Jökull - 01.12.1982, Page 82

Fig. 1. A topographic map of
the Laugaland area showing
the location of the drillholes.
Mynd 1. Kort af borsvebinu á
Laugalandi í Öngulsstaðahreppi,
sem synir staðsetningu borholanna í
þversniðum á myndum 2 og 3.
gyrolite, stilbite, heulandite, chabazite and platy
calcite. In olivine tholeiite basalts gyrolite, mesolite,
analcime, heulandite and calcite areabundant. An-
alcime is often in well developed crystals (up to 4
mm in diameter). Gyrolite is rarely in a close associ-
ation with the zeolites and is often the single occu-
pant ofa cavity. In tholeiite lavas chalcedony, cela-
donite and mordenite are common at this level.
SUBSURFACE STRÁTIGRAPHY
In Fig. 2 and 3 areshown simplified crosssections
through the drillholes. The N-S section in Fig. 2
spans over 1200 m of the Laugaland area (Fig. 1)
and is extendcd to 2 km by a drillhole on Ytri-
Tjarnir further to the North (not shown on the
map). The VV-E section spans over 400 m. The
reason for the elongated form of the drilled area is
that the upílow zones of the geothermal water fall
along a N-S going strip. The stratigraphic section is
composed of lava flows with mostly thin interlayers
of scoria and sediments. A few thick sediment beds
are encountered in the drillholes, but their horizont-
al extension appears to be small.
I n the upper part of the protíle olivine — tholeiitc
basalt lavas are significant, making up 30-40% of
the profile. Below 400-600 m depth tholeiite lavas
are predominant. Porphyric lavas are not vol-
uminous, less than 5% of the pile, but they are
important marker horizons in the stratigraphy. The
frequency of dykes is variable from one drillhole to
another. In some drillholes apparently nearly no
dykes are crossed, whereas inothersover30% ofthe
penetrated rocks are dykes. Evaluation of dyke int-
ensity is rather uncertain from drillhole data. The
location of the drillholes is not random within the
area. Further, the drill can followdyke margins for
tens and even hundreds of meters. The dykes are
mostly crossed under low angles giving highly ex-
aggerated thicknesses. On the other hand a reliable
distinction between dykes and lavas is not always
possible. The coarse grained dykes are readily iden-
tified but some of the finegrained ones can be over-
looked. On average an increased frequency ofdykes
is observed by depth. The intrusive rocks crossed in
78 JÖKULL 32. ÁR