Effects of depositional environment, asssemblage taphonomy and diagenesis on vertebrate skeletal preservation in a crevasse splay sandstone, upper Cretaceous Hell Creek Formation, eastern Montana
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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, taphonomy and diagenesis of an assemblage of vertebrate skeletal elements from a sandstone in the Late Cretaceous Hell Creek Formation are investigated in order to decipher relations between sandstone diagenesis and skeletal element fossilization. Fieldwork included taphonomic data collection, section measurement and description as well as sample collection. The rock and fossil bone samples were 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 sandstone including the fossil bones indicates physical compaction, fossil bones do not exhibit signs of compaction. Barite is an unusual authigenic mineral found in fossil bones, suggesting collagen degradation during early diagenesis facilitated unique diagenetic microenvironment within bones. 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. In order to maximize the information, not only the fossil bones, but surrounding rocks need to be analyzed because 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.