ICHNOFABRIC OF THE MIDDLE CAMBRIAN SPENCE SHALE: A SNAPSHOT OF THE AGRONOMIC REVOLUTION

The agronomic revolution that took place as bioturbation increased during the Cambrian radiation played a critical role in the development of subsequent seafloor substrates, and is vital to our understanding of the evolution of both infaunal and epifaunal organisms during the Cambrian. Most studies of early-to-middle Cambrian ichnofabric and paleocommunities from multiple localities suggest that the pace of the agronomic revolution was uneven, progressing at different paces at various locations on the seafloor. The resolution on this irregularity is very coarse, however. Here we present preliminary data from a study designed to examine the degree of local control on the agronomic revolution through investigation of the middle Cambrian Spence Shale Lagerstätte of southern Idaho and northern Utah. Samples totaling 2.32 meters of Spence Shale were collected from four localities in Utah and Idaho during the 2009 field season. These samples were analyzed on a millimeter scale for bioturbation levels (ii), as well as any other indicators of substrate condition at the time of deposition (i.e. presence of microbial mats, trace fossils). Preliminary results suggest that the agronomic revolution was well underway in shallow subtidal marine environments, although “patches” of Proterozoic-style substrate likely still remained. The deeper water environment represented by the Oneida Narrows section remained largely unbioturbated (ii ~1), which is consistent with the prediction of an onshore-offshore progression of the agronomic revolution. Future work includes expanding the number of localities sampled, in addition of incorporating both geochemical and paleoecological data to paint a more complete picture of the Spence Shale deposition environment.