Studies on the effects of volcanic ash on fauna of the terrestrial realm have been numerous, however, this phenomenon has gone less well studied in the marine realm. Five localities within the Puerto Madryn Formation (Valdes Peninsula, Chubut Province, Argentina) were investigated in order to assess the effects of distal volcanic ash deposition on shallow marine communities during the middle Miocene (Tortonian). All five localities expose shallow marine deposits, and contain invertebrate fossils including abundant decapod crustaceans and bivalve mollusks. Sediments and fossils were analyzed using multiple geochemical and visual methods including scanning electron microscopy (SEM), electron dispersive spectroscopy (EDS), X-ray diffraction spectroscopy (XRD), petrographic microscopy, and reflected light microscopy. The sediments were found to uniformly contain a high percentage of very fine volcanic ash. Three previously unstudied localities including two paleosurfaces and a stratigraphic section which record a single mass mortality event were documented. Two paleosurfaces previously described by three of the authors (SC, RMF, CES) were revisited in order to assess the cause of death of the fossil assemblages at those sites. Although the field locality lies a minimum distance of 650 km from the Andean volcanic front, the effects of ash were nonetheless catastrophic; the crustaceans are interpreted to have been the victims of acute respiratory failure due to the introduction of volcanic ash to the branchial chamber.