Jökull - 01.01.2009, Síða 40

Leó Kristjánsson
area, during a short field course supervised by B.S.
Harðarson. Through the kind cooperation of the stu-
dents, descriptions of some of these profiles were
made available to the author (Á.R. Hjartardóttir, pers.
comm. 2004).
Grímsson and Símonarson (2008) review results
from detailed research on plant fossils and paleo-
environments in sediments at localities on the south
side of Arnarfjörður.
OBJECTIVES AND PROCEDURES OF
THE PRESENT STUDY
The previous research projects on paleomagnetism re-
ferred to above, and most others of that type in Ice-
land, have been carried out in cooperation between
geologists and geophysicists. For the duration of the
sampling in the present project (2004–2008), no geol-
ogist was however available to provide mapped pro-
files for sampling or to advise on stratigraphic corre-
lations. In spite of this, a project was embarked upon,
largely based on the geological information provided
by J. Preston but several new profiles were also sam-
pled. It must be emphasized here that the present sur-
vey is only a reconnaissance study, intended to pro-
vide a tentative framework for future comprehensive
multi-disciplinary work in the area.
One prime objective of this survey was to add new
information to current knowledge of the long-term
secular variation of the geomagnetic field. Studies on
basalts in Iceland have already provided a valuable set
of remanence direction data from the last 15 m.y. In
their continuous coverage as well as in their quantity
and reliability, these surpass comparable results from
any other region in that interval of time. Some results
and conclusions from the present survey concerning
this aspect will be reported elsewhere.
Another objective of the survey was to locate
boundaries of some polarity zones (such as that at the
top of the profile AB of Kristjánsson et al. (1975)),
in order to facilitate any subsequent work on strati-
graphic correlations in the three promontories of the
Arnarfjörður- Breiðafjörður area, and possibly farther
afield.
In connection with both the above objectives, it
was expected to find records of major excursions of
the geomagnetic pole. These and the accompanying
changes in intensity of the field are currently of con-
siderable interest to researchers in paleomagnetism.
The accessibility of sampling sites in the
Arnarfjörður-Breiðafjörður area is quite variable.
Many of the mountains in the area are below 400 m
in height, rounded and scree-covered by erosion so
that it is not common to find continuous profiles of
more than 20–30 lavas in these. On the other hand,
many higher mountainsides are too steep for negotiat-
ing with core-drilling gear. For this reason and others
such as availability of cooling water for the drills, the
profiles sampled in the present survey tend to be short
compared to those in previous surveys.
MAPPING, SAMPLING AND
MEASUREMENT METHODS
The coordinates of the lowest lava in each sampling
profile are given in Table 1. Note that in profile MH
the sampling began at flow 26, some 600 m southeast
of flow 1. Altitudes of lava boundaries were always
recorded by pocket GPS receivers, usually to an ac-
curacy of 5–10 m. On steep hillsides where less ac-
curacy was available, the altitudes were also obtained
with the aid of a digital aneroid altimeter or by visual
estimates of lava thicknesses.
For each lava, its general lithologic character was
recorded in the field using the conventional field-
work criteria and checked by later inspection of the
cores collected. These criteria include the presence of
feldspar or olivine phenocrysts, grain size, flow struc-
tures, and weathering features. The method was first
applied in East Iceland by Walker (1959, p. 370) who
classified the basalts of the lava pile into three cate-
gories, i.e. tholeiites, olivine tholeiites and feldspar-
porphyritic basalt. However, it should be noted that
such estimates are known to vary from person to per-
son, and the character of a lava is also sometimes
seen to change laterally. From the author’s experi-
ence, the classification of lavas in Northwest Iceland
into these three main types is not as clear-cut as is
the case in East Iceland. In particular, the amount of
40 JÖKULL No. 59