A Late Eocene age proposal for the Loreto Formation (Brunswick Peninsula, southernmost Chile), based on fossil cartilaginous fishes, paleobotany and radiometric evidence Una edad eocena tard赤a propuesta para la Formaci車n Loreto (pen赤nsula de Brunswick, extremo sur de Chile), basada en peces cartilaginosos f車siles, paleobot芍nica y evidencia radi車gena

作者：Rodrigo A Otero; Teresa Torres; Jacobus P Le Roux; Francisco Herve

来源：Andean Geology, 2012.

• Peces cartilaginosos fosiles
• Paleobotanica
• Shrimp U-Th-Pb
• Priaboniano
• Formacion Loreto
• Extremo sur de Chile
• Fossil cartilaginous fishes
• Paleobotany
• Shrimp U-Th-Pb
• Priabonian
• Loreto Formation
• Southernmost Chile

We present new data on the paleoichthyology, paleobotany and radiometric results of the Loreto Formation in the Brunswick Peninsula of southernmost Chile, that allow us to propose a Late Eocene age. The rich diversity of fossil cartilaginous fishes (Chondrichthyes, Elasmobranchii) recognized in upper levels of this unit includes the taxa Carchar赤as aff. %26apos;hopef (Agassiz), Odontaspis sp., Carcharoides catticus (Philippi), Striatolamia macrota (Agassiz), Anomotodon sp., Macrorhizoduspraecursor (Leriche), Galeorhinus sp., Abdounia sp., Hexanchus sp., Squatina sp., Hexanchidae indet.,Myliobatis sp., Myliobatoidea indet., and Ischyodus dolloi Leriche. This assemblage has clear ecological affinities with Eocene Tethyan fauna previously described in the Northern Hemisphere, and also has common elements with Eocene cartilaginous fishes from Antarctica. Additionally, a paleobotanic study of this unit identified leaf imprints ofAsplenium sp., Pteris sp., Podocarpus sp., and abundant angiosperms including Nothofagus lanceolata Dus谷n, N. simplicidens Dus谷n, N. variabilis Dus谷n, N. cf. alessandri Espinosa, N. subferruginea (Dus谷n), Hydrangea sp. and Phyllites spp. Wood remains of Nothofagoxylon scalariforme Gothan and Araucariaceae cf. Araucarioxylon Kraus were also identified. Additionally, pollen grains indicate gymnosperms and angiosperms: Podocarpidites otagoensis Couper, Retitricolpites sp., Tricolpites sp., Liliacidites sp., Polyporina sp., Nothofagidites cincta Cookson, and Nothofagidites cranwellae Couper, having affinities with Eocene florae, and being consistent with the age of the fossil fishes. Finally, a SHRIMP U-Th-Pb analysis of two samples collected from the studied beds provided thirty-eight and sixty zircon grains, indicating a clear main peak at 36.48㊣0.47 Ma (MSWD=1.5) and 36,73㊣0.50 Ma (MSWD=0.65). The integrated results indicate that the upper part of the Loreto Formation has a minimum Priabonian age, supporting previous reassignations of this part of the formation into the Late Eocene, and differing from the Oligocene age proposed in its original definition. El presente estudio integra nueva evidencia paleoictiol車gica, paleobot芍nica y resultados radiom谷tricos que en conjunto permiten proponer una edad eocena tard赤a para la Formaci車n Loreto, en la pen赤nsula de Brunswick del extremo sur de Chile. La rica diversidad de peces cartilaginosos f車siles (Chondrichthyes, Elasmobranchii) reconocida en niveles superiores de la mencionada unidad incluye los taxa Carcharias aff. hopei (Agassiz), Odontaspis sp., Carcharoides catticus (Philippi), Striatolamia m