Abstract

The Cosmelli Plateau, south of lake General Carrera, includes the most extensive sedimentary infill of the Cenozoic retroarc in Aysén. The early to middle Miocene Santa Cruz Formation is the thickest of its record, encompassing more than 800 m of monotonous continental deposits, bearing abundant fossil mammals. Two localities, Pampa Castillo and Pampa Guadal, host the majority of these occurrences but the temporal range and biostratigraphic correlation between exposures remains largely unexplored. Sedimentological analysis of five lithostratigraphic sections at both localities allowed the identification of eleven lithofacies and two facies association, characterizing sandy meandering fluvial systems with subordinated fine-grained deposits. Fossil remains were exhumed from five of these lithofacies, interpreted as massive and laminated sand sheets, and fine-grained siltstones of the floodplain deposit association. In Pampa Castillo, the total thickness of the unit reaches up to 254 m with higher concentrations of fossiliferous exposures at the lowermost 5 to 15 m. A published U-Pb zircon age from a reworked tuff bed gives an MDA of 18.7±0.3 Ma for these basal levels, but geochronologic control for the top of the succession is still unavailable. The composited faunal list, based on previous collections and our new unpublished materials, suggests the presence of forty-four species, although some of them demand future corroboration. Interesting new additions includes Microbiotherium tehuelchum, Stilotherium dissimile (Bostelmann, this congress), Peltephilus sp., Stenotatus sp., and Interatherium cf. I. robustum. At Pampa Guadal, the section reaches 250 m with fossiliferous levels mostly occurring in the middle portion, between 120 and 150 m. A total of nineteen species have been identified including the most complete Chilean remains of Astrapotherium magnum, Nesodon imbricatus and Theosodon sp. New U-Pb detrital zircon data yielded MDAs of 18.73 ± 0.49 Ma for the top of the underlying Guadal Formation, and 17.50 ± 0.17 to 17.18 + 0.12/-0.14 Ma constraining the fossiliferous levels. Cluster analysis using the Jaccard index confirms the Santacrucian age signature of both faunal assemblages as previously described, discarding the artificial “Notohippidian'' age suggested by Folguera et al. (2019) for the Pampa Castillo fauna. An undetermined Propalaeohoplophorinae, and Nesodon imbricatus, Adinotherium ovinum, Neoreomys australis and Proeutatus oenophorus are the most frequent species recovered at both localities but a small fraction of western endemic elements like Stenotatus planus (Pampa Guadal) and Picturotherium sp. (Pampa Castillo), attest for a marginal biogeographic provincialism. Acknowledgements: We thank the Díaz-Seguel family and Benjamín Molina (CONAF) for their logistic support in Puerto Guadal. CONAF and Tompkins Conservation facilitates access to the localities inside Patagonia National Park. Esteban Salazar gives valuable assistance with the calibration of the U-Pb ages. Financial support comes from the ANID national doctoral grant to J.E.B.T., and projects FONDECyT 1130006 and FNDR 40007715-0 of the Aysén Regional government.