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THE STRATIGRAPHY OF CRETACEOUS M

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					60    Revista de la Asociación Geológica Argentina 64 (1): 60 - 69 (2009)




     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 Univer-
     sitaria, C1428EGA Buenos Aires.


     ABSTRACT
          The stratigraphy of Cretaceous marine mudstones in the Fuegian Andes, roughly equivalent to Charles Darwin's clay-slate for-
          mation, remains a still unsolved problem. Previous records of Albian, Turonian-Coniacian, and Santonian-Campanian bival-
          ves are combined with new findings of the Late Albian inoceramid Inoceramus anglicus Woods, and the Maastrichtian ammoni-
          tes 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 consis-
          tent 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 bio-
          turbated 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 condi-
          tions, 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 An-
          des Fueguinos la estratigrafía de las fangolitas del Cretácico marino (clay-slate formation de Charles Darwin), constituye un pro-
          blema 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 consis-
          tente 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, densa-
          mente 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.
                                                                               The stratigraphy of Cretaceous mudstones in the eastern Fuegian Andes...   61




INTRODUCTION                                           addition, new records of characteristic           the consequent origination of the Rocas
                                                       trace fossil assemblages with distinctive         Verdes Marginal Basin. The earliest Late
The stratigraphic division of Cretaceous               ichnofabrics, restricted to particular sedi-      Cretaceous ductile deformation, isoclinal
marine mudstones in the Fuegian Andes                  mentary successions of the Yahgan For-            folding, and low-grade regional meta-
(Fig. 1) is a major and still unsolved pro-            mation, Beauvoir Formation, Bahía The-            morphism of these rocks indicate a com-
blem. These mudstones correspond                       tis Formation, Policarpo Formation, and           pressional tectonic regime that resulted
roughly to the clay-slate formation of                 "Estratos de Buen Suceso", constitute             in the closure of the marginal basin. The
Charles Darwin (1846); a very extensive                meaningful data for the stratigraphic             Fuegian Andes were uplifted by the Late
formation of folded, low-grade meta-                   division of the relatively homogeneous,           Campanian; subsequent propagation of
morphic rocks with scarce marine fossils.              mudstone-rich Cretaceous deposits.                the compressional deformation and sub-
In the course of the voyage of HMS                                                                       sidence by tectonic loading along the
Beagle Darwin noted that "The great clay-              STRATIGRAPHIC                                     northern orogenic margin resulted in the
slate formation of Tierra del Fuego being Cre-         FRAMEWORK                                         formation of the Austral and Malvinas
taceous, is certainly a very interesting fact,--whe-                                                     foreland basins. The Turonian-Lower
ther we consider the appearance of the country,        The stratigraphy of southernmost South            Campanian "Estratos de Buen Suceso"
which, without the evidence afforded by the fos-       America has been controlled since the             represent the final stages of the closure
sils, would form the analogy of most known dis-        Mesozoic by a series of contrasting tec-          of the marginal basin and/or the begin-
tricts, probably have been considered as belonging     tonic regimes, represented by Late Juras-         ning of the foreland basins. Conglo-
to the Palaeozoic series,…"(Darwin 1846, p.            sic-Early Cretaceous extension, Late Cre-         merate beds in the Bahía Thetis Forma-
152).                                                  taceous-Paleogene compression, and la-            tion bear foliated clasts of Andean-deri-
The evidence mentioned by Darwin in-                   test Paleogene-Neogene and present day            ved rocks and thus they record uplifting
cluded a few fossils from Mount Tarn,                  strike-parallel transcurrent motion (Katz         and subaerial erosion of the Fuegian
on the western side of the Strait of Ma-               1972, Dalziel et al. 1974, Suárez et al.1985,     Andes. The youngest Cretaceous rocks
gellan, Brunswick Peninsula in Chile (Fig.             Kohn et al. 1995, To-rres Carbonell et al.        are included in the Policarpo Formation,
2d), particularly the ammonites described              2008). Accordingly, Cretaceous rocks in           which includes the Maastrichtian/Danian
by Forbes (in Darwin 1846) as "Ancyloce-               the Fuegian Andes are distributed in              boundary at its top (Olivero et al. 2003).
ras simplex" d'Orbigny, from Mount Tarn                three tectonostratigraphic domains: 1)            Figure 3 summarizes the paleontological
(Fig. 2f), and "Hamites elatior" Sowerby,              the magmatic arc, located along the axis          information, including partly new body
from the coastal area nearby Port Fami-                of the southern archipelago, includes             and trace fossil data, used in this study to
ne. We now know that these fossils are                 Lower Cretaceous andesitic (Hardy For-            differentiate particular horizons within
not Albian hamitids; the "A. simplex" is a             mation) and Upper Cretaceous plutonic             these lithologically homogeneous strati-
Maastrichtian kossmaticeratid, probably                rocks (Beagle Channel Plutonic Group);            graphic units. The stratigraphic implica-
a deformed fragment of Maorites, and                   2) the Rocas Verdes Marginal Basin in-            tions of these data are described for each
"H. elatior" is a Diplomoceras, also of Maas-          cludes Lower Cretaceous, low-grade me-            formation in the next section.
trichtian age (Spath 1953, Lahsen and                  tamorphic rocks, mostly slates and sands-
Charrier 1972). However, the correction                tones of the Yahgan and Beauvoir for-             LOWER CRETACEOUS
in the age of these ammonite-bearing                   mations, and Upper Jurassic-Lower Cre-
strata does not change the fact that in the            taceous ophiolitic rocks; and 3) the              On the main island of Tierra del Fuego,
Fuegian Andes, Albian to Maastrichtian                 Austral and Malvinas foreland basins,             Darwin (1846) recorded only Cenozoic
fine-grained rocks have quite similar li-              separated in the northern area by the             fossils in the vicinity of Cape San Sebas-
thological and structural features and                 Dungeness or Rio Chico Arch, include              tian, including Nothofagus leaves and
consequently they are very difficult to                Upper Cretaceous rocks dominated by               mollusks. The first fossils from the
distinguish in the field (Fig. 2).                     slaty mudstones (Fig. 1).                         region, which are now accepted as upper-
In this study, previous records of Albian,             According to a recent review (see Oli-            most Jurassic or Cretaceous in age, were
Turonian-Coniacian, and Santonian-                     vero and Malumián 2008, and the biblio-           recorded in 1839 near Nassau Bay (Na-
Campanian bivalves, mostly inoceramids                 graphy cited therein), the Yahgan For-            varino Island) by James Dwight Dana, a
(Olivero and Martinioni 1996a, Olivero                 mation represents a volcaniclastic apron          geologist of Charles Wilkes` United Sta-
and Medina 2001) are combined with                     of mudstones and deep-marine andesite-            tes Exploring Expedition (cf. Andersson
new findings of latest Albian inocera-                 rich turbidites and the Beauvoir Forma-           1906, Kranck 1932). The first Cretaceous
mids and Maastrichtian ammonites to                    tion records basin plain and slope muds-          fossils from the main island of Tierra del
further constrain the Cretaceous strati-               tones. Both formations reflect Late Ju-           Fuego were found in 1882 nearby
graphy of the eastern Fuegian Andes. In                rassic-Early Cretaceous extension and             Ushuaia by Lovisato. They include trace
62    E . B. O L I V E RO, F. A . M E D I N A , A N D M . I . L Ó P E Z C.




     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.
     fossils (and calcareous algae?) described            mudstones, and andesite-rich greywackes        dominated by lithic andesitic fragments
     by Richter (1925), together with Creta-              exposed in Mount Olivia and Ushuaia.           and plagioclase, suggesting derivation
     ceous radiolaria from the Staten and                 The Formation is well exposed along the        from the volcanic arc, located to the
     New Year islands (also collected by Lo-              northern margin of the Beagle Channel,         South in the Fuegian Archipelago (Winn
     visato) and several fossils, including be-           from Ushuaia to Bahía Sloggett (Fig. 1);       1978, Suárez et al. 1985, Olivero and Mar-
     lemnites from the Hito XIX area, collec-             and in Hoste, Navarino, Nueva, Lennox,         tinioni 1996b). This is well documented
     ted by the Expedition of University of               and Picton islands (Katz and Watters           with paleocurrent data on the island of
     Buenos Aires to Tierra del Fuego (Doe-               1966, Dott et al. 1977, Winn 1978, Suárez      South Georgia, now displaced along the
     llo Jurado 1922).                                    et al. 1985). Dominant sedimentary facies      North Scotia Ridge to the east (Dalziel et
                                                          are: a) black mudstones, fine-grained,         al. 1974, Macdonald 1986).
     Yahgan Formation                                     thin-bedded turbidites (Fig. 2a) and tuff;     Body and trace fossils are very scarce in
                                                          b) classical turbidites; and c) massive to     the Yahgan Formation (Fig. 3). On Nava-
     The Yahgan Formation was established                 graded sandstones. Petrographic compo-         rino Island Tithonian-Neocomian am-
     by Kranck (1932) for the slates, slaty               sition of the sandstones is uniform and        monites and belemnites (Aguirre-Urreta
                                                                            The stratigraphy of Cretaceous mudstones in the eastern Fuegian Andes...    63




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 fine-
grained 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.

and Suárez 1985); Aptian-Albian corals,            ved due to the strong tectonic deforma-            cribed by Richter (1925) from the vicinity
gastropods, and bivalves (Dott et al.              tion of the rocks (Kranck 1932, Olivero            of Ushuaia. Biostratigraphically diagnos-
1977); and indeterminate inoceramids               and Martinioni 1996b). The trace fossil            tic Late Albian fossils are known from
(Katz and Watters 1966) were reported.             Chondrites targioni (Brongniart) and the           Moat, where Actinoceramus concentricus
Microfossils, particularly radiolaria, are         presumed fossil calcareous algae "Litho-           (Parkinson) and Inoceramus carsoni Mc Coy
apparently common, but not well preser-            caulon antarcticum" Bornemann were des-            were recorded (Olivero and Martinioni
64    E . B. O L I V E RO, F. A . M E D I N A , A N D M . I . L Ó P E Z C.




                                                                                                          just west of Good Success (Buen Suce-
                                                                                                          so) 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 defi-
                                                                                                          ned 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 marls-
                                                                                                          tones contain abundant specimens of the
                                                                                                          Aptian-Albian bivalves Aucellina "radiotos-
                                                                                                          triata" Bonarelli, A. "andina" Feruglio
                                                                                                          (Macellari 1979), and A. striata Richter
                                                                                                          (Richter 1925). Additional fossils include
                                                                                                          serpulids, brachiopods, inoceramids, pro-
                                                                                                          bably Inoceramus anglicus Woods or I.
                                                                                                          sutherlandi MacCoy (cf. Medina 2007) and
                                                                                                          abundant rostra of the belemnite Parahi-
                                                                                                          bolites 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 (Well-
                                                                                                          man), of which the nominal species "Ino-
                                                                                                          ceramus" urius Wellman and "I. kapuus"
                                                                             Figure 3: Stratigraphic
                                                                                                          Wellman are now considered as syno-
                                                                             distribution of characte-    nyms (see Crampton 2004). Associated,
                                                                             ristic body and trace fos-   very thin beds include abundant well-
                                                                             sils in the Cretaceous of    preserved, articulate shells of the Late
                                                                             the Fuegian Andes
                                                                             (explanation in the text).   Albian Aucellina euglypha Woods (Fig. 5b)
                                                                                                          covered by a thin pyrite coating (Olivero
     1996a). Fine-grained turbidites and mud-             isp. (Fig. 3). A similar trace fossil assem-    and Medina 2001). Rare, complete but
     stones nearby Ushuaia and adjoining                  blage, with the addition of Phycosiphon in-     badly deformed echinoids are associated
     Mount Martial bear scarce and ill-preser-            certum, was recorded in the Yahgan For-         with the latter bivalves. Near the locality
     ved belemnites.                                      mation by Winn (1978).                          of Knokeke Hill, black slates record rela-
     Trace fossils are rare and restricted to a                                                           tively well preserved and articulated
     few horizons; Chondrites targioni (Fig. 4b)          Beauvoir Formation                              shells of Inoceramus anglicus (Fig. 5a) and
     and Zoophycos isp. are the most common                                                               delicate back-filled burrows of Nereites
     trace fossils and are locally abundant in            The Beauvoir Formation (Camacho                 missouriensis (Weller) (Fig. 4a).
     particular beds. A few localities, e.g. on           1967) consists predominantly of homo-           In the subsurface, the stratigraphically
     the coast of the Beagle Channel just east            geneous black or bluish, massive to fain-       equivalent mudstone-dominated rocks of
     from Ushuaia, bear a more diversified                tly laminated dark slates and gray tuffs        the Nueva Argentina Formation and Arro-
     ichnoassemblage, including Chondrites tar-           mainly exposed in Sierra de Beauvoir, in        yo Alfa Formation contain Early-Mid Al-
     gioni, Helminthopsis tenuis, Stelloglyphus isp.,     the belt stretching from the eastern part       bian and Late Albian foraminiferal as-
     Ophiomorpha cf. annulata, and Zoophycos              of Lake Fagnano to the Montes Negros,           semblages, respectively (Flores et al. 1973).
                                                                 The stratigraphy of Cretaceous mudstones in the eastern Fuegian Andes...                    65




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.

                                                    gascar and New Zealand, and to the                   differ from "I."? nukeus, which is almost
UPPER CRETACEOUS                                    Upper Turonian-Lower Coniacian in                    equivalve and much less inflated than T.
                                                    Antarctica (cf. Olivero and Medina                   madagascariensis.
"Estratos de Buen Suceso"                           2001). The exact age and identity of ino-            The upper part of the succession, expo-
                                                    ceramids assigned to "Inoceramus" (=Te-              sed along the northern margin of the
These beds include a thick sedimentary              thyoceramus) madagascariensis is now deba-           bay, is dominated by dark slaty mudsto-
succession of folded dark gray and black,           ted. Walaszczyk et.al (2004) argue that the          nes and silty sandstones, with some inter-
slaty mudstones, marlstones, and sandy              New Zealand material is not true "I."                bedded light gray, thin tuff beds (Fig. 2c).
siltstones with common quartz veins.                madagascariensis and should be referred to           The slaty mudstones contain occasional,
The lower part of the succession, expo-             "I." nukeus Wellman. However, Crampton               large specimens (up to 50 cm high) of
sed in the southern part of the bay, is             (writ. comm. 2008) believes that some                complete shells of Inoceramus (Platyce-
dominated by marlstones, impure limes-              New Zealand material is indistinguisha-              ramus) sp. and relatively small, well-pre-
tones, and fine-grained silty sandstones.           ble from topotype T. madagascariensis (e.g.,         served specimens of Sphenoceramus sp.
The slaty micritic limestones and marly             compare Crampton 1996 pl. 16M and pl.                The association described by Olivero and
mudstone record well-preserved, articu-             17E, with Walaszczyk et al. 2004, Fig. 13B           Medina (2001) was referred to the San-
lated shells of Tethyoceramus madagascarien-        and fig. 14E, respectively). The Tierra del          tonian-Lower Campanian.
sis (Heinz). This species has been refe-            Fuego material is indistinguishable from             The silty sandstones within the Santo-
rred to the Middle Coniacian in Mada-               the Antarctic specimens, and appears to              nian-Lower Campanian beds record se-
66    E . B. O L I V E RO, F. A . M E D I N A , A N D M . I . L Ó P E Z C.




     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.

     veral fully bioturbated horizons. The                died the Bahía Thetis Formation and re-           of dysoxic environments and are charac-
     trace fossils Chondrites, Palaeophycus, Plano-       cognized three packages, more than 250            terized by Rzehakina epigona (Rzehak), R.
     lites, Rhizocorallium (Fig. 4c), and Zoophycos       m thick, of hard, highly deformed rocks           lata Cushman and Jarvis and R. fissistoma-
     are recognized within a heavily bioturba-            including: 1) dark, organic-rich, lamina-         ta (Grzybowski) (Caramés and Malumián
     ted background.                                      ted mudstones and tuffs with incipient            2006). In the second package, the con-
                                                          cleavage; 2) resedimented conglomerates           glomerates include large clasts of radiola-
     Bahía Thetis Formation                               and pebbly mudstones; and 3) turbidite            rian-bearing slates and foliated rhyolites,
                                                          sandstones and slaty mudstones.                   derived from the Beauvoir-Yahgan For-
     The dominant slaty mudstones and sand-               The lowest organic-rich package bears             mations and Jurassic volcanics, respecti-
     stones cropping out in the Thetis Bay                abundant radiolaria and more restricted           vely. In the third package, the turbidites
     area were first recognized by Furque and             foraminifera. The latter comprise a low           (Fig. 2e) bear scarce, ill-preserved ammo-
     Camacho (1949). Olivero et.al (2003) stu-            diversity, agglutinated assemblage typical        nites, including flattened fragments of
                                                                     The stratigraphy of Cretaceous mudstones in the eastern Fuegian Andes...   67




Diplomoceras sp. and kossmaticeratids.        Relatively abundant invertebrate body            a strong feeling of a Paleozoic age for
Trace fossils are very rare, and only a few   fossils are common in certain concretio-         most of the clay-slate formation still
specimens of Stelloglyphus were recovered.    nary horizons, but they are very difficult       grew. This was based mainly on compari-
The absence of trace fossils is consistent    to recover due to the extreme hardness           sons of the regional features of the low-
with the high organic matter content and      of the rocks. Irregular echinoids are very       grade metamorphic rocks with similar
the preservation of delicate sedimentary      common near the faulted contact with             features in Paleozoic orogenic belts, par-
banding in the slaty mudstones. Based on      the Bahía Thetis Formation near Cabo             ticularly the basement in the British Isles
the recorded foraminifera and ammoni-         San Vicente. Solitary corals, scaphopods,        and in Alpine areas, which at that time
tes the Bahía Thetis Formation was assig-     gastropods, bivalves, and ammonites are          was thought to be of Paleozoic age (see
ned to the Late Campanian-?Early Maas-        less abundant. One horizon at the con-           Kranck 1932), and on the age interpreta-
trichtian (Olivero et al. 2003, Caramés       tact with the Río Bueno Formation near           tions of the dubious fossils found by Lo-
and Malumián 2006).                           Puesto Río Bueno, bears relatively well-         visato in 1881 in Staten Island (see
                                              preserved specimens of Diplomoceras sp.,         Harrington 1943). However, the presu-
Policarpo Formation                           Anagaudryceras sp., Maorites densicostatus       med Cambrian quartzites and schists
                                              (Kilian and Reboul), Maorites sp. and large      from Staten Island turned out to be
The Policarpo Formation is very well ex-      Pachydiscus (Neodesmoceras) sp. (Fig. 5c-f).     Jurassic acidic tuffs and volcaniclastic,
posed along the Atlantic shore of Penín-      Near Puesto Donata, very well-preserved          foliated rocks, and the dubious Paleozoic
sula Mitre, between Policarpo Cove and        specimens of the gastropod Stru-thioptera        fossils were probably part of the fibrous
San Vicente Cape (Furque and Camacho          cf. gregaria Wilckens were recovered.            structure of Cretaceous inoceramid
1949, Olivero et al. 2002, 2003). The ex-     The foraminifera are dominated by                shells (Richter 1925) or belemnite rostra
tensive, but less well-exposed rocks for-     agglutinated cosmopolitan forms that in-         (Harrington 1943).
ming the NW belt of strata in the Sierra      clude the oldest record in the Austral Ba-       As has been confirmed by additional fos-
de Noguera and Sierra de Apen are also        sin of Spiroplectammina spectabilis (Olivero     sil discoveries in the clay-slate formation
included in the Policarpo Formation           et al. 2002, 2003, Olivero and Malumián          of the main island of Tierra del Fuego,
(Fig. 1).                                     2008). Dinocysts are scarce and not well-        including the first locality visited by
In the type area of Península Mitre the       preserved, but two assemblages of Maas-          Darwin on the island (Good Success
Policarpo Formation includes a mini-          trichtian and Danian age were tentatively        Bay), we now know that all these rocks
mum thickness of 350 m, and probably          recognized (Olivero et al. 2003).                are of Cretaceous age (Olivero and Mar-
more than 700 m, of bioturbated, tuffa-                                                        tinioni 1996a, Olivero and Medina 2001).
ceous, monotonous organic-rich, dark          CONCLUDING REMARKS                               The relatively homogeneous lithology is
gray, very hard sandy mudstones and silty                                                      a major obstacle to stratigraphic subdivi-
sandstones. Fresh-pyroclastic material is     The first fossils to be discovered in the        sion; nonetheless, particular combina-
relatively abundant, and some fine-grai-      clay-slate formation were Cretaceous cri-        tions of fossil content, ichnofabric, and
ned tuffaceous sandstones are almost ex-      noids, gastropods, bivalves, and ammoni-         lithological features make it possible to
clusively composed of zoned, euhedral         tes found by Darwin in 1834 on Mount             recognize characteristic sedimentary suc-
plagioclase crystals and volcanic glass.      Tarn (Darwin 1846). This discovery was           cessions in the Lower and Upper Creta-
Most part of the Policarpo Formation          followed soon by new records of Cre-             ceous rocks.
consists of a crude alternation of tuffa-     taceous invertebrates, notably among             In the Lower Cretaceous, only Aptian-
ceous mudstones and silty sandstones          these were the findings of additional fos-       Albian fossils have been recovered;
but the stratification is not always appa-    sil mollusks in Mount Tarn by Hombron            however, the Yahgan Formation has
rent due to the intense bioturbation. To-     and Grange in 1837; belemnites in Nas-           thick horizons with massive sandstones
tally bioturbated thick packages occur        sau Bay by Dana in 1839; and bivalves            and sandy turbidites, a lithological asso-
repeatedly, and they are commonly cha-        and ammonites in 1887-1890 in the                ciation that is not found in the homoge-
racterized by a high abundance of sandy,      islands of Saint Peter and Saint Paul,           nous, black slaty mudstones and tuffs of
agglutinated tubes of Schaubcylindrichnus     Magellan Channel, reported by White              the Beauvoir Formation. The Yahgan
(previously assigned to "Terebellina"). In    (see Andersson 1906, Bonarelli 1917,             Formation also has localized horizons
addition to a dense background mottling,      Kranck 1932). Despite these discoveries          dominated by the trace fossils Chondrites
the trace fossils Tasselia, Rhizocorallium,   of Cretaceous fossils in the clay-slate for-     (Fig. 4b) and Zoophycos, an ichnoassem-
Phycodes, Teichichnus and small Chondrites    mation in distant localities, stretching         blage that is consistent with the inferred
are occasionally recorded (Fig. 4d). Some     more than 500 km between Navarino                setting of a low-oxygenated slope (cf.
horizons bear a dense concentration of        Island and Brunswick Peninsula south of          Uchman 2007) in a volcaniclastic apron
large Zoophycos burrows.                      Punta Arenas on the Strait of Magellan,          environment (Olivero and Martinioni
68    E . B. O L I V E RO, F. A . M E D I N A , A N D M . I . L Ó P E Z C.




     1996b, Olivero and Malumián 2008). The               clastic material was uplifted Andean         ACKNOWLEDGMENTS
     black, fine-grained rocks of the Beauvoir            rocks, including foliated, Jurassic acidic
     Formation contain localized horizons                 volcanics and radiolarian-rich Cretaceous    We thank N. Malumián (SEGEMAR-
     with Nereites missouriensis (Fig. 4a). This          slates. These coarse-grained clastic rocks   CONICET) for fruitful discussion over
     ichnofossil is a typical component of the            probably reflect deposition in submarine     the years on the geology of Tierra del
     Nereites ichnosubfacies, which is distribu-          fans, with a source area in the hinterland   Fuego. D. Martinioni (CADIC-CONI-
     ted in distal flysch facies and characteri-          part of the growing Fuegian Andes. The       CET) helped with part of the fieldwork
     zed by the dominance of deposit-feeding              associated dark, organic-rich slaty muds-    and analysis of information in German
     invertebrates, such as the tracemaker of             tones (Fig. 2e) are thought to have origi-   papers. We thank P. Torres Carbonell
     Nereites (cf. Seilacher 1974, Uchman                 nated in channel margin or more distant      (CADIC-CONICET) for a critical re-
     2007). Accordingly, these fine-grained               basin settings. The foraminiferal content    view of an early manuscript. M.B. Agui-
     deposits probably reflect distal basin               and the generalized absence of trace fos-    rre-Urreta (UBA-CONICET) provided
     plain settings, occupying an intermediate            sils are consistent with the interpreted     the pictures of the figured original mate-
     position between the Pacific volcaniclas-            dysoxic bottom conditions (Olivero et al.    rial collected by Darwin in Brunswick
     tic apron of the Yahgan Formation and                2003, Caramés and Malumián 2006). The        Peninsula and housed in the Natural His-
     the typical, South-American cratonic slo-            Bahía Thetis Formation probably re-          tory Museum, London. We thank the re-
     pe settings of the Lower Cretaceous                  cords the last occurrence of inoceramid      viewers A. Crame, J.S. Crampton, I.W.D.
     rocks in the Austral Basin (Wilson 1991).            bivalves in the Austral Basin (Olivero et    Dalziel, and M.D. Suárez for constructive
     In the Upper Cretaceous, four distinctive            al. 2003, 2004).                             observations that improved the original
     successions are recognized: 1) the lower             The Maastrichtian-Danian Policarpo For-      manuscript. This study was financed by
     part (Turonian-Coniacian) of the "Estra-             mation records a marked, regional chan-      PIP 5100 CONICET and PICTO 36315
     tos de Buen Suceso", 2) the upper part               ge of paleoceanographic conditions in        FONCYT.
     (Santonian-Lower Campanian) of the                   the basin as is evidenced by the tho-
     "Estratos de Buen Suceso"; 3) the Upper              roughly bioturbated ichnofabric that cha-    WORKS CITED IN THE TEXT
     Campanian-?Lower Maastrichtian Bahía                 racterizes thick sedimentary packages.
     Thetis Formation; and 4) the Maastri-                This change in the ichnofabric occurs in     Aguirre Urreta, M.B. and Suárez, M. 1985. Be-
     chtian-Danian Policarpo Formation (Oli-              the Maastrichtian Policarpo Formation           lemnites de una secuencia turbidítica volcano-
     vero and Medina 2001, Olivero et al. 2002,           just above the inoceramid extinction            clástica de la Formación Yahgan-Titoniano-
     2003, Olivero and Malumián 2008).                    level in the Fuegian Andes. The inocera-        Cretácico inferior del extremo sur de Chile. 4°
     The lower part of the "Estratos de Buen              mids were apparently adapted to warm            Congreso Geológico Chileno (Antofagasta),
     Suceso" is characterized by marlstones,              and poorly oxygenated waters and their          Actas 1: 1-16.
     impure limestones, and fine-grained silty            global extinction pulses, during the mid-    Andersson, J.G. 1906. Geological fragments from
     sandstones with T. madagascariensis, whe-            Maastrichtian, were related to cooling          Tierra del Fuego. Bulletin of the Geological
     reas the upper part is characterized by              and enhanced bottom ventilation, pro-           Institution, University of Upsala 8: 169-183.
     dark slaty mudstones, tuffs, and silty               moted by circulation of deep Antarctic       Bonarelli, G. 1917. Tierra del Fuego y sus turbe-
     sandstones. The latter beds appear to                waters (MacLeod et al. 1996). The strong        ras. Anales del Ministerio de Agricultura de la
     record the earliest successions with                 contrast in the intensity of bioturbation       Nación, Sección Geología, Mineralogía y Mi-
     extensively bioturbated horizons in the              between the Upper Campanian-Lower               nería 12(3): 1-119, Buenos Aires.
     basin, probably reflecting for the first             Maastrichtian Bahía Thetis Formation,        Camacho, H.H. 1967. Las transgresiones del
     time prolonged periods with well-oxyge-              almost devoid of trace fossils, and the         Cretácico superior y Terciario de la Argentina.
     nated bottom conditions (Fig. 4c). These             highly bioturbated Maastrichtian-Danian         Revista de la Asociación Geológica Argentina
     beds alternate with those showing evi-               Policarpo Formation reflects a change           22: 253-280.
     dence for more oxygen-deficient bottom               from dysoxic-anoxic to well ventilated       Caramés, A. and Malumián, N. 2006. La Familia
     conditions, lacking trace fossils and                bottom conditions, probably associated          Rzehakinidae (Foraminifera) en el Cretácico
     populated locally by Inoceramus                      with a cooling trend of bottom waters in        superior-Paleógeno de la cuenca Austral y la
     (Platyceramus) sp. and Spheno-ceramus sp.            the austral deep oceans (Olivero et al.         plataforma continental atlántica adyacente,
     (Olivero and Medina 2001).                           2003, 2004). In addition, the Policarpo         Argentina. Ameghiniana 43: 649-668.
     The resedimented conglomerates, pebbly               Formation also records a distinctive pe-     Crampton, J.S. 1996. Inoceramid bivalves from
     mudstones, and sandy turbidites of the               trographic composition, which is domi-          the Late Cretaceous of New Zealand. Insti-
     Bahía Thetis Formation record a very                 nated by fresh volcaniclastic material          tute of Geological and Nuclear Sciences Mo-
     distinctive petrographic composition,                implying a coeval volcanic pulse in the         nograph 14: 1-188.
     with clear evidence that the source of               magmatic arc.                                Crampton, J.S. 2004. Shell composition, cryptic
                                                                                The stratigraphy of Cretaceous mudstones in the eastern Fuegian Andes...         69




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