EFFECTS OF DEPOSITIONAL ENVIRONMENT, ASSEMBLAGE TAPHONOMY AND DIAGENESIS ON VERTEBRATE SKELETAL PRESERVATION IN A CREVASSE SPLAY DEPOSIT, UPPER CRETACEOUS HELL CREEK FORMATION, EASTERN MONTANA

Although a great deal of paleontological information is derived from analyzing fossilized skeletal remains, the fossilization process of vertebrate skeletal remains is poorly understood. Depositional environment, assemblage taphonomy and diagenesis of a fossil assemblage within Late Cretaceous Hell Creek Formation are investigated in order to decipher relationships between sandstone diagenesis and fossilization. Rock and fossil bone samples were collected in Glendive, MT and analyzed by petrography, cathodoluminescence, scanning electron microscopy (SEM) and X-ray diffractometry (XRD).

The depositional environment including the fossil assemblage is interpreted as a crevasse splay based on the facies association of repeating sequences of mudrock and fine sandstone overlying channel and levee deposits. Taphonomic data suggest the bones were exposed on the surface up to 15 years before burial, and the assemblage is time-averaged. Although sandstones that hosts the fossil bones indicates physical compaction, bones do not exhibit signs of compaction. Barite authigenic mineral found in fossil bones suggests collagen degradation during early diagenesis facilitated unique diagenetic microenvironment within bones, such as sulfate reduction. Calcite and siderite cements were precipitated multiple times, suggesting alkaline pore-fluid conditions were common during diagenesis. However, oversized pores and highly altered grains indicate that porefluids were acidic during late diagenesis; acidic pore-fluid conditions are likely due to the organic acid produced by degradation of organic molecule at higher diagenetic temperatures. Fossil bone minerals are altered due to 1) early pore-fluid infiltration combined with dehydration/rehydration processes, 2) saline pore-fluid infiltration and 3) increased temperature and pressure during deeper burial.

This study demonstrates that petrography, XRD and SEM/EDS together can help decipher the diagenetic history of fossilized bones. This study also illustrates how, surrounding rocks need to be analyzed, in addition to bone since surrounding rocks record different sets of diagenetic processes such as calcite precipitation in the vadose zone, siderite precipitation in the phreatic zones, acidic pore-fluid condition during mesogenesis.