Jökull - 01.12.1993, Blaðsíða 38

Figure 3. The Þistilfjörður area with raised shoreline
features and major sediment terraces. Contour interval
is 100 m. —Fornfjörumörk, sethjallar og jökulgarð-
ar við Þistilfjörð. Hœðarlínur eru á 100 m bili.
initial state of a maximum glaciation when glaciers
reached an unknown position on the Iceland shelf.
Subsequently to a considerable glacial retreat the
glaciers readvanced and an increased glacial overbur-
den load caused a subsidence of the crust that exceeded
the eustatic lowering of global sea-level generated by
increased volume of ice masses around the world.
Consequently, relative sea-level transgressed and a
marine limit was formed when sea-level reached a tem-
porary maximum position. Furthermore, the MAD-
model accounts for a subsequent glacial retreat, re-
duced crustal overburden load and a glacio-isostatic
recovery exceeding the eustatic rise of global sea-level.
Consequently, the isostatic uplift caused a regression
ofrelative sea-level. During afollowing but less exten-
sive glacial readvance and a concurrent transgression
of relative sea-level a new set of shorelines was formed
somewhat below the earlier marine limit. This rela-
tionship between glacier extent and position of rela-
tive sea-level has enabled us to recognize successively
lower and younger shorelines within the study area.
This paper follows the chronostratigraphical termi-
nology of Mangerud et al. (1974), and the age of 14C-
dated marine molluscs mentioned in the text has been
corrected for reservoir effect according to Hákansson
(1983).
MARINE LEVELS
THE APPARENT 65 m LEVEL
The outermost identifiable Lateglacial position of a
glacier margin in the study area is shown by a conspic-
uous lateral moraine situated below the steep mountain
slope above the Viðarvíkcove in the Þistilfjörðurarea
(Figures 2 and 3). The moraine continues in a complex
of kames and kettles south and east of the cove. A sed-
imentary terrace, just outside the moraine (Figure 2)
is made of semi-lithified sand overlain by a coarse
grained, poorly sorted ablation till or gelifluction sed-
iment. The formation of the terrace in this exposed
position was probably controlled by sea-level, which
at that time may have reached as high as 65 m a.s.l.
SHORELINES BETWEEN 50 AND 40 m
A 30-35 m thick sediment sequence was formed at
or close to a glacier margin inside the above mentioned
lateral moraine in Viðarvík (Figure 3), when relative
sea-level stood at 40-45 m a.s.l. The delta sequence
consists of at least two sedimentary units underlain by
a compact sandy diamicton, probably a till. The lower
unit - a few metres thick - comprises thinly bedded
and laminated silt and fine sand, while the upper unit
is made of coarser sand and gravel sediments. Near
the middle of the upper unit, cross-bedded sediments
have been slightly distorted by ice-push or melting of
buried glacier ice. On the inside of the lateral moraine
with its associated 65 m terrace we find shorelines at
50 and 40 m a.s.l. (Figures 2 and 3).
A shoreline at about 50 m a.s.l. on the Digra-
nes peninsula in the Bakkaflói area, just outside
the Bakkafjörður village (Figure 4), was most likely
formed close to a glacier margin, when the glaciers
reached just beyond the present coastline and into the
Bakkaflói bay.
36 JÖKULL,No. 43, 1993