Jökull - 01.12.1979, Síða 12

appear unexpectedly low if compared to the age of
the sea bottom from magnetic anomaly identifica-
tion (see chapter 1). The explanation may lie in the
very significant overlap of subaerial lavas, these
ages being derived from the uppermost 1000 m of
the pile below which another 2—5 km of lavas at
least must be expected. Southward and eastward
displacement of rift zones with time combined with
simultaneous crustal spreading along two parallel
axes appears more important though. The unequal
disposition of the Tertiary series which is nowhere
exposed south of lat 64° but occupies the greater
part of Iceland north of lat 65° (Fig. 1) is probably
the result ofífuch displacements.
STRATIGRAPHY AND GENERAL TEC-
TONICS
The predominantly volcanic pile of Iceland,
which ranges in age back to about 16. m.y., is con-
ventionally divided into four stratigraphic groups
or series. This division is based on climatic evidence
from inter-lava sediment or volcanic breccias and
on palaeomagnetic reversal patterns supported by
absolute age data. The four groups are (Fig. 4):
Postglacial: last 9.000 to 13.000 y.
Upper Pleistocene: back to 0.7 m.y.; corresponding
to the present normal geomagnetic epoch, Brunhes.
Plio-Pleistocene: 0.7—3.1 m.y.; includes the
Matuyama epoch and the Gauss epoch upwards of
the Mammoth event.
Tertiaiy: rocks older than 3.1 m.y.
Tertiary.
The Tertiary areas include the classical plateau
basalt series typical of the fjord landscapes of
eastern Iceland and much of northern and western
Iceland. Altogether Tertiary rocks cover about
50.000 km2 or about 1 / 2 of the total area of Iceland.
The Tertiary sequences are made up of subaerial
tholeiitic lavas and genetically associated inter-
mediate and acidic rocks. The Tertiary lava pile
shows little variation in lithology; the stratigraphy
is generally very regular with about 5—15 m thick
lavas separated by minor clastic interbeds of vol-
canic origin. The monotony is interrupted where
oentral volcanoes occur with their buried palaeo-
topography, acid rocks, hydrothermal alteration
and irregular dips. About 15 such have been
PALEOMAGNETIC POLARITY
SERIES / SYSTEM
- 5
10 -
5A
‘,5C
POLARITY EVENT POLARITY EPOCH
BRUNHES 1
JARAMILLO Olduvai MATU- YAMA 2
KAENA MAMMOTH GAUSS 3
COCHITl THVERA GILBERT 4
5.
6
7
8
9
10
II
12
14
15
16
HOLOCENE
PLEISTO-
CENE
PLIO-
CENE
l
QUATER-
NARY
T
E
R
T
I
A
R
Y
LOCAL
USA6E
UPPER 'Pa'
PLEISTOCENf
PLIO-
PLEIST0-
CENE
T
E
R
T
I
A
R
Y
Fig. 4. Stratigraphic time table (LaBrecque et al.,
1977). Modified version that is conventionally used
in Iceland is included. There is a confusion about
nomenclature of the normally magnetized events in
the lower Matuyama. In Iceland two events occur
in many sections studied to date (see for instance
Fig. 9). In this review Gilsá is still retained. It would
correlate in time approximately to the Olduvai of
LaBrecque’s et al. time table.
defined and mapped in the Tertiary but another 40
are suspected from the occurrence of acidic rocks.
In early geological work the establisment of time
relations within the lava pile presented a major
difficulty since it had to be based on palaeonto-
logical (mainly palaeobotanical) evidence.
Radiometric dating techniques and palaeomag-
netic research have improved this situation. The
lavas themselves are now used for determining the
ages thus providing a firm background for studying
changes in flora and climate. Over 100 radiometric
ages are now available from various parts of the
Tertiary areas. The palaeomagnetic reversal pat-
tern provides a means of stratigraphic mapping of
lava groups independent of lithological character-
istics. Mapping of lithological units was first
applied on a large scale by G. P. L. Walker in
eastem Iceland in the nineteen-fifties. During this
10 JÖKULL 29. ÁR