7-8 Preliminary characterization of microfossils in the Middle Devonian Charlevoix and Petoskey formations, Northern Michigan

Session: Undergraduate Research, Part I (Posters)

Presenting Author:

Authors:

Abstract:

Samples of the Charlevoix and Petoskey formations were collected with the initial goal of extracting conodonts for refining the biostratigraphy of these units, which are currently understood to be Middle Devonian. Although none of the samples processed yielded conodonts, a microfossil assemblage dominated by scolecodonts was recovered and is preliminarily described herein.

The outcrop at Bay Front Park, formerly Waterfront Park, in Petoskey, Michigan, is a well-studied succession, with detailed stratigraphic examination by various researchers undertaken since at least 1866; indeed, it is the type section for the Petoskey Formation. Up to 2.35m of the Charlevoix Formation is exposed at the base of the outcrop, and is unconformably overlain, with 0.75m of relief on the contact surface, by ~5.5m of the Petoskey Formation. The Charlevoix Formation is composed of gray and brown limestone and dolostone, with a macrofauna that includes clams and gastropods. Black shale interbeds occur in the upper part of the unit as well as small atrypid brachiopod molds. Previous workers, who presumably had access to a better-exposed outcrop, describe a bed of varying thickness up to ~1m thick containing reworked angular limestone clasts at the contact between the Charlevoix and Petoskey formations; this bed was not observed as part of the present study. The lower ~5m of the Petoskey Formation observed consists of overturned stromatoporoids in a matrix of brown dolostone with atrypid molds and coral fragments. The upper ~0.5m of the Petoskey Formation consists of light gray limestone with abundant corals and some stromatoporoids, and in the uppermost part, black shale partings.

Microfossils of the Charlevoix Formation include a well-preserved, diverse assemblage of scolecodonts found within various samples. A sample from a bed of black shale within the Charlevoix contains scolecodonts as well as sponge spicules and small crinoid ossicles. Scolecodonts were only recovered from the lowest Petoskey Formation. Microfossils were relatively scarce in the remainder of the Petoskey Formation except for small crinoid ossicles. The results of this study provide a framework for future investigation of Devonian scolecodonts from the Michigan Basin.

© Copyright 2026 The Geological Society of America (GSA), all rights reserved.