DIATOM FOSSILS FROM THE ASHFALL FOSSIL BEDS, INDICATE AN ALLUVIAL MEGA-FAN BURIED DURING THE WET SEASON BY THE BRUNEAU-JARBIDGE ERUPTION (∼11.86 MA)

Abstract Diatom fossils offer reliable means of evaluating biotic, climatic, and ecological conditions of past environments. Here we present new insights in reconstructing the mid-Miocene paleoenvironment of the Ashfall Fossil Beds in Nebraska, USA. Extensive excavation of megafauna and flora at the Ashfall site has provided a well-preserved glimpse into the Clarendonian Land Mammal age of the North American Plains. From sedimentary evidence the environment at the site has been described as a small waterhole in a seasonally wet depression located in a stream bed. Here we report a diatom fossil assemblage containing 38 species spanning 30 genera found in the ash and underlying sand formations at the Ashfall site. We also provide insights into environmental conditions prior to and during the eruption and deposition of the ash, including water chemistry. Diatom assemblages from the underlying sand and the ash deposits indicate drastically different depositional environments. We find that the diatom fossils robustly separate across the sand-ash transition and propose two possible explanations for the differences observed in the assemblages. Either the diatom assemblages across the sand-ash transition represent a dry to wet season preservation, or the ash from the eruption changed the water chemistry and the diatom assemblage changed in response to changing water conditions. It is also possible both scenarios have some influence on diatom composition.