CHANGES IN FOSSIL BIOAPATITE AS A RESULT OF STRATIGRAPHIC AND PALEOENVIRONMENTAL VARIATIONS

Rare Earth Element (REE) composition and the Unit Cell Dimensions (UCD) in fossil bioapatite are dependent upon and controlled by the composition of diagenetic waters. The changes in chemical composition and structure of the bioapatite record the composition of depositional and diagenetic fluids. Bioapatite then becomes a “flight recorder” for the environment of diagenesis and may be used to interpret paleoenvironments. REE signature and UCD differences in bioapatite identify distinct intervals in lithologic units. Since these distinct intervals represent an averaging of periods of depositional environments, they can be correlated over significant areas. REE and UCD analyses of bioapatite were used to interpret changes occurring in paleoenvironments over time and for stratigraphic correlation in the marine deposits of the Pierre Shale. Fossil vertebrate samples were obtained from the Sharon Springs (lower, middle and upper), Gregory, Crow Creek, DeGrey (lower and upper) and Verendrye members in South Dakota at localities along 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 have been used successfully to interpret paleoenvironmental conditions. Specific REE signatures and UCD extend laterally and act as markers for their units. Paleoenvironmental interpretations and stratigraphic correlation from REE and UCD analyses in the Pierre Shale were consistent with those from other geologic data. REE and UCD analyses resulting from this research provides a finer scale of resolution for stratigraphic correlation and a proxy for paleoenvironmental interpretation.