The stratigraphy of cretaceous mudstones in the eastern Fuegian Andes: new data from body and trace fossils
Eduardo B. Olivero1, Francisco A. Medina2, and María I. López C.1
1 Centro Austral de Investigaciones Científicas (CADIC-CONICET), B.A. Houssay 200, 9410 Ushuaia, Tierra del Fuego.
Email: emolivero@gmail.com
2 Departamento de Ciencias Geológicas, Facultad de Ciencias Exactas y Naturales, UBA, Intendente Güiraldes 2160, Ciudad Universitaria,
C1428EGA Buenos Aires.
ABSTRACT: The stratigraphy of Cretaceous marine mudstones in the Fuegian Andes, roughly equivalent to Charles Darwin's clay-slate formation, remains a still unsolved problem. Previous records of Albian, Turonian-Coniacian, and Santonian-Campanian bivalves are combined with new findings of the Late Albian inoceramid Inoceramus anglicus Woods, and the Maastrichtian ammonites Diplomoceras sp., Anagaudryceras sp., Maorites densicostatus (Kilian and Reboul), Maorites sp., and Pachydiscus (Neodesmoceras) sp. to further constrain the Cretaceous stratigraphy of the eastern Fuegian Andes. In addition, new records of distinctive trace fossils and ichnofabric are meaningful for stratigraphic division and delineation of paleoenvironmental settings in these Cretaceous mudstones. The Lower Cretaceous ichnoassemblage of Chondrites targioni (Brongniart) and Zoophycos isp. is consistent with the inferred slope-volcaniclastic apron settings of the Yahgan Formation; Nereites missouriensis (Weller) reflects distal basin plain depositional settings for the Beauvoir Formation. In the Upper Cretaceous, the "Estratos de Buen Suceso" record the earliest extensively bioturbated horizons, reflecting prolonged well-oxygenated bottom conditions. In the Bahía Thetis Formation, organic-rich, channel margin or distal basin slaty mudstones record the last occurrence of inoceramid bivalves in the Austral Basin; the generalized absence of trace fossils is consistent with dysoxic bottom conditions. The thoroughly bioturbated Policarpo Formation, records a marked change in paleoceanographic conditions. The strong contrast in the intensity of bioturbation between the Upper Campanian-Maastrichtian Bahía Thetis Formation, almost devoid of trace fossils, and the highly bioturbated Maastrichtian-Danian Policarpo Formation reflects a change from dysoxic-anoxic to well ventilated conditions, probably associated with a cooling trend of bottom waters in the austral deep oceans.
Keywords: Clay-slate formation; Inoceramid; Ammonites; Cretaceous; Trace fossils; Fuegian Andes.
RESUMEN: Estratigrafía de las fangolitas del Cretácico en los Andes Fueguinos orientales: Nuevos datos de cuerpos y trazas fósiles. En los Andes Fueguinos la estratigrafía de las fangolitas del Cretácico marino (clay-slate formation de Charles Darwin), constituye un problema no enteramente resuelto. Datos previos de bivalvos del Albiano, Turoniano-Coniaciano, y Santoniano-Campaniano, junto con nuevos hallazgos de inoceramidos del Albiano tardío, Inoceramus anglicus Woods, y amonites maastrichianos, Diplomoceras sp., Anagaudryceras sp., Maorites densicostatus (Kilian y Reboul), Maorites sp., y Pachydiscus (Neodesmoceras) sp. se utilizan para obtener una mejor resolución estratigráfica. Además, nuevas asociaciones distintivas de trazas fósiles permiten ajustar la interpretación paleoambiental. La asociación de Chondrites targioni (Brongniart) y Zoophycos isp. del Cretácico inferior es consistente con el modelo inferido de rampa volcaniclástica para la Formación Yahgan. En la Formación Beauvoir la presencia de Nereites missouriensis (Weller) refleja condiciones de depositación en planicies distales de la cuenca. En el Cretácico superior, los "Estratos de Buen Suceso" registran los primeros horizontes bioturbados asociados a fondos con prolongados periodos de condiciones de buena oxigenación. En la Formación Bahía Thetis, fangolitas pizarreñas ricas en materia orgánica depositadas en márgenes de canales o en planicies distales, registran la última aparición de bivalvos inocerámidos en la Cuenca Austral. La ausencia generalizada de trazas fósiles es consistente con condiciones de fondos anóxicos. La Formación Policarpo, densamente bioturbada, marca un profundo cambio en las condiciones paleoceanográficas. El fuerte contraste en la intensidad de la bioturbación entre las Formaciones Bahia Thetis, Campaniano-Maastrichtiano, con escasa presencia de trazas fósiles y Policarpo, Maastrichiano-Daniano, altamente bioturbada, refleja el cambio de condiciones de fondo disóxico-anóxico a óxico, probablemente asociado al comienzo del enfriamiento de las aguas profundas del océano austral.
Palabras clave: Clay-slate formation; Inoceramidos; Amonites; Trazas fósiles; Cretácico; Andes Fueguinos.
INTRODUCTION
The stratigraphic division of Cretaceous marine mudstones in the Fuegian Andes (Fig. 1) is a major and still unsolved problem. These mudstones correspond roughly to the clay-slate formation of Charles Darwin (1846); a very extensive formation of folded, low-grade metamorphic rocks with scarce marine fossils. In the course of the voyage of HMS Beagle Darwin noted that "The great clayslate formation of Tierra del Fuego being Cretaceous, is certainly a very interesting fact,--whether we consider the appearance of the country, which, without the evidence afforded by the fossils, would form the analogy of most known districts, probably have been considered as belonging to the Palaeozoic series,…"(Darwin 1846, p. 152).
Figure 1: Locality map, tectonic settings, and distribution of Lower and Upper Cretaceous rocks in the Rocas Verdes Marginal basin and Austral/Malvinas Foreland basins.
The evidence mentioned by Darwin included a few fossils from Mount Tarn, on the western side of the Strait of Magellan, Brunswick Peninsula in Chile (Fig. 2d), particularly the ammonites described by Forbes (in Darwin 1846) as "Ancyloceras simplex" d'Orbigny, from Mount Tarn (Fig. 2f), and "Hamites elatior" Sowerby, from the coastal area nearby Port Famine. We now know that these fossils are not Albian hamitids; the "A. simplex" is a Maastrichtian kossmaticeratid, probably a deformed fragment of Maorites, and "H. elatior" is a Diplomoceras, also of Maastrichtian age (Spath 1953, Lahsen and Charrier 1972). However, the correction in the age of these ammonite-bearing strata does not change the fact that in the Fuegian Andes, Albian to Maastrichtian fine-grained rocks have quite similar lithological and structural features and consequently they are very difficult to distinguish in the field (Fig. 2).
Figure 2: Structure, lithology, and paleontology of Cretaceous rocks. a) Thin-bedded turbidites, Yahgan Formation, Lower Cretaceous, Beagle Channel
close to Ushuaia. b) Black mudstones, Beauvoir Formation, Lower Cretaceous (Albian), Knokeke Hill area. c) Black mudstones and interbedded light-gray
tuffs, Estratos de Buen Suceso, Upper Cretaceous (Santonian-Lower Campanian). d) Upper Cretaceous (Maastrichtian) interbedded siltstones and finegrained
sandstones, Mount Tarn, Brunswick Peninsula, Chile. e) Highly folded, black mudstones and fine-grained sandstones, Bahía Thetis Formation,
Thetis Bay. f) Crushed ?Maorites sp. from Mount Tarn. This is the original specimen, housed at the Natural History Museum, London (B.M. No. 2612),
collected by Darwin and described by Forbes as "Ancyloceras simplex" d´Orbigny. Scale bar: 1 cm.
In this study, previous records of Albian, Turonian-Coniacian, and Santonian- Campanian bivalves, mostly inoceramids (Olivero and Martinioni 1996a, Olivero and Medina 2001) are combined with new findings of latest Albian inoceramids and Maastrichtian ammonites to further constrain the Cretaceous stratigraphy of the eastern Fuegian Andes. In addition, new records of characteristic trace fossil assemblages with distinctive ichnofabrics, restricted to particular sedimentary successions of the Yahgan Formation, Beauvoir Formation, Bahía Thetis Formation, Policarpo Formation, and "Estratos de Buen Suceso", constitute meaningful data for the stratigraphic division of the relatively homogeneous, mudstone-rich Cretaceous deposits.
STRATIGRAPHIC FRAMEWORK
The stratigraphy of southernmost South
America has been controlled since the
Mesozoic by a series of contrasting tectonic
regimes, represented by Late Jurassic-Early Cretaceous extension, Late Cretaceous-Paleogene compression, and latest
Paleogene-Neogene and present day
strike-parallel transcurrent motion (Katz
1972, Dalziel et al. 1974, Suárez et al.1985,
Kohn et al. 1995, To-rres Carbonell et al. 2008). Accordingly, Cretaceous rocks in
the Fuegian Andes are distributed in
three tectonostratigraphic domains: 1)
the magmatic arc, located along the axis
of the southern archipelago, includes
Lower Cretaceous andesitic (Hardy Formation)
and Upper Cretaceous plutonic
rocks (Beagle Channel Plutonic Group);
2) the Rocas Verdes Marginal Basin includes
Lower Cretaceous, low-grade metamorphic
rocks, mostly slates and sandstones
of the Yahgan and Beauvoir formations,
and Upper Jurassic-Lower Cretaceous
ophiolitic rocks; and 3) the
Austral and Malvinas foreland basins,
separated in the northern area by the
Dungeness or Rio Chico Arch, include
Upper Cretaceous rocks dominated by
slaty mudstones (Fig. 1).
According to a recent review (see Olivero
and Malumián 2008, and the bibliography
cited therein), the Yahgan Formation
represents a volcaniclastic apron
of mudstones and deep-marine andesiterich
turbidites and the Beauvoir Formation
records basin plain and slope mudstones.
Both formations reflect Late Jurassic-Early Cretaceous extension and
the consequent origination of the Rocas
Verdes Marginal Basin. The earliest Late
Cretaceous ductile deformation, isoclinal
folding, and low-grade regional metamorphism
of these rocks indicate a compressional
tectonic regime that resulted
in the closure of the marginal basin. The
Fuegian Andes were uplifted by the Late
Campanian; subsequent propagation of
the compressional deformation and subsidence
by tectonic loading along the
northern orogenic margin resulted in the
formation of the Austral and Malvinas
foreland basins. The Turonian-Lower
Campanian "Estratos de Buen Suceso" represent the final stages of the closure
of the marginal basin and/or the beginning
of the foreland basins. Conglomerate
beds in the Bahía Thetis Formation
bear foliated clasts of Andean-derived
rocks and thus they record uplifting
and subaerial erosion of the Fuegian
Andes. The youngest Cretaceous rocks
are included in the Policarpo Formation,
which includes the Maastrichtian/Danian
boundary at its top (Olivero et al. 2003). Figure 3 summarizes the paleontological
information, including partly new body
and trace fossil data, used in this study to
differentiate particular horizons within
these lithologically homogeneous stratigraphic
units. The stratigraphic implications
of these data are described for each
formation in the next section.
Figure 3: Stratigraphic
distribution of characteristic
body and trace fossils
in the Cretaceous of
the Fuegian Andes
(explanation in the text).
LOWER CRETACEOUS
On the main island of Tierra del Fuego, Darwin (1846) recorded only Cenozoic fossils in the vicinity of Cape San Sebastian, including Nothofagus leaves and mollusks. The first fossils from the region, which are now accepted as uppermost Jurassic or Cretaceous in age, were recorded in 1839 near Nassau Bay (Navarino Island) by James Dwight Dana, a geologist of Charles Wilkes` United States Exploring Expedition (cf. Andersson 1906, Kranck 1932). The first Cretaceous fossils from the main island of Tierra del Fuego were found in 1882 nearby Ushuaia by Lovisato. They include trace fossils (and calcareous algae?) described by Richter (1925), together with Cretaceous radiolaria from the Staten and New Year islands (also collected by Lovisato) and several fossils, including belemnites from the Hito XIX area, collected by the Expedition of University of Buenos Aires to Tierra del Fuego (Doello Jurado 1922).
Yahgan Formation
The Yahgan Formation was established
by Kranck (1932) for the slates, slaty
mudstones, and andesite-rich greywackes
exposed in Mount Olivia and Ushuaia.
The Formation is well exposed along the
northern margin of the Beagle Channel,
from Ushuaia to Bahía Sloggett (Fig. 1);
and in Hoste, Navarino, Nueva, Lennox,
and Picton islands (Katz and Watters
1966, Dott et al. 1977, Winn 1978, Suárez
et al. 1985). Dominant sedimentary facies
are: a) black mudstones, fine-grained,
thin-bedded turbidites (Fig. 2a) and tuff;
b) classical turbidites; and c) massive to
graded sandstones. Petrographic composition
of the sandstones is uniform and
dominated by lithic andesitic fragments
and plagioclase, suggesting derivation
from the volcanic arc, located to the
South in the Fuegian Archipelago (Winn
1978, Suárez et al. 1985, Olivero and Martinioni
1996b). This is well documented
with paleocurrent data on the island of South Georgia, now displaced along the
North Scotia Ridge to the east (Dalziel et
al. 1974, Macdonald 1986).
Body and trace fossils are very scarce in
the Yahgan Formation (Fig. 3). On Navarino
Island Tithonian-Neocomian ammonites
and belemnites (Aguirre-Urreta and Suárez 1985); Aptian-Albian corals,
gastropods, and bivalves (Dott et al. 1977); and indeterminate inoceramids
(Katz and Watters 1966) were reported.
Microfossils, particularly radiolaria, are
apparently common, but not well preserved
due to the strong tectonic deformation
of the rocks (Kranck 1932, Olivero
and Martinioni 1996b). The trace fossil
Chondrites targioni (Brongniart) and the
presumed fossil calcareous algae "Lithocaulon
antarcticum" Bornemann were described
by Richter (1925) from the vicinity
of Ushuaia. Biostratigraphically diagnostic
Late Albian fossils are known from
Moat, where Actinoceramus concentricus (Parkinson) and Inoceramus carsoni Mc Coy
were recorded (Olivero and Martinioni 1996a). Fine-grained turbidites and mudstones
nearby Ushuaia and adjoining Mount Martial bear scarce and ill-preserved
belemnites.
Trace fossils are rare and restricted to a
few horizons; Chondrites targioni (Fig. 4b)
and Zoophycos isp. are the most common
trace fossils and are locally abundant in
particular beds. A few localities, e.g. on
the coast of the Beagle Channel just east
from Ushuaia, bear a more diversified
ichnoassemblage, including Chondrites targioni,
Helminthopsis tenuis, Stelloglyphus isp.,
Ophiomorpha cf. annulata, and Zoophycos isp. (Fig. 3). A similar trace fossil assemblage,
with the addition of Phycosiphon incertum,
was recorded in the Yahgan Formation
by Winn (1978).
Figure 4: Typical trace fossils and ichnofabrics of Cretaceous rocks in the Fuegian Andes. a) Nereites missouriensis (Weller) from Albian beds of the
Beauvoir Formation near Knokeke Hill, CADIC PI 88. b) Dense ichnofabric of Chondrites targioni (Brongniart) and associated Ophiomorpha cf. annulata (Op), Lower Cretaceous Yahgan Formation, Beagle Channel near Ushuaia, field specimens. c) Localized horizons with a relatively dense ichnofabric in
mudstones and isolated specimens of Rhizocorallium isp. (Rh), Estratos de Buen Suceso, Santonian-Lower Campanian, Buen Suceso Bay, field specimens.
d) Typical ichnofabric affecting thick packages in the Maastrichtian Policarpo Formation, Zoophycos isp. (Zo) and Schaubcylindrichnus ("Terebellina") isp.
(Sc) are the only recognizable structures in the intensely mottled background, field specimens. Scale bar: 1 cm.
Beauvoir Formation
The Beauvoir Formation (Camacho
1967) consists predominantly of homogeneous
black or bluish, massive to faintly
laminated dark slates and gray tuffs
mainly exposed in Sierra de Beauvoir, in
the belt stretching from the eastern part
of Lake Fagnano to the Montes Negros,
just west of Good Success (Buen Suceso)
Bay (Figs. 1 and 2b). Locally, the black
slates are rhythmically interbedded with
thin, fine-grained sandstones. Stratification
is only visible in the latter beds, whereas
in the massive slates the bedding is defined
only by the presence of thin tuff beds.
The fossiliferous marlstones cropping
out near Hito XIX in the Argentinean-Chilean border are provisionally included
in the Beauvoir Formation. These marlstones
contain abundant specimens of the
Aptian-Albian bivalves Aucellina "radiotostriata" Bonarelli, A. "andina" Feruglio
(Macellari 1979), and A. striata Richter
(Richter 1925). Additional fossils include
serpulids, brachiopods, inoceramids, probably
Inoceramus anglicus Woods or I.
sutherlandi MacCoy (cf. Medina 2007) and
abundant rostra of the belemnite Parahibolites
fuegensis (Stolley) in interbedded
sandstone beds (Richter 1925) and large,
poorly preserved ammonites.
The massive, dark slates of the Beauvoir
Formation near Knokeke Hill and Buen
Suceso Bay contain several horizons with
abundant but ill-preserved inoceramids,
which normally are concentrated in very
thin beds. Part of this material includes
the Albian Mytiloides cf. ipuanus (Wellman),
of which the nominal species "Inoceramus" urius Wellman and "I. kapuus" Wellman are now considered as synonyms
(see Crampton 2004). Associated,
very thin beds include abundant wellpreserved,
articulate shells of the Late
Albian Aucellina euglypha Woods (Fig. 5b)
covered by a thin pyrite coating (Olivero
and Medina 2001). Rare, complete but
badly deformed echinoids are associated
with the latter bivalves. Near the locality
of Knokeke Hill, black slates record relatively
well preserved and articulated
shells of Inoceramus anglicus (Fig. 5a) and
delicate back-filled burrows of Nereites
missouriensis (Weller) (Fig. 4a). In the subsurface, the stratigraphically
equivalent mudstone-dominated rocks of
the Nueva Argentina Formation and Arroyo
Alfa Formation contain Early-Mid Albian
and Late Albian foraminiferal assemblages,
respectively (Flores et al. 1973).
Figure 5: Cretaceous ammonites and bivalves from the Fuegian Andes. a) Inoceramus anglicus Woods, Upper Albian, Beauvoir Formation, right valve, CADIC
PI 87; b) Black mudstones with pyritized shells of Aucellina euglypha Woods, Upper Albian, Beauvoir Formation, CADIC PI6 a; c) Fragment of the body
chamber of Diplomoceras sp., Maastrichtian, Policarpo Formation, CADIC PI 85; d) Pachydiscus (Neodesmoceras) sp., Maastrichtian, Policarpo Formation,
CADIC PI 84; e) and f) Fragments of the body chamber of crushed Maorites spp., Maastrichtian, Policarpo Formation; e) Maorites densicostatus (Kilian and
Reboul) CADIC PI 86; f) Maorites sp. CADIC PI 89. Scale bar: 1 cm.
UPPER CRETACEOUS
"Estratos de Buen Suceso"
These beds include a thick sedimentary
succession of folded dark gray and black,
slaty mudstones, marlstones, and sandy
siltstones with common quartz veins.
The lower part of the succession, exposed
in the southern part of the bay, is
dominated by marlstones, impure limestones,
and fine-grained silty sandstones.
The slaty micritic limestones and marly mudstone record well-preserved, articulated
shells of Tethyoceramus madagascariensis (Heinz). This species has been referred
to the Middle Coniacian in Madagascar
and New Zealand, and to the
Upper Turonian-Lower Coniacian in
Antarctica (cf. Olivero and Medina
2001). The exact age and identity of inoceramids
assigned to "Inoceramus" (=Tethyoceramus) madagascariensis is now debated.
Walaszczyk et.al (2004) argue that the
New Zealand material is not true "I." madagascariensis and should be referred to "I." nukeus Wellman. However, Crampton
(writ. comm. 2008) believes that some
New Zealand material is indistinguishable
from topotype T. madagascariensis (e.g.,
compare Crampton 1996 pl. 16M and pl.
17E, with Walaszczyk et al. 2004, Fig. 13B
and fig. 14E, respectively). The Tierra del
Fuego material is indistinguishable from
the Antarctic specimens, and appears to
differ from "I."? nukeus, which is almost
equivalve and much less inflated than T. madagascariensis.
The upper part of the succession, exposed
along the northern margin of the
bay, is dominated by dark slaty mudstones
and silty sandstones, with some interbedded
light gray, thin tuff beds (Fig. 2c).
The slaty mudstones contain occasional,
large specimens (up to 50 cm high) of
complete shells of Inoceramus (Platyceramus) sp. and relatively small, well-preserved
specimens of Sphenoceramus sp.
The association described by Olivero and
Medina (2001) was referred to the Santonian-Lower Campanian.
The silty sandstones within the Santonian-Lower Campanian beds record several fully bioturbated horizons. The
trace fossils Chondrites, Palaeophycus, Planolites,
Rhizocorallium (Fig. 4c), and Zoophycos are recognized within a heavily bioturbated
background.
Bahía Thetis Formation
The dominant slaty mudstones and sandstones
cropping out in the Thetis Bay
area were first recognized by Furque and
Camacho (1949). Olivero et.al (2003) studied
the Bahía Thetis Formation and recognized
three packages, more than 250
m thick, of hard, highly deformed rocks
including: 1) dark, organic-rich, laminated
mudstones and tuffs with incipient
cleavage; 2) resedimented conglomerates
and pebbly mudstones; and 3) turbidite
sandstones and slaty mudstones.
The lowest organic-rich package bears
abundant radiolaria and more restricted
foraminifera. The latter comprise a low
diversity, agglutinated assemblage typical
of dysoxic environments and are characterized
by Rzehakina epigona (Rzehak), R.
lata Cushman and Jarvis and R. fissistomata (Grzybowski) (Caramés and Malumián
2006). In the second package, the conglomerates
include large clasts of radiolarian-bearing slates and foliated rhyolites,
derived from the Beauvoir-Yahgan Formations
and Jurassic volcanics, respectively.
In the third package, the turbidites
(Fig. 2e) bear scarce, ill-preserved ammonites,
including flattened fragments of Diplomoceras sp. and kossmaticeratids.
Trace fossils are very rare, and only a few
specimens of Stelloglyphus were recovered.
The absence of trace fossils is consistent
with the high organic matter content and
the preservation of delicate sedimentary
banding in the slaty mudstones. Based on
the recorded foraminifera and ammonites
the Bahía Thetis Formation was assigned
to the Late Campanian-?Early Maastrichtian
(Olivero et al. 2003, Caramés
and Malumián 2006).
Policarpo Formation
The Policarpo Formation is very well exposed
along the Atlantic shore of Península
Mitre, between Policarpo Cove and
San Vicente Cape (Furque and Camacho
1949, Olivero et al. 2002, 2003). The extensive,
but less well-exposed rocks forming
the NW belt of strata in the Sierra
de Noguera and Sierra de Apen are also
included in the Policarpo Formation
(Fig. 1).
In the type area of Península Mitre the
Policarpo Formation includes a minimum
thickness of 350 m, and probably
more than 700 m, of bioturbated, tuffaceous,
monotonous organic-rich, dark
gray, very hard sandy mudstones and silty
sandstones. Fresh-pyroclastic material is
relatively abundant, and some fine-grained
tuffaceous sandstones are almost exclusively
composed of zoned, euhedral
plagioclase crystals and volcanic glass.
Most part of the Policarpo Formation
consists of a crude alternation of tuffaceous
mudstones and silty sandstones
but the stratification is not always apparent
due to the intense bioturbation. Totally
bioturbated thick packages occur
repeatedly, and they are commonly characterized
by a high abundance of sandy,
agglutinated tubes of Schaubcylindrichnus (previously assigned to "Terebellina"). In
addition to a dense background mottling,
the trace fossils Tasselia, Rhizocorallium,
Phycodes, Teichichnus and small Chondrites are occasionally recorded (Fig. 4d). Some
horizons bear a dense concentration of
large Zoophycos burrows.
Relatively abundant invertebrate body
fossils are common in certain concretionary
horizons, but they are very difficult
to recover due to the extreme hardness
of the rocks. Irregular echinoids are very
common near the faulted contact with
the Bahía Thetis Formation near Cabo
San Vicente. Solitary corals, scaphopods,
gastropods, bivalves, and ammonites are
less abundant. One horizon at the contact
with the Río Bueno Formation near
Puesto Río Bueno, bears relatively wellpreserved
specimens of Diplomoceras sp.,
Anagaudryceras sp., Maorites densicostatus (Kilian and Reboul), Maorites sp. and large
Pachydiscus (Neodesmoceras) sp. (Fig. 5c-f).
Near Puesto Donata, very well-preserved
specimens of the gastropod Stru-thioptera
cf. gregaria Wilckens were recovered.
The foraminifera are dominated by
agglutinated cosmopolitan forms that include
the oldest record in the Austral Basin
of Spiroplectammina spectabilis (Olivero
et al. 2002, 2003, Olivero and Malumián
2008). Dinocysts are scarce and not wellpreserved,
but two assemblages of Maastrichtian
and Danian age were tentatively
recognized (Olivero et al. 2003).
CONCLUDING REMARKS
The first fossils to be discovered in the
clay-slate formation were Cretaceous crinoids,
gastropods, bivalves, and ammonites
found by Darwin in 1834 on Mount
Tarn (Darwin 1846). This discovery was
followed soon by new records of Cretaceous
invertebrates, notably among
these were the findings of additional fossil
mollusks in Mount Tarn by Hombron
and Grange in 1837; belemnites in Nassau
Bay by Dana in 1839; and bivalves
and ammonites in 1887-1890 in the islands of Saint Peter and Saint Paul,
Magellan Channel, reported by White
(see Andersson 1906, Bonarelli 1917,
Kranck 1932). Despite these discoveries
of Cretaceous fossils in the clay-slate formation
in distant localities, stretching
more than 500 km between Navarino
Island and Brunswick Peninsula south of
Punta Arenas on the Strait of Magellan,
a strong feeling of a Paleozoic age for
most of the clay-slate formation still
grew. This was based mainly on comparisons
of the regional features of the lowgrade
metamorphic rocks with similar
features in Paleozoic orogenic belts, particularly
the basement in the British Isles
and in Alpine areas, which at that time
was thought to be of Paleozoic age (see
Kranck 1932), and on the age interpretations
of the dubious fossils found by Lovisato
in 1881 in Staten Island (see
Harrington 1943). However, the presumed
Cambrian quartzites and schists
from Staten Island turned out to be
Jurassic acidic tuffs and volcaniclastic,
foliated rocks, and the dubious Paleozoic
fossils were probably part of the fibrous
structure of Cretaceous inoceramid
shells (Richter 1925) or belemnite rostra
(Harrington 1943).
As has been confirmed by additional fossil
discoveries in the clay-slate formation
of the main island of Tierra del Fuego,
including the first locality visited by
Darwin on the island (Good Success
Bay), we now know that all these rocks
are of Cretaceous age (Olivero and Martinioni
1996a, Olivero and Medina 2001).
The relatively homogeneous lithology is
a major obstacle to stratigraphic subdivision;
nonetheless, particular combinations
of fossil content, ichnofabric, and
lithological features make it possible to
recognize characteristic sedimentary successions
in the Lower and Upper Cretaceous
rocks.
In the Lower Cretaceous, only Aptian-Albian fossils have been recovered;
however, the Yahgan Formation has
thick horizons with massive sandstones
and sandy turbidites, a lithological association
that is not found in the homogenous,
black slaty mudstones and tuffs of
the Beauvoir Formation. The Yahgan
Formation also has localized horizons
dominated by the trace fossils Chondrites (Fig. 4b) and Zoophycos, an ichnoassemblage
that is consistent with the inferred
setting of a low-oxygenated slope (cf.
Uchman 2007) in a volcaniclastic apron
environment (Olivero and Martinioni 1996b, Olivero and Malumián 2008). The
black, fine-grained rocks of the Beauvoir
Formation contain localized horizons
with Nereites missouriensis (Fig. 4a). This
ichnofossil is a typical component of the
Nereites ichnosubfacies, which is distributed
in distal flysch facies and characterized
by the dominance of deposit-feeding
invertebrates, such as the tracemaker of
Nereites (cf. Seilacher 1974, Uchman
2007). Accordingly, these fine-grained
deposits probably reflect distal basin
plain settings, occupying an intermediate
position between the Pacific volcaniclastic
apron of the Yahgan Formation and
the typical, South-American cratonic slope
settings of the Lower Cretaceous
rocks in the Austral Basin (Wilson 1991).
In the Upper Cretaceous, four distinctive
successions are recognized: 1) the lower
part (Turonian-Coniacian) of the "Estratos
de Buen Suceso", 2) the upper part
(Santonian-Lower Campanian) of the "Estratos de Buen Suceso"; 3) the Upper
Campanian-?Lower Maastrichtian Bahía
Thetis Formation; and 4) the Maastrichtian-Danian Policarpo Formation (Olivero
and Medina 2001, Olivero et al. 2002,
2003, Olivero and Malumián 2008).
The lower part of the "Estratos de Buen
Suceso" is characterized by marlstones,
impure limestones, and fine-grained silty
sandstones with T. madagascariensis, whereas
the upper part is characterized by
dark slaty mudstones, tuffs, and silty
sandstones. The latter beds appear to
record the earliest successions with
extensively bioturbated horizons in the
basin, probably reflecting for the first
time prolonged periods with well-oxygenated
bottom conditions (Fig. 4c). These
beds alternate with those showing evidence
for more oxygen-deficient bottom
conditions, lacking trace fossils and
populated locally by Inoceramus
(Platyceramus) sp. and Spheno-ceramus sp.
(Olivero and Medina 2001).
The resedimented conglomerates, pebbly
mudstones, and sandy turbidites of the
Bahía Thetis Formation record a very
distinctive petrographic composition,
with clear evidence that the source of
clastic material was uplifted Andean
rocks, including foliated, Jurassic acidic
volcanics and radiolarian-rich Cretaceous
slates. These coarse-grained clastic rocks
probably reflect deposition in submarine
fans, with a source area in the hinterland
part of the growing Fuegian Andes. The
associated dark, organic-rich slaty mudstones
(Fig. 2e) are thought to have originated
in channel margin or more distant
basin settings. The foraminiferal content
and the generalized absence of trace fossils
are consistent with the interpreted
dysoxic bottom conditions (Olivero et al. 2003, Caramés and Malumián 2006). The
Bahía Thetis Formation probably records
the last occurrence of inoceramid
bivalves in the Austral Basin (Olivero et
al. 2003, 2004).
The Maastrichtian-Danian Policarpo Formation
records a marked, regional change
of paleoceanographic conditions in
the basin as is evidenced by the thoroughly
bioturbated ichnofabric that characterizes
thick sedimentary packages.
This change in the ichnofabric occurs in
the Maastrichtian Policarpo Formation
just above the inoceramid extinction
level in the Fuegian Andes. The inoceramids
were apparently adapted to warm
and poorly oxygenated waters and their
global extinction pulses, during the mid-Maastrichtian, were related to cooling
and enhanced bottom ventilation, promoted
by circulation of deep Antarctic
waters (MacLeod et al. 1996). The strong
contrast in the intensity of bioturbation
between the Upper Campanian-Lower
Maastrichtian Bahía Thetis Formation,
almost devoid of trace fossils, and the
highly bioturbated Maastrichtian-Danian
Policarpo Formation reflects a change
from dysoxic-anoxic to well ventilated
bottom conditions, probably associated
with a cooling trend of bottom waters in
the austral deep oceans (Olivero et al. 2003, 2004). In addition, the Policarpo
Formation also records a distinctive petrographic
composition, which is dominated
by fresh volcaniclastic material
implying a coeval volcanic pulse in the
magmatic arc.
ACKNOWLEDGMENTS
We thank N. Malumián (SEGEMARCONICET) for fruitful discussion over the years on the geology of Tierra del Fuego. D. Martinioni (CADIC-CONICET) helped with part of the fieldwork and analysis of information in German papers. We thank P. Torres Carbonell (CADIC-CONICET) for a critical review of an early manuscript. M.B. Aguirre- Urreta (UBA-CONICET) provided the pictures of the figured original material collected by Darwin in Brunswick Peninsula and housed in the Natural History Museum, London. We thank the reviewers A. Crame, J.S. Crampton, I.W.D. Dalziel, and M.D. Suárez for constructive observations that improved the original manuscript. This study was financed by PIP 5100 CONICET and PICTO 36315 FONCYT.
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Recibido: 14 de agosto de 2008
Aceptado: 22 de octubre de 2008