geology and stratigraphy of sierra baguales, Última...
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Geology and stratigraphy of Sierra Baguales, Última Esperanza
Province, Magallanes, Chile.
Néstor M. Gutiérrez*1, Jacobus P. Le Roux
1, Enrique Bostelmann
2,3, José Luis
Oyarzún4, Raúl Ugalde
1, Ana Vásquez
1, Rodrigo Otero
3, José Araos
1, Catalina
Carreño1, C. Mark Fanning
5, Teresa Torres
6, Francisco Hervé
1,7
(1)Departamento de Geología, Universidad de Chile / Centro de Excelencia en
Geotermia de los Andes, Casilla 13518, Santiago, Chile.
(2)Museo Nacional de Historia Natural, CC. 399, 11.000. Montevideo, Uruguay.
(3)Red Paleontológica U-Chile. Laboratorio de Ontogenia y Filogenia, Departamento
de Biología, Universidad de Chile, Av. Las Palmeras 3425, Santiago, Chile.
(4) Callejón Pedro Méndez - Huerto N° 112, Puerto Natales, Chile.
(5) Research School of Earth Sciences, The Australian National University, Mills Road,
Canberra, ACT 0200, Australia. (6) Departamento de Producción Agrícola, Facultad de Ciencias Agronómicas, Universidad de
Chile, Correo 1004, Santiago, Chile.
(7)Departamento de Geología, Universidad Andrés Bello, Santiago, Chile.
Sierra Baguales is a unique region in the Austral (Magallanes) Basin of southern South
America, where one of the most complete, uninterrupted Mesozoic-Cenozoic
sedimentary sequences crops out. The approximately 1300 m thick succession includes
the Tres Pasos and Dorotea Formations, both of Late Cretaceous age, as well as
Cenozoic Formations such as the Middle-Late Eocene Man Aike Formation, the the late
Oligocene to Early Miocene? Río Leona Formation, the Early Miocene Estancia 25 de
Mayo Formation, and the Early Miocene Santa Cruz Formation. In spite of previous
studies (Piatnitzky, 1938; Feruglio, 1938; Cecioni, 1957; Hoffstetter et al., 1957;
Furque, 1973; Malumián, 1990; Marenssi et al. 2000, 2002, 2005; Le Roux et al., 2010;
Malumián and Náñez, 2011; Cuitiño et al., 2012; Le Roux, 2012; Bostelmann et al., in
press) in this area no consensus has been reached on the geographical distribution of the
different stratigraphic units proposed for the area and their contact relationships. In
Sierra Baguales, the most detailed map of the region is at a scale of only 1:100,000,
showing obsolete stratigraphic names such as the Río Bandurrias, Río Baguales and Las
Flores Formations (Muñoz, 1981; Le Roux et al., 2010). In this study we present a new
1:10,000 scale geological map showing the distribution and stratigraphy of the Tres
Pasos, Dorotea, Man Aike, Río Leona, Estancia 25 de Mayo, and Santa Cruz
Formations in the Sierra Baguales area. Additionally, we discuss the age and origin of
intrusions in the Cenozoic units. The Tres Pasos Formation (Campanian-Maastrichtian)
in the region of Las Chinas and Cerro Guido shows intercalations of shale and siltstones
with lower flow regime horizontal lamination, indicating a deep marine environment at
the base that shallows towards the top. The Tres Pasos Formation underlies the Dorotea
Formation, which in the study area reaches a thickness of 200 m, consisting of medium-
to very coarse-grained sandstones deposited in a transitional, shallow marine to
continental environment. It contains invertebrates, vertebrates, and thin continental beds
with fossil leaves. We recently dated the Dorotea Formation in Sierra Baguales by
detrital zircons, which yielded 71±1.2 Ma. The Man Aike Formation, incorrectly
known as the Río Baguales Formation, unconformably overlies the Dorotea Formation
and was dated by detrital zircons at 40-36 Ma. It corresponds to 300 m of medium-
grained sandstones and conglomerates deposited in an estuary environment (Le-Roux.
et al., 2010). Shark teeth of Late Eocene age, mantas, fishes and invertebrates represent
the characteristic fossil assemblage of the Man Aike Formation (Otero et al.,
submitted). In Sierra Baguales, the Río Leona Formation (Late Oligocene – Early
Miocene) paraconformably overlies the Man Aike Formation and has an approximate
thickness of 200 m, consisting of mudstones and medium-grained sandstones with
lignite beds near the base. It was deposited in meandering and anastomosing rivers as
well as overbank marshes (Marenssi et al., 2000, 2005). Tree trunks and leaves are the
typical fossils of the Río Leona Formation (Barreda et al., 2009; Torres et al., 2009).
The Estancia 25 de Mayo Formation overlies the Río Leona Formation and represents
the Patagonian/-Superpatagonian transgression of Florentino Ameghino during the
Early Miocene, between 20-19 Ma (Cuitiño et al., 2013, Parras et al., 2012, Bostelmann
et al., in press). Its fossil assemblage includes oyster reefs of Ostrea hatcheri, soft-
bottom bivalves, typical Leonian gastropods like Perissodonta ameghinoi, and crab
fossils Chaceon peruvianum. A 2 m thick pyroclastic horizon with a rhyodacytic
composition is present in the basal part of the succession, which has also been
recognized in Argentina as the LPL (Cuitiño et al., 2013). U-Pb ages of 19.14±0.5 have
been obtained by Cuitiño et al. (2013) for this explosive volcanic episode. The Santa
Cruz Formation concordantly overlies the Estancia 25 de Mayo Formation (Bostelmann
et al., in press), reaching a thickness of 100 m and consisting of medium- to coarse-
grained sandstones, conglomerates and mudstones deposited in a fluvial environment
with meandering rivers and overbank lakes. Vertebrate terrestrial fossils indicating a
post-Colhuehuapian, pre-Santacrucian (19 to 17.8 Ma) age are supported by a detrital
zircon date of 18.23±0.26 Ma (Bostelmann et al., in press). The Eocene-Early Miocene
sedimentary rocks have been intruded by laccolite-type plutons and olivine-gabbro sills,
referred to as the La Cumbre Sill by Bostelmann et al. (in press) and as the Los Leones,
Cerro Donoso and Cerro Negro gabbro sills by Muñoz (1981). None of these sills
intrudes the Santa Cruz Formation, and the only units affected by this intrusive event
are the Dorotea, Man Aike, Río Leona and Estancia 25 de Mayo Formations. This
indicates that the intrusive event took place around 19 Ma, before the deposition of the
Santa Cruz Formation. Late Pliocene andesitic lavas (Muñoz, 1981) overlie the Santa
Cruz Formation. In addition, the whole Cretaceous-Late Miocene succession is intruded
by dioritic and basaltic dykes in an E-W direction, also traversing Pliocene lavas, so that
they have a late Pliocene Pleistocene age.
Acknowledgements: Project Anillos de Investigación en Ciencia Antártica (ATC-105)
Project Conicyt/Fondap 15090013, and Project Fondecyt 1130006. We express our
gratitude to J. Maclean, C. Münchmeyer and P. Quercia..
References. Barreda, V.D.; Palazzesi, L.; Marenssi, S., 2009. Palynological record of the Paleogene Río
Leona Formation (southernmost South America): Stratigraphical and
paleoenvironmental implications. Review of Palaeobotany and Palynology 154 (1-
4): 22-33. Bostelmann, E.; Le Roux, J.P.; Vasquez,A.; Gutiérrez, N.M.; Oyarzún, J.L.; Carreño, C.;
Torres,T.; Otero, R.A.; Llanos, A.; Fanning, C. Mark.; Hervé, F., (in press) Burdigalian deposits of the Santa Cruz Formation in the Sierra Baguales, Austral
(Magallanes) Basin: Age, depositional environment and vertebrate fossils. Andean
Geology. Cecioni, G., 1957. Età della flora del Cerro Guido e stratigrafía del Departamento Última
Esperanza. Bollettino della Società Geologica Italiana 76: 3-16.
Cuitiño, J.I.; Pimentel, M.M.; Ventura Santos, R.; Scasso, R.A., 2012. High resolution
isotopic ages for the early Miocene “Patagoniense” transgression in Southwest
Patagonia: Stratigraphic implications. Journal of South American Earth Sciences 38:
110-122.
Feruglio, E., 1938. El Cretácico superior del lago San Martín (Patagonia) y de las regiones
adyacentes. Physis 12: 293-342.
Furque, G., 1973. Descripción geológica de la Hoja 58b Lago Argentino. Boletín del
Servicio Nacional Minero y Geológico 140: 1-49, Buenos Aires.
Hoffstetter, R.; Fuenzalida, H.; Cecioni, G., (1957). Lexique Stratigraphique Internacional.
Vol. V, Amérique Latine, Fascicule 7, Chile-Chili. Centre Nacional de la Recherche
Scientifique, Paris, 444 pp.
Le Roux, J.P., 2012. A review of Tertiary climate changes in southern South America and
the Antarctic Peninsula. Part 1: Oceanic conditions. Sedimentary Geology 247: 1-
20.
Le Roux, J.P.; Puratich, J.; Mourgues, A.; Oyarzún, J.L.; Otero, R.A.; Torres, T.; Hervé, F.,
2010. Estuary deposits in the Río Baguales Formation (Chattian-Aquitanean),
Magallanes Province, Chile. Andean Geology 37: 329-344.
Malumián, N., 1990. Foraminíferos de la Formación Man Aike (Eoceno, sureste Lago
Cardiel) provincia de Santa Cruz. Revista de la Asociación Geológica Argentina
45(3-4): 364-385.
Malumián, N.; Náñez, C., 2011. The Late Cretaceous–Cenozoic transgressions in Patagonia
and the Fuegian Andes: foraminifera, paleoecology and paleogeography. Biological
Journal of the Linnean Society 103: 269–288.
Marenssi, S.A.; Casadío, S.; Santillana, S., 2002. La Formación Man Aike al sur de El
Calafate (Provincia de Santa Cruz) y su relación con la discordancia del Eoceno
medio en la cuenca Austral. Revista de la Asociación Geológica Argentina 57: 341-
344.
Marenssi, S.A.; Limarino, C.O.; Tripaldi, A.; Net, L.I., 2005. Fluvial systems variations in
the Rio Leona Formation: Tectonic and eustatic controls on the Oligocene evolution
of the Austral (Magallanes) Basin, southernmost Argentina. Journal of South
American Earth Sciences 19: 359-372.
Marenssi, S.A.; Santillana, S.N.; Net, L.I.; Rinaldi, C.A., 2000. Facies conglomerádicas
basales para la Formación Río Leona al sur del lago Argentino, Provincia de Santa
Cruz, Argentina. II Congreso Latinoamericano de Sedimentología y VIII Reunión
Argentina de Sedimentología. Mar del Plata. Resúmenes: 109-110.
Muñoz, J., 1981 Geología y petrología de las rocas ígneas e inclusiones ultramáficas del
sector SW de Meseta Las Vizcachas, Ultima Esperanza, Magallanes, XII Región,
Chile. Santiago: Universidad de Chile, Departamento de Geología.
Otero, R.A.; Oyarzún, J.L.; Soto-Acuña, S.; Yury-Yáñez, R.E.; Gutiérrez, N.M.; Le Roux,
J.P.; Torres, T.; Hervé, F., (submitted) Neoselachians and Chimaeriformes
(Chondrichthyes) from the latest Cretaceous-Paleogene of Sierra Baguales,
southernmost Chile. Chronostratigraphic, paleobiogeographic and
paleoenvironmental implications. Journal of South American Earth Sciences.
Piatnitzky, A., 1938. Observaciones geológicas en el oeste de Santa Cruz (Patagonia).
Boletín de Informaciones Petroleras 165: 45-85.
Parras, A.; Dix, G.R.; Griffin, M., 2012. Sr-isotope chronostratigraphy of Paleogene-
Neogene marine deposits: Austral Basin, southern Patagonia (Argentina). Journal of
South American Earth Sciences 37:122-135.
Torres, T.; Cisterna, M.; Llanos, A.; Galleguillos, H.; Le Roux, J.P., 2009. Nuevos registros
de Nothofagus Bl. en Sierra Baguales, Última Esperanza, Patagonia, Chile. XII
Congreso Geológico Chileno, Santiago, Chile. Extended Abstracts S12-S19.