Abstract
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 Carcharias aff. 'hopei' (Agassiz), Odontaspis sp., Carcharoides catticus (Philippi), Striatolamia macrota (Agassiz), Anomotodon sp., Macrorhizodus praecursor (Leriche), Galeorhinus sp., Abdounia sp., Hexanchus sp., Squatina sp., Hexanchidae indet., Myliobatis sp., Myliobatoidea indet., and Ischyodus dolloi Leriche. This assemblage has clear eco- logical 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 of Asplenium sp., Pteris sp., Podocarpus sp., and abundant angiosperms including Nothofagus lanceolata Dusen, N. simplicidens Dusen, N. variabilis Dusen, N. cf. alessandri Espinosa, N. subferruginea (Dusen), 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.
Highlights
The Loreto Formation (Hoffstetter et al, 1957) was previously known by several different informal names such asAraucaria Stufe’, ʻArenaense’, ʻBanco de Gastrópodos’, ʻBanco de Venus’, ʻCapas de Loreto’, ʻFagus Stufe’, ʻMagallaniano’, ʻMagallaniense’, ʻMagellanian beds’, etc
In addition to the relative age obtained from the paleontological evidence, an absolute age of approximately 36-37 Ma was obtained by SHRIMP U-Th-Pb dating of detrital zircons
The presence of Carcharias aff. ‘hopei’ (Agassiz), Striatolamia macrota (Agassiz), Macrorhizodus praecursor (Leriche), Abdounia sp. and Ischyodus dolloi Leriche supports an Eocene age for the hosting strata, while the presence of Carcharoides catticus (Philippi), previously known in Oligocene beds of the Northern Hemisphere, suggests the relative age of the assemblage closer to the Eocene/Oligocene boundary. This diversity seems to be typical of cold to temperate, shallow waters not deeper than 30 m, being consistent with climate and seaway models during the Eocene and previous to the major changes experienced in the Southern Hemisphere during the Oligocene
Summary
The Loreto Formation (Hoffstetter et al, 1957) was previously known by several different informal names such asAraucaria Stufe’, ʻArenaense’, ʻBanco de Gastrópodos’, ʻBanco de Venus’, ʻCapas de Loreto’, ʻFagus Stufe’, ʻMagallaniano’, ʻMagallaniense’, ʻMagellanian beds’, etc. Fasola (1969) was the first to propose a type locality and paralocalities for the Loreto Formation in the Los Ciervos River, Lynch River and Las Minas River valleys This author measured a stratigraphic column in each of these areas and constructed a composite section showing sandstones with variable grain size and hardness, together with sporadic clay, conglomerate, concretionary and fossiliferous beds including carbonized wood, assigned by this author to the Oligocene based on palynomorphs. An abundant fossil assemblage of cartilaginous fish teeth hosted by one particular bed in the studied locality, made it possible to obtain enough samples to interpret the different dental shapes belonging to certain genera and species In this way, identifications based on isolated elements that usually lead to dubious taxonomical interpretations could be avoided. In addition to the relative age obtained from the paleontological evidence, an absolute age of approximately 36-37 Ma was obtained by SHRIMP U-Th-Pb dating of detrital zircons
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