Jökull - 01.12.1982, Page 81

Subsurface Stratigraphy and Alteration of the
Tertiary Flood Basalt Pile in the Laugaland Area,
Eyjafjörður, northern Iceland
HREFNA KRISTMANNSDÓTTIR
National Energy Authority, Grensásvegur 9, 108 Reykjavík, lceland
ABSTRACT
In the Laugáland area, Eyjajjörður, eight deep drillholes,
reaching down to a maximum oj 2820 m deþth, have been
drilled jor geothermal water. So far, this is the thickest
section of the Tertiary flood basalt þile in lceland open for
study.
The rock alteration corresponds mostly to the zeolite meta-
morphicfacies. The successionfrom the chabazite, analcime,
and the mesolite — scolecite zones to the lowest laumontite
Zeolite zone is encountered in the section. In the lowest
300-800 m of the section the rock alteration is grading into
the greenschist metamorphicfacies.
INTRODUCTION
Drilling for geothermal water in Eyjafjördur,
northern Iceland has been going on intermittently
since 1933. During the last years it has been con-
centrated to the area regarded as the center of hot
water upflow, Laugaland. (Fig. 1). The Laugaland
area is today the main supply area of hot water for
the municipal heating system of the town Akureyri,
Eyjarfjördur. Eight drillholes 1 100-28(X) m deep
have been drilled in the area during the last ten
years. The area is considered to be the main upflow
zone of hot groundwater in the region. The main
aquifers in the drillholes are found at 470-1500 m
depth yielding water at temperature 70-96°C. The
regional geothermal gradient in the area is about
60°C/km. The underground rocks are rather im-
permeable and upflow zones are coníined to tect-
°ntc features such as crossing dykes and faults.
Erosion at sea level in Eyjafjördur amounts to
about 1500 m. A section of the crust exeeding 4000
m is thus available for study by adding the drillhole
sections.
Only one short core has been taken from the
drillholes and the petrographic and mineralogical
studies are done on cuttings. The cuttings are sam-
pled at every second meter of penetration.
Penetration rate, corrections for travel time and
partly also inhole loggings are used for location of
contacts.
GEOLOGY OF THE AREA
The mountains of Eyjafjördur are built ofTerti-
ary volcanic formations of the age 3-10 m.y. (Björns-
son and Stemundsson 1975, Bjómsson et al. 1978).
The rocks are mainly basalt lavas with mostly
thin scoriaceous and sedimentary interlayers. The
Iava pile dips about 4-7° to south and southeast.
Thick sediment beds are found locally. According to
Sæmundsson three central volcanoes are known in
Eyjafjördur (Bjömsson et al. 1978). Two of them are
far from the Laugaland area and the third is in the
youngest part of the Eyjafjördur area. The surface
rocks at Laugaland are thus not expected to have
gone through previous high temperature alteration.
A dyke swarm (6-7% dyke density) lies along the
Eyjafjördur Valley (Bjömsson et al. 1978). Laugaland
is located within this zone. The predominant dyke
direction in the area is NNE-SSW. Most of the dykes
dip gently (1-7°) to the West or are near vertical.
The lavas are zeolitized up to about 1000 m
elevation (Bjömsson et al. 1978). The top of the
mesolite—scolecite alteration zone (see Walker 1960)
is approximately 300 m elevation.
Exposures are rather scanty in the Laugaland
area. Vesicles and crack fillings from exposures at
the surface have not been studied in much detail.
In porphyritic lavas in the mesolite— scolecite
zone a common mineral assemblage is mesolite,
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