Jökull - 01.12.1990, Page 34

at Reykjavíkurflugvöllurpostdated the marine limit in
Reykjavík.
DOUBLE ADVANCE DEGLACIATION
(DAD) MODEL
The SAD-model remained unchanged only for a
relatively short while, or until Th. Einarsson (1960)
correlated the formation of two moraines; one on
Álftanes in the vicinity of Reykjavík (Tryggvason
and Jónsson, 1958) and another in the outer parts
of Hvalfjörður (Bárðarson, 1923), with a glacier
readvance preceding the Búði-Hólkot stage (Fig. 1).
He referred to this older advance of the inland ice
sheet as the Álftanes advance and compared it with
Pjeturss’ (1910) ”Langanesstadium“ (Th. Einarsson,
1961). Later Th. Einarsson (1967, 1968) correlated
these moraines with terminal moraines in the up-
per Borgarfjörður, Húnaflói, and Eyjafjörður areas
(Fig. 1). Furthermore, the margin of the ”Álftanes
ice-sheet“ between Álftanes in Southwest Iceland and
Langanes in Northeast Iceland was reconstructed on
basis of morphological correlations (Th. Einarsson,
1971). Chronostratigraphically the Álftanes advance
occurred during the Álftanes stage which was pre-
sumed to be of an Older Dryas age (Th. Einarsson,
1967).
14C dates of subfossil molluscs from Stóri-
Sandhóll and Melar in West Iceland (Fig. 1) yielded ap-
parent ages of 12,270 ± 150 B.P. 2) (1-1824) (Ashwell
1967, 1975) and 11,925 ± 160 B.P. (U-641) (Ols-
son et al., 1969), respectively. Still unaware of the
effect of sea-water correction, Th. Einarsson (1968)
concluded that the apparent late Bplling age of the
Melar sample (U-641) predated the Álftanes advance,
which according to Th. Einarsson (1967, 1968) ter-
minated at the mouth of upper Borgarfjörður tributary
valleys.
This new concept of the mode of deglaciation, the
double advance deglaciation (DAD) model display-
ing two Late Weichselian readvances of the inland
ice sheet, also accounted for the areal extent of the
”Álftanes and Búði ice-sheets“ (Fig. 1). According
to Th. Einarsson (1968, 1985) the main ice-divide of
these two glaciers was extended in a northeastward
direction from the Tungnaáröræfi area in South Ice-
land to East Iceland, and towards the north from the
Kverkfjöll area to Melrakkaslétta in Northeast Ice-
land (Fig. 1). Furthermore, the maximum extent of
the Weichselian glaciation in Iceland, was most likely
reached concurrently with the European Weichselian
maximum glaciation at about 18,000 B.P. This tim-
ing of the maximum glaciation was first proposed by
Kjartansson (1962). An approximately 100 km long
and 20-30 m high ridge of probable glacial origin oc-
curs at a depth between 200 and 250 m below sea-level
and close to the edge of the shelf about 130 km west
of Breiðafjörður. This ridge has been interpreted as a
marginal moraine representing the Weichselian max-
imum glacier extent in Northwest Iceland at about
18,000 B.P. (Ólafsdóttir, 1975).
Gravel terraces at 70-90 m a.s.l. in the upper
Borgarfjörður area in West Iceland were formed when
relative sea-level reached approximately that height
(Bárðarson, 1923). Th. Einarsson (1961) determined
a regional marine limit in this area at about 70 m
a.s.l. According to Th. Einarsson (1968) this height
of relative sea-level was reached at about 11,000 B.P.
Generally speaking, the DAD-model implies that the
marine limit in Iceland was normally reached at the
beginning of the Búði advance at about 11,000 B.P.
Sediments of an apparent Allerpd age were first
described when Kjartansson (1966) reported a date
of 11,255 ± 240 B.P.1) (U-2019) for subfossil mol-
luscs at Ekruhorn in Saurbær in Northwest Iceland
(Fig. 1). Additional 14C dates yielding Allerpd ages
were later obtained for marine molluscs at Kaldá in
West Iceland and at Félagsstofnun in the Univer-
sity area in Reykjavík (Fig. 1). The Kaldá sample
yielded an age of 11,265 ± 160 B.P. (U-2227) and
the Félagsstofnun sample an age of 11,255 ± 255
B.P. (U-2596) (Thorleifur Einarsson; personal com-
munication 1991). Sediments of apparent Bolling age
were first described when Th. Einarsson (1971) re-
ported a date of 12,465 ± 170 B.P.1) (U-2225) (Ols-
son et al., 1972)forsubfossilmolluscscollectedfrom
the Röndin sediments at Kópasker in Northeast Ice-
land (Fig. 1). According to Th. Einarsson (1971),
the Röndin sediments were overridden during a sub-
sequent glacier advance, which he correlated with the
32 JÖKULL, No. 40, 1990