Fossils to Fertilizer: Taphonomic Implications of Uranium Roll Fronts

Abstract

Wyoming is famous for its deep structural basins containing both abundant vertebrate fossils and uranium-ore deposits in the form of roll fronts. Research conducted in The Breaks, an area of badlands in the northeastern corner of the Hanna Basin, south-central Wyoming, shows that these two disparate phenomena have an important relationship. Vertebrate bones and teeth, composed of bioapatite, are chemically stable under most diagenetic conditions. The chemical changes that occur at the leading edge of a propagating roll front, however, are capable of eliminating these fossils. This happens as pyrite is oxidized, which releases sulfuric acid to the ground water. Sulfuric acid reacts vigorously with apatite, releasing soluble phosphate. Phosphate liberated from the dissolved bones and teeth provides a valuable, often limiting, nutrient to plants or lichens once the roll front is exposed on the ground surface. In The Breaks, bedrock permeability is the most important factor controlling the distribution of roll fronts and, therefore, also of vertebrate fossils. Thus, patterns of roll-front distribution, observed from differing lithologic characteristics or floral assemblages growing on the rock, might provide a useful prospecting tool for paleontologists.