A Search for Iridium Abundance Anomalies at Two Late Cambrian Biomere Boundaries in Western Utah

Iridium concentrations have been measured in samples taken across two Late Cambrian biomere boundaries (crisis zones) in search of evidence for possible elemental abundance anomalies similar to the one observed at the Cretaceous-Tertiary boundary. Sampling was performed in uplifted marine limestone deposits in the House Range of western Utah. Although the two trilobite-brachiopod extinction boundaries could be assigned to $\pm $4 millimeters of vertical section by laboratory examination of the rocks, only background amounts of iridium (2 $\times $ 10$^{-12}$ to 17 $\times $ 10$^{-12}$ gram per gram of whole rock) were observed.