TAPHONOMY AND DIAGENESIS OF FOSSIL-BEARING CONCRETIONS IN VOLCANIC ASH FROM SOUTHERN PATAGONIA, SANTA CRUZ PROVINCE, ARGENTINA

A diverse fauna of crustaceans including representatives from eight families of decapods and a new species of isopod (Crustacea: Peracarida) is preserved in concretions with a volcanic ash matrix from southern Patagonia, Argentina. The concretions occur in the Estancia 25 de Mayo Formation (early Miocene, ~20 Ma), Santa Cruz Province, Argentina, and are composed almost entirely of volcanic glass shards, with minor amounts of volcanic mineral fragments. The volcanic source in the nearby Andean chain is interpreted to have decimated the crustacean fauna. The very fine-grained and abrasive nature of volcanic ash coupled with its rapid deposition caused acute respiratory distress and mass mortality. It also facilitated remarkable preservation in many instances, while its high reactivity set the stage for complex and heterogenetic taphonomic and diagenetic pathways. Delicate distal appendage elements, as well as microscopic cuticle architecture are preserved in some instances, while other structures are obliterated by the formation of secondary zeolites. Biological signals, including evidence of respiratory distress, are also preserved. The unusual gaping position of the third maxillipeds in the decapod fossils described by Crawford (2008) and Maguire (2015) is observed, making this the third such occurrence in Cenozoic volcanic rocks in Argentina. This study details the unique taphonomic and diagenetic pathways which led to the formation of fossil-bearing concretions in volcanic ash. Additionally, the biological and ecological effects of volcanic ash deposition on shallow marine communities in a setting proximal to the Andean volcanic front document a major impact on the benthic fauna of the Estancia 25 de Mayo Formation.