CONODONT FAUNA CONSTRAINS THE DEPOSITION OF THE DELAWARE LIMESTONE FROM THE MIDDLE OF THE AUSTRALIS ZONE INTO THE UPPER PORTION OF THE KOCKELIANUS ZONE

The inability of biostratigraphers to establish a well-constrained conodont zonation for the Delaware Limestone in Central Ohio has hindered our understanding of sea-level fluctuations and depositional sequences in the Middle Devonian on the Wabash Platform and adjacent basins. Lacking such control, geologists relying on various sequence-stratigraphic interpretations have placed the upper boundary of the Delaware Limestone into one of three successive zones: the kockelianus Zone, the ensensis Zone, or even into the hemiansatus Zone in the Givetian.

The small cliffs along Deep Run (Powell, Ohio) offer a rare and complete exposure of the entire Delaware Limestone; interestingly, geologists have only published preliminary (1874-1909) descriptions of this section. Four samples from the Columbus Limestone and ten samples from the Delaware Limestone contain a conodont assemblage representing a low-diversity, shallow-water fauna dominated by Icriodus and Polygnathus that lacks most zone-defining Polygnathus species: a fauna characteristic of the Wabash Platform in the Middle Devonian.

The fauna (I. michiganus, I. stephensoni, P. costatus, P. linguiformis linguiformis γ) in the top 5.8 meters of the Columbus Limestone, places it within the costatus Zone, an assignment consistent with previous work. The base of the Delaware Limestone contains the first appearance of I. angustus, yet the top of the Columbus Limestone contains both I. latericrescens robustus and P. costatus. Elsewhere, I. angustus coexists with both I. latericrescens robustus and P. costatus. Such a faunal discontinuity between the Columbus and Delaware limestones, suggests a gap extending from the upper portion of the costatus Zone into the lower portion of the australis Zone, which supports other work in Central Ohio. The presence of I. michiganus throughout the Delaware Limestone (with the possible exception of the top 1.5 m, which includes an 87 cm unsampled interval followed by a 68 cm sampled, barren interval) constrains the Delaware Limestone to residing no higher than the kockelianus Zone, as I. michiganus does not range up into the overlying ensensis Zone. In sum, the Deep Run conodont fauna constrains the deposition of the Delaware Limestone from the middle of the australis Zone into the upper portion of the kockelianus Zone.