Early Eocene Ar/ Ar age for the Pampa de Jones plant, pdw3/2010_Wilf_et_al_   p³sitos asociados

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  • Introduction

    A significantly improved geochronologic frame-work for classic Eocene fossil deposits in ArgentinePatagonia is emerging from paleomagnetic stratigra-phy and analytically precise 40Ar/39Ar radioisotopic

    dating (Kay et al., 1999; Wilf et al., 2003, 2005a; Gosses,2006). The improved age control provides the neces-sary precision for diverse topics to be investigated, in-cluding clade dating, biogeographic patterns, andcomparisons of climate change and biodiversity withdisparate regions of South America and other conti-nents (Petrulevicius and Nel, 2005; Wilf et al., 2005b,2009; Zamaloa et al., 2006; Barreda and Palazzesi, 2007;Cione and Bez, 2007; Crisp et al., 2009; Petrulevicius,2009; Sarzetti et al., 2009; Tejedor et al., 2009).

    The Pampa de Jones fossil locality of NeuqunProvince, Patagonia, Argentina (also in the literatureas Nahuel Huapi and Nahuel Huapi Este), con-tains an informative biota but is not reliably dated,limiting its usefulness in a broader evolutionary andstratigraphic framework. The site is an accessible butstratigraphically isolated roadcut outcrop of the

    AMEGHINIANA (Rev. Asoc. Paleontol. Argent.) - 47 (2): 207-216. Buenos Aires, 30-06-2010 ISSN 0002-7014

    Asociacin Paleontolgica Argentina AMGHB2-0002-7014/10$00.00+.50

    1Department of Geosciences, Pennsylvania State University,University Park, Pennsylvania 16802, USA. pwilf@psu.edu2Department of Geoscience, University of Wisconsin, Madison,Wisconsin 53706, USA. bsinger@geology.wisc.edu3Facultad de Ciencias Exactas y Naturales, Universidad deBuenos Aires, Intendente Giraldes 2620, 1428 Buenos Aires,Argentina. mzamaloa@ege.fcen.uba.ar4Department of Earth Sciences, Denver Museum of Nature &Science, Denver, Colorado 80205, USA. KJohnson@dmns.org5Museo Paleontolgico Egidio Feruglio, CONICET, 9100 Trelew,Chubut, Argentina. rcuneo@mef.org.ar

    Early Eocene 40Ar/39Ar age for the Pampa de Jones plant,frog, and insect biota (Huitrera Formation, NeuqunProvince, Patagonia, Argentina)Peter WILF1, Brad S. SINGER2, Mara del Carmen ZAMALOA3, Kirk R. JOHNSON4 and N.Rubn CNEO5

    Key words. Early Eocene. Huitrera Formation. Geochronology. Paleobotany. Patagonia. Neuqun. Argentina.

    Palabras clave. Eoceno temprano. Formacin Huitrera. Geocronologa. Paleobotnica. Patagonia. Neuqun. Argentina.

    Abstract. The Pampa de Jones fossil site, a stratigraphically isolated roadcut near the northeastern shore ofNahuel Huapi Lake in Neuqun Province, Argentina, holds a rich fossil biota including a macroflora, a mi-croflora, insects, and most famously, an ontogenetic series of pipid frogs. The site exposes tuffaceous mudstoneand sandstone beds of probable lacustrine origin, considered to belong to the volcanic Huitrera Formation.However, there have been no reliable age constraints for the fossil assemblage. We undertook laser fusion analy-ses of sanidine and biotite crystals occurring in a tuff layer found 4.4 m above the main fossil horizon. Twenty-eight sanidine crystals yielded an 40Ar/39Ar age of 54.24 0.45 Ma that is preferred over our biotite age of 53.64 0.35 Ma. Pampa de Jones is thus the oldest well-dated Eocene fossil site in Patagonia, predating two other re-cently 40Ar/39Ar-dated sites: Laguna del Hunco (51.91 0.22 Ma) and Ro Pichileuf (47.46 0.05 Ma). The im-proved age control makes possible a finer scale of evolutionary hypothesis testing and turnover analysis in theregion. The age is concordant with the sites placement in the Huitrera Formation and a depositional origin re-lated to Early Paleogene arc volcanism, and it correlates to an interval of significant climate fluctuations follow-ing the Paleocene-Eocene boundary.Resumen. EDAD 40AR/39AR PARA LA BIOTA DE PLANTAS, ANUROS E INSECTOS DEL EOCENO TEMPRANO DE PAMPA DEJONES (FORMACIN HUITRERA, PROVINCIA DEL NEUQUN, ARGENTINA). La localidad de Pampa de Jones es un aflo-ramiento estratigrficamente aislado, cercano a la costa noreste del Lago Nahuel Huapi en la Provincia delNeuqun, Argentina. Contiene una rica biota fsil que incluye macroflora, microflora, insectos y una reconoci-da serie ontogentica de ppidos. La secuencia estratigrfica consiste de fangolitas y areniscas tufceas de prob-able origen lacustre, asignada a la Formacin Huitrera. La ausencia de datos geocronolgicos directos ha impe-dido la estimacin de edades confiables para esta paleobiota. En este trabajo se analizan por fusin lser loscristales de sanidina y biotita presentes en un nivel de toba ubicado a 4.4 m por encima del principal horizontefosilfero. Veintiocho cristales de sanidina arrojaron una edad 40Ar/39Ar de 54.24 0.45 Ma, la cual se prefiere ala edad de 53.64 0.35 Ma estimada a partir de la biotita. La biota de Pampa de Jones es la ms antigua delEoceno de Patagonia datada radiomtricamente, y precede a las dos localidades Eocenas datadas en la regin:Laguna del Hunco (51.91 0.22 Ma) y Ro Pichileuf (47.46 0.05 Ma). El control cronolgico ajustado permi-tir evaluar hiptesis evolutivas y analizar recambios en la regin con una mayor resolucin temporal. La edadobtenida concuerda con la ubicacin de la secuencia dentro de la Formacin Huitrera y con el origen de los de-psitos asociados al volcanismo de arco del Palegeno temprano, y se correlaciona con un lapso de significati-vas fluctuaciones climticas ocurridas con posterioridad al pasaje Paleoceno-Eoceno.