RARE EARTH ELEMENT (REE) AND TRACE ELEMENT (TE) GEOCHEMISTRY OF LATE JURASSIC MORRISON FORMATION FOSSIL LOCALITIES IN THE NORTHERN BLACK HILLS: STRATIGRAPHIC AND PALEOENVIRONMENTAL IMPLICATIONS

Rare earth and trace element analysis of vertebrate fossil bone in four sites from the Morrison Formation of the northern Black Hills proves the utility of REE and trace element geochemistry as a tool for stratigraphic and paleoenvironment interpretation within terrestrial systems.

REE signatures of the Morrison Formation are shown to have a high degree of similarity over a wide geographic area. Other fossil-bearing formations in the same area are significantly different, indicating a possible new tool for determining fossil provenience. Furthermore, fossil bones from each site display similar patterns, but statistically distinctive REE signatures. Therefore, it may be possible to correlate Morrison Formation fossil sites over short distances, yet discriminate among differing paleoenvironments within the same overall depositional system.

REE signatures at the four Morrison Formation sites are the result of influence from fluvial and circum-neutral ground waters, a result consistent with current interpretations of depositional environment. REE signatures also reveal evidence of reworking and slightly elevated sedimentation rates.

Uranium concentrations within fossil bone apatite are systematically different at each site, a result of either slight stratigraphic or local depositional environment influences at those sites. Because uranium reactions are heavily influenced by redox conditions in aqueous chemistry, uranium concentrations may be used to infer paleoenvironmental conditions. High uranium values indicate relatively oxic environments, whereas low uranium concentrations indicate anoxic environments.