Multi-proxy analysis of the enigmatic site Arenas Blancas (Buenos Aires Province, Argentina) and its repercussion on the Pampean biostratigraphy

TOMASSINI, RODRIGO; MONTALVO, CLAUDIA; BEILINSON, ELISA; BARASOAIN, DANIEL; SCHMIDT, GABRIELA; CERDEÑO, ESPERANZA; ZURITA, ALFREDO; BONINI, RICARDO; MIÑO-BOILINI, ANGEL; RASIA, LUCIANO; GASPARINI, GERMÁN

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Jornada; XXXV Jornadas Argentinas de Paleontología de Vertebrados; 2023

Museo Egidio Feruglio

Arenas Blancas is a poorly known site located in the lower reach of the Chasicó creek (Buenos Aires Province, Argentina), with great relevance from a biostratigraphic viewpoint. The Macrochorobates scalabrinii Biozone was defined from this site, proposed as the biostratigraphic basis of the early Huayquerian Stage/Age, late Miocene. However, the geological context and faunal record of this site were never studied in detail. Now, we perform a multi-proxy analysis of the Arenas Blancas site, as well as a nearby site so-called Curva de la Vaca (without fossils), and provide new interpretations on their origin, age, and biostratigraphy. Sedimentological, stratigraphic, and geomorphological characteristics of both sites suggest that the sequences include fluvial/alluvial deposits that represent Quaternary terraces; it is highlighted that, at the moment, no Quaternary fossils were founded in these sites. The taxonomic analysis of the Arenas Blancas faunal assemblage reveals 14 mammal taxa, together with some fishes, reptiles, and birds. This assemblage is correlated with the fauna from the Cerro Azul Formation assigned to the Chasicoan Stage/Age (late Miocene), also recovered from the lower reach of the Chasicó creek. Some mammals are abundant (e.g. Chasicotatus ameghinoi, Vetelia perforata, Typotheriopsis chasicoensis) and one taxon (Cardiatherium chasicoense) is considered as typical of the Chasicoan Stage/Age in the Pampean region. The taphonomic evaluation indicates that the assemblage is constituted by reworked specimens. It is proposed that fluvio/alluvial events, occurred during the late Pleistocene-Holocene, eroded the upper Miocene substrate and reworked fossil remains, which results in a significant temporal difference between the sedimentation of the deposits and the fossil content of Arenas Blancas site. The updating data on the taxa used to characterize the Macrochorobates scalabrinii Biozone (including this species) show their low biostratigraphic value, considering their taxonomic status and/or temporal distribution. The available geological and paleontological evidence leads to reject this biozone as a valid biostratigraphic unit. This type of comprehensive study, involving multiple lines of evidence, is of great interest and necessary to make integral evaluations of the vertebrate assemblages preserved in fluvial environments. Particularly, the identification of reworking, a common process in different fluvial/alluvial systems, is essential when biochronological and/or biostratigraphic interpretations are proposed.