A COMPARATIVE STUDY OF AUTHIGENIC MINERALIZATION AND RARE EARTH ELEMENT GEOCHEMISTRY OF VERTEBRATE MICROFOSSIL ASSEMBLAGES IN THE CAMPANIAN JUDITH RIVER FORMATION OF MONTANA

The Judith River Formation (JRF) of north-central Montana consists of shallow marine and coastal plain deposits that accumulated on the western margin of the Cretaceous Interior Seaway. JRF facies preserve abundant vertebrate microfossil assemblages that are ideal for taphonomic studies. This study provides a comprehensive analysis of vertebrate microtaphonomy within the JRF, focusing on the microscopic mineralogical details of bone and highlighting the differences in authigenic mineralization and REE geochemistry among several environments. Fossil bones derived from JRF microfossil assemblages record the succession of authigenic minerals formed during early diagenesis in primary void spaces. Physical and chemical alteration of the bone mineral provides further insight into the dynamic diagenetic environment that operated in the burial environment.

Samples were collected from two paralic sites and one shoreface site to explore and compare preservational patterns related to variations in microbial activity, salinity, Eh, pH, and porosity and permeability. Using a combined analytical approach, general microtaphonomic features (e.g., weathering, bioerosion), authigenic cements filling cracks and voids, and bone replacement in ~80 bone specimens from three vertebrate microfossil sites were examined in thin section using both polarized light microscopy and SEM-EDS. REE concentrations and fractionation patterns were analyzed using LA-ICP-MS.

Preliminary findings indicate that both REE signature and the nature and amount of authigenic cementation vary among the sampled sites, suggesting that these traits can be used to track environments and link vertebrate bioclasts to specific depositional settings. Bones recovered from coastal mudstones are MREE enriched, and voids are characterized by multiple rinds of detrital clay and a paucity of authigenic cements. Bones preserved in shallow marine sandstones show abundant pyritization and phosphatization and are HREE enriched. Authigenic Ca-phosphate, framboidal pyrite, and calcite are the most common fills. These observations suggest that REE concentrations and authigenic cements can be used together to provide valuable insights into the conditions of early diagenesis within the richly fossiliferous Judith River Formation.