Cara L. Harwood
Advisor: Ray Rogers
Senior Honors Thesis: 2006
Authigenic Mineralization and Geochemical Taphonomy of Vertebrate Microfossils from the Upper Cretaceous Judith River Formation of Montana
Fossil bone is a unique medium in the study of early diagenesis because bone tissue becomes enriched in rare earth elements and includes void space that can readily accommodate authigenic mineralization. Previous studies of bone diagenesis, however, have either focused on vertebrate remains from an archaeological perspective, or have been limited to diagenesis within one paleoenvioronment, resulting in a gap in understanding of early diagenetic and taphonomic processes acting on vertebrate material across environments. The objective of this study was to compare vertebrate material from marine and terrestrial localities in the Upper Cretaceous Judith River Formation, with an emphasis on authigenic cementation and rare earth element concentration and fractionation patterns, with the goal of clarifying the relationships between bone mineral, trace elements, authigenic minerals, and water in the enclosing sediment during diagenesis.
The richly fossiliferous Campanian Judith River Formation (JRF) is widely exposed across much of north-central Montana, and is composed of coastal lowland and marine lithofacies. The ˜180 m thick unit comprises the distal reaches of an eastward-thinning clastic tongue that accumulated on the western margin of the Cretaceous Western Interior Seaway. The JRF is a heterolithic composite of silty claystones, siltstones, and fine- to medium-grained sandstones of fluvial, tidal, and shallow marine origin. Authigenic cements and rare earth element (REE) signatures in ˜110 fossil bones from two terrestrial sites and one marine site were analyzed using LA-ICP-MS, petrographic microscopy, and SEM-EDS to explore and compare preservational signatures.
Results suggest that rare earth elements and authigenic cements provide site-specific signatures in fossils bones, characterize the chemistry of pore-waters during diagenesis, and allow determination of the degree of mixing of a fossil assemblage. Fossil material preserved in shallow marine sandstones displays extensive phosphatization and pyritization, and is interpreted to be highly mixed (high variance in REE fractionation patterns). Fossil material from one terrestrial locality has a paucity of authigenic cements, and is consistently MREE enriched (low variance). The other terrestrial locality preserves fossil material with a more diverse suite of authigenic minerals including calcite, Ba/Sr sulfates, pyrite, and ankerite. Fossils are MREE/ HREE enriched, and also are not interpreted to have undergone significant pre-burial mixing.
Differences in authigenic cementation patterns and REE signatures are interpreted to represent a corresponding difference in physical and chemical conditions at and below the sediment-water interface. The authigenic cements and REE signatures indicate contrasting geochemical conditions, suggesting that they record different stages of fossilization, with rare earth elements recording the signature of the environment of deposition and authigenic cements indicative of the burial environment. In addition to characterizing the environment of fossilization, REE and authigenic cements used together may provide information about the formative history (i.e. degree of exhumation, reworking) of a fossil assemblage. Investigation of the relationships between preservation of vertebrate fossil material, in terms of geochemical taphonomy and authigenic mineralization, and preservational environment provides valuable information regarding geochemisty, diagenesis, and taphonomy in the Campanian Judith River Formation of Montana.