Stratigraphic and paleoenvironmental variations in Rare Earth Element (REE) composition and crystallography of fossil bioapatite
Rare Earth Elements (REE) and unit cell dimension (UCDA) analyses of fossil bioapatite from Cretaceous, Tertiary and Pleistocene age vertebrates were used to identify and interpret changes occurring in the paleoenvironment over time and for stratigraphic correlation. REE composition and the UCD in bioapatite are dependent upon the composition of diagenetic waters, concentrations of REE and other species incorporated into bioapatite during permineralization. REE signature and UCD differences in bioapatite identify distinct intervals in lithologic units. These differences reflect changes in the depositional environment, and can be used to interpret paleoenvironments. Because these distinct intervals represent an averaging of periods of certain depositional environments, they can be correlated over significant areas.
Fossil vertebrate samples were obtained from the Upper Cretaceous, Campanian and Maastrictian deposits of the Pierre Shale, from the Sharon Springs, Gregory, Crow Creek, DeGrey and Verendrye members. These samples were collected in an area between Chamberlain and Pierre, South Dakota, at localities along the banks of the Missouri River. REE signatures and UCD variations were found to be consistent within individual lithostratigraphic units, but are significantly different between these units. Variations in REE signatures and UCD in the Pierre Shale represent changes in the depositional environment and have been used successfully to interpret events during diagenesis. Specific REE signatures and UCD have been shown to extend laterally and act as markers for their units. These markers can be used to discriminate among units for purposes of stratigraphic correlation in the Pierre Shale.
Analysis of REE in fossils from the Pleistocene Fossil Lake, Oregon, also revealed that REE signatures are laterally constant over many km, but differ among the thin lithostratigraphic units. Paleoenvironmental interpretations from REE analysis were consistent with those from other geologic data.
Preliminary research was conducted in the fluvial deposits of the Upper Cretaceous Judith River and Lance Formations and the Tertiary deposits of the Fort Union Formation in Wyoming. Unique REE signatures and UCD variations were found for each tested lithostratigraphic unit.
REE and UCD analyses resulting from this research provides a finer scale of resolution for stratigraphic correlation and a proxy for paleoenvironmental interpretation.