Jökull - 01.12.2006, Page 57

A paleomagnetic study of stratigraphic relations in lava pile
local plateau level. On the plateau there are coarse
sedimentary layers of at least 20 m thickness; lava
flow 86 is within the exposed sediment outcrops and
86A overlies them. These sedimentary rocks are most
probably the Merkidalur sediment at the top of the
Merkidalur formation: for the location of Merkidalur,
see the legend of Figure 1b.
PH is located on the western side of the Mosgil
gully in Austurdalur, beginning on the banks of the
Austari Jökulsá river (Figure 2a). It was mapped by
Á. G. and H. H. in 2003. Coordinates: 65◦21’.74N,
19◦02’.90W. Tilt decreases from 12◦ at base to 7◦ at
top, downdip 160◦E. There are some faults evident in
the profile, notably one of at least 60 m throw above
flow 39 (Figure 3). Coarse sedimentary rocks occur
between flows 43 and 45 in a reversely magnetized
group of lavas and again in the normally magnetized
group above flow 49, but no sediments are seen at
the polarity boundary between flows 47 and 48. The
character of the sediments varies somewhat laterally,
with localized occurrences of acid tuff. They resem-
ble most likely the Merkidalur sediment.
PG was mapped by Á. G. and J. Helgason and
sampled already in 1976. It lies on the west side of
an unnamed gully in the Bakkadalur tributary valley
(Figure 2b). Coordinates: 65◦21’.47N, 18◦59’.20W.
The lava-mean directions in this profile, published
in microform with the paper of Saemundsson et al.
(1980), had all been corrected for an average tilt of 7◦
in a downdip direction of 135◦E. The profile consists
mainly of tholeiite lavas although some of the flows
may be intermediate in appearance between tholei-
ites and olivine-tholeiite or feldspar-porphyritic lava
types. Coarse sedimentary rocks which occur be-
tween some of the lavas PG 9 to 18 are seen also to
crop out at various points along both sides of the val-
ley. In places, one of these sediments overlies an acid
tuff. The lower part of the profile has suffered mod-
erate to high geothermal alteration, probably due to
proximity to the central volcano.
PM lies along a small brook named Fjóslækur in
Austurdalur north of Austari Jökulsá, 1 km west of
a bridge over the river. Coordinates: 65◦19’.31N,
18◦57’.02W. Tilt decreases from 8◦ at base to 6◦ at
top, down-dip 180◦E. PM is underlain by sedimentary
rocks of which about 10 m are exposed. The top part
of the sediments is coarse and similar to some of those
already mentioned in profiles PH and PG, the lower
part is more fine-grained and partly layered. This pro-
file was first mapped and described by Hjartarson et
al. (1997), remapped and described by Á. G. and H.
H. in 2002.
PN, mapped by Hjartarson et al. (1997), is on the
mountain slope near the Miðhús farm ruins some 5-
600 m west of profile PO. Coordinates: 65◦18’.98N,
18◦53’.56W. Tilt is 7◦ towards 180◦E. The profile
consists of eight lavas and thick sedimentary layers.
The top couple of flows are fine-grained, possibly an-
desitic. Exposures between PN and PO are insuffi-
cient to allow direct tracing of any units from one pro-
file to the other.
PO follows the western side of a gully named
Brennigil, opposite the abandoned Skatastaðir farm in
Austurdalur. Coordinates: 65◦19’.04N, 18◦52’.62W,
the tilt gradually changes from 7◦ towards 180◦E at
base to 2◦ towards 200◦E at top. PO was first mapped
and described by Á. G. and J. Helgason in 1975 (un-
published), and remapped by Hjartarson et al. (1997).
There are several horizons of sedimentary rocks in
this profile, including an acid tuff at a normal-to-
reverse polarity boundary above PO 22 that can be
traced along the mountainside to the Skati Rhyolite
dome, the main body of the Tinná Central Volcano.
The acid tuff and the rhyolite dome are monogenetic
(Hjartarson et al. 1997, Hjartarson 2003, 2005). Mea-
surements in the field indicate that the dome is re-
versely magnetized. In many localities, although not
in the PO-profile in Brennigil, the acid tuff is accom-
panied by a prominent sedimentary layer, up to 30 m
thick. This layer which includes lignite seams, is
called the Tinná lignite sediment (Ti1). The acid tuff,
along with the Tinná lignite sediment, forms an im-
portant marker horizon in the strata pile of the Skaga-
fjörður valleys.
JÖKULL No. 56 55