Lower Silurian (Aeronian) megafaunal and conodont biofacies of the northwestern Michigan Basin

Abstract

Lower Silurian (Llandovery: Aeronian) carbonates of the Burnt Bluff Group, northwestern Michigan Basin, represent a transect along a southward-dipping ramp that extends from tidal-flat to basin environments. Benthic megafaunas include an ostracod biofacies (tidal flat), stromatoporoid–coral biofacies (very shallow subtidal), pentamerid, crinozoan, and crinozoan–stromatoporoid biofacies (deeper subtidal), and a crinozoan–sponge biofacies (distal ramp and basin). The crinozoan–sponge biofacies, which includes diverse, small crinozoan ossicles, 19 types of siliceous sponge spicules, and at least 65 other taxa, has a biota that is similar to those of Silurian continental margins. Megafaunal biofacies indicate an Early Silurian gradient going from a shoreline in the north to water depths of perhaps 60 m in the south. The Burnt Bluff Group contains conodonts of the Icriodella discreta – Icriodella deflecta zone as an onshore biofacies with Panderodus, Kockelella, Ozarkodina, Icriodella, and Oulodus, and an offshore biofacies with Panderodus, Walliserodus, and Aspelundia.