Jökull - 01.01.2005, Page 40

Árni Hjartarson
Figure 5. The Hvítárdalir dome with fossil screes at its sides buried in younger lavas. Fossá porphyritic basalt
with intercalated sediments is at the top. Nýibær andesite in the middle. Nýibær tholeiites at the base. N and R
show the polarity. – Hvítárdalagúll með fornum skriðum á báða bóga kaffærður af yngri hraunum. Fossárdals-
dílabasalt og setlög efst. Nýjabæjarandesít um miðbikið. Nýjabæjarþóleiít neðst. N og R sýna rétta og öfuga
bergsegulstefnu.
The location of this unit inside the geological col-
umn is not clear and the Keldudalur dacite formation
might even represent an intrusion. Further investiga-
tions are needed to determine this.
The Nýibær tholeiite formation is very similar
to the Tinná tholeiite in all respects except polar-
ity. From a petrological point of view, they would
be regarded as the same member. These lavas fi-
nally immersed the Skati Dome totally. Remnants of
the Nýibær tholeiite lava cap can still be seen near
the summit of the former acid dome in the mountain
Skatastaðafjall.
The Nýibær andesite and the Nýibær rhyolite for-
mations. At the end of the Nýibær period, a inter-
mediate phase of acidic volcanic activity started up in
the area, forming the Nýibær andesite and the Nýibær
rhyolite formations. In the Vesturdalur valley there is
a hill of andesitic hyaloclastite and fragmental basalt
lavas with remnants of pseudo-craters. This might in-
dicate an eruption inside a lake and high groundwater
level and wetlands. The acidic rock consists mostly
of lavas; tephra layers have not been reported. Chem-
ical analysis shows that at least some of the lavas
are in fact dacite, with 65% SiO2 (Hjartarson 2003).
Tholeiite lavas are also found in this pile, the thick-
ness of which reaches 180 m. Several cone sheets
(cone dykes) indicate caldera formation during the fi-
nal stage of the volcano, although no distinct caldera
fault has been found. The location and form of the
caldera shown on the map (Figure 1) is mainly based
on the existence of cone sheets near the Fossá river.
The upper limit of the Nýibær formations is in
many places marked by the major Skagafjörður un-
conformity. The Nýibær acid and intermediate lavas
are the youngest known formations of the Tinná Cen-
tral Volcano but it cannot be stated that they indicate
the extinction of the volcano. Still younger forma-
tions have been eroded away, as seems to be indicated
by cone sheets and acid dykes cutting the topmost lay-
ers of the Nýibær formations below the unconformity.
Major caldera faults are not found.
The Fossárdalur group. This group overlies the
Tinná Group. It is dominated by porphyritic basalt
lavas and prominent sandstone layers. It is about
40 JÖKULL No. 55