SEDIMENTOLOGY AND TAPHONOMY OF THE DAKOTA ROSE BONEBED OF THE CRETACEOUS CARLILE SHALE NEAR MILBANK, SOUTH DAKOTA

The Dakota Rose bonebed preserves densely concentrated fossil material in the Turonian Carlile Shale of eastern South Dakota. A combined approach was used to characterize the bonebed and associated rocks in hand sample and in thin section, with a focus on the paleontology and taphonomy of the bonebed, as well as the geochemistry and local relief of the underlying granite. This study seeks to reconstruct the formative history of the bonebed in the immediate vicinity of the Dakota Granite Quarry near Milbank, South Dakota. During the course of our study the bonebed was divided into two discrete regions (hereafter designated areas A and B) on the basis of readily apparent macroscopic differences.

Geochemical and petrographic analyses of the underlying Milbank Granite reveal a relatively thin weathered surface layer enriched in mobile elements with clays filling cracks. Fossiliferous deposits immediately overlying the granite are characterized by conglomeratic limestone that preserves bones and teeth of fishes, bones of marine reptiles, shark teeth, phosphatic pebbles, granite clasts, and carbonaceous material. However, areas A and B are distinctly different in sedimentology, fossil content, and taphonomy. Area A is interpreted to represent a lower-energy anoxic environment due to its finer-grained matrix and the presence of large pieces of pyritized wood and abundant disseminated pyrite. Area B is interpreted to represent a slightly higher-energy setting prone to oxidation, as evidenced by better sorting and abundant oxidized plant debris. An associated fossiliferous shale (designated area C) is potentially the source of fossil material found in the bonebed conglomerates.

The Dakota Rose bonebed is interpreted to be a complex ravinement lag that formed following, or in association with, the removal of the Cretaceous weathering profile during transgression. The topography of the underlying Milbank Granite strongly influenced the nature of this deposit, resulting in local differences in sedimentology and taphonomy. Landward migration of the sea is interpreted to have eroded and reworked the associated shale facies, leaving bones and teeth behind as a residual lag deposit.