A STRATIGRAPHIC REVISION OF THE MINUENS SHALE AND OTHER CINCINNATIAN BUTTER SHALES

The Cincinnatian Series of the Upper Ordovician in the Cincinnati Arch is noted for claystone Lagerstätten deposits known as “butter shales.” These intervals are famed for their abundance of well-preserved bivalves, cephalopods, and trilobites. The best documented instances of the Cincinnatian butter shales are the Harpers Run (Treptoceras duseri) shale and the Oldenburg shale intervals of the Richmondian Age Waynesville Formation. However, there are several more that are far less documented. While collectors and paleontologists alike are aware of their existence and regularly collect from these shales, they have been variously conflated and misplaced stratigraphically. In fact, each has distinctive faunal and sedimentological features.

The minuens shale is a butter shale known for its abundance of well-preserved instances of articulated and enrolled specimens of the diminutive trilobite Flexicalymene retrorsa minuens. While the existence of F. r. minuens has been acknowledged since 1919 (originally as Calymene retrorsa minuens), the butter shale interval was not described until 2007. Since 2013, the understanding has been that the minuens shale lies within the middle Liberty Formation, but in earlier publications both the minuens and the underlying “crinoid shales” were misidentified as upper Waynesville units. The origin of this confusion can be traced back to the 1920s, when different shale beds, like the “crinoid shale” and two thicker, fossiliferous beds, were noted within the Waynesville Formation. Work from 2013 onwards has refined the stratigraphy and paleoecology of Waynesville butter shales but not those of the Liberty Formation. Indeed, the general ambiguity of the minuens shale’s placement within the Liberty is still prevalent in the paleontological community. The present study revises the stratigraphic placement of the “crinoid” and minuens shales and provides a documentation of the microstratigraphy and paleontology/paleoecology of the latter. A comparison of the minuens shale with other butter shales indicates strong similarities in the sedimentology and taphonomy but also differences; notably, the diminutive F. r. minuens is apparently confined to the single band in the Liberty Formation and perhaps a single fifth order cycle.