GEOCHEMICAL AND STRATIGRAPHIC ANALYSIS OF PYRITIC DECOMPOSITION WITH RESPECT TO BONE PRESERVATION QUALITY: EXAMPLES FROM THE LANCE FORMATION OF EASTERN WYOMING

The Late Cretaceous (Maastrichtian) fossil assemblages of the Lance Formation in eastern Wyoming exhibit varying degrees of preservation quality. Commonly, fossil vertebrate bone, in particular, is degraded by the decomposition of iron sulfides, including pyrite. Corrosive sulfuric acid production from the oxidation of iron sulfides in or near the bones, causes an increased degradation of dinosaur fossils and other organic material. This decay, however, is not uniform and the distribution of well-preserved and poorly-preserved fossils suggests that another factor affects preservation quality. In some locations the presence of fluvial/estuarine invertebrate fossils with calcareous shells, such as bivalves and gastropods occur abundantly in distinct layers. These act as a buffer to reduce the acidity of water flowing through the sediments. This produces a discontinuous distribution of preservation quality of the Lance Formation fossils. A Niton XRF 3T GOLDD portable analyzer was used to examine the geochemical properties of dinosaur bone specimens and the surrounding matrix to assess elemental properties that support increased decomposition of organic material post deposition. In addition, a detailed description of the stratigraphic section suggests short term alternation of a regressive-transgressive systems tract that accounts for environmental oscillation between fluvial and estuarine conditions, producing vertical and lateral facies variation of interbedded layers containing terrestrial and aquatic or estuarine organisms that support the buffering of water moving through the sediments.