RARE EARTH ELEMENT COMPARISON OF VERTEBRATE ASSEMBLAGES IN THE MORRISON AND CLOVERLY FORMATIONS

Rare earth element (REE) concentrations were analyzed from two Upper Jurassic Morrison Formation fossil assemblages, and one Lower Cretaceous Cloverly Formation (Little Sheep Member) fossil locality in the Bighorn Basin near Shell, Wyoming. REE concentrations were used to investigate the environment of deposition and early terrestrial weathering at the various fossil localities. The two Morrison Formation localities (designated upper and lower quarries) are dominated by sauropod dinosaur elements in fluvial and overbank deposits, whereas the Cloverly locality contains indeterminate dinosaur remains preserved in micrite nodules. Neutron Activation Analysis (NAA) was used to determine REE signatures.

Preliminary analysis of NAA results indicate distinctive REE patterns for each of the localities tested. A comparison of light and heavy REE (LREE and HREE) allowed for the determination of early weathering conditions. The ratio of LREE and HREE has been shown to fractionate during terrestrial weathering. These ratios correspond to unique early depositional environments. The upper quarry of the Morrison Formation shows an overall enrichment of LREE and fewer HREE when compared to either the lower quarry or Cloverly localities. These ratios suggest a floodplain as the early depositional environment for the upper quarry, whereas the Cloverly and lower quarry ratios correspond to fossils exposed to river channel pore waters. This initial REE analysis is consistent with sedimentology and geology studies of the area. Further REE analysis will show if the fossils have been reworked and moved to a new depositional environment after initial burial.