AUTHIGENIC CEMENTS IN FOSSIL BONES FROM THE UPPER CRETACEOUS TWO MEDICINE AND JUDITH RIVER FORMATIONS, MONTANA

The Campanian Two Medicine (TMF) and Judith River (JRF) Formations of northwestern and north-central Montana (respectively) preserve coeval deposits of the semi-arid alluvial uplands (TMF) and the more humid coastal plain lowlands (JRF). Previous field-based taphonomic studies have served to clarify the abundance and quality of vertebrate fossils preserved in both formations. In this study we shift the focus to the inside of TMF and JRF bones and investigate the development and distribution of authigenic cements. Samples were collected from a variety of terrestrial and shallow marine depositional environments in an effort to explore potential preservational patterns related to variations in salinity, redox potential, pH, and porosity and permeability. Authigenic cements filling cracks and voids in ~50 bone specimens were examined in thin section using both polarized light microscopy and SEM-EDS.

Preliminary findings indicate that the nature and amount of void infill varies among the sampled sites, suggesting that authigenic cements track environments at both the facies and formation scale. Fossil bones collected from fine- to medium-grained fluvial sandstone bodies in the TMF tend to display complete fills dominated by calcite or iron oxides. Void spaces in bones recovered from nearby fine-grained floodplain deposits (mudstones) typically display detrital clay rinds and are only partially filled with authigenic sulfates (Ba,Sr) or iron oxide cements. Authigenic chlorite has only been found in TMF bones preserved in terrestrial mudstones. Bones recovered from a fine-grained TMF locality in coastal plain facies situated near the paleo-shoreline of the Claggett Sea are characterized by voids predominantly filled with calcite and pyrite. In contrast, the voids in bones recovered from coastal mudstones of the JRF are characterized by multiple rinds of detrital clay and a paucity of authigenic cements. Bones preserved in shallow marine sandstones of the JRF show abundant pyritization, and exhibit no evidence of calcite cementation. These observations suggest a correlation between depositional environment and authigenic mineralization.