Gamachian Echinoderm Fauna
The famous limestones and shales of our region lend their name to the Cincinnatian Series, the North American term for the upper part of the Upper Ordovician (~450 – 445 million years ago). This series is subdivided into several stages: the Edenian, Maysvillian, and Richmondian. The members of this trio are roughly equal to one another in terms of thickness and outcrop extent. However, this pattern is not universal, and other areas may preserve more or less of a given time interval depending on the local conditions.
One such location is Anticosti Island, a grande île in the Gulf of Saint Lawrence of northeastern Quebec with breathtaking sea cliffs and gorges that reveal a spectacular package of Upper Ordovician and Lower Silurian rock, making it a bucket list destination for paleontologists and stratigraphers. Anticosti’s fossiliferous grey shales and limestones immediately invite comparison with those of the Cincinnatian and, indeed, they partially share the same age and some of the same fossils.
However, there are key differences as well. The Vauréal Formation is approximately equivalent to our Richmondian Stage. Yet our local Richmondian formations’ total thickness is about 100 meters, whereas the Vauréal is nearly a kilometer! But the differences don’t end here, for the underlying Macasty Formation (accessible via drill core) records the Edenian-Maysvillian interval in less than 30 meters, about a quarter of its thickness in our area. Therefore, the Anticosti basin preserves both more and less of the Cincinnatian Series than the Cincinnati region itself, hinting at an intriguing depositional story that is beyond the scope of this article.
More topically, Anticosti also preserves something missing in our area: the uppermost Ordovician Hirnantian Stage. The Hirnantian was a time of significant climate upheaval and the infamous end-Ordovician mass extinction event. Expanding glaciers and ice caps locked up massive volumes of water, drawing down global sea levels and subjecting many formerly flooded areas to erosion or at least non-deposition. Such was the case in the Cincinnati region, where Hirnantian strata are not present (or perhaps exceptionally rare). However, Anticosti Island preserves tens of meters of Hirnantian-age rock in the Ellis Bay Formation, sandwiched between the Vauréal Formation and the basal Silurian Becscie Formation. Stratigraphers have recognized this interesting interval for over a century and originally named it the Gamachian Series (later: Gamachian Stage) after the Gamache River/Bay on the western part of the island. The Gamachian is essentially equivalent to the Hirnantian, a more modern term used in global stratigraphic correlation.
The fauna of the Ellis Bay Formation is simultaneously familiar yet unusual, with an eclectic mix of Cincinnatian holdovers like the square-columned camerate crinoid Xenocrinus, uniquely Hirnantian forms such as the draboviid brachiopod Hirnantia, and even Silurian precursors like lumpy diploporitan cystoid Holocystites. These creatures lived among shallow water bioherm mounds, oncolite beds, and shifting carbonate sands, bridging the gap between the Ordovician and Silurian.