Fossil invertebrates of the Patagonian beds in Sierra Baguales, Última Esperanza Province, Magallanes, Chile

GRIFFIN, M.; UGALDE, R.; GENTA ITURRERÍA, S. F.; BOSTELMANN, E.; PARRAS, A.; OYARZÚN, J. L.

Montevideo

Congreso; 3º Congreso Uruguayo de Zoología Prof. Dr. Raúl Vaz-Ferreyra; 2014

Sociedad Zoológica de Uruguay

During the early Miocene an extended transgression referred to as the Patagonian, flooded part of southern South America. In Chile this event is represented by the Guadal Formation in Aysén, and the recently identified Estancia 25 de Mayo Formation in Magallanes. Good exposures of the later occur along the Alto Río Bandurrias in Sierra Baguales, reaching 145 meters thick. Current work in this unit has allowed to: 1) recognize the occurrence of the two members previously described in nearby Argentina (Quién Sabe and Bandurrias members); 2) identify a 1.5 meter thick dacitic tuff (lower pyroclastic level of Cuitiño and Scasso, 2010) as a regionally stratigraphic marker; and 3) collect and identify abundant marine fossils. Preliminary paleoenvironmental analysis suggests subtidal marine environment at the bottom of the unit and intertidal to marginal marine/estuarine environments towards the top. Over 35 identified species includes characteristic biostratigraphic taxa like: Modiomytilus argentinensis, Modiolus arctus, Neopanis cf. N. quadrisulcata, Zygochlamys geminata, Crassostrea hatcheri, Trachycardium puelchum, Patagonicardium philippii, Pleuromeris elegantoides, Pitar? sp., Panopea bagualesia, Offadesma sp., Solariella dautzenbergi, Valdesia dalli, Turritella cf. T. ambulacrum, Ficus (Diconoficus) posadasensis, Calyptraea sp., Terebra quemadensis, Perissodonta ameghinoi, Proscaphella cossmanni, Cominella annae and Kaitoa? patagonica. The biogeographic affinity of the fossil assemblage reveals a typically Atlantic origin, with all species also recorded in the classical upper Patagonian outcrops of the Estancia 25 de Mayo and/or Monte León formations in Argentina. Following isotopic and radiometric dating of the homologous sequences south of Calafate, a 20-19 Ma age is proposed for the unit. The collected specimens are housed in the Invertebrate Paleontological collection of the National Museum of Natural History in Santiago de Chile.