IMPLICATION OF STABLE ISOTOPE SIGNATURE ON THE DIAGENESIS OF SANDSTONE AND ENCLOSED FOSSIL BONES; THE UPPER CRETACEOUS HELL CREEK FORMATION NEAR GLENDIVE, MONTANA

Although fossilized vertebrate skeletal remains have been utilized to interpret the ancient life, processes of such preservation remain poorly understood. Since fossilized skeletal remains and enclosing sandstone experience similar diagenetic environment (i.e. pressure, temperature, and pore-fluid interaction), sandstone diagenesis is the key to understand bone diagenesis. Previous studies indicate that carbonate precipitations are important processes for preservation of skeletal remains. Such carbonate precipitates exhibit various fabric, suggesting that multiple carbonate precipitation events took place throughout diagenesis. Since fossilized skeletal remains contain carbonate phase, the effect of carbonate precipitation needs to be investigated. The objectives of this study are two-fold; 1) to distinguish the origins of various carbonate phases using stable isotope compositions, and 2) to assess the diagenetic alteration of structural carbonate in fossil bone.

The fossil bone and surrounding sandstone samples were collected from the Hell Creek Formation near Glendive, MT. In order to distinguish the timing/origin of various carbonate precipitation events, stable isotope compositions of structural carbonate of fossil bone and two phases of permineralizing concretion were analyzed using Gas Bench II at the University of Arkansas Stable Isotope Laboratory. Additionally, a concretion from channel sandstone 3m below the bone bearing sandstone was analyzed. Preliminary results indicate that siderite phase of permineralizing concretion has significantly higher δ¹³C and δ¹⁸O values (+2.1‰ VPDB for δ¹³C and +25.93‰ VSMOW for δ¹⁸O). The δ¹⁸O value of fossil bone carbonate was 4.21‰ higher than adjacent concretion, suggesting that the diagenetic effect on bone carbonate isotope composition is minimal. The δ¹³C value for the concretion from channel sandstone was lower than the fossil bone and associated concretion (-22.39‰ VPDB), suggesting the origin of the concretion in the lower sandstone is not related to that of the bone bearing sandstone.

This study demonstrates the use of the isotopic composition for the study of diagenetic history of sandstones and enclosed fossilized skeletal remains.