PALEOENVIRONMENTAL RECONSTRUCTION OF THE ELY BASIN BASED ON THE DISTRIBUTION OF CHAETETES

The Ely Basin, formed along the western margin of North America during the Middle Pennsylvanian, experienced periods of quiescence and deformation as tectonism continued along the margin. During the Atokan, the Ely Basin provided a unique environment for Chaetetes, a coralline demosponge, and species of Syringopora to thrive producing extensive carbonate accumulation. Detailed lithologic and biostratigraphic studies above and below the Chaetetes/Syringopora assemblage have provided important information for paleoenvironmental reconstruction of the basin.

The major rock unit preserving the Chaetetes/Syringopora assemblage is the Ely Group. Along the western margin of the Ely Basin, the Ely Group is comprised of the Moleen and Tomera Formations. These units record deposition along a dynamic margin with a high influx of siliciclastic and carbonate debris. Chaetetes occurs sporadically with minor traces of Syringopora in thin units within the upper Moleen. This suggests that a brief period of quiescence allowed Chaetetes to initially develop but never producing stable carbonate complexes. Further east into the Ely Basin the character of deposition changes as recorded in the carbonate-dominated units of the Ely Limestone. Fieldwork has revealed a laterally extensive network of the Chaetetes/Syringopora assemblage. Two Chaetetes growth forms, domical and columnar, have been identified suggesting a shallow, high-energy environment.

The occurrences of the Chaetetes/Syringopora assemblage and detailed stratigraphy from sections studied within the Ely Basin have allowed for a detailed paleoenvironmental reconstruction. The variations in the development of the Chaetetes/Syringopora assemblage and lithologies suggest a western area where high levels of fluvial input limited the full development of the assemblage while the eastern margins with lower energy and stable slope characteristics allowed them to fully develop.