 10.000 years a substantial addition has
been through sedimentary deposits, deposited on
the South Icelandic lowlands in Finiglacial time
when the sealevel was much higher than at
present time and through lava flows coming from
the Veiðivötn area which have built up the valley
bottom east of Búrfell about 100 m. which create
the head utilized at Búrfell. These lava flows
have changed the drainage pattern of southern
Iceland substantially as shown on fig. 6. Another
volcanism in postglacial time is in the Hekla
region only 12 km. away from Búrfell. It has
formed enormous layers of pumice and ash
which forms thick overburden in the Búrfell
area.
Dating of the formations has mainly been
through measurements of paleomagnetic polarity
of the rocks. In figure 7 is a scheme showing
the polarity epochs and events as they are now
dated and tentative correlations of own polarity
measurements.
The usual dip is towards east to north and
clearly increases with increasing age. The old-
est basalt formation is dipping 4°-5° but the
youngest formations have practically no dip.
There are two types of faults in the area: normal
faults with vertical movements and strike slip
faults with horizontal movements. At some of
the strike slip faults a horizontal movement of
the order of magnitude 500-800 m can be
suspected. This is indicated by the offset of the
other set of faults crossing it. The fault lines
are not seen on the postglacial lava flows nor
in the youngest móberg.
The geological investigation was done with:
drilling permeability testing, ground water level
measurements, seismic sounding, Borro sounding,
test pits, and exploration tunnelling.
The drilling is the most extensive investigation.
On the whole 128 holes have been drilled with