AUTHIGENIC MINERALOGY AND ELEMENT DISTRIBUTIONS OF VERTEBRATE MICROFOSSILS FROM THE HELL CREEK (LATEST CRETACEOUS) AND TULLOCK (PALEOCENE) FORMATIONS OF EASTERN MONTANA

The Hell Creek and Tullock formations of eastern Montana together record terrestrial sedimentation across the Cretaceous-Paleogene (K-Pg) boundary and consequently play a critical role in understanding how biological communities responded to the K-Pg mass extinction event on land. Despite their importance and long history of paleobiological research, little is known about the diagenetic history and mineralogical make-up of the vertebrate fossils recovered from these two formations. Here we present a preliminary investigation into the pore-filling mineralogy and element distributions of vertebrate microfossils from two Hell Creek and two Tullock localities. A minimum of four bones per locality were cut into ~30 µm thin sections and polished. Minerals present within voids were specifically targeted using energy-dispersive X-ray spectroscopy (EDS) and the element distributions of both the voids and the bone itself were mapped using micro X-ray fluorescence (µXRF).

Primary voids in bones from both formations were usually empty, although many exhibited detrital infills consisting of clay and silt-sized grains of quartz and feldspar. Authigenic minerals appear to be limited to pyrite and iron oxide framboids, which vary in abundance both within and between localities, and occur as globular clusters along the edge of bone pores. These framboids, particularly those which have altered to iron oxide, often occur in close association with clay minerals. Neither authigenic mineralogy nor bulk geochemistry show a noticeable change across the K-Pg boundary, which may suggest that fossil vertebrates from both the Hell Creek and Tullock formations experienced similar diagenetic conditions on the path to fossilization. Though these results are preliminary, they represent the first attempt to characterize the diagenetic history of vertebrate fossils across the K-Pg boundary from the Hell Creek region of eastern Montana, and they set the stage for future research into post-depositional changes that influence our understanding of the K-Pg mass extinction.