THE ICHNOTAXONOMY OF THE CAMBRIAN SPENCE SHALE OF UTAH: PRELIMINARY RESULTS

The Series 3 Cambrian Spence Shale Member of the Langston Formation is a well-known Lagerstätte that has produced numerous well-preserved trilobites, other arthropods, and soft-body tissues. The Spence Shale is the oldest unit with Burgess Shale-type preservation (BST) from the middle Cambrian of North America. The paleoenvironmental conditions that caused the BST are still unclear. The Spence Shale is a unique BST deposit as it contains BST soft tissues in the same stratigraphic intervals as ichnofossils. Ichnofossils are important tools to help reconstruct ancient environments and can indicate paleoenvironmental conditions during and after deposition as well as the associated paleoecology of a deposit, even when body fossils are absent. Although numerous body fossil studies have been done, an ichnotaxonomic treatment of the Spence Shale has never before been conducted. The Spence Shale is a calcareous shale with intervals of peloidal and oolitic limestone and was deposited on a carbonate ramp on the northwestern edge of Laurentia. To date, eleven ichnogenera have been identified from the Spence Shale: Bergaueria Prantl, Cochlichnus Hitchcock, Cruziana d’Orbigny, Diplichnites Dawson, Didymaulichnus Young, Gyrophyllites Glocker, Monomorphichnus Crimes, Neonereites Seilacher, Planolites Nicholson, Rusophycus Hall, and Treptichnus Miller. Ichnocoenoses will be assigned via the ichnofossil assemblages that are the result of single ecological communities of tracemaking organisms and will be used to understand the interactions and controls on the epi- and endobenthic communities. A currently tentative ichnocoenoses is the Cruziana ichnocoenoses, which includes (in order of abundance) Cruziana, Rusophycus, Planolites, and Treptichnus. The members of the Cruziana ichnocoenoses all occur on a single laminated shale slab with discontinuous laminations of fine-grained carbonate sand. Ichnofacies have yet to be established for the Spence Shale but they will be used to define recurring ichnofossil associations in specific lithofacies. The use of ichnocoenoses and ichnofacies will help delineate environments of deposition, changes in shoreline position, and physicochemical controls including depositional energy, sedimentation rate, and oxygenation.