DIAGENETIC AND TAPHONOMIC IMPLICATIONS OF AUTHIGENIC MINERALIZATION IN FOSSIL BONES FROM THE UPPER CRETACEOUS TWO MEDICINE AND JUDITH RIVER FORMATIONS, MONTANA

The Campanian Judith River (JRF) and Two Medicine (TMF) Formations of north-central and northwestern Montana (respectively) contain concurrent deposits of upland terrestrial (TMF) and coastal lowland and marine lithofacies (JRF). Previous studies have explored the abundance and quality of vertebrate fossils preserved within these formations. This study focuses on authigenic mineralization within the primary and secondary void spaces of said vertebrate fossil bones in an attempt to examine taphonomic and diagenetic processes. Samples were collected from lacustrine, fluvial and shallow marine depositional environments and examined in thin section using polarized light microscopy and SEM-EDS. Presence/absence of void infill was recorded and analyzed using binary similarity coefficients in an attempt to establish any congruencies between sites. Additionally, previously collected Rare Earth Element (REE) concentration data were re-examined in an attempt to explore any relationships that might exist between groundwater geochemistry and authigenic cement formation. Bones collected from fine-grained lacustrine deposits (mudstones) in the TMF exhibit an abundance of authigenic cements, though the nature of these cements vary between sights; bones recovered from terrestrial mudstones contain chlorite cements while bones collected near the paleo-shoreline of the Claggett Sea contain calcite cements. Both sites exhibit voids containing small amounts of iron oxides, authigenic sulfates (Ba,Sr) and fine detritus (clays). Samples collected from lacustrine sites in the JRF exhibit many empty voids lined with minimal amounts of calcite, iron oxides and/or fine detritus. In contrast, fine to medium-grained sandstone bodies in the TMF tend to exhibit complete fills dominated by calcite or iron oxides while fluvial deposits of similar lithology in the JRF are dominated by coarse to fine detrital fill (sands to clays) and hematite/iron oxide cements. The marine site in the JRF is dominated by authigenic phosphate and exhibits a high degree of pyritization in association with iron oxides. Calcite and small amounts of authigenic sulfates are also present. These observations indicate that authigenic mineralization preserves paleoenvironmental as well as taphonomic and diagenetic information.