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Tjörnes - Pliocene and Pleistocene sediments and faunas

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On the western side of the Tjornes Peninsula in North Iceland a long sequence of fossiliferous marine sediments, basalts, and diamicitites records the climatic history of the North Atlantic during the Pliocene and Lower Pleistocene. The Pliocene Tjornes beds are divided in three biozones; the Tapes Zone (oldest), the Mactra Zone, and the Serripes Zone (youngest). The Tjornes beds consist mainly of marine silt- and sandstones but there art, also several fossiliferous terrestrial beds in the lower part. The marine faunas in the Tapes and Mactra Zones are mainly boreal, but during the deposition of the Serripes Zone the fauna greatly diversified with immigration of Pacific molluscan species with more arctic elements. They reached the North Atlantic at 3.6 Ma after migration through the Bering Strait coeval with closing of the Central American Seaway. Marine molluscs of Pacific ancestry it? the Tapes and Mactra Zones post-date also the first opening of the Bering Strait. In the Breioavik Group, diamictite beds alternate with volcaniclastic mudrocks and sandstones, and basaltic lava flows. Fourteen lithological cycles are identified in the Breidavik Group each one starting with a diamictite interpreted as lodgement tillite and ending with terrestrial sediments and lava flows. Interbedded marine fossiliferous mudrocks and sandstones contain arctic to boreal faunal assemblages. The oldest cycle in the Breioavik group was probably deposited about 2.5 Ma, just after the Gauss/Matuyama polarity reversal.
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Reviewed research article
Tjörnes Pliocene and Pleistocene sediments and faunas
Leifur A. Símonarson and Jón Eiríksson
Institute of Earth Sciences, University of Iceland, Askja, Sturlugata 7, 101 Reykjavík
leifuras@raunvis.hi.is jeir@raunvis.hi.is
Abstract On the western side of the Tjörnes Peninsula in North Iceland a lo ng sequence of fossiliferous
marine sediments, basalts, and diamictites records the climatic history of the North Atlantic during the Pliocene
and Lower Pleistocene. The Pliocene Tjörnes beds are divided in three biozones; the Tapes Zone (old est), the
Mactra Zone, and the Serripes Zone (youngest). The Tjörnes beds consist mainly of marine silt- and sandsto nes,
but there are also several fossiliferous terrestrial beds in the lower part. The marine faunas in the Tapes and
Mactra Zones are mainly boreal, but during the deposition of the Serripes Zone the fauna greatly diversied
with immigration o f Pacic molluscan species with more arctic elements. They reached the North Atlantic at
3.6 Ma after migration through the Be rin g Strait coeval with closing of the Central American Seaway. Marine
molluscs of Pacic ancestry in the Tapes and Mactra Zone s post-date also the rst o pening of the Bering
Strait. In the Brei ðavík Group, diamictite beds alternate with volcaniclastic mudrocks and sandstone s, and
basaltic lava ows. Fourteen lithological cycles are ide ntied in the Breiðavík Gro up each one starting with
a diamictite interpreted as lodgement tillite a nd ending with terrestrial sediments and lava ows. Interbedded
marine fossilif erous mudrocks and sandstones contain arctic to boreal faunal a ssemblages. The oldest cycle in
the Breiðavík gro up was probably deposited ab out 2.5 Ma, just after the Gauss/Matuyama polarity reversal.
INTRODUCTION
On the Tjörnes Peninsula in northern Iceland there
is a well-exposed sequence of Pliocene and Pleis-
tocene fossiliferous marine and non-marine sedimen-
tary rocks intercalated with basaltic lava ows and,
in the higher part, with tillite layers recording at least
fourteen glaciations (Eiríksson e t al., 1992). The Tjör-
nes sequence contains a unique record of environmen-
tal and faunal changes from the Upper Tertiary to
the Quaternary in the shallow-marine and terrestrial
North Atlantic region. This is reected in lithologi-
cal and faunal variations in the sequence where ma-
rine and terrestrial sediments are intercalated between
lava ows and pyroclastic rocks. In fact the proxim-
ity to the Arctic Polar Front makes North Iceland a
key area for understanding climatic variations in the
North Atlantic (Eiríksson et al., 1992; Buchardt and
Símonarson, 2003).
The abrupt appearance of marine invertebrates of
North Pacic origin in the lowermost part of the Ser-
ripes Zone of the Tjörnes beds at 3.6 Ma is partic-
ularly noteworthy. This event has often been inter-
preted as reecting the opening of the Bering Strait,
but it is probably more related to the closing of the
Central American Seaway at this time (Backman,
1979; Marincovich, 2000). New evidence indicates
that the Bering Strait rst opened at 5.5–4.8 Ma and
then the initial phase of faunal interchange took place
and some Pacic species reached North Iceland when
the lowermost part of the marine sediments of the
Tjörnes sequence were deposited (Durham and Mac-
Neil, 1967, Marincovich, 2000, Buchardt and Símon-
arson, 2003).
The present paper gives an overview of the faunal
and sedimentological changes observed on the Tjör-
nes Peninsula, from relatively stable Pliocene condi-
tions, to the highly variable and periodically harsh cli-
matic conditions of repeated glaciations.
JÖKULL No. 58, 2008
331
L. A. Símonarson and J. Eiríksson
Figure 1. Geologic al map of Tjörnes
Peninsula showing the major lithological
units of the Tjörnes sequence : 1. Kalda-
kvísl lavas. 2. Tjörnes beds. 3. sk-
uldsvík Group. 4. Breiðavík Grou p (after
Eiríksson, 1981). Helstu jarðlagasyr-
pur á Tjörnesi: 1. Köldukvíslarhraunlög.
2. Tjörneslög. 3. Höskuldsvíkurhó pur.
4. Breiðuvíkurhópur (frá Jóni Eiríkssyni,
1981).
GEOLOGICAL SETTING
The Tjörnes peninsula is located near the intersection
between the axial zone of rifting and volcanism and
the Tjörnes Fracture Zone in northern Iceland ( Figure
1). Tectonically, T jörnes has been an active area dur-
ing the Pliocene and Pleistocene and was described as
a horst by Thoro ddsen (1902). The uplift relative to
the area south of Tjörnes was estim ated as over 700 m
by T. Einarsson (1958). According to Th. Einarsson
et al. (1967) the uplift in the southea stern part of the
Tjörnes area amounts to 500–600 m. Sæmundsson
(1974) estimated the subsidence in the Öxarfjörður
depression east of Tjörnes to be at least 1000 m.
The Tjörnes Peninsula exposes a thick sequence
of Miocene to Pleistocene lava ows as well as alter-
nating terr estrial, transitional, and marine sediments
of Pliocene and Pleistocene ages. The stratigraphic
sequence on Tjörnes was divided into four lithostrati-
graphic units by Eiríksson (1981), the informal Kalda-
kvísl lavas and Tjörnes beds, the Hö sk uldsvík Group,
and the Breiðavík Group (Figure 1).
332 JÖKULL No. 58, 2008
Tjörnes Pliocene and Pleistocene sediments and faunas
The Tjörnes beds
The informal lithostratigraphic term Tjörnes beds will
be used here as a synonym for the Tjörnes sedimen-
tary unit between the Kaldakvísl and Höskuldsvík
Group basalts. The Tjörnes beds and the uppermost
Kaldakvísl lava ows dip 5–15
to the northwest. T he
sediment sequence is exposed in river canyons and
sea cliffs for about 6 km along the coast on the west
side of Tjörnes (Bárðarson, 1925; Th. Einarsson et al.,
1967). The bulk of the sediments is made up of fossil-
iferous marine sandstones with intermittent terrestrial
or transitional lignites and muddy sandstones (Figure
2). Bárðarson (1925) divided the Tjörnes beds into
25 distinct shell bearing horizons, which he numbered
1–25, and 10 terrestrial or transitional horizons des-
ignated A-J. Furthermore, he grouped the shell hori-
zons into three biozones: the Tapes Zone (oldest),
the Mactra Zone, and th e Cardium or Serripes Zone
(youngest).
The sed imentary sequence of the Tjörnes beds be-
gins with sandstone and conglomerate containing ma-
rine and littor al epifauna l mollu scs in the bottom of
the Tapes Zone (Figure 2). The stratigraphic relation-
ship and contact with the Kaldakvísl lavas is compli-
cated by the presence of a ser ies o f WNW trending
faults, some of which displa y right-lateral transform
movement. The strata on each side o f these faults do
not match. To the north of the river Kaldakvísl, the
sediments rest on the erosional surface of the Kalda-
kvísl lavas and are overlain by a thin lignite seam
formed by plant remains that most pro bably accumu-
lated in swamps close to the coast. The sandstone
overlying the lignite has infaunal molluscan assem-
blages that preferably lived in tidal at areas. How-
ever, the scarcity of mudrocks indicates sedimenta-
tion in areas with rather limited tidal range . Today
the tidal range is only about 1.5 m for the mean spring
tide and 0.5 m for the mean neap tide in Northeast
Iceland (Stefánsson, 1962). After the outpouring of a
thin subaerial lava sheet, tidal at sands accumu lated
again and they are overlain by a conglomera te with
littoral epifaunal molluscs about 50 m from the bot-
tom of the Tapes Zo ne. Subsequently, the upper part
of the Tapes Zone and the lower part of the Mactra
Zone were deposited partly as tidal at sediments and
partly as plant remains (lignite beds) in swamps close
to the coast.
The middle part of the Mactra Zone consists of
cross-bedded sandstone and conglomerates dissected
by current channels in bed E (Figure 2). They are
almost devoid of marine fossils, with only very frag-
mented mollusc shells fou nd in the lowermost part.
This indicates the inner part of littoral bar deposits.
The b ed is overlain by a thick lignite layer wh ich is in
turn overlain b y a conglomerate with littoral epifaunal
molluscs. No lignite beds are found in the upper part
of the Mactra Zone and the lower part of the Serripes
Zone which consists of alternating layers of sand- and
siltstones deposited in shallow water sublittoral envi-
ronments (Figure 3).
In the middle part of the Serripes Zone conglom-
erate with littoral epifaunal molluscs appears again
overlain b y alternating layers of sand- and mudstones
apparen tly fo rmed in estuarian e nvironment, as indi-
cated by the mollusc fauna and fossil wood remains
found in the sediments. A lignite bed is resting on
the estuarian seri es and the sedimentary Tj örnes se-
quence terminates in sandstone with littoral epifaunal
molluscs.
The skuldsvík Group
The Höskuldsvík Group consists o f six lava ows with
intercalated uvial sediments and tuffs, conformably
overlying the Tjörnes beds. No marine fossils have
been found in the Höskuldsvík Group sediments, and
the lithofacies reects a regression in the Tjörn es area
with incre a sed volcanic activity evidenced by the sub-
aerial lava ows.
The Breiðavík Group
The youngest lithostratigraphical group on Tjörnes
is the Breiðavík Group, which is characterized by
recurring diamictite units interpreted as glacial de-
posits. The Breiðavík Group contains a r ecord of
Pliocene and Pleistoce ne glaciations with fourteen
lithological cycles in six formations showing alter-
nating ice cover and ice free conditions. The low-
est one, th e Furuvík Formation, displays nor thwest-
erly dip conformable with the underlyin g Höskulds-
vík Group rocks, but subsequent formations exhibit
decreasing northerly dip. The aggregate thickness of
JÖKULL No. 58, 2008
333
L. A. Símonarson and J. Eiríksson
Figure 1. Geologic al map of Tjörnes
Peninsula showing the major lithological
units of the Tjörnes sequence: 1. Kalda-
kvísl lavas. 2. Tjörnes beds. 3. sk-
uldsvík Group. 4. Breiðavík Grou p (after
Eiríksson, 1981). Helstu jarðlagasyr-
pur á Tjörnesi: 1. Köldukvíslarhraunlög.
2. Tjörneslög. 3. Höskuldsvíkurhó pur.
4. Breiðuvíkurhópur (frá Jóni Eiríkssyni,
1981).
GEOLOGICAL SETTING
The Tjörnes peninsula is located near the intersection
between the axial zone of rifting and volcanism and
the Tjörnes Fracture Zone in northern Iceland ( Figure
1). Tectonically, T jörnes has been an active area dur-
ing the Pliocene and Pleistocene and was described as
a horst by Thoro ddsen (1902). The uplift relative to
the area south of Tjörnes was estim ated as over 700 m
by T. Einarsson (1958). According to Th. Einarsson
et al. (1967) the uplift in the southea stern part of the
Tjörnes area amounts to 500–600 m. Sæmundsson
(1974) estimated the subsidence in the Öxarfjörður
depression east of Tjörnes to be at least 1000 m.
The Tjörnes Peninsula exposes a thick sequence
of Miocene to Pleistocene lava ows as well as alter-
nating terr estrial, transitional, and marine sediments
of Pliocene and Pleistocene ages. The stratigraphic
sequence on Tjörnes was divided into four lithostrati-
graphic units by Eiríksson (1981), the informal Kalda-
kvísl lavas and Tjörnes beds, the Hö sk uldsvík Group,
and the Breiðavík Group (Figure 1).
332 JÖKULL No. 58, 2008
Tjörnes Pliocene and Pleistocene sediments and faunas
The Tjörnes beds
The informal lithostratigraphic term Tjörnes beds will
be used here as a synonym for the Tjörnes sedimen-
tary unit between the Kaldakvísl and Höskuldsvík
Group basalts. The Tjörnes beds and the uppermost
Kaldakvísl lava ows dip 5–15
to the northwest. T he
sediment sequence is exposed in river canyons and
sea cliffs for about 6 km along the coast on the west
side of Tjörnes (Bárðarson, 1925; Th. Einarsson et al.,
1967). The bulk of the sediments is made up of fossil-
iferous marine sandstones with intermittent terrestrial
or transitional lignites and muddy sandstones (Figure
2). Bárðarson (1925) divided the Tjörnes beds into
25 distinct shell bearing horizons, which he numbered
1–25, and 10 terrestrial or transitional horizons des-
ignated A-J. Furthermore, he grouped the shell hori-
zons into three biozones: the Tapes Zone (oldest),
the Mactra Zone, and th e Cardium or Serripes Zone
(youngest).
The sed imentary sequence of the Tjörnes beds be-
gins with sandstone and conglomerate containing ma-
rine and littor al epifauna l mollu scs in the bottom of
the Tapes Zone (Figure 2). The stratigraphic relation-
ship and contact with the Kaldakvísl lavas is compli-
cated by the presence of a ser ies o f WNW trending
faults, some of which displa y right-lateral transform
movement. The strata on each side o f these faults do
not match. To the north of the river Kaldakvísl, the
sediments rest on the erosional surface of the Kalda-
kvísl lavas a nd are overlain by a thin lignite seam
formed by plant remains that most pro bably accumu-
lated in swamps close to the coast. The sandstone
overlying the lignite has infaunal molluscan assem-
blages that preferably lived in tidal at areas. How-
ever, the scarcity of mudrocks indicates sedimenta-
tion in areas with rather limited tidal range . Today
the tidal range is only about 1.5 m for the mean spring
tide and 0.5 m for the mean neap tide in Northeast
Iceland (Stefánsson, 1962). After the outpouring of a
thin subaerial lava sheet, tidal at sands accumu lated
again and they are overlain by a conglomera te with
littoral epifaunal molluscs about 50 m from the bot-
tom of the Tapes Zo ne. Subsequently, the upper part
of the Tapes Zone and the lower part of the Mactra
Zone were deposited partly as tidal at sediments and
partly as plant remains (lignite beds) in swamps close
to the coast.
The middle part of the Mactra Zone consists of
cross-bedded sandstone and conglomerates dissected
by current channels in bed E (Figure 2). They are
almost devoid of marine fossils, with only very frag-
mented mollusc shells fou nd in the lowermost part.
This indicates the inner part of littoral bar deposits.
The b ed is overlain by a thick lignite layer wh ich is in
turn overlain b y a conglomerate with littoral epifaunal
molluscs. No lignite beds are found in the upper part
of the Mactra Zone and the lower part of the Serripes
Zone which consists of alternating layers of sand- and
siltstones deposited in shallow water sublittoral envi-
ronments (Figure 3).
In the middle part of the Serripes Zone conglom-
erate with littoral epifaunal molluscs appears again
overlain b y alternating layers of sand- and mudstones
apparen tly fo rmed in estuarian e nvironment, as indi-
cated by the mollusc fauna and fossil wood remains
found in the sediments. A lignite bed is resting on
the estuarian seri es and the sedimentary Tj örnes se-
quence terminates in sandstone with littoral epifaunal
molluscs.
The skuldsvík Group
The Höskuldsvík Group consists o f six lava ows with
intercalated uvial sediments and tuffs, conformably
overlying the Tjörnes beds. No marine fossils have
been found in the Höskuldsvík Group sediments, and
the lithofacies reects a regression in the Tjörn es area
with incre a sed volcanic activity evidenced by the sub-
aerial lava ows.
The Breiðavík Group
The youngest lithostratigraphical group on Tjörnes
is the Breiðavík Group, which is characterized by
recurring diamictite units interpreted as glacial de-
posits. The Breiðavík Group contains a r ecord of
Pliocene and Pleistoce ne glaciations with fourteen
lithological cycles in six formations showing alter-
nating ice cover and ice free conditions. The low-
est one, th e Furuvík Formation, displays nor thwest-
erly dip conformable with the underlyin g Höskulds-
vík Group rocks, but subsequent formations exhibit
decreasing northerly dip. The aggregate thickness of
JÖKULL No. 58, 2008
333
L. A. Símonarson and J. Eiríksson
Figure 2. Generalised lithostratig raphic section of the Tjör nes beds. The Bár ðarson (1925) subdivisions in beds
and molluscan biozone are also shown. Palaeoenvironmental interpretations are from the authors. Berg-
jarðlagafræðilegt snið úr Tjörneslögum . Skipting set- og skeljalaga er frá Guðmundi G. Bárðarsyni (1925).
Fornvistfræðileg túlkun er höfunda.
unit strato types of the Breiðavík Group is over 600 m
(Eiríksson et al., 1992). T he Furuvík, rgi, Þreng-
ingar, and Máná Formations consist mainly of sedi-
mentary roc ks with a few interca lated lava ows (Fig-
ure 4). The sediments accumulated in nearshore and
terrestrial environments while the uppermost forma-
tions, the Grasafjöll and Húsavík Formations consist
of series of lava ows with in tercalated terrestrial sed -
iments (Eiríksson, 1985).
334 JÖKULL No. 58, 2008
Tjörnes Pliocene and Pleistocene sediments and faunas
DISCUSSION AND AGE
Strauch (1963) suggested that the Tjör nes beds were
accumulated in a slight depression and that sedimen-
tation took p lace in a shallow fjord open towards the
north with sediment supply from the south. When the
supply exceeded the rate of subsidence, the fjord was
rapidly lled and tidal at sediments, lignite seams
and lacustrine beds were deposited in the Tapes Zone
and the lower part of the Mactra Zone. During times
of increased subsidence rate the fjord deepened and
sublittoral sediments wer e deposited in the upper part
of the Mactra Zone and the lower pa rt of the Serripes
Zone.
With a decreasin g rate of subsiden ce in the upper
part of the Serripes Zone, estuarine and littoral sed-
iments were deposited. The lavas of the Höskulds-
vík Group mark the end of sediment accumulatio n in
the Pliocene Tjörnes basin. As repeated glaciation s
set in, a system of valleys and fjords was established
within the Tjörnes Fracture Zone, and the sedimen-
tary basins envisaged by Eiríksson et al. (1 992) fo r the
glacial-interglac ial deposits of the Breiðavík Group
were partly of tectonic origin and partly glac ially
eroded valleys inundated by marine transgressions as-
sociated with glacio -isostasy combined wi th marginal
uplift of the Tjörnes peninsula. This scenario is anal-
ogous to the modern Öxarfjörður trough to the east of
Tjörnes.
Samples from the basal units of lava ows in the
Kaldakvísl ar ea (Figures 1 an d 5) have been dated
radiometrical ly and yield ages from 9.9±1.8 Ma to
8.6±0.4 Ma (Aronson and Saemundsson, 1975). The
Tertiary Kaldakvísl lavas are overlain by the Tjör nes
Figure 3. Shell accumulations in the Mactra Zone of the Tjörnes beds at 3.7–3.8 Ma. The shell beds are formed
by c urrents that r emoved the sediment grains but left the heavier shells and shell fragments that accumulated on
the bottom. Skeljalög í tígulskeljalögum á Tjörnesi. Einstök skeljalög hafa myndast við samansöfnun skelja
og skeljabrota þegar straumar léku um botnsetið og skoluðu burtu setkornum, en skeljarnar sátu eftir þar sem
þær eru þyngri. Aldur laganna er 3,7–3,8 milljónir ára.
JÖKULL No. 58, 2008 335
L. A. Símonarson and J. Eiríksson
Figure 2. Generalised lithostratig raphic section of the Tjör nes beds. The Bár ðarson (1925) subdivisions in beds
and molluscan biozone are also shown. Palaeoenvironmental interpretations are from the authors. Berg-
jarðlagafræðilegt snið úr Tjörneslögum . Skipting set- og skeljalaga er frá Guðmundi G. Bárðarsyni (1925).
Fornvistfræðileg túlkun er höfunda.
unit strato types of the Breiðavík Group is over 600 m
(Eiríksson et al., 1992). T he Furuvík, rgi, Þreng-
ingar, and Máná Formations consist mainly of sedi-
mentary roc ks with a few interca lated lava ows (Fig-
ure 4). The sediments accumulated in nearshore and
terrestrial environments while the uppermost forma-
tions, the Grasafjöll and Húsavík Formations consist
of series of lava ows with in tercalated terrestrial sed -
iments (Eiríksson, 1985).
334 JÖKULL No. 58, 2008
Tjörnes Pliocene and Pleistocene sediments and faunas
DISCUSSION AND AGE
Strauch (1963) suggested that the Tjör nes beds were
accumulated in a slight depression and that sedimen-
tation took p lace in a shallow fjord open towards the
north with sediment supply from the south. When the
supply exceeded the rate of subsidence, the fjord was
rapidly lled and tidal at sediments, lignite seams
and lacustrine beds were deposited in the Tapes Zone
and the lower part of the Mactra Zone. During times
of increased subsidence rate the fjord deepened and
sublittoral sediments wer e deposited in the upper part
of the Mactra Zone and the lower pa rt of the Serripes
Zone.
With a decreasin g rate of subsiden ce in the upper
part of the Serripes Zone, estuarine and littoral sed-
iments were deposited. The lavas of the Höskulds-
vík Group mark the end of sediment accumulatio n in
the Pliocene Tjörnes basin. As repeated glaciation s
set in, a system of valleys and fjords was established
within the Tjörnes Fracture Zone, and the sedimen-
tary basins envisaged by Eiríksson et al. (1 992) fo r the
glacial-interglac ial deposits of the Breiðavík Group
were partly of tectonic origin and partly glac ially
eroded valleys inundated by marine transgressions as-
sociated with glacio -isostasy combined wi th marginal
uplift of the Tjörnes peninsula. This scenario is anal-
ogous to the modern Öxarfjörður trough to the east of
Tjörnes.
Samples from the basal units of lava ows in the
Kaldakvísl ar ea (Figures 1 an d 5) have been dated
radiometrical ly and yield ages from 9.9±1.8 Ma to
8.6±0.4 Ma (Aronson and Saemundsson, 1975). The
Tertiary Kaldakvísl lavas are overlain by the Tjör nes
Figure 3. Shell accumulations in the Mactra Zone of the Tjörnes beds at 3.7–3.8 Ma. The shell beds are formed
by c urrents that r emoved the sediment grains but left the heavier shells and shell fragments that accumulated on
the bottom. Skeljalög í tígulskeljalögum á Tjörnesi. Einstök skeljalög hafa myndast við samansöfnun skelja
og skeljabrota þegar straumar léku um botnsetið og skoluðu burtu setkornum, en skeljarnar sátu eftir þar sem
þær eru þyngri. Aldur laganna er 3,7–3,8 milljónir ára.
JÖKULL No. 58, 2008 335
L. A. Símonarson and J. Eiríksson
beds, which include abou t 500 m of fossiliferous sed-
iments on the west side of Tjörnes (Bárðarson, 1 925).
A basaltic lava ow immediat ely benea th th e sedi-
ments to the north of the river Kaldakvísl was ra-
diometrically dated as 4.3±0.17 Ma old (Albertsson,
1976). Substantial decomposition of the uppermost
Kaldakvísl lava, probably due to severe weathering,
and th e signicant age difference of the Kalda kvísl
lava s on each side of the river suggest a consid erable
time gap between the formation of the two units.
According to Hospers (1953), the lowermost 160
m of the Tjörnes beds are reversely magnetized,
whereas the uppe r part is normally magnetized. How-
ever, Gladenkov and Gurari (1976) re ported two re-
versed polarity events within the Serripes Zone, sep-
arated by a normal polarity episode. Pillow lavas of
reverse remanent magnetism occur at the river Skeifá
just above the Mactra/Serripes Zone boundary (Th.
Einarsson et al., 1967). Kristjánsson (2004) found the
sediments of the Serripes Zone to be normally magne-
tized up to the ter restrial horizon I, where it changes
to reverse up to the top of th e zone. A lower reversed
polarity interval at the pillow lava level was also con-
rmed.
Th. Einarsson et al. (1967) attempted a correla-
tion with the geomagnetic polarity scale and accor d-
ing to their p referred alternative the pillow lavas were
erupted close to the Gilbert/Gauss reversal. Alberts-
son (197 6) suggested that the pillow lavas should most
probably be correlated with either the Kaena or Mam-
moth event of the Gauss epoch.
Figure 4. Marine fossilife rous sedime nts between kame conglomerates in t he Hörgi Formation of the Bre iðavík
Group at Tjörnes. The marine sediments were deposited at 2.10 Ma when the area was inundated by a marine
transgression durin g deglaciation. The marine sediments contain the oldest occurrence of the arctic bivalve
Portlandia arctica in Iceland known so far. Setlög í Hörgamyndun í austurhluta Breiðuvíkur á Tjörnesi.
Lagskipt sjávarset með skeldýraleifum hefur sest í dældirnar milli malarkeila, sem mynduðust þegar ísa leysti í
lok jökulskeiðs fyrir um það bil 2,10 milljónum ára. Í sjávarsetinu eru elstu menjar um kultoddu í íslenskum
setlögum.
336 JÖKULL No. 58, 2008
Tjörnes Pliocene and Pleistocene sediments and faunas
The normally magnetized Höskuldsvík lava ow
directly capping the Tjörnes beds yielded a radiomet-
ric age of 2.55±0 .27 Ma, while an overlying, reversed
polarity lava bed separ ated from the rst by a cross-
bedded sandstone was dated to 2.36±0.16 Ma (Al-
bertsson, 1976, 1978; Eiríksson et al., 1990).
Eiríksson et al. (1990) reviewed the age esti-
mates for the Upper Tjörnes sequence and supported
the suggestion of Th. Einarsson et al. (1967) that
the reversely magnetiz ed lower part of the overlying
Breiðavík Group accum ulated during the Mat uyama
epoch (Figure 5). According to the preferred age
model of Eiríksson et al. (1990), the polarity re-
versal from no rmal to reverse at the ba se of the
Breiðavík Group in Furuvík is correlated with the
Gauss/Matuyama bo undary (2.6 Ma, cf. Ogg and
Smith, 2004). Therefore the r everse-magnetized
Höskuldsvík lava in Hvalvík should most probably
be correlated with the Mammoth event. If that is
the case, then the reversed interval immedi ately be-
low the Höskuldsvík Group would correspond to the
Kaena sub chron, and the r eversed interval at the level
of the Skeifá pil low lavas would corre spond to the up-
permost Gilbert chron. This would place the Mac-
tra/Serripes bou ndary within the uppermo st Gilbert
chron, the Gilbert/Gauss boundary being at 3.6 Ma
(cf. Ogg a nd Smith, 2004).
Only normal polarity rocks have been found in
the Húsavík and Grasafjöll Formations, which post-
date the Brunhes/Matuyama chron boundary, identi-
ed within the Breiðavík Group both on the east coast
of Tjör nes and in central Tjörnes ( Th. Einarsson et
al., 1967; Albertsson, 1976, 1978; Kristjánsson et al.,
1988; Eir íksson et al., 1990).
Both radi ometric dates and palaeomagnetic mea-
surements indicate a late Early Pliocene to early Late
Pliocene age for the Tjörnes beds. This time interval
is chara cterized by several second order eustatic sea
level ch anges (Haq et al., 19 87), probably reecting
changes in global ice volumes (Ch en et al., 1995).
Figure 5. Ages and correlation between the Tjörnes se-
quence and the palaeomagnetic polarity time scale. Aldur
jarðlaga á Tjörnesi og tenging við segultímakvarða.
JÖKULL No. 58, 2008 337
L. A. Símonarson and J. Eiríksson
beds, which include abou t 500 m of fossiliferous sed-
iments on the west side of Tjörnes (Bárðarson, 1 925).
A basaltic lava ow immediat ely benea th th e sedi-
ments to the north of the river Kaldakvísl was ra-
diometrically dated as 4.3±0.17 Ma old (Albertsson,
1976). Substantial decomposition of the uppermost
Kaldakvísl lava, probably due to severe weathering,
and th e signicant age difference of the Kalda kvísl
lava s on each side of the river suggest a consid erable
time gap between the formation of the two units.
According to Hospers (1953), the lowermost 160
m of the Tjörnes beds are reversely magnetized,
whereas the uppe r part is normally magnetized. How-
ever, Gladenkov and Gurari (1976) re ported two re-
versed polarity events within the Serripes Zone, sep-
arated by a normal polarity episode. Pillow lavas of
reverse remanent magnetism occur at the river Skeifá
just above the Mactra/Serripes Zone boundary (Th.
Einarsson et al., 1967). Kristjánsson (2004) found the
sediments of the Serripes Zone to be normally magne-
tized up to the ter restrial horizon I, where it changes
to reverse up to the top of th e zone. A lower reversed
polarity interval at the pillow lava level was also con-
rmed.
Th. Einarsson et al. (1967) attempted a correla-
tion with the geomagnetic polarity scale and accor d-
ing to their p referred alternative the pillow lavas were
erupted close to the Gilbert/Gauss reversal. Alberts-
son (197 6) suggested that the pillow lavas should most
probably be correlated with either the Kaena or Mam-
moth event of the Gauss epoch.
Figure 4. Marine fossilife rous sedime nts between kame conglomerates in t he Hörgi Formation of the Bre iðavík
Group at Tjörnes. The marine sediments were deposited at 2.10 Ma when the area was inundated by a marine
transgression durin g deglaciation. The marine sediments contain the oldest occurrence of the arctic bivalve
Portlandia arctica in Iceland known so far. Setlög í Hörgamyndun í austurhluta Breiðuvíkur á Tjörnesi.
Lagskipt sjávarset með skeldýraleifum hefur sest í dældirnar milli malarkeila, sem mynduðust þegar ísa leysti í
lok jökulskeiðs fyrir um það bil 2,10 milljónum ára. Í sjávarsetinu eru elstu menjar um kultoddu í íslenskum
setlögum.
336 JÖKULL No. 58, 2008
Tjörnes Pliocene and Pleistocene sediments and faunas
The normally magnetized Höskuldsvík lava ow
directly capping the Tjörnes beds yielded a radiomet-
ric age of 2.55±0 .27 Ma, while an overlying, reversed
polarity lava bed separ ated from the rst by a cross-
bedded sandstone was dated to 2.36±0.16 Ma (Al-
bertsson, 1976, 1978; Eiríksson et al., 1990).
Eiríksson et al. (1990) reviewed the age esti-
mates for the Upper Tjörnes sequence and supported
the suggestion of Th. Einarsson et al. (1967) that
the reversely magnetiz ed lower part of the overlying
Breiðavík Group accum ulated during the Mat uyama
epoch (Figure 5). According to the preferred age
model of Eiríksson et al. (1990), the polarity re-
versal from no rmal to reverse at the ba se of the
Breiðavík Group in Furuvík is correlated with the
Gauss/Matuyama bo undary (2.6 Ma, cf. Ogg and
Smith, 2004). Therefore the r everse-magnetized
Höskuldsvík lava in Hvalvík should most probably
be correlated with the Mammoth event. If that is
the case, then the reversed interval immedi ately be-
low the Höskuldsvík Group would correspond to the
Kaena sub chron, and the r eversed interval at the level
of the Skeifá pil low lavas would corre spond to the up-
permost Gilbert chron. This would place the Mac-
tra/Serripes bou ndary within the uppermo st Gilbert
chron, the Gilbert/Gauss boundary being at 3.6 Ma
(cf. Ogg a nd Smith, 2004).
Only normal polarity rocks have been found in
the Húsavík and Grasafjöll Formations, which post-
date the Brunhes/Matuyama chron boundary, identi-
ed within the Breiðavík Group both on the east coast
of Tjör nes and in central Tjörnes ( Th. Einarsson et
al., 1967; Albertsson, 1976, 1978; Kristjánsson et al.,
1988; Eir íksson et al., 1990).
Both radi ometric dates and palaeomagnetic mea-
surements indicate a late Early Pliocene to early Late
Pliocene age for the Tjörnes beds. This time interval
is chara cterized by several second order eustatic sea
level ch anges (Haq et al., 19 87), probably reecting
changes in global ice volumes (Ch en et al., 1995).
Figure 5. Ages and correlation between the Tjörnes se-
quence and the palaeomagnetic polarity time scale. Aldur
jarðlaga á Tjörnesi og tenging við segultímakvarða.
JÖKULL No. 58, 2008 337
L. A. Símonarson and J. Eiríksson
THE FAUNAL ASSEMBLAGES AND
THEIR ENVIRONMENTAL INDICATION
The Tjörnes beds
In the lowermost part of the Tapes Zone the intertidal
sediments contains littoral epifaunal molluscs belong-
ing to the Mytilus edulis assemblage. The overlying
marine sediments of the Tapes Zon e contain infaunal
molluscan assemblages that preferably lived in tidal
at ar e a s with Venerupis rhomboide s and Cerasto-
derma decorticata as the most characteristic species
(Bárðarso n, 1925; Norto n, 1975; Gladenkov et al.,
1980). The boreal-lusitanian fauna of the Tapes Zone
is Atlantic with a few species of Pacic ancestry, and
similar to that of th e Coralline Crag of East Anglia
in England (Baden-Powell, 1955; Áskelsson, 1960;
Durham and MacNeil, 1967). The fossil assemblages
are c onsidered to be death assemblages of the pa-
rautochthonous type where burrowing bivalve species
are more often found with articulated valves and in
life position. The Tapes Zone sediments contain sev-
eral thin lignite seams. The pollen ora indicates a
mixed coniferous and hardwood forest vegetation and
a temperate climate (Schwarzbach and Pu g, 1957;
Akhmetiev e t al., 1978).
The infaunal tidal at sediments in the lower part
of the The Mactra Zone contain boreal molluscan
assemblages with Spisula arcuata as a ch a r a c t e r i stic
species and some Venerupis species that strongly de-
crease in numbe r ( Bárðarson, 1925; Norton, 1975;
Gladenkov et al., 1980). The epifaunal assemblages
are of the Littorina squalida- Mytilus edulis type with
the barnacle Balanus hopkinsi as a common partic -
ipant. After the transgression upon bed E the envi-
ronment became clearly more sublittoral as indic ated
by a mollusc fauna, consistent with shallow water,
immediately outside the tidal zone. S pisula arcuata
and Arctica islandica b ecome the dominant species
together with Lentidium comp lanatum, occurring in
beds dep osited under low salinity conditions caused
by r elatively high inux of fresh water from land.
The marine mollusc fauna in the lower part o f the
Serripes Zone is greatly diversied through immigra-
tion of Pacic and suba rctic elements and more su blit-
toral assemblages are represented, especially by gas-
tropods (Durham and MacNeil, 1967; Norton, 1975;
Gladenkov et al., 1980). The most characteristic mol-
luscs are e pifaunal gastropod species belo nging to Lit-
torina squalida, Neptunea de cemcostata, Sipho olavii
and Propebela as well as
Oenopo ta species. Among
the infaunal bivalves, S erripes groenlandicus, Arctica
islandica, Macoma obliqua, Lentidium co mplanatum
and Cyrtodaria angusta are the most common. The
bival ve shells are generally not f ound in life position,
with disarticulated and broken valves, even the thick
and strong Arctica islandica. This strongly indicates
post-mortem transport and crushing an d death assem-
blages.
Biostratigraphically the Se rripes Zone has been
consider d to be of the same age as the Red Crag For-
mation in England (Baden-Powell, 1955; Áskelsson,
1960).
The ostracod fauna of the Tjörnes b eds has been
thoroughly studied b y Cronin (1991), who divided
the sediments into thre e ostracod zones. The low-
ermost Hemicythere-Leptocythere Zo ne corresponds
to the molluscan Tapes Zone and the Sarsicyheridea-
Thaerocythere Zone to the Mactra Zone and low-
ermost Serripes Zone. Finally, the uppermost
Cytheridea Zone corresponds to the mid and upper
parts of the Serripes Zone (horizon 14–19). The os-
tracod assemblages contain many thermophilic gen era
that do not inhabit Iceland today and indicate early
to middle Pliocene winter a nd summer bottom-water
temperatures that averaged 5–6
C and 14–16
C, re-
spectively (Cronin, 1991).
Immigration of Pacic species
There are two d isctinct molluscan faunal changes in
the Tjörnes beds. The older one in the middle p art
of the Mactra Zone is clearly connected to environ-
mental changes in the area reecting a change from
an intertidal or tidal at environmen t to a more sub-
littoral one. At the same time, the Venerupis species
gradually disappeared and Spisula arcuata and Arc-
tica islandica became common.
The younger faunal change is of different char-
acter and not posed by changing environments in the
Tjörnes area. T he Pliocene molluscan assemblag es in
the Tjörnes beds include up to 22% extinct species
(Norton, 1975). Th e boreal-lusitanian assemblages
338 JÖKULL No. 58, 2008
Tjörnes Pliocene and Pleistocene sediments and faunas
in the Tapes Zone and the borea l assemb la ges in the
Mactra Zone have a distinct Atlantic character with a
few spe cies of Pacic ance stry. At the boundary be-
tween the Mactra and Serripes Zones, about 3.6 Ma
ago, there is an abrupt appearance of several boreal-
subarctic molluscs in the sequen ce (Durham and Mac-
Neil, 1967; Th. Einarsson et al., 1967; Eiríksson et al.,
1990). Neptunea despecta, Buccinum undatum, Ser-
ripes groenlandicus, Clinocardium ciliatum, Macoma
calcarea, Hiatella arctica, and several other species
of North Pacic origin obviously migrated into the
Arctic Ocean and the North Atlantic during the de-
position of the lowermost part of the Serripes Zone
(Durham and MacNeil, 1967). Some of these species
have since been among the domin ants in arctic and
subarctic assemblages within marine Quaternary fau-
nas in the North Atlantic area.
The closing of the Central American seaway in-
duced a ow of surface water from the Pacic thro ugh
the Bering Strait and the Arctic Ocean and brought a
tide of Pacic molluscs to the North Atlantic and Ice-
land at 3.6 Ma (Backman, 1979; Marincovich, 2000).
Furthermore, it has been sugg ested that the closing
of the Panama isthm us considerably increased the
transport activity of the Gulf Stream System (Brun-
ner, 1978). The initiatio n of the ow might also
have triggered the onset of the Labrador Current and
very strongly affected the ocean current pattern bot h
west and east of Greenland (Backman, 1979). Ap-
parently the Bering Strait rst opened at 5.5–4.8 Ma
and the initial phases of the migration took place
when the sediments of the Tapes and Mactra Zones
of the Tjö rnes sequence were deposited, as indicated
by several Pacic molluscs in the se zones (Símonar-
son et al., 1998, Marincovich and Gladenkov, 1999;
Marincovich, 2000). They include common specie s
such as Mytilus edulis, Modiolus modiolus, and Zir-
faea crispata considered to have migrated through the
Bering Strait and the Arctic Ocean (cf. Durham and
MacNeil, 1967; Bárðarso n, 1925).
Among the rst species to migrate into the North
Atlantic fro m the Pacic were those shallow water and
littoral spe c i e s that reached Iceland during the deposi-
tion of the Tapes and Mactra Zo nes of the Tjörnes se-
quence. Then the sudden in ux came at the boundary
between the Mactra and Serripes Zones mainly con-
sisting of sublittoral spec ies. Although they display a
more boreal-subarctic distribution than the species in
the Tapes and
Mactra Zones, they are not consid ered
to indicate decreasing sea temperatures in the Tjör-
nes area a nd the North Atlantic. The migration must
have taken place when the Arctic Oce an was ice free
and warmer than at present, as some of the migrating
species no longer range that far north. However, the
sea temperatures in the Arctic Ocean dur ing the mi-
gration were obviously a few degrees lower than those
in the North Pacic and North Atlantic, although the
differences were not as striking a s today. The Arctic
Ocean was probab ly acting as a lter to migration and
the species best adapted to the conditions in the Ar ctic
Ocean came through rst. These species dominate in
the assemblage that represent a sudden inux of Pa-
cic species at the boundary between the Mactra and
Serripes Zones. Although the faunal change might
seem to indicate a change in sea temperature, this is
not supported by oth er da ta (Buchardt and Símonar-
son, 2003). The migrating fauna is, therefo re, not con-
sidered to reec t changes in sea temperatu res in the
Tjörnes area while the Serripes Zone was deposited,
but rather the temperatur e conditions farther north in
the Arctic Ocean during the migration. This would
explain why we do not nd any signicant drop in
the isotope temperature prole in the lower part of the
Serripes Zon e.
The Breiðavík Group
The lithological cycles of the Breiðavík Group from
glacial to proglacial and then to shallow marine en-
vironments coincide with faunal succession reecting
a change from arctic to boreal-arctic or even boreal
conditions. This is quite different from what we nd
in the Tjörnes beds. Only two mollusc species have
been iden tied in the Furu vík Formation sediments
just above the Matuyama/Gauss chron boundary (Mya
truncata and Hiatella cf. rugosa). T he Hörgi Forma-
tion dated to about 2.10 Ma (Figure 5) reveals changes
from sublittoral arctic Macoma-Portlandia molluscan
assemblages with Portlandia arctica in the lower part
to semilittoral boreal arctic Mytilus-Balanus assem-
blages in the upper part. The siltstone in the lower
part, with lateral contact to thick bodies of conglom-
JÖKULL No. 58, 2008
339
L. A. Símonarson and J. Eiríksson
THE FAUNAL ASSEMBLAGES AND
THEIR ENVIRONMENTAL INDICATION
The Tjörnes beds
In the lowermost part of the Tapes Zone the intertidal
sediments contains littoral epifaunal molluscs belong-
ing to the Mytilus edulis assemblage. The overlying
marine sediments of the Tapes Zon e contain infaunal
molluscan assemblages that preferably lived in tidal
at a r e a s with Venerupis rhomboides and Cerasto-
derma decorticata as the most characteristic species
(Bárðarso n, 1925; Norto n, 1975; Gladenkov et al.,
1980). The boreal-lusitanian fauna of the Tapes Zone
is Atlantic with a few species of Pacic ancestry, and
similar to that of th e Coralline Crag of East Anglia
in England (Baden-Powell, 1955; Áskelsson, 1960;
Durham and MacNeil, 1967). The fossil assemblages
are c onsidered to be death assemblages of the pa-
rautochthonous type where burrowing bivalve species
are more often found with articulated valves and in
life position. The Tapes Zone sediments contain sev-
eral thin lignite seams. The pollen ora indicates a
mixed coniferous and hardwood forest vegetation and
a temperate climate (Schwarzbach and Pu g, 1957;
Akhmetiev e t al., 1978).
The infaunal tidal at sediments in the lower part
of the The Mactra Zone contain boreal molluscan
assemblages with Spisula arcuata as a ch a r a c t e r i stic
species and some Venerupis species that strongly de-
crease in numbe r ( Bárðarson, 1925; Norton, 1975;
Gladenkov et al., 1980). The epifaunal assemblages
are of the Littorina squalida- Mytilus edulis type with
the barnacle Balanus hopkinsi as a common partic -
ipant. After the transgression upon bed E the envi-
ronment became clearly more sublittoral as indic ated
by a mollusc fauna, consistent with shallow water,
immediately outside the tidal zone. S pisula arcuata
and Arctica islandica b ecome the dominant species
together with Lentidium comp lanatum, occurring in
beds dep osited under low salinity conditions caused
by r elatively high inux of fresh water from land.
The marine mollusc fauna in the lower part o f the
Serripes Zone is greatly diversied through immigra-
tion of Pacic and suba rctic elements and more su blit-
toral assemblages are represented, especially by gas-
tropods (Durham and MacNeil, 1967; Norton, 1975;
Gladenkov et al., 1980). The most characteristic mol-
luscs are e pifaunal gastropod species belo nging to Lit-
torina squalida, Neptunea de cemcostata, Sipho olavii
and Propebela as well as
Oenopo ta species. Among
the infaunal bivalves, S erripes groenlandicus, Arctica
islandica, Macoma obliqua, Lentidium co mplanatum
and Cyrtodaria angusta are the most common. The
bival ve shells are generally not f ound in life position,
with disarticulated and broken valves, even the thick
and strong Arctica islandica. This strongly indicates
post-mortem transport and crushing an d death assem-
blages.
Biostratigraphically the Se rripes Zone has been
consider d to be of the same age as the Red Crag For-
mation in England (Baden-Powell, 1955; Áskelsson,
1960).
The ostracod fauna of the Tjörnes b eds has been
thoroughly studied b y Cronin (1991), who divided
the sediments into thre e ostracod zones. The low-
ermost Hemicythere-Leptocythere Zo ne corresponds
to the molluscan Tapes Zone and the Sarsicyheridea-
Thaerocythere Zone to the Mactra Zone and low-
ermost Serripes Zone. Finally, the uppermost
Cytheridea Zone corresponds to the mid and upper
parts of the Serripes Zone (horizon 14–19). The os-
tracod assemblages contain many thermophilic gen era
that do not inhabit Iceland today and indicate early
to middle Pliocene winter a nd summer bottom-water
temperatures that averaged 5–6
C and 14–16
C, re-
spectively (Cronin, 1991).
Immigration of Pacic species
There are two d isctinct molluscan faunal changes in
the Tjörnes beds. The older one in the middle p art
of the Mactra Zone is clearly connected to environ-
mental changes in the area reecting a change from
an intertidal or tidal at environmen t to a more sub-
littoral one. At the same time, the Venerupis species
gradually disappeared and Spisula arcuata and Arc-
tica islandica became common.
The younger faunal change is of different char-
acter and not posed by changing environments in the
Tjörnes area. T he Pliocene molluscan assemblag es in
the Tjörnes beds include up to 22% extinct species
(Norton, 1975). Th e boreal-lusitanian assemblages
338 JÖKULL No. 58, 2008
Tjörnes Pliocene and Pleistocene sediments and faunas
in the Tapes Zone and the borea l assemb la ges in the
Mactra Zone have a distinct Atlantic character with a
few spe cies of Pacic ance stry. At the boundary be-
tween the Mactra and Serripes Zones, about 3.6 Ma
ago, there is an abrupt appearance of several boreal-
subarctic molluscs in the sequen ce (Durham and Mac-
Neil, 1967; Th. Einarsson et al., 1967; Eiríksson et al.,
1990). Neptunea despecta, Buccinum undatum, Ser-
ripes groenlandicus, Clinocardium ciliatum, Macoma
calcarea, Hiatella arctica, and several other species
of North Pacic origin obviously migrated into the
Arctic Ocean and the North Atlantic during the de-
position of the lowermost part of the Serripes Zone
(Durham and MacNeil, 1967). Some of these species
have since been among the domin ants in arctic and
subarctic assemblages within marine Quaternary fau-
nas in the North Atlantic area.
The closing of the Central American seaway in-
duced a ow of surface water from the Pacic thro ugh
the Bering Strait and the Arctic Ocean and brought a
tide of Pacic molluscs to the North Atlantic and Ice-
land at 3.6 Ma (Backman, 1979; Marincovich, 2000).
Furthermore, it has been sugg ested that the closing
of the Panama isthm us considerably increased the
transport activity of the Gulf Stream System (Brun-
ner, 1978). The initiatio n of the ow might also
have triggered the onset of the Labrador Current and
very strongly affected the ocean current pattern bot h
west and east of Greenland (Backman, 1979). Ap-
parently the Bering Strait rst opened at 5.5–4.8 Ma
and the initial phases of the migration took place
when the sediments of the Tapes and Mactra Zones
of the Tjö rnes sequence were deposited, as indicated
by several Pacic molluscs in the se zones (Símonar-
son et al., 1998, Marincovich and Gladenkov, 1999;
Marincovich, 2000). They include common specie s
such as Mytilus edulis, Modiolus modiolus, and Zir-
faea crispata considered to have migrated through the
Bering Strait and the Arctic Ocean (cf. Durham and
MacNeil, 1967; Bárðarso n, 1925).
Among the rst species to migrate into the North
Atlantic fro m the Pacic were those shallow water and
littoral spe c i e s that reached Iceland during the deposi-
tion of the Tapes and Mactra Zo nes of the Tjörnes se-
quence. Then the sudden in ux came at the boundary
between the Mactra and Serripes Zones mainly con-
sisting of sublittoral spec ies. Although they display a
more boreal-subarctic distribution than the species in
the Tapes and
Mactra Zones, they are not consid ered
to indicate decreasing sea temperatures in the Tjör-
nes area a nd the North Atlantic. The migration must
have taken place when the Arctic Oce an was ice free
and warmer than at present, as some of the migrating
species no longer range that far north. However, the
sea temperatures in the Arctic Ocean dur ing the mi-
gration were obviously a few degrees lower than those
in the North Pacic and North Atlantic, although the
differences were not as striking a s today. The Arctic
Ocean was probab ly acting as a lter to migration and
the species best adapted to the conditions in the Ar ctic
Ocean came through rst. These species dominate in
the assemblage that represent a sudden inux of Pa-
cic species at the boundary between the Mactra and
Serripes Zones. Although the faunal change might
seem to indicate a change in sea temperature, this is
not supported by oth er da ta (Buchardt and Símonar-
son, 2003). The migrating fauna is, therefo re, not con-
sidered to reec t changes in sea temperatu res in the
Tjörnes area while the Serripes Zone was deposited,
but rather the temperatur e conditions farther north in
the Arctic Ocean during the migration. This would
explain why we do not nd any signicant drop in
the isotope temperature prole in the lower part of the
Serripes Zon e.
The Breiðavík Group
The lithological cycles of the Breiðavík Group from
glacial to proglacial and then to shallow marine en-
vironments coincide with faunal succession reecting
a change from arctic to boreal-arctic or even boreal
conditions. This is quite different from what we nd
in the Tjörnes beds. Only two mollusc species have
been iden tied in the Furu vík Formation sediments
just above the Matuyama/Gauss chron boundary (Mya
truncata and Hiatella cf. rugosa). T he Hörgi Forma-
tion dated to about 2.10 Ma (Figure 5) reveals changes
from sublittoral arctic Macoma-Portlandia molluscan
assemblages with Portlandia arctica in the lower part
to semilittoral boreal arctic Mytilus-Balanus assem-
blages in the upper part. The siltstone in the lower
part, with lateral contact to thick bodies of conglom-
JÖKULL No. 58, 2008
339
L. A. Símonarson and J. Eiríksson
erate of suprag lacial outwash origin, was probably de-
posited when th e sea transgressed glacial landscape in
front of the re treating glacier. Conglomerate lenses
and tongues internger with the siltstone containing
molluscs that have suffered some transport from the
lateral conglomera te.
During accumulation of the 1.5 Ma Svarthamar
Member, the foraminiferal assemblages changed from
the Islandiella helenae-Cassidulina teretis Zone in
the lower part to Cibicides lobatulus-Elp hidium hal-
landense Zone in the up per part, reecting changes
from arctic to boreal-arctic m arine environments in
the lower part to temperature conditions even warmer
than at present on the south coast of Iceland in the
upper part (Eiríksson et al., 1992). Similar changes
have been found in the distribution of marine mol-
luscs in the Svarthamar Member. While arctic m ollus-
can assemblages with Portlandia arctica dominates
the lowermost layers the Arctica islan dica-Cyrtodaria
angusta assemblages in the upper part reect b oreal
marine conditions (Vilhjálmsson, 1985 ).
These changes coincide with lithological changes
from a lateglacial to an interglaci al scenar io. When
the marine part of the 1.1 Ma M á Formation was
deposited, arctic spec ies such as Portlan dia arctica
did not reach northern Ic eland probably because of a
rapid shift of the Polar Front across the n orth coast
which minimized the inuence of the East Icelandic
Current during deglaciation (Símonarson and Leifs-
dóttir, 2007).
SUMMARY AND CONCLUSIONS
On the Tjörnes Peninsula in northern Iceland there
is a well-exposed sequence of Pliocene and Pleis-
tocene fossiliferous marine and non-marine sedimen-
tary ro cks intercalated with basaltic lava ows and,
in the higher part, with tillite layers recording a t least
fourteen glac iations.
(1) The Pliocene Tjörnes beds are divided in three
biozones; the Tapes Zone (oldest), the Mac-
tra Zone, and the Serripes Zone (youngest).
The Tjörnes b eds consist mainly of fossilifer-
ous marine silt- and sandstones, but there are
also several fossiliferous terrestrial beds in the
lower part. The marine faunas in the Tapes
and Mactra Zones are mainly boreal, but during
the deposition of the Serripes Zone the fauna
greatly diversied with an abrupt immigration
of Pacic molluscan species with more arctic
elements.
(2) An abrupt appearanc e of marine molluscs of
Pacic ancestry in the North Atlantic took place
at 3.6 Ma after migration through the Bering
Strait coeval with closing of the Centra l Amer-
ican Seaway. North Pacic molluscs in the
Tapes and Mactra Zones also post-date the rst
opening of the Bering Strait at 5.5–4.8 Ma
and reached no rthern Iceland during the initial
phases of the interchange.
(3) In the Bre iðavík Group, diamictite bed s alter-
nate with volcaniclastic mudrocks and sand-
stones, and basaltic lava ows. Fourteen litho-
logical cycles are identied in the Breiðavík
Group each one starting with a diamictite in-
terpreted as lodgemen t ti llite and ending with
terrestrial sedimen ts and lava ows. Interbed -
ded marine fossiliferous mudrocks and sand-
stones contain arctic to boreal faunal assem-
blages. The olde st cycle in the Breiðavík Group
was probably deposited about 2.5 Ma, just after
the Gauss/Matuyama polarity reversal.
Acknowledgements
We would like to thank Dr. Lovísa Ásbjörnsdóttir, Ice-
landic Institute of Natural History, Dr. Karen Luise
Knudsen, Department of Earth Sciences, University
of Aarhus, and Dr. Leó Kristjánsson, Institute of Earth
Sciences, University of Iceland, for valuable c om-
ments on the ma nuscript. The University of Iceland
Research Fund and Rannís are thanked for nancia l
suppor t.
ÁGRIP
Á vestanverðu Tjörnesi er þykk lagasyrpa úr hraun-
lögum, sjávarseti, setlögum myndu ðum á landi og
jökulbergi. Í setlö gunum hefur fundist mikið af stein-
gervingum og ha fa þeir geð þýðingarmiklar upplýs-
340 JÖKULL No. 58, 2008
Tjörnes Pliocene and Pleistocene sediments and faunas
ingar um loftslag á norðanverðu Atlantsha. Jarðlög-
um á Tjörnesi er skipt í Köldukvíslarhraunlög, Tjör-
neslög, Höskuldsvík urhóp og Breiðuvíkurhóp. Tjör-
neslögin skiptast í þrjú lífbelti. Gár uskeljalögin eru
elst, síðan tígulskeljalög og yngst eru krókskeljalög,
en öll beltin eru tal in mynd uð á plíósentíma. Lögin
eru mestu mynduð úr sjávarseti, siltsteini og sand-
steini, en í neðr i hluta þeirra eru áberandi lög sem hafa
myndast á landi, þar á meðal brúnkolalög. Þessi hluti
laganna hefur því myndast ýmist á grunnsævi eð a o f-
an sjávarmáls. Sjávardýraleifar í gáru- og gulskelja-
lögum be nda til þess að estar tegundirnar ha lifað
í mun hlýrri sjó en er hér við land. Í kr ókskelja-
lögum eru dýrasamfé lögin líkari þeim sem lifa v
Ísland, en sjávarhiti hefur þó verið lítið eitt hærri en
er. Neðst í krókskeljalögunum koma fram allmarg-
ar tegundir skeldýra sem eru ættaðar úr Kyrraha, en
þær tóku þátt í miklum sædýrautni ngum mil li Kyrra-
hafs og Atlantshafs fyrir um það bi l 3,6 milljónum ára
þegar sjávarsundið um Mið-Ameríku (Panama) lok-
aðist og straumaker hafanna br eyttist verulega. Set-
lögin í krókskeljalögum hafa nær eingöngu myndast á
grunnsævi, en efst í þeim e ru þunn brúnkolalög.
Í Breiðuvíkurhópi skiptast á hraunlög, setlög
mynduð á landi, jökulberg og sjávarset m stein-
gervingum. Fjórtán jarðlagasyrpur hafa verið greind-
ar í Breiðuvíkurhópnum, og hefst hver syrpa á jök-
ulbergslagi, en síðan koma setlög og hraun lög, sem
ýmist hafa sest til á landi a í sjó. Í Breiðuvík eru
einstæðar myndanir þa r sem rekja umhversbreyt-
ingar frá jökulskeiðum yr til hlýskeiða á ísöld, bæði í
ásýnd setlaga og steingervingum. Fyrstu merki um ís-
aldarjökla á Tjörnesi eru varðveitt í Fu ruvík, og liggja
þau rétt ofan v Gauss - Matuyama segulskiptin, sem
urðu fyrir 2,6 milljónum ára.
REFERENCES
Akhmetiev, M. A., G. M. Bratseva, R. E. Giterman, L.
V. Golubeva and A. I . Moiseyeva 1978. Late Ceno-
zoic Stratigraphy and ora of Iceland. Trudy Geologis-
cheskogo Instituta Academia Nauk SSSR 316, 1–188
(in Russian).
Albertsson, K. J. 1976. K/Ar ages of Pliocene-Pleistocene
glaciations in Iceland with special reference to the
Tjörnes sequence, northern Iceland. Ph. D. thesis.
University of Cambridge, 268 pp.
Albertsson, K. J. 1978. Um aldur jarðlaga á Tjörnesi. Nátt-
úrufræðingurinn 48, 1–8 (in Icelandic).
Aronson, J. L. and K. Saemundsson 1975. Relatively old
basalts from structurally high areas in central Iceland.
Earth Planet. Sci. Lett. 28, 83–97.
Áskelsson, J. 1960. Fossiliferous xenoliths in the Móberg
Formation of South Iceland. Acta Naturalia Islandica
2 (2), 30 pp.
Backman, J. 1979. Pliocene biostratigraphy of DSDP sites
111 and 116 from the North Atlantic Ocean and the
age of Northern Hemisphere glaciation. Stockholm
Contributions in Geology 32, 115–137.
Baden-Powell, D. F. W. 1955. The correlation of the
Pliocene and Pleistocene marine beds of Britain and
the Mediterranean (with discussion). Proceedings of
the Geologists’ Association of London 66, 271–292.
Bárðarson, G. G. 1925. A stratigraphical survey of the
Pliocene deposits at Tjörnes, in northern Iceland.
Det Kongelige Danske Videnskabernes Selskab, Biol-
ogiske Meddelelser 4 (5), 118 pp.
Brunner, C. A. 1978. Late Neogene history recorded by
sedimentation in the Straits of Florida. Geological So-
ciety of America Abstracts 1978, 373.
Buchardt, B. and L. A. Símonarson 2003. Isotope
palaeotemperatures from the Tjörnes beds in Ice-
land: evidence of Pliocene cooling. Palaeogeography,
Palaeoclimatology, Palaeoecology 189, 71–95.
Chen, J., J. W. Farrell, D. W. Murray and W. L. Prell
1995. Timescale and paleoceanographic implications
of a 3.6 m. y. oxygen isotope record from the north-
east Indian Ocean (Ocean Drilling Program site 758).
Paleoceanography 10, 21–47.
Cronin, T. 1991. Late Neogene marine ostracoda from
Tjörnes, Iceland. Journal of Paleontology 65, 767–
794.
Durham, J. W. and F. S. MacNeil 1967. Cenozoic mi-
grations of marine invertebrates through the Bering
Strait Region. In: D. M. Hopkins, ed. The Bering Land
Bridge. Stanford University Press, Stanford, 326–349.
Einarsson, T. 1958. A survey of the geology of the area
Tjörnes-Bárðardalur in northern Iceland, including pa-
leomagnetic studies. Vísindafélag Íslendinga Rit 32,
79 pp.
Einarsson, Th., D. M. Hopkins and R. R. Doell 1967. The
stratigraphy of Tjörnes, northern Iceland, and the his-
tory of the Bering Land Bridge. In: D. M. Hopkins, ed.
JÖKULL No. 58, 2008 341
L. A. Símonarson and J. Eiríksson
erate of suprag lacial outwash origin, was probably de-
posited when th e sea transgressed glacial landscape in
front of the re treating glacier. Conglomera te lenses
and tongues internger with the siltstone containing
molluscs that have suffered some transport from the
lateral conglomera te.
During accumulation of the 1.5 Ma Svarthamar
Member, the foraminiferal assemblages changed from
the Islandiella helenae-Cassidulina teretis Zone in
the lower part to Cibicides lobatulus-Elp hidium hal-
landense Zone in the up per part, reecting changes
from arctic to boreal-arctic m arine environments in
the lower part to temperature conditions even warmer
than at present on the south coast of Iceland in the
upper part (Eiríksson et al., 1992). Similar changes
have been found in the distribution of marine mol-
luscs in the Svarthamar Member. While arctic m ollus-
can assemblages with Portlandia arctica dominates
the lowermost layers the Arctica islan dica-Cyrtodaria
angusta assemblages in the upper part reect b oreal
marine conditions (Vilhjálmsson, 1985 ).
These changes coincide with lithological changes
from a lateglacial to an interglaci al scenar io. When
the marine part of the 1.1 Ma M á Formation was
deposited, arctic spec ies such as Portlan dia arctica
did not reach northern Ic eland probably because of a
rapid shift of the Polar Front across the n orth coast
which minimized the inuence of the East Icelandic
Current during deglaciation (Símonarson and Leifs-
dóttir, 2007).
SUMMARY AND CONCLUSIONS
On the Tjörnes Peninsula in northern Iceland there
is a well-exposed sequence of Pliocene and Pleis-
tocene fossiliferous marine and non-marine sedimen-
tary ro cks intercalated with basaltic lava ows and,
in the higher part, with tillite layers recording a t least
fourteen glac iations.
(1) The Pliocene Tjörnes beds are divided in three
biozones; the Tapes Zone (oldest), the Mac-
tra Zone, and the Serripes Zone (youngest).
The Tjörnes b eds consist mainly of fossilifer-
ous marine silt- and sandstones, but there are
also several fossiliferous terrestrial beds in the
lower part. The marine faunas in the Tapes
and Mactra Zones are mainly boreal, but during
the deposition of the Serripes Zone the fauna
greatly diversied with an abrupt immigration
of Pacic molluscan species with more arctic
elements.
(2) An abrupt appearanc e of marine molluscs of
Pacic ancestry in the North Atlantic took place
at 3.6 Ma after migration through the Bering
Strait coeval with closing of the Centra l Amer-
ican Seaway. North Pacic molluscs in the
Tapes and Mactra Zones also post-date the rst
opening of the Bering Strait at 5.5–4.8 Ma
and reached no rthern Iceland during the initial
phases of the interchange.
(3) In the Bre iðavík Group, diamictite bed s alter-
nate with volcaniclastic mudrocks and sand-
stones, and basaltic lava ows. Fourteen litho-
logical cycles are identied in the Breiðavík
Group each one starting with a diamictite in-
terpreted as lodgemen t ti llite and ending with
terrestrial sedimen ts and lava ows. Interbed -
ded marine fossiliferous mudrocks and sand-
stones contain arctic to boreal faunal assem-
blages. The olde st cycle in the Breiðavík Group
was probably deposited about 2.5 Ma, just after
the Gauss/Matuyama polarity reversal.
Acknowledgements
We would like to thank Dr. Lovísa Ásbjörnsdóttir, Ice-
landic Institute of Natural History, Dr. Karen Luise
Knudsen, Department of Earth Sciences, University
of Aarhus, and Dr. Leó Kristjánsson, Institute of Earth
Sciences, University of Iceland, for valuable c om-
ments on the ma nuscript. The University of Iceland
Research Fund and Rannís are thanked for nancia l
suppor t.
ÁGRIP
Á vestanverðu Tjörnesi er þykk lagasyrpa úr hraun-
lögum, sjávarseti, setlögum myndu ðum á landi og
jökulbergi. Í setlö gunum hefur fundist mikið af stein-
gervingum og ha fa þeir geð þýðingarmiklar upplýs-
340 JÖKULL No. 58, 2008
Tjörnes Pliocene and Pleistocene sediments and faunas
ingar um loftslag á norðanverðu Atlantsha. Jarðlög-
um á Tjörnesi er skipt í Köldukvíslarhraunlög, Tjör-
neslög, Höskuldsvík urhóp og Breiðuvíkurhóp. Tjör-
neslögin skiptast í þrjú lífbelti. Gár uskeljalögin eru
elst, síðan tígulskeljalög og yngst eru krókskeljalög,
en öll beltin eru tal in mynd uð á plíósentíma. Lögin
eru mestu mynduð úr sjávarseti, siltsteini og sand-
steini, en í neðr i hluta þeirra eru áberandi lög sem hafa
myndast á landi, þar á meðal brúnkolalög. Þessi hluti
laganna hefur því myndast ýmist á grunnsævi eð a o f-
an sjávarmáls. Sjávardýraleifar í gáru- og gulskelja-
lögum be nda til þess að estar tegundirnar ha lifað
í mun hlýrri sjó en er hér við land. Í kr ókskelja-
lögum eru dýrasamfé lögin líkari þeim sem lifa v
Ísland, en sjávarhiti hefur þó verið lítið eitt hærri en
er. Neðst í krókskeljalögunum koma fram allmarg-
ar tegundir skeldýra sem eru ættaðar úr Kyrraha, en
þær tóku þátt í miklum sædýrautni ngum mil li Kyrra-
hafs og Atlantshafs fyrir um það bi l 3,6 milljónum ára
þegar sjávarsundið um Mið-Ameríku (Panama) lok-
aðist og straumaker hafanna br eyttist verulega. Set-
lögin í krókskeljalögum hafa nær eingöngu myndast á
grunnsævi, en efst í þeim e ru þunn brúnkolalög.
Í Breiðuvíkurhópi skiptast á hraunlög, setlög
mynduð á landi, jökulberg og sjávarset m stein-
gervingum. Fjórtán jarðlagasyrpur hafa verið greind-
ar í Breiðuvíkurhópnum, og hefst hver syrpa á jök-
ulbergslagi, en síðan koma setlög og hraun lög, sem
ýmist hafa sest til á landi a í sjó. Í Breiðuvík eru
einstæðar myndanir þa r sem rekja umhversbreyt-
ingar frá jökulskeiðum yr til hlýskeiða á ísöld, bæði í
ásýnd setlaga og steingervingum. Fyrstu merki um ís-
aldarjökla á Tjörnesi eru varðveitt í Fu ruvík, og liggja
þau rétt ofan v Gauss - Matuyama segulskiptin, sem
urðu fyrir 2,6 milljónum ára.
REFERENCES
Akhmetiev, M. A., G. M. Bratseva, R. E. Giterman, L.
V. Golubeva and A. I . Moiseyeva 1978. Late Ceno-
zoic Stratigraphy and ora of Iceland. Trudy Geologis-
cheskogo Instituta Academia Nauk SSSR 316, 1–188
(in Russian).
Albertsson, K. J. 1976. K/Ar ages of Pliocene-Pleistocene
glaciations in Iceland with special reference to the
Tjörnes sequence, northern Iceland. Ph. D. thesis.
University of Cambridge, 268 pp.
Albertsson, K. J. 1978. Um aldur jarðlaga á Tjörnesi. Nátt-
úrufræðingurinn 48, 1–8 (in Icelandic).
Aronson, J. L. and K. Saemundsson 1975. Relatively old
basalts from structurally high areas in central Iceland.
Earth Planet. Sci. Lett. 28, 83–97.
Áskelsson, J. 1960. Fossiliferous xenoliths in the Móberg
Formation of South Iceland. Acta Naturalia Islandica
2 (2), 30 pp.
Backman, J. 1979. Pliocene biostratigraphy of DSDP sites
111 and 116 from the North Atlantic Ocean and the
age of Northern Hemisphere glaciation. Stockholm
Contributions in Geology 32, 115–137.
Baden-Powell, D. F. W. 1955. The correlation of the
Pliocene and Pleistocene marine beds of Britain and
the Mediterranean (with discussion). Proceedings of
the Geologists’ Association of London 66, 271–292.
Bárðarson, G. G. 1925. A stratigraphical survey of the
Pliocene deposits at Tjörnes, in northern Iceland.
Det Kongelige Danske Videnskabernes Selskab, Biol-
ogiske Meddelelser 4 (5), 118 pp.
Brunner, C. A. 1978. Late Neogene history recorded by
sedimentation in the Straits of Florida. Geological So-
ciety of America Abstracts 1978, 373.
Buchardt, B. and L. A. Símonarson 2003. Isotope
palaeotemperatures from the Tjörnes beds in Ice-
land: evidence of Pliocene cooling. Palaeogeography,
Palaeoclimatology, Palaeoecology 189, 71–95.
Chen, J., J. W. Farrell, D. W. Murray and W. L. Prell
1995. Timescale and paleoceanographic implications
of a 3.6 m. y. oxygen isotope record from the north-
east Indian Ocean (Ocean Drilling Program site 758).
Paleoceanography 10, 21–47.
Cronin, T. 1991. Late Neogene marine ostracoda from
Tjörnes, Iceland. Journal of Paleontology 65, 767–
794.
Durham, J. W. and F. S. MacNeil 1967. Cenozoic mi-
grations of marine invertebrates through the Bering
Strait Region. In: D. M. Hopkins, ed. The Bering Land
Bridge. Stanford University Press, Stanford, 326–349.
Einarsson, T. 1958. A survey of the geology of the area
Tjörnes-Bárðardalur in northern Iceland, including pa-
leomagnetic studies. Vísindafélag Íslendinga Rit 32,
79 pp.
Einarsson, Th., D. M. Hopkins and R. R. Doell 1967. The
stratigraphy of Tjörnes, northern Iceland, and the his-
tory of the Bering Land Bridge. In: D. M. Hopkins, ed.
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342 JÖKULL No. 58, 2008
Reviewed research article
Late Weichselian and Holocene envir onmental history of Ice land
Hreggviður Norðdahl
1
, Ólafur Ingólfsson
1
, Halldór G. Péturss on
2
and M argrét Hallsdóttir
3
1) Institute of Earth Sciences, University of Iceland, Askja, Sturlugata 7, 101 Reykjavík.
2) Icelandic Institute of Natural History, Borgum Norðurslóð, IS-600 Akureyri, Iceland
3) Icelandic Institute of Natural History, Hlemmi 3, IS-105 Reykjavík, Iceland
hreggi@hi.is, oi@hi.is,
Abstract At the Last Glac ial Maximum (LGM) glaciers extended out towards the shelf break around Iceland
and ice thickness over most of Iceland was 2000 m. Rapidly rising global sea level at 13.0–12.5 ka BP caused
extremely fast deglaciation and collapse of the marine based part of the Icelandic ice sheet. Raised shorelines
at very high altitudes, dated to about 12.6 ka BP, signify the rapid glacial retreat and transgression of relative
sea level in coastal areas. A readvance of the Icelandic ice sheet culminated in Younger Dryas times, about
10.3 ka BP, when the ice margin was close to the present coastline. Following ice retreat and regression of
relative sea level at the end of the Younger Dryas, the ice sheet advanced again and reached a spatial extent
close to its Younger Dryas maximum at about 9.8 ka BP. Subsequently, the ice sheet retreated rapidly and
at about 9.4 ka BP relative sea level fell towards and eventually below present sea level. Relative sea level
had regressed to about 40 m below present sea level at about 8.9 ka BP, when the rate of isostatic rebound
was slowing down and eustatic sea level rise caused the onset of a transgression. At about 8.0 ka BP Icelandic
glaciers were of a similar o r a little lesser extent than at present, and during the mid-Holocene thermal optimum
some of the presen t-day ice caps may have been signic antly reduced or absent. Th e onset of Neoglaciation
occurred after 6–5 ka BP and most Icelandic glaciers reached their Holocene maximum during the Little Ice
Age. The vegetation history of Iceland reects the glacial history in that the earliest evidence of plant succession
is from mid Allerød times as grass and dwarf shrub tundra d eveloped in the wake of the initial ice retreat. The
younger Dryas cooling is evident in the biostratigraphical re cord as discontinuous vegetation cover and tund ra
environments developed. At the beginning of the Holocene, dwarf shrub, and later on, shrub heath with Salix,
Dwarf birch, and Juniperus became established. Birch woodland, the climax vegetation during the Holocene,
most probably had its greatest extent during the Atlantic Chronozone, at about 7–6 ka BP. The woodland showed
a retrogressive succession towards more open landscape during the second half of the Holocene as widespread
mires, dwarf shrub heaths an d grassland took over in the landscape evolutio n. The Norse settlement of Ic eland
in the 9th Century AD caused rapid vegetation changes in the wake of the settlement, where primarily intensive
grazing caused woodland destruction and expansion of grass heath, dwarf shrub heath and mires.
INTRODUCTION
Our und erstanding of the Late Weichselian and
Holocene environmental history of Iceland has de-
veloped rapidly d uring the past two decades, as
new litho- and biostratigraphical data together with
chronological data from terrestrial a nd shelf environ-
ments have acc umulated. One outstanding aspect of
the new idea on the pattern and succession of envi-
ronmental changes is a new insight into the dynamics
of large-scale changes: where we previously thought
development was slow, we now un derstand it to have
JÖKULL No. 58, 2008
343
... For the purposes of this paper, we define the pre-invasion section as the Tapes and Mactra zones, which contain fewer species of Pacific origin (four and five mollusc species with Pacific affinities, respectively; Durham & MacNeil 1967), and the invasion section as the Serripes zone, marked by the arrival of many more Pacific species. Durham & MacNeil (1967) reported 22 mollusc species with Pacific affinities in the Serripes zone; see also Símonarson & Eiríksson (2008). We hypothesize that naticid predation on bivalves will (1) be low in the Tapes and Mactra zones; ...
... beds 14, 15, 16, 18). Environmental interpretations from Símonarson & Eiríksson (2008). Thickness of lithostratigraphical sections from Gladenkov et al. (1980). ...
... Because the Tjörnes beds, with their record of the TAI, also represent a changing depositional environment, care must be taken to disentangle effects of the invasion from those of environmental change. Símonarson & Eiríksson (2008) recognized two intervals of molluscan faunal change in the Tjörnes beds: one in the middle Mactra zone and another at the Mactra-Serripes boundary. The faunal change within the Mactra zone is accompanied by a shift from intertidal environments to sublittoral shallow water environments (Norton 1975;Símonarson & Eiríksson 2008; Fig. 2), with the loss or decreased abundance of dominant genera such as Venerupis and Spisula and changes in dominance and diversity. ...
Article
The fossil record provides a long‐term perspective to better understand the impacts of species invasions in their environmental contexts. Temporal analyses of predator–prey interactions from the Tjörnes deposits, Iceland, track naticid gastropod drilling predation across the trans‐Arctic invasion (TAI: ~3.5 Ma). These deposits represent three zones subdivided into 25 marine fossil‐bearing beds that correspond with the stages of invasion: Tapes (1–5) and Mactra (6–12) zones are pre‐invasion, whereas the Serripes zone (13–25) represents the invasion. Bivalve and naticid gastropod specimens were analysed from the Bárðarson (1925) samples at the Icelandic Institute of Natural History, which consisted of pre‐invasion and invasion samples; we also bulk‐sampled the Serripes zone. Height and length of specimens were measured to assess size changes, and the occurrence of complete and incomplete drill holes and drill hole diameter were recorded for whole bivalves and naticids. Drilling frequency (DF) and prey effectiveness (PE, the incidence of failed drilling) were calculated to track predator–prey interactions. Genus‐level diversity increased through the Tjörnes deposits, in part related to a shift from intertidal to sublittoral environments. DF increased and PE decreased significantly between the pre‐invasion and Serripes zones. However, DF decreased from the early to the late Serripes zone, which could signify stabilization of the Tjörnes community. An increase in competition among predators through the invasion is supported by an increase in abundance of naticids relative to bivalves, especially invasive species, a switch to smaller‐sized bivalve prey, a decrease in naticid mean size and an increase in confamilial predation.
... Two distinct faunal changes have been observed in the molluscan assemblages of the Tjörnes sequence (Símonarson & Eiríksson, 2008). The rst one occurs in the middle part of the Mactra Zone and is apparently connected to environmental changes re ecting a change from a relatively sheltered infralittoral environment to a more open sublittoral one . ...
... The second faunal change is of quite different character and does not appear to be related only to environmental changes in the Tjörnes area. This change involves the migration of a number of species of North Paci c origin into the North Atlantic, which have since been among dominants in arctic, subarctic, and even boreal assemblages of marine faunas in the northern part of the Atlantic Ocean (Durham & MacNeil, 1967;Símonarson & Eiríksson, 2008). Some of these species had already reached the Tjörnes area when the oldest part of the beds were deposited, being of terminal Miocene age (Eiríksson et al., 2020c;Verhoeven et al., 2011). ...
... Some of these species had already reached the Tjörnes area when the oldest part of the beds were deposited, being of terminal Miocene age (Eiríksson et al., 2020c;Verhoeven et al., 2011). However, the tide of the migration seems to have taken place during the deposition of the lowermost part of the Serripes Zone at about 3.8 Ma (Durham & MacNeil, 1967;Símonarson & Eiríksson, 2008). The Paci c species include Neptunea species, Buccinum undatum Linné, Mytilus edulis Linné, Serripes groenlandicus (Mohr), Ciliatocardium ciliatum (Fabricius), Macoma species, and species belonging to the genus Mya. ...
Book
This volume sheds new light on the marine fauna and geological setting of the Tjörnes Sequence, North Iceland, which is a classic site for the Pliocene and Pleistocene stratigraphy of the North Atlantic region. Readers will discover descriptions of new data collected by the editors over a period of over three decades on marine faunal assemblages and sedimentology available for palaeoenvironmental reconstructions, as well as the tectonic and stratigraphical relationships on Tjörnes Peninsula. The book includes a comprehensive account of all the collections of marine fossil invertebrate macrofossils and foraminifera known to the editors from the Tjörnes Sequence. It is expected to elucidate sedimentological and faunal changes from relatively stable Pliocene conditions to highly variable and periodically harsh climatic conditions of recurring Quaternary glaciations. The distribution, recent or fossil, of various species is recorded and pertinent ecological and biological features are also discussed. The Tjörnes Sequence records the Neogene migration of Pacific species into the North Atlantic. Researchers in geology, climate science, environmental science and earth science will find this book particularly valuable.
... At that time, at about 3.8-3.9 Ma, several mollusc species of North Paci c origin migrated into the North Atlantic and have since been among dominants in arctic, subarctic, and even boreal assemblages within marine faunas in the northern part of the Atlantic (Durham & MacNeil, 1967;Símonarson & Eiríksson, 2008). They include Neptunea species, Buccinum undatum, Serripes groenlandicus, Ciliatocardium ciliatum, Macoma species, and species belonging to the genus Mya. ...
... In the Skeifá Formation, both in Bárðarson's units 10 and 11 (Brunngil and Ytri Svarthamar Members, which are separated by a lignite seam), there are distinct shell beds mainly consisting of fragmented shells of Arctica islandica ( Fig. 9.8). These beds are interpreted as having formed by currents that removed the ner sediment grains but left the heavier shells, shell fragments, and pebbles that accumulated on (Símonarson & Eiríksson, 2008). The beds of shell fragments of strong and heavy Arctica islandica indicate high-energy environments when the crushing took place. ...
... 15%) dominate the signal and indicate a recovery of the marshes on the coastal plain. The sediments towards the top of the Serripes Zone are interpreted by Símonarson and Eiríksson (2008) as deposited in a uvial in uenced coastal environment, a rather unfavourable habitat for the preservation of microfossils. ...
Chapter
The Barmur Group of the Tjörnes sequence in North Iceland contains data which are interpreted as up to 20 transgression-regression cycles between 5.3 and 3.2 Ma. The Barmur Group fauna is unique among Cainozoic marine faunas in the North Atlantic region north of the British Isles containing a Pliocene mollusc fauna. The Tjörnes Basin was presumably an early stepping-stone for the invertebrate species migrating from the Pacific into the North Atlantic since the Miocene-Pliocene boundary. A marked faunal change, mainly visible in the lowermost part of the Serripes Zone at about 3.8 Ma, was not only connected to environmental changes in the Tjörnes region but rather due to migration of at least 34 mollusc species of North Pacific origin into the North Atlantic. These species have since been among dominants in marine arctic, subarctic, and even boreal assemblages in the northern part of the Atlantic. We interpret our data as reflecting an overall change in sea-water temperature from summer temperatures between 10 and 15 °C during the deposition of the Tapes Zone and the lower part of the Mactra Zone to colder water conditions with summer temperatures between 5 and 10 °C at the top of the Serripes Zone.
... Two distinct faunal changes have been observed in the molluscan assemblages of the Tjörnes sequence (Símonarson & Eiríksson, 2008). The rst one occurs in the middle part of the Mactra Zone and is apparently connected to environmental changes re ecting a change from a relatively sheltered infralittoral environment to a more open sublittoral one . ...
... The second faunal change is of quite different character and does not appear to be related only to environmental changes in the Tjörnes area. This change involves the migration of a number of species of North Paci c origin into the North Atlantic, which have since been among dominants in arctic, subarctic, and even boreal assemblages of marine faunas in the northern part of the Atlantic Ocean (Durham & MacNeil, 1967;Símonarson & Eiríksson, 2008). Some of these species had already reached the Tjörnes area when the oldest part of the beds were deposited, being of terminal Miocene age (Eiríksson et al., 2020c;Verhoeven et al., 2011). ...
... Some of these species had already reached the Tjörnes area when the oldest part of the beds were deposited, being of terminal Miocene age (Eiríksson et al., 2020c;Verhoeven et al., 2011). However, the tide of the migration seems to have taken place during the deposition of the lowermost part of the Serripes Zone at about 3.8 Ma (Durham & MacNeil, 1967;Símonarson & Eiríksson, 2008). The Paci c species include Neptunea species, Buccinum undatum Linné, Mytilus edulis Linné, Serripes groenlandicus (Mohr), Ciliatocardium ciliatum (Fabricius), Macoma species, and species belonging to the genus Mya. ...
Chapter
The Tjörnes sequence documents an exchange of molluscs between the North Pacific and the Arctic and North Atlantic Oceans. Before the Pliocene, the main open passage between the Atlantic and the Pacific was through the Central American Seaway. A Beringian land bridge prevented exchange of marine biota in the northern region. The Bering Strait was probably opened towards the termination of the Miocene at 5.3 Ma, and the final closure of the Central American Seaway took place at about 2.7 Ma. At least 34 molluscan species of Pacific ancestry reached Iceland during the Pliocene and three species reached Iceland, while the Lower Pleistocene part of the Breiðavík Group on Tjörnes was deposited. The major migration of Pacific species into the Tjörnes area is manifested at the base of the Serripes biozone of the Barmur Group of the Tjörnes sequence at about 3.8 Ma. This was preceded by earlier appearances of mollusc species close to the Miocene-Pliocene boundary. Mollusc migration into the Tjörnes area was not a single, abrupt event, but occurred at various times during the Pliocene and Lower Pleistocene. Some of these species of Pacific origin have subsequently dominated boreal, subarctic, and arctic molluscan assemblages in the North Atlantic.
... Photographs of steep coastal cliffs were used to analyse inaccessible and indistinct coarse-grained bedding structures. The interpretation of sedimentary facies presented in this chapter is to a great extent digested and compiled from work published previously by the present authors (Eiríksson, 1981a(Eiríksson, , 1985Eiríksson et al., 1992;Símonarson & Eiríksson, 2008). ...
... Nevertheless, some sedimentary or other facies may be missing due to these faults. Casts of three bivalve species (Table 10.3) have been found in the massive siltstone facies (Eiríksson, 1985;Símonarson & Eiríksson, 2008), suggesting a marine environment during the siltstone deposition. Deposition in quiet water is indicated by ne grain and massive structure. ...
Chapter
Thick Quaternary sediments of the Breiðavík Group are intercalated between lava flows and tuffs on Tjörnes, North Iceland. The present chapter is a compilation of lithostratigraphic and fossil data, combined with an interpretation of the sedimentary and volcanic facies. The vertical column of strata displays a rhythmic character, and a model for sedimentation during glacial-interglacial cycles in a volcanic environment is presented. The Quaternary part of the Tjörnes sequence contains evidence of 14 separate glaciations with tillite horizons. The tillites are typically associated with kame and outwash conglomerates, followed at first by glacio-lacustrine and glaciomarine mudrocks and diamictites, and then by deltaic and/or bar-lagoon assemblages, fluvial sediments, and subaerial lava flows. Depositional environments are reconstructed for several steps in the geological history of the Breiðavík Group. The oldest tillite has been dated to c. 2.6 Ma. Several of the tillite beds are succeeded by marine sediments with fossils, especially in the lower part of the sequence. During the first million years of the accumulation, the deposition took place in a transitional environment while the area was subsiding, but the latter part of the geological record reflects terrestrial deposition and increased volcanism in the region.
... The over-arching project targeted the Tjörnes Beds of northeastern Iceland, a predominantly coastal exposure of Late Neogene sedimentary rocks. The Tjörnes Beds have historical significance, are the thickest sedimentary section in Iceland, contain a useful palaeoclimate archive, and expose a variety of sedimentological and structural features suitable for teaching (Símonarson and Eiríksson, 2008;Eiríksson et al., 2021). We aimed to capture the entire coastal exposure of c. 7 km length within a single virtual outcrop model for the V3Geo platform with uniform cmscale visible resolution. ...
Preprint
In 2021, a photogrammetric model of a 7 km coastal cliff section of the Tjörnes Peninsula, NE Iceland was constructed to provide a “virtual geological outcrop” at a spatial resolution of approximately 5 cm or less. Field expedition planning and post-expedition processing were somewhat hampered by a lack of freely accessible guiding resources. This document presents a first-hand case study of the complete project workflow, including expedition planning, field data acquisition, and data processing within Agisoft Metashape software to create an interactive “Structure from Motion” photogrammetric model for uploading to the V3Geo cloud-based platform. Drone-based photogrammetry is a burgeoning tool, and this account is intended to allow safer, more effective and more efficient use of the technology in creating virtual outcrops for use in Earth science teaching and research.
... This species, however, is abundantly present in late Pliocene and Pleistocene faunas in western and northern Alaska (Hopkins and MacNeil, 1960;Mac-Neil, 1957). The oldest specimens of H. arctica in lower Pliocene beds on Iceland occur along with the oldest undisputed Pacific mollusks there (Gladenkov and Pokrovskiy, 1980;Símonarson and Eiríksson, 2008;. There is no record of H. arctica in the well-studied Eocene to Miocene molluscan faunas of the Panamanian Isthmus (Woodring, 1982), and although some molecular studies have suggested there must have been interchange before the opening of the Bering Strait, they considered the Bering Strait to have first opened well within the Pliocene (Laakkonen et al., 2015). ...
Chapter
The Beaufort Formation records extraordinary details of Arctic environments and amplified temperatures at approximately modern levels of atmospheric CO2. It was deposited during the Neogene on the western side of what is now the Canadian Arctic Archipelago. Meighen Island is a key locality for studying this formation because marine sediments there are interbedded with terrestrial fossiliferous sands. The biostratigraphic succession, fossils from the marine beds, and paleomagnetic data from the Bjaere Bay region of the island suggest two potential ages for the studied exposures: either continuous deposition at ca. 3.0 Ma, or a sequence of deposits at ca. 4.5 Ma and 3.4 Ma. The sediments appear to encompass at least two eustatic highstands of sea level and a particularly warm climate interval of the Pliocene Arctic.
Chapter
Glacial sediments and landforms older than the Late Weichselian are rare in Iceland but alternating sedimentary and volcanic formations in the bedrock stratigraphy indicate earlier glacials and interglacials, even as far back as 5–7 million years. This is particularly prominent in the stratigraphy of the Tjörnes Peninsula, N-Iceland, from around 2.6 million years ago, with five extensive glaciations until the Eemian Interglacial. Within the northern and western volcanic zones, similar climatological cyclicity is revealed by the stratigraphy of different volcanic systems where alternating hyaloclastites and lava flows are formed during glacial and ice-free conditions. Likewise, hyaloclastite ridges and tuyas (table mountains) are indicative of volcanic eruptions under ice as opposed to shield volcanoes and extensive lava flows formed during ice-free conditions. Based on relative ages, it has been speculated that some of these formations represent a cyclicity from Marine Isotope Stage (MIS) 12 (the Elsterian glacial), through the Holsteinian Interglacial (MIS 11) and the Saalian glacial (MIS 6), to the Eemian Interglacial (MIS 5e), but the lack of absolute ages makes this inference only hypothetical. Indications of glacial oscillations during the Early Weichselian occur in the bedrock stratigraphy but are poorly constrained in age. Formations from the Middle Weichselian (MIS 3) are best preserved in Southwest Iceland where they have been studied and dated, suggesting that the ice did not extend onto the southwestern shelf during the latter part of MIS 3.
Chapter
Iceland is situated in the North Atlantic Ocean covering an area of 102,775 km². It is located at a rifting plate boundary of the Mid-Atlantic Ridge that separates the North American and Eurasian tectonic plates and controls the geological structure of Iceland in conjunction with the Iceland mantle plume. As a result, the youngest rocks occur within the neovolcanic zones and the oldest rocks in the western and eastern extremities. Due to high volcanic activity, interglacial lavas typically protect glacial sediments and hyaloclastites from erosion, resulting in excellent preservation of the Neogene and Quaternary stratigraphic record, which reveals over 20 glaciations during the past 4–5 million years. Iceland is mountainous with 47% of the land area situated about 500 m above sea level and the highest peaks reaching 1500–2100 m. The climate of Iceland is characterised by relatively mild winters and cool summers. It classifies as arctic tundra or subpolar oceanic and is largely affected by the surrounding ocean currents and Iceland’s position near the polar front. Any shift in these currents significantly influences the climate of Iceland. Warm and moist southerly winds prevail causing mean annual precipitation to vary from 5000–7000 mm on ice caps in the south to 400–600 north of the main ice caps. About 10% of Iceland is currently covered by glaciers with the Vatnajökull ice cap as the largest one.
Chapter
Foraminiferal assemblages have been studied in sediments from the Barmur Group (Tjörnes beds), which is part of the most complete geological Plio-Pleistocene section in Iceland, situated on the west side of the Tjörnes Peninsula in northern Iceland. The Barmur Group comprises the three biozones, the Tapes Zone, the Mactra Zone, and the Serripes Zone, which represent a major part of the Pliocene. The sediments in the Tjörnes section are lithified, and the registered foraminiferal assemblages represent a paleocoenosis, consisting of a residual of the most chemically and mechanically resistant tests. The foraminiferal taxa found in the three biozones of the Barmur Group are presented with their systematic affiliations, as well as their ecological and stratigraphic indication, when possible. Despite the relatively bad preservation state of the foraminiferal tests, it has been possible to attain a rough estimate of the stratigraphy and the paleoenvironmental development through each of the biozones.
Article
Four marine molluscan species that migrated to Iceland during the deposition of sedimentary sequences on the north side of Snaefellsnes, western Iceland, at about 1.1 Ma, are not living in Iceland today. Three of these species are arctic and reached the area during an Early Pleistocene deglaciation. The fourth is thermophilic and arrived during the following interglacial, together with several littoral species now living in Iceland. The arctic species probably migrated to Iceland from the west or northwest due to a southward shift of the cold and euhaline East Greenland Current to the Icelandic west coast. At that time the Polar Front was lying considerably south of Iceland, but then followed a periodic northward shift of the front. The fact that the arctic species did not reach northern Iceland at this time may indicate a rapid shift of the Polar Front across the north coast of Iceland that minimized the influence of the East Icelandic Current during the deglaciation. Several thermophilic littoral species that migrated to western Iceland during the following interglacial did not reach northern Iceland. They came from the south or southeast during strengthening of the warm Irminger Current. However, the current's influence on the Icelandic north and northeast coasts was probably limited because of mixing with colder water masses with reduced salinity from the East Icelandic Current.