Jökull - 01.12.1993, Síða 41

SHORELINES BELOW 30 m
In a few places in the Þistilfjörður and Bakkaflói
areas, a series of lower, mostly erosional shorelines is
found at different elevations below the 30 m regional
marine limit; e.g. above Gunnólfsvík in the Bakkaflói
area where two distinct shorelines occur at 25 m and
10 m a.s.l., respectively (Figure 4).
A sedimentary terrace at Krossavík in the Þistil-
fjörður area (Figure 3) consists of about 18 m thick
cross-bedded sand and gravel sediments, overlain by
an about 2 m thick diamicton interpreted as a till (Fig-
ure 6B). Small-scale folds in the uppermost part of
the cross-bedded sediments reveal an eastward direc-
tion of the overriding glacier. The terrace is topped
by about 1 m of cross-bedded littoral sand and gravel
reflecting a relative sea-level at about 20 m a.s.l.
The age of the sub-till sediments is unknown,
whereas shoreline features in Krossavík and Gunn-
ólfsvík were probably formed when the glaciers had
retreated on to present-day dry land and relative sea-
level was situated below the regional 30 m marine
limit. During this regression all the sediment terraces
in the Þistilfjörðurand Bakkaflói areas were gradually
eroded leaving conspicuous degradational terraces and
water passages at successively lower altitudes.
THE MIÐFJÖRÐUR TEPHRA
Approximately in the middle of the Miðfjörður ter-
race in the Bakkaflói area, where its surface reaches
about 30 m a.s.l. (Figure 5), a fluvially eroded channel
has been filled with younger sediments. The chan-
nel was formed after relative sea-level had been low-
ered below the channel floor. The deposition of the
channel-fill sediments, therefore, clearly postdates the
regression of relative sea-level from the regional 30 m
marine limit.
The following succession of sediments is exposed
in a wedge-shaped sediment body at the edge of the
Miðfjörðurterrace (Figure7). Discordantly overlying
the coarse grained fluvioglacial sediments is a 12 cm
thick bed of stratified sand overlain by an approx-
unately 15 cm thick layer of light coloured tephra,
the Miðfjörður Tephra with a distinct lower boundary
but not so clear upper surface (Figure 8). Geochem-
A B
Miðfjöröur Krossavík
section section
— 25

E ö-'T’-cs ö-'°ö Sand and gravel
— 20 cs'Acs sand and gravel — 20 o
E Till
cs°cs cs-T-ci
cs'r'cs
— 15 — 15
Cross-bedded sand cs'Acs cs'r’cs
-10 — 10 ö'Aci Cross-bedded
cs'f’cs sand and gravel
— Laminated =><?Í3'
T_'.T. silt and fine sand cs-'r’cs
— 5 11—-l — 5 CS-T-CS
CS-'t’-Ci
l— 0 ®lll |Mí{l« Bedrock — 0 Ö.°-CS csT-ci
Figure 6. Stratigraphical succession at two coastal
sites: A. The Miðfjörður terrace. B. The Krossavík
terrace. For location see Figures 3 and 4. — Jarð-
lagasnið: A. Sethjalli við Miðfjörð við Bakkaflóa. B.
Sethjalli í Krossavík við Þistilfjörð.
ical analyses show that the glass shards have a rhy-
olitic composition (Table I) with a distinct Askja affin-
ity (Karl Grönvold, personal communication, 1994).
Above the Miðfjörður Tephra is 3 cm thick black
coloured basaltic tephra, which in turn is overlain by
an about 15 cm thick loessial soil (Figure 8), the ear-
liest known Postglacial soil in the study area. These
sediments are c.overed by a few metres of eolian sand
(Figure 7).
DISCUSSION AND CONCLUSIONS
As evidenced by the distribution of glacial striae
and the extent of the till-cover the whole of the present
study area, except for some of the highest coastal
mountains, was ice-covered during the Weichselian
glaciation maximum. The glaciers originated in the
mountains south and south-west of Þistilfjörður and
Bakkaflói and in the southern Melrakkaslétta area,
JÖKULL, No. 43, 1993 39