Jökull - 01.12.2007, Side 13

Early Pleistocene molluscan migration to Iceland
THE MÁNÁ FORMATION, BREIÐAVÍK
GROUP, NORTHERN ICELAND
The Máná Formation of the Breiðavík Group is ex-
posed in the eastern part of Breiðavík Bay on the
Tjörnes Peninsula, northern Iceland. It consists of
sediments capped by reverse polarity lava flows, the
Máná basalts. The lowermost part of the basalts has
been K/Ar dated to !1.13 Ma (Albertsson, 1978).
As we have not found a glacially striated erosional
contact, erosional unconformity, glacial sediments or
glacial pyroclastic products (palagonite) between the
sediments and the overlying lava, it is concluded that
they were piled up during the same interglacial pe-
riod and therefore the bottom lava is considered to
be of the same age as the uppermost sediments (cf.
Eiríksson, 1981). The Breiðavík Group has been thor-
oughly mapped by Eiríksson (1981, 1985) and the
faunal assemblages were described by Vilhjálmsson
(1985). The sediments and the lavas of the Breiðavík
Group were piled up in a sedimentary basin and the
Máná Formation was probably formed during the final
stages of the infilling of this basin (Eiríksson, 1981).
Obviously the sedimentary basin on the north side of
the Snæfellsnes Peninsula in western Iceland and the
Breiðavík basin on the Tjörnes Peninsula in northern
Iceland were contemporaneous for a while, when the
infilling of the basin was almost at an end in Breiða-
vík and was in its initial phases on Snæfellsnes. This
is unique for Pleistocene sedimentary basins in Ice-
land, although the location in two different volcanic
zones is not surprising.
The lowest part of the Máná Formation, divided
into the Stapavík Member and the Torfhóll Mem-
ber, consists of sediments corresponding to Bárðar-
son’s horizon 13 and 14 (Bárðarson, 1925; Eiríksson,
1981). The base of the formation consists of a di-
amictite bed up to 4 m in thickness, which is trun-
cated towards the south by an erosional unconformity
(Figure 3). The diamictite is succeeded by an up to
18 m thick conglomerate with sandstone lenses and
common high angle and folded bedding. It also con-
tains angular diamictite boulders redeposited from the
underlying diamictite. This strongly indicates that the
diamictite was already lithified when the conglomer-
ate was deposited and that the underlying unconfor-
mity apparently represents a considerable hiatus. This
conglomerate is overlain by an up to 10 m thick cross-
bedded and flat-bedded conglomerate that grades lo-
cally upwards into sandstone and rests on progres-
sively older sediments towards the south. In places
this bed is severely folded. The upper conglomerate is
succeeded by massive siltstone, faintly laminated near
the base and up to 4 m in thickness. It is fossiliferous
and contains marine molluscs belonging to the Ma-
coma assemblage. The top of the siltstone is grada-
tional and no sharp contact is seen with the overlying
fine-grained sandstone which is up to 14 m in thick-
ness. It is faintly cross-bedded, with rare erratics and
abundant marine molluscan fossils of the Macoma-
Serripes-Clinocardium assemblage (Table 2). The
sandstone is overlain by pillowy lava, the lowermost
flow of the Máná basalt, that here obviously reached
the sea.
The marine siltstone has been considered to be la-
goonal sediment, which became covered by bar sed-
iments during a following transgression (Eiríksson,
1985).
THE MARINE FAUNAL ASSEMBLAGES
The marine faunal assemblages in the Búlandshöfði
Formation contain 43 species of marine molluscs and
barnacles as shown in Table 1 (also Leifsdóttir, 1999).
In the Búland Member the bivalve shells are gener-
ally broken and disarticulated and not in situ, a condi-
tion that strongly indicates death assemblage (thana-
tocoenosis), probably due to wave and current action
and considerable environmental energy. On the other
hand, in the Höfði Member the shells are generally
articulated and undamaged and in situ, strongly indi-
cating life assemblage (biocoenosis) due to lower en-
vironmental energy. There are three layers yielding
marine molluscs in the Búland Member. The mol-
luscs belong to theMacoma-Portlandia bottom infau-
nal assemblage with the bivalve taxodont P. arctica as
a prominent species (Table 1). The number of species
decreases strongly upward in the Búland Member, es-
pecially the more arctic species.
Four layers yielding marine molluscs are found
from the Höfði Member. The layers contain ther-
mophilic molluscs such as Littorina littorea, Nu-
JÖKULL No. 57 11