Jökull


Jökull - 01.12.1982, Side 81

Jökull - 01.12.1982, Side 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, JÖKULL 32. ÁR 77
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