Jökull - 01.12.1987, Page 22

that the temperature drops fairly sharply to about 180°C
where a deeper water saturated zone is entered, but
increases again reaching about 250°C at 1900 m depth.
GEOLOGICAL INVESTIGATIONS
Investigations of subsurface lithology at Krafla are
based on the analysis of drill cuttings from the 24 wells.
Information has been obtained on the geological struc-
ture, the distribution of individual lithological units, the
correlation of aquifers with these, and the degree of rock
alteration. All the drilling fields are within the caldera.
The general subsurface geological and alteration model
of the Leirbotnar field based on the first eleven wells was
established in 1978 (Kristmannsdóttir 1978, Stefánsson
1980 and 1981). Drilling in new fields during the years
1980-1984 provided valuable additional information on
the lithology. The locations of two cross sections, A-A’
and B-B’, are shown in Fig. 8 along with well positions
and the main tectonic features of the area. The cross
section A-A’ extends from Well KJ-11 in Leirbotnar to
well KJ-18 in Suðurhlíðar (Fig. 9). At least five stra-
tigraphical horizons have been distinguished in the up-
permost 1200-1300 m (Guðmundsson, Á. 1983), but in-
trusives are the dominant features below that depth.
The approximate geological time scale is as follows; The
top horizon (B-l) is a postglacial lava formation
(<10.000 years old), next is a hyaloclasite formation
(M-l) probably dating from last glacial period. Below
that is another group of lavas (B-2) dating from the latter
part of the last interglacial period. This is underlain by
the second hyaloclastite formation (M-2) which is con-
sidered to represent the bottom of the caldera filling.
Below this level there are lavas (B-3) from the first part
of the last interglacial period followed by basaltic and
doleritic intrusions which dominate the strata down to
2000-2200 m depth in the Leirbotnar field, but in Suður-
hlíðar gabbro intrusions become dominant below 1800-
2000 m depth. In wells KJ-14, KJ-17 and KJ-19 there are
some acid intrusions above the gabbro and at the bottom
of the hyaloclastite formation, M-2, there are also acid
intrusions which are characteristic of the Suðurhlíðar
field.
The surface area of the Krafla caldera extends over 64
km\ The caldera filling is considered to be 1 km thick
and shaped like a reverse cone. Thus the total produc-
tion of eruptives during and after the caldera formation
is estimated to be 0.42 km3 per thousand years.
Well KJ-20 was drilled directionally towards NE and
cuts through faults, which are seen on the surface in the
Suðurhlíðar field. At 1270 m depth the geology changes
compared with the situation in neighbouring wells. The
fault has an apparent dip of 8° N, a north throw of some
hundred metres and a WNW-ESE direction. The exist-
ence of a subsurface horst inside the caldera is also
confirmed (Fig. 10), but evidence for the horst had al-
ready been observed in wells KG-4, KJ-6 and KJ-15.
Some faults may also exist between the Hvíthólar field
(in the southern part of the caldera rim) and well KJ-6
(Fig. 10).
A prograde hydrothermal alteration pattern is ob-
served with depth and correlates well with measured
temperatures at Hvíthólar and Suðurhlíðar, but in the
Leirbotnar field an invasion of 190-200°C water from the
north has overprinted the earlier alteration mineral as-
semblages by calcite down to a depth of 1000-1200 m.
Cross sections which show the alteration zones in the
Fig. 9. Geological cross section of the Leirbotnar and
Suðurhlíðar wellfields. — Mynd 9. Jarðlagasnið af Leir-
botnum og Suðurhlíðum.
20