Jökull - 01.12.1963, Page 7

in which sparse shell fragments occur through-
out. Strauch thinks that the undulating surface
of this gravel reflects strandwalls.
A new submergence finally produced the
sandstone H14, the Cardium-Mya deposit of
Bárðarson. This layer is directly covered by
basalt lavas of reverse magnetic polaritv, cor-
responding to the lower magnetic group of
Búrfelh In the lowest lava, resting on H14,
such a glassy base has been formed that a wet
ground is suggested but there are no grounds
for assuming submarine flow.
II.
The sediments of Breiðavík stretch far in-
land, and especially Hs appears to dominate
in the inland area. Outcrops are seen along
the gullies of Tröllagil and Fossgil but there
are doubts about the correlation of the strata.
As a general remark it may be stated that
conglomerates occur at several places at the
base of finer sediments, and as these conglome-
rates are nearly devoid of the brown sediment
material, Strauch assumed that they always cor-
respond to H2a, as he also assumed for Svart-
hamar. But we have seen that within Hs, and
especially at the top of H0/7, there are conglo-
merates of this character, and it best fits the
facts that these are occasionally seen at the
base of Hs inland, while the postulation of
windows down into H2a is cjuite unwarranted.
Hs is predominant along rnost of the Trölla-
gil gully. Below it can be seen light clay and a
conglomerate and this we consider as a part
of Hs or perhaps the top of H7. In the lower
part of the Fossgil gully we usually see H3,
covered by H4. H5 soon thins out and is dif
ficult to identify. Above the main road, crossing
the gully, we have: a) dark-grey conglomerate
overlain by b) light brown mudstone, and in
rny earlier study I took these to be H3 and
H4. But tliese layers are also quite similar to
the basal rocks in Tröllagil, a few hundred
metres farther east. On this basis I suggested
in my earlier study that H5 and Hs corre-
spondecl perhaps to each other. But we have
now excluded that possibility and I now assume
with Strauch that a) and b) in the upper Foss-
gil correspond to the base of Hg, but not to
H3 and H4.
The Fossgil section continues with c) con-
glomerate and sandstone. No pebbles frorn the
deeper sediments are founcl here. We recon it
as part of Hg. It is conformably overlain by
d) normally polarized volcanic breccia and two
lava flows. There is no cover, so that the
stratigraphic position of d) is not seen. However,
rich cementation and amygdales make it pret-
ty certain that the rocks are older than the
Fleistocene (and last) normal magnetic group
(which hardly ever contains any notable in-
fillings in Iceland) which means that we must
put them below the reverse lavas Ri, that
cover tlie Breiðavík deposits. We have found
above that after deposition of H0/7 the Breiða-
vík deposits were subjected to much cement-
ation. This most likely happened under the
cover of the lava pile whose main remnant is
left in Búrfell. This then agrees with the as-
sumecl position of d).
At the heacl of Tröllagil is a section with
Hs—Hin- Apparently on this rests a reverse
volcanic breccia covered by a conglomerate
(corresponding perhaps to 12x) and sandstone
which Strauch thinks is H12. Directly on this
rest here the lavas of Ri.
Thus, it woulcl seem that faint volcanism
began in Hg, with normal polarity, continuing
above Hio after change of polarity, and finally
gathering full force after H14. This would
agree with a trend which I have founcl in a
number of cases: that magnetic polarity changes
soon after the beginning of a volcanic periocl.
According to recent absolute datings (Cox,
Doell, Dalrymple 1963) the last three rnagne-
tic periods (Ni, Ri, N2) each lasted close to 1
million years. The top of the BreiðaVík sedi-
ments is then close to 2 million years old. We
shall assume that the (normal) Stangarhorn
basalts, Hi, are early N2 or older. The Breiða-
vík sediments then cover a time of l/2— 1 million
years, perhaps more.
III
The Breiðavík deposits contain 110 clear
evidence of a glaciation of the area, and it
seems also to bc in goocl agreement with their
age that they are older than the first major
Pleistocene glaciation. But the deposits indi-
cate repeated changes of sealevel, and changes
of climate may also be inferred; the instability
in several strata may be due to changes of sea-
JOKULL 1963
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