THE SEQUENCE STRATIGRAPHY AND BIOSTRATIGRAPHY OF THE MIDDLE CAMBRIAN SPENCE SHALE

The Middle Cambrian Spence Shale Member of the Langston Formation of northern Utah and southern Idaho is a fossil-rich unit that exhibits distinct cyclicity at the parasequence scale. At least seven discrete parasequences, or cycles, can be found at the Miners Hollow and Antimony Canyon localities, each composed of calcareous shale capped by limestone. Within this cyclicity, predictable patterns of faunal distribution are evident. Sampling and identification of fossils from two localities have revealed that observed changes in fauna track changes in sea level throughout the section. Through cluster and principle components analyses it has been determined that those rocks of the Spence Shale that represent a transgressive systems tract are home to a particular community of organisms while those rocks of the highstand systems tract are home to another. It logically follows that the rocks of the transgressive tract represent a distinct biofacies, while those of the highstand tract represent another. The transgressive biofacies is composed of species that are commonly found in deeper ramp settings such as agnostoid and oryctocephalid trilobites, while the highstand biofacies is distinguished by such taxa as Zachanthoides and other larger trilobite genera such as Glossopleura and Kootenia, among others.