A COMPARATIVE INVESTIGATION OF BONE DIAGENESIS FROM MARINE AND TERRESTRIAL FACIES IN THE CAMPANIAN JUDITH RIVER FORMATION OF NORTH-CENTRAL MONTANA

The richly fossiliferous Judith River Formation (JRF) is widely exposed in the Missouri Breaks of north-central Montana. Vertebrate fossils in the marine portion of the JRF are locally concentrated on discontinuity surfaces that separate back-stepping fourth-order sequences. Because bones include abundant primary void spaces that can readily accommodate authigenic mineralization, fossil bone provides a unique opportunity to study early diagenesis. This study seeks to (1) compare authigenic signatures in skeletal material from a variety of paleoenvironments, and (2) to characterize authigenesis on a fourth-order marine sequence boundary.

This study focuses on a discontinuity surface that is extremely fossiliferous with regard to vertebrate hardparts, and is marked by abundant shark teeth, bones of marine reptiles, fish, and dinosaurs. Authigenic cements in cracks and primary voids in ~100 bone specimens were examined using polarized light microscopy and SEM-EDS. Fossil material associated with the sequence boundary exhibits a complex history of mineralization. Authigenic phosphate is present in most bones, and on occasion fills entire voids to the exclusion of other authigenic cements. Calcite and pyrite are also abundant. Fossil material from associated terrestrial localities tends to show less extensive cementation, with one terrestrial assemblage virtually devoid of authigenic cements. Bones from terrestrial localities also do not show the extensive phosphatization seen in marine fossils, and calcite and pyrite are present to a limited extent in only one of the nonmarine localities.

Authigenic cements in the JRF record track environments and allow for correlation between fossil bones and depositional settings. Moreover, cements in fossil bone provide a novel means of characterizing authigenesis associated with a marine sequence boundary. To determine whether the authigenic signature of this surface is distinct from the background marine signature, it will be necessary to characterize authigenesis in contemporaneous fossil material not associated with the discontinuity surface (collection is on-going). This in turn will yield critical insights into the formative history of the surface (e.g., degree of exhumation/reworking, spatial fidelity of bioclasts, etc.).