A TAPHONOMIC ANALYSIS OF EXCEPTIONALLY PRESERVED BIVALVES FROM THE LATE ORDOVICIAN OF NORTHERN KENTUCKY

Identifying circumstances that lead to high quality versus typical or poor preservation of fossils can provide insight into the process of taphonomy. In this study, we examine the taphonomy of a layer of extremely well-preserved bivalves from the Late Ordovician (Katian) Arnheim Formation near Weedonia, Kentucky.

The Oregonia Formation is part of the Cincinnati Series, which includes highly fossiliferous deposits of Late Ordovician marine fauna in the Kentucky, Ohio, and Indiana region. During the Late Ordovician, the area that now comprises the Cincinnatian outcrop area was located south of the equator within a strong storm belt. These storms often churned the water of the shallow sea, covering the organisms that inhabited the seafloor with sediment. Variation between rapid storm deposition and background sedimentation rates, changes in grain size or changes in geochemical conditions may have produced variations in local taphofacies.

Samples for this study were collected from a single layer of the Arnheim Formation near Weedonia Cemetery, Mason County, northern Kentucky. The focal bed is approximately 10 cm thick and includes common bivalve fossils that preserved shell material with detailed structure, which is atypical as most bivalves from the Cincinnati series represent internal molds. Over 50 bivalve specimens were collected from this layer. The dominant genera are Carotidens and Ambonychia.

A series of geochemical, taphonomic and petrographic analyses were used to determine the unique conditions that permitted such high fidelity preservation. Samples were analyzed via SEM, taphofacies and thin-section analyses. Three working hypotheses were investigated. The first is that the remarkable preservation is due to rapid burial in sediment due to the strong storms common in the Cincinnatian. The second is that a local shift in the elemental geochemistry caused a temporary or local faunal shift from calcitic organisms (brachiopods) to aragonitic organisms (bivalves). The final hypothesis is that a layer of fine-grained sediment was deposited immediately around the bivalves, thus forming a seal and promoting high fidelity preservation. The outcome of this project will further constrain the process of fossilization, and contribute to the working knowledge of how organisms are preserved in the fossil record.