Jökull - 01.01.2009, Page 36

Leó Kristjánsson
Previous work in the Arnarfjörður-Breiðafjörður
area (approx. 65.6◦N, 23.7◦W)
Friedrich (1966) mapped the lithology and magnetic
polarity of lavas in a small area (F in Figure 1) near
the Breiðafjörður coast south-east of Patreksfjörður.
His study was mostly concerned with plant fossils at
a sediment site in that area. The sediment is found
above lava JF 48 in the youngest profile of the com-
posite section 1 in Figure 1 (McDougall et al., 1984).
J. Preston carried out stratigraphic field mapping
of the lava pile between Arnarfjörður and Patreks-
fjörður in 1970–1973. Many of the main results
of his work, along with those of a paleomagnetic
study on one lava profile (AB of Figures 1, 2), were
published by Kristjánsson et al. (1975). That pro-
file consisted of about 40 normally magnetized lava
flows overlain by two reversely magnetized flows just
below a hyaloclastite/lignite horizon. Preston’s de-
tailed maps of individual profiles, petrographic stud-
ies, and much other information have not appeared in
print, but copies of some of his notes and sketches
were made available to the present author at the
time. The reader is referred to Preston’s map of the
Arnarfjörður-Patreksfjörður area in the above paper.
A central volcano which is exposed north of the
central part of Arnarfjörður may be the source of some
of the extrusives and dikes seen south of the fjord.
On the whole however, the area between Arnarfjörður
and Breiðafjörður is relatively free of central-volcano
manifestations such as intrusions, lavas or sediments
of intermediate to acid composition, hydrothermal al-
teration, large local variations in dip, and thick series
of pahoehoe flow-units.
In East Iceland, the presence of extensive tuff
layers from explosive eruptions as well as groups
of feldspar-porphyritic lavas (and to a less extent of
olivine tholeiite lavas) within the mainly tholeiitic
lava pile has greatly aided in stratigraphic correlations
(Walker, 1959). Such groups seem to be rarer and less
persistent in Northwest Iceland. Instead, J. Preston in
his mapping made use of the occurrence of lava flows
which are crowded with large (even up to 2–3 cm)
feldspar phenocrysts. These flows which were called
by him “cumulate plagioclase basalts” are found as
isolated cases widely here and elsewhere in Northwest
Iceland (cf. McDougall et al., 1984; Kristjánsson et
al., 2003). Preston traced two such lavas for several
kilometers in the fjord area and noted many additional
occurrences at two other levels in the succession. For
stratigraphic connections, Preston also made use of
ankaramitic lavas and of hyaloclastite layers, as well
as a couple of series of thin lava “flow units”. He con-
sidered the total stratigraphic thickness from sea level
at the Kópur promontory (Figure 1) to the mountains
southeast of Patreksfjörður to exceed 2 kilometers.
L. Kristjánsson (unpublished work) sampled in
1987 some 30 lavas in the upper part of a hillside
profile (MH) west of the area of Friedrich (1966).
This profile has been mapped during undergraduate
projects, and paleomagnetic results from it will be
reported below. In 2002 a group of students at the
University of Iceland mapped some 15 profiles at
scattered locations in the Arnarfjörður-Patreksfjörður
Figure 2. Sketches of the strata and magnetic polarities in all sampling profiles except AH (a short profile close
to and overlapping with the lower part of AJ). The profiles in each area are arranged in no particular order.
All altitudes are in meters above sea level. Lavas having intermediate or variable character are shown by split
signatures. Small arrowheads indicate the presence of thin interbasaltic sediments. The sketch for AB is mostly
based on a manuscript drawing by J. Preston (pers. comm. 1973), and that for MH on information from G.Ö.
Bragason (pers. comm. 2008). In profiles TE, VS, FH, KV, HM and HV, notes from previous mapping by
geology students were used for reference. Profile AC was sampled along a level road, and the altitudes given
are estimated for the case of a vertical profile. All the magnetic polarity information shown is based on labora-
tory measurements. – Skissur af þykkt laga í sýnatökusniðunum og gerð þeirra, ásamt upplýsingum um stöðu
jarðsegulskauta fyrir hvert hraun. Í nokkrum sniðanna voru niðurstöður úr kortlagningarnámskeiði við H.Í.
hafðar til hliðsjónar.
36 JÖKULL No. 59