(Sangamon?) in South America. First record of upraisedPleistocene marine deposits in Isla Navarino (Beagle Channel, Southern Chile)Marine beach shell deposits recording a pre-Holocene marine transgression have been found at the southern shore of the Beagle Channel, Isla Navarino, Chile. These shelly deposits were dated by AMS at 41,700 14C years B.P., which clearly indicates a Pleistocene age. A sample of wood underlying the marine deposits yielded an infinite age (>46.1 14C ka B.P.). If the date on the shells is considered as a minimum, infinite age, together with the elevation of these marine units above present mean tide sea level (at least 10 m a.s.l.) they may be considered as deposited during the Last Interglacial, of Sangamon age (Marine Isotope Stage -MIS- 5e) or during a younger phase of MIS 5. The fossil content of this unit is similar to the fauna living in this region today, supporting also an Interglacial palaeoenvironment interpretation. If this interpretation and the dating proposal are correct, this is the first reported record of Sangamon deposits in the Beagle Channel and the southernmost Last Interglacial site (MIS 5) in South America.
Geologica Acta, Vol.6, Nº 3, September 2008, 251-258
Available online at www.geologica-acta.com
© UB-ICTJA 251
ABSTRACT
(1)J. RABASSA S.(2) GORDILLO,(3) C. OCAMPO and(3) P. RIVAS HURTADO
(1)Centro Austral de Investigaciones Científicas, CADIC, CONICET and
Universidad Nacional de la Patagonia San Juan Bosco
C.C. 92, 9410 Ushuaia, Tierra del Fuego, Argentina. E-mail: jrabassa@infovia.com.ar
(2)Centro de Investigaciones Paleobiológicas (CIPAL),
Universidad Nacional de Córdoba and CONICET
Av. Vélez Sársfield 299, 5000 Córdoba, Argentina. E-mail: sgordillo@efn.uncor.edu
(3)Ciprés Consultores Ltda. and Fundación Wulaia
Sioux 2075, Vitacura, Santiago, Chile. C. Ocampo E-mail: cocampo@cipresconsultores.cl
P. Rivas E-mail: privas@cipresconsultores.cl
KEYWORDS Interglacial. Marine beach shell deposits. Mollusks. Tierra del Fuego. Southernmost South America.
INTRODUCTION
The Beagle Channel (Tierra del Fuego, Argentina and Chile; lat. 55º S, long. 67º-70º W; Fig. 1) is a sea flooded glacial trough, which was occupied by marine waters
after deglaciation in Late Glacial or earliest Holocene times, that is, sometime in between 15,000 and 9,000 14C years ago (Porter et al., 1984; Rabassa et al., 2000; J. RABASSA et al. Pleistocene interglacial marine deposits in Isla Navarino, Chile
Geologica Acta, Vol.6, Nº 3, September 2008, 251-258 252
Location map of the Corrales Viejos Site. Note the position of the site close to the town of Puerto Williams. The arrows indicate the ice flow direction of the ancient Beagle Glacier
Geologica Acta, Vol.6, Nº 3, September 2008, 251-258
Available online at www.geologica-acta.com
Location map of the Corrales Viejos Site. Note the position of the site close to the town of Puerto Williams. The arrows indicate the ice
flow direction of the ancient Beagle Glacier.
FIGURE 1
Bujalesky, 2007; Bartole et al., 2008). This glacial valley was formed by a discharge outlet glacier, the “Beagle Glacier”, descending from the Darwin Cordillera mountain ice cap (lat. 54º30’ S, long. 69º-71º W; Chile). This
still surviving ice body was the southernmost portion of the Patagonian Ice Sheet during the Pleistocene (Rabassa et al., 1992, 2000). The “Beagle Glacier” occupied this trough during at least the last two major glaciations. These glacial episodes were originally identified by Caldenius (1932) and later named as Lennox Glaciation(Middle Pleistocene, Marine Isotope Stage -MIS- 6 or older) and Moat Glaciation (Late Pleistocene, MIS 4-2) by Rabassa et al. (1992, 2000).
Both the northern (Argentina) and southern (Chile) shores of the Beagle Channel have extensive outcrops of Holocene marine terraces at various altitudes (Rabassa
et al., 2000; Bujalesky, 2007) but no Pleistocene marine deposits had yet been discovered. In previous papers, Rabassa et al. (1990, 1992, 2000) reported very scarce, fragmentary marine shells in the lower till unit at Isla Gable (lat. 55º S, long. 67º30’ W; Argentina). These authors interpreted them as coming from Late Pleistocene marine deposits that had been overriden by the “Beagle Glacier” during the Last Glaciation advance (MIS 4-2), which incorporated them as part of its sedimentary
load, but the original marine deposits were never
found. The Last Glaciation Maximum (LGM) in the
region would have peaked around 25 ka B.P., based
upon a correlation with the Magellan Straits sequence
(McCulloch et al., 2005), and not later than 15 ka 14C
B.P., based on the radiocarbon age of the basal peat at
the Harberton Bog (Argentina, lat. 54º52’ S, long.
67º53’W; 14,640 14C years B.P.; Heusser and Rabassa,
1987; Heusser, 1989). Thus, the existence of a Pleistocene
marine environment along the Beagle Channel
depression had been already suggested based on reasonable
evidence (Rabassa et al., 2000).
During recent archaeological studies at Isla Navarino
(October-November 2005), two of us (C. Ocampo and P.
Rivas) found a new locality of marine upraised beaches at
the northern shore of Isla Navarino (lat. 55º S, long. 67º15’
W; Chile), surveyed the section and sampled the identified
units. The marine deposits were exposed by the construction
of a new road along the coast, east of the town of Puerto
Williams (lat. 55º S, long. 67º30’W; Chile; Fig. 1).
This contribution deals with the above-mentioned
findings and the investigations that confirmed the existence
of a Pleistocene marine environment record in the
Beagle Channel (Fig. 1). Although a full systematic
account of the whole Pleistocene fauna of this site will
require additional studies, the available data justify their
publication together with our interpretations. The primary
goal of this paper is to provide an overview of this interesting
fossiliferous site, which constitutes a new record for the
marine Pleistocene of southernmost South America.
GEOLOGICAL SETTING. THE CORRALES VIEJOS
SECTION
The geology of Tierra del Fuego, where Isla Navarino
is located, has been the subject of research from long time
ago (see Menichetti and Tassone, 2007, 2008 and cites
therein). The characterization of the late Paleozoic-Mesozoic
metamorphic complexes, the study of the Mesozoic-
Cenozoic stratigraphy and of the ancient to recent tectonic
processes in the region (Hervé et al. 2008, Olivero and
Malumián, 2008, Menichetti et al., 2008 and other papers
therein) have resulted in a noticeable increase of the geological
knowledge on this region. The Pleistocene to
Holocene record has also been the subject of many studies
that have focused on the recent quaternary evolution
of this remote southernmost South America area (Rabassa
et al., 1992, 2000).
Holocene successions and their related faunal
assemblages occur in many places along the northern
and southern coasts of the Beagle Channel (e.g., Porter
et al., 1984, Rabassa et al., 1986, 2000; Gordillo, 1992;
Gordillo et al., 1992, 2005). However, deposits corresponding to the Pleistocene marine transgressions seemed to have not been preserved in the Beagle Channel
region due to the intense erosive effect of the Last Glaciation (MIS 4 to 2; Rabassa et al., 2000). Though the exact age of the Pleistocene Beagle glacial valley
formation is still unknown, it is herein assumed that during glacial periods the ice excluded much, if not all, of the benthic marine fauna inhabiting the marine environment in the present Beagle Channel valley or its original depression. During such glacial events, sea shore was located at least several tens of km eastwards
due to glacioeustatic sea level lowering.
Only two previous poorly preserved fossil records recovered from till deposits in the vicinity of the city of Ushuaia (lat. 54º50’ S, long. 68º W; Argentina;
Rabassa et al., 1986) and in Isla Gable (Rabassa et al., 1990; Gordillo, 1990), indicated that the Beagle Channel had been occupied by seawater at least once before
the Last Glaciation. A different situation occurs along the northeastern Atlantic coast of the Isla Grande of Tierra del Fuego, where several lithostratigraphic units
represent different Pleistocene interglacial episodes (Bujalesky et al., 2001; Bujalesky, 2007).
Among them, La Sara Formation (at 14 m a.s.l.), located near
the city of Río Grande (lat. 53º45’ S; long. 67º 30’W; Argentina), is attributed to the Late Pleistocene (Codignotto and Malumián, 1981), and it has been correlated
with the Last Interglacial period, Sangamon Stage, MIS 5e (Bujalesky et al., 2001; Bujalesky, 2007).
Stratigraphy
The Corrales Viejos Site is located at approximately lat. 55º S, long. 67º15’ W (Fig. 1). Mean tide amplitude in the area is 2-3 m. The base of the section is at an elevation of 7.3 m above high tide level (Fig. 2). Mean tide amplitude in the area is 2-3 m.
There is no field evidence of post-depositional glaciotectonic deformation or lateral displacement which could have been forced as the ice overrun this site
after the deposition of the marine layers.
Likewise, there is no evidence that landsliding or slumping would have affected this locality. Nevertheless, even if any of these latter processes would have affected the area, the original topographical position of the marine sedimentswould have been even higher in the landscape than today.
Depositional environment
The visible base of the section is composed of continental sediments, probably of fluvial, lacustrine and marshy origin (Units 1 to 4), nearby a fully developed
Nothofagus forest. These layers are covered by Unit 5, which represents an upraised marine beach, corresponding to a marine transgression. When sea level receded
from this site, a terrestrial environment was established J. RABASSA et al. Pleistocene interglacial marine deposits in Isla Navarino, Chile
Geologica Acta, Vol.6, Nº 3, September 2008, 251-258 253
Stratigraphic section of the raised Pleistocene beach deposits at Corrales Viejos.1: Base of the section. Visible base is below 6.30 m from the top. Dark greenish, greyish sandy beds. 2: 0.20 m. Greyish clayey silt which breaks in small blocks. 3: 0.25 m.
A silty-sandy layer including tree trunks and Nothofagus spp. wood
fragments.
(Sample 4). 4: 0.15 m. Greyish silty gravels.
(Sample 3). 5: 0.9 m. Marine beach deposits, a layer composed entirely of broken
and rounded marine shell fragments, reduced to fine gravel size
by wave action. (Sample 2). 6: 0.30 m. Greyish clayey-silty beds
which separates in small blocks.
(Sample 1). 7: 0.5 m. Greyish,laminated, fine grained beds, containing decomposed wood fragments.
8: 4.0 m. Till, composed of a medium sized gravel, with a sandy-clayey matrix, showing no internal stratification.
The cobbles and pebbles are irregularly distributed in the unit, showing a distinctive yellowish orange color, due to weathering.
FIGURE 2
again, with soil development and forest recovery (Units 6
and 7). Finally, an advancing glacier covered the section,
partially eroding the top of it and burying the marine
beach units (Unit 8). Most likely, the ice thickness was
smaller at the margins of the ancient glacial trough, which
reduced its erosive force, thus allowing preservation of
the marine beds.
Radiocarbon dating and age discussion
A radiocarbon date on selected fragments of marine shells obtained from the sample of Unit 5 (Fig. 2, Sample 2) was measured by AMS 14C technique at the NSF-Arizona
AMS Laboratory (University of Arizona). It yielded an age of 41,700 ± 1,500 years BP (AA 69648), with a ∂13C value of +0.6. Likewise, a sample of Nothofagus sp. wood coming from Unit 3 was also dated at the same laboratory and using the same technique (AA 75295), obtaining and age of >46,100 years B.P., with a ∂13C value
of -28.5. Considering that the dated materials in the first sample are old marine shell fragments and the obtained age is close to the accepted reliability boundary of the AMS dating method, the given age could be interpreted as (a) a correct absolute age or (b) if contaminated with a very small proportion of modern C, as an infinite age, beyond the lowest limit of the radiocarbon dating technique. In
any case, the dated shells are of undoubtedly of pre-Holocene age, thus corresponding to the Late Pleistocene or even an older age. The second date on a wood sample clearly goes beyond the radiocarbon method dating boundary, and it is considered as infinite.
PALEOBIOLOGICAL CONTENT
Taxonomy
The fossil fauna identified in the shelly bed sample is quite diverse and comprises at least 25 different mollusk species (13 bivalves and 12 gastropods) and other invertebrate groups as bryozoans, echinoids and cirripeds. Many taxa are represented by fragments of macrofossils or small tiny shells sometimes difficult to identify. A preliminary list of this fauna is reported in Table 1 and part of the material collected is illustrated in Fig. 3.
The paleontological material mentioned here is housed in the Centro de Investigaciones Paleobiológicas (CIPAL), Universidad Nacional de Córdoba, Argentina, under the prefix CEGHUNC.
J. RABASSA et al. Pleistocene interglacial marine deposits in Isla Navarino, Chile
Geologica Acta, Vol.6, Nº 3, September 2008, 251-258 254
A) Pectinidae (?Zygochlamys patagonica), fragment (CEGH-UNC 22786). B) Mytilidae ( ?Mytilus edulis chilensis), fragment (CEGH-UNC
22796). C) Veneridae ( ?Venus antiqua), fragment (CEGH-UNC 22768). D) Neolepton sp. (CEGH-UNC 22820). E) Hiatella sp. (CEGH-UNC 22826). F)
Aulacomya atra, juvenile specimen (CEGH-UNC 22821). G) Rissoiform gastropod, sp1 (CEGH-UNC 22777). H) Rissoiform gastropod, sp2 (CEGH-UNC
22776) I) Pareuthria ?plumbea (CEGH-UNC 22775). J) Rissoiform gastropod, sp3 (CEGH-UNC 22783). K) ?Margarella violacea (CEGH-UNC 22782).
L) Cerithiella sp. (CEGH-UNC 22828). M) Glypteuthria sp (CEGH-UNC 22723). N) Xymenopsis muriciformis (CEGH-UNC 22823). O) ?X. muriciformis
(CEGH-UNC 22774). P) Trophon geversianus (CEGH-UNC 22785). Q) Crepidula cf. dilatata (CEGH-UNC 22780). R) Turbonilla cf. smithi (CEGH-UNC
22784). S-T) Echinoid fragments, test elements (CEGH-UNC 22791). U-V) Echinoid fragments, isolated spines (CEGH-UNC 22790). W-X) Bryozoans
(CEGH-UNC 22789). Y-AB) Cirripeds Y. (CEGH-UNC 22794). Z) (CEGH-UNC 22795). AA) (CEGH-UNC 22793). AB) (CEGH-UNC 22792). Scale: 1 mm
(except A, B, C and Z). Scale: 5 mm (A, B, C and Z).
FIGURE 3
MOLLUSCA
Bivalvia
Nucula sp.
Aulacomya atra (Molina, 1782)
Mytilidae (?Mytilus edulis chilensis Hupé in Gay,
1854)
Pectinidae (?Zygochlamys patagonica (King and
Broderip, 1832))
Rochefortia rochebrunei Dall, 1908
Neolepton concentricum (Preston, 1912)
Neolepton spp. (2)
Hiatella sp.
Veneridae sp 1, fragments, (?Venus antiqua (King
and Broderip, 1832))
Veneridae sp2, fragments
Indeterminable bivalves
Gastropoda
Trochidae (?Margarella violacea (King and Broderip,
1832))
Rissoiform gastropods (3)
Crepidula cf. dilatata Lamarck, 1822
Cerithiella sp.
Trophon geversianus (Pallas, 1769)
Xymenopsis muriciformis (King and Broderip,
1832)
Pareuthria ?plumbea (Philippi, 1844)
Glypteuthria sp.
Turbonilla cf. smithi Strebel (Pfeffer, MS), 1905
Indeterminable gastropods
ECHINODERMATA
Echinoidea
Isolated spines and test elements (?Loxechinus
albus)
ARTHROPODA (CRUSTACEA)
Cirripedia
BRYOZOA (undetermined bryozoans)
Preliminary list of taxa identified from the Corrales Viejos
site, Isla Navarino, Chile.
TABLE 1
J. RABASSA et al. Pleistocene interglacial marine deposits in Isla Navarino, Chile
Geologica Acta, Vol.6, Nº 3, September 2008, 251-258 255
Mytilids dominate over other mollusks, and together
with the cirripeds represent more that the 75% of the fossil
materials. Many other taxa are represented by low
number of specimens, sometimes broken, that makes their
identification difficult. They belong to different families,
including pectinids, venerids, muricids and rissoiform
gastropods, among others. The rissoiform gastropods
-very difficult to classify on shell characters alone (see
Ponder and Worsfold, 1994) - include at least 3 different
species. The specific assignment of Neolepton specimens
will require a description using scanning electronic microscope
(MEB) to be performed in the future. The echinoids
are represented by isolated spines and test elements. They
do not show differences when comparing with those
belonging to one of the living species in the region and
probably represent the same taxa (i.e., Loxechinus albus).
Taphonomy
The marine shelly beds (Unit 5) yielded an abundant
fossil fauna dominated by calcareous macro- and microfossils.
The macrofossils show frequent signals of fragmentation
but low levels of abrasion (Figs. 3A, B and C).
These characteristics suggest that the fossils that compose
this assemblage have moved only a short distance away
from their original life habitats. Cirripeds (Figs. 3Y-AB)
dominate over other macroinvertebrates, followed by
macromollusks (especially mytilids). The microfossils
recovered from the marine shelly beds (Unit 4) also show
signs of fragmentation (e.g., Figs. 3L, O and P-V) and
may represent reworked shallow-marine faunas. A
detailed study of the microfossil content will be performed
in the future, based on more extensive sampling.
Palaeoenvironmental remarks
All identified species still live today in the Beagle
Channel. The macrofauna represented in the fossil assemblage
is strongly dominated by sessile suspension feeder
epifauna (i.e., cirripeds, mytilids), intermixed with some
infaunal elements (i.e., fragments of venerids). This situation
suggests the availability of hard substrate which permitted
the development of the epifauna, and soft subenvironmental
conditions, which allowed the existence of
burrowing clams. This biota is typical of modern environments
in this region.
We have compared this site, dominated by epifaunal
elements, with the La Sara Formation (Fm), a marine unit
of Last Interglacial age (MIS 5e) which is a likely time
equivalent to the section studied herein. The La Sara Fm.
is quite homogeneous with a low number of species and
dominated by infaunal bivalves (i.e., venerids; see Gordillo,
2006). It may be interpreted that these differences can
be related to the prevalence of different regional conditions
(e.g., bottom geomorphology, rock substratum, current
velocity) in both regions which allow for the development
of different local communities under similar climatic
conditions.
The presence of barnacles also suggests the existence
of strong bottom currents and shallow waters. However,
most of the mollusk species recovered are able to distribute
over a wide depth range from few to several meters.
At the time of deposition of the marine shell unit of
Corrales Viejos, the Beagle Channel was occupied by the
sea at least in its easternmost portion. It is still impossible
to estimate the extent of westward penetration of the sea,
and even more difficult to conclude if it was a fjord or
channels open to both austral seas. The coeval deeper
water marine deposits in the channel, if they ever existed,
were mostly likely wiped away by the advancing Last
Glaciation ice. Additional work is needed to understand
these paleogeomorphological circumstances.
DISCUSSION AND CONCLUDING REMARKS
From a paleontological viewpoint, the Beagle Channel
is of great interest for biogeographic and paleobiogeographic
studies because this region represents a transitional
area between the Atlantic and Pacific oceans, and also
because of its proximity to the Drake Passage and the Circumpolar
Antarctic Current. The study of the fossil Quaternary
biota in the region can be a clue to understand the
origin and migration routes of the fauna living today in
the area and how it was affected by past positional
changes of the Circumpolar Current.
The Corrales Viejos Section is the first reported record
of in-situ Pleistocene marine sediments in the Beagle
Channel region. All pre-existing geological information
about marine beds in the area is strictly related to
Holocene raised beaches and other coastal deposits.
A Pleistocene age for these sediments is inferred from
the following evidence:1. An AMS 14C date of 41.7 ± 1.5 ka B.P. on marine
shells, which may be correct or contaminated by younger
carbon, in the latter condition suggesting an infinite age,
but in any case of undoubtedly pre-Holocene age.
2. An AMS 14C infinite date of >46.1 ka B.P. on fossil
wood underlying the marine beds but clearly forming part
of the same transgressive sedimentary sequence.
3. The elevation of the shelly layers at >10.0 m a.s.l. is
too high to be assigned to the Holocene, as shown by pre-
J. RABASSA et al. Pleistocene interglacial marine deposits in Isla Navarino, Chile
Geologica Acta, Vol.6, Nº 3, September 2008, 251-258 256
vious studies in the eastern portion of the Beagle Channel
(Gordillo et al., 1992; Rabassa et al., 2004). Therefore,
these marine units are undoubtedly of Pleistocene age.
4. The studied section is covered by till, which could
have been deposited only by a Pleistocene glacier
(Holocene glaciation was restricted in this region only to
the mountain summits; Rabassa et al., 2000), most likely
during the Moat Glaciation (MIS 2 or even MIS 4). Glaciers
had already vanished from this area during Late
Glacial times (Rabassa et al., 2000).
5. Considering the available radiocarbon ages, these
marine deposits could be assigned to an interstadial event
of the Late Pleistocene, either to the beginning of the
Mid-Wisconsin Interstadial (MIS 3) or most likely, to the
Last Interglacial (Sangamon, ca. 125 ka B.P., MIS 5e) or
to other warmer events during MIS 5. However, sea level
was during MIS 3 clearly below present sea level, perhaps
at around the -40/-50 m isobath. If this should be the case,
it would have required a very strong, fast and steady tectonic
or glacioisostatic uplift of Navarino Island since
MIS 3, for which there is no evidence within the entire
region.
6. Alternatively, assuming contamination of the marine
shells with younger radiocarbon, an infinite absolute
14C age of > 41 ka B.P. may be interpreted for these units
and, most likely, a Last Interglacial age corresponding to
the MIS 5e (Sangamon Interglacial) or other later times
during MIS 5. This is fully supported by the infinite age
of the dated Nothofagus wood fragment. During the Sangamon
Interglacial epoch sea level was basically at the
same elevation as today, and the present elevation of these
deposits, mostly due to seismotectonic uplifting, is coherent
with what we know about the La Sara Fm. (of
undoubtedly Sangamon age) along the Atlantic Ocean
coast of Isla Grande de Tierra del Fuego (Rabassa et al.,
2000; Bujalesky et al., 2001; Bujalesky, 2007, and other
papers therein).
7. There is not any kind of available evidence in this
region to suggest a pre-Sangamon age for these units at
the present state of our knowledge.
For all these reasons, a Last Interglacial age (Sangamon
Stage; MIS 5e, or any of the later events during MIS
5) is favoured for the sediments found in this section.
At the moment these shelly marine deposits represent
the richest and most diverse fossil marine Pleistocene
record of Southernmost South America and the closest
locality to the Drake Passage, the Antarctic Peninsula and
the Circumpolar Antarctic Current. This new finding
opens important windows on the paleoclimatic and the
faunal history of the Beagle Channel during the Pleistocene.
Further integrated studies, with additional surveying
and sampling and including other proxy elements
(diatoms, pollen and phytoplancton analysis, micropaleontology,
dendrochronology, etc.), will give a more complete
and precise information over these high-stand sea
level deposits and the knowledge of the biota that inhabited
this region during Pleistocene times.
ACKNOWLEDGEMENTS
Field work at Isla Navarino by C.O. and P.R. was supported
by funding provided by several Chilean academic organizations.
Radiocarbon dates were funded by the project PICT 00067/2002
(ANPCYT-FONCYT, Argentina) to J.R. The field information
and sedimentary samples were sent to CADIC, Ushuaia,
Argentina, thanks to the worthy collaboration of Ernesto Piana
(CADIC) who kindly put both research groups in contact. The
authors are greatly indebted to Professor Katrin Linse, Professor
David B. Scott and other anonymous reviewers for very valuable
suggestions on earlier versions of this manuscript.
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Manuscript received September 2007;
revision accepted February 2008;
published Online May 2008.
