Jökull - 01.12.1987, Blaðsíða 59

The Late Weichselian glacial geology of the
Melabakkar-Ásbakkar coastal cliffs,
Borgarfjördur, W-Iceland
ÓLAFUR INGÓLFSSON
Department of Quaternary Geology, University of
Lund, Sölvegatan 13, S-223 62 Lund, Sweden
ABSTRACT
A 5 km long coastal section in the lower Borgarfjördur
region, W-Iceland, was investigated with regard to strati-
graphical, sedimentological ánd structural features, in
order to clarify Late Weichselian events. Fossil molluscs
were sampled for dating purposes and as environmental
indicators. The investigation revealed a 145 m thick se-
quence of glacial and littoral sediments, deposited in an
isostatically depressed basin, where waterdepth andprox-
imity to an ice margin and source ofmeltwater input were
the major controls of lithofacies distribution and stra-
tigraphical associations. Three major units of glacioma-
rine sediments make up more than half the strata. They
are separated by meltwater sediments and tills. Sediment-
ological and structural data indicate two separate glacial
advances out of the Borgarfjördur main valley into the
basin. The first advance occurred around 12.000 ,4C
years BP, the second one some time after 11.400 BP.
Some time prior to 10.000 BP the glacier retreated and the
sea transgressed the isostatically depressed lowlands.
These results suggest a more extensive glaciation in W-
Iceland during the latestpart ofthe Late Weichselian than
hitherto assumed. Some recent models for subarctic gla-
ciomarine fjord sedimentation, and for glaciotectonic de-
formations, are discussed in the light ofthe present study.
INTRODUCTION
The coastal lowlands of the lower Borgarfjördur area,
W-Iceland (Fig. 1) are blanketed by Late Weichselian
glacial and glaciomarine sediments and Late Weich-
selian to early Holocene littoral sands and gravels
(Bárðarson 1923, Ashwell 1975, Ingólfsson 1985). The
regional marine limit is represented by marine terraces
and shorelines at 60 to 70 m above present sea level.
There are indications in the outer coastal areas of an
older marine maximum at 80-90 m a.s.l. The Melabak-
kar-Asbakkar coastal cliffs are the most continuous sec-
tion in the area. They are roughly 5 km long and up to
30 m high, and transect a terminal moraine zone, the
Skorholtsmelar moraines (Fig. 1). A survey of the ge-
ological literature (Ingólfsson 1984) showed that opin-
ions on the Late Weichselian geology and glacial history
Fig. 1. Locality map. Legend: (1) Terminal moraine
ridges, (2) ancient strandlines at 60-70 m above present
sea level, (3) glacial striae: a: oldest, b: younger, c:
youngest, (4) ice-marginal delta, (5) farm, (6) present
coastline, (7) lake, (8) areas above 100 m a.s.l. The
coastal profile in Fig. 2 runs between points N and S.
1. mynd. Staðsetningarkort
JÖKULL
No. 37,1987