CONODONT BIOSTRATIGRAPHY OF THE EAST LIBERTY BONE BED AND SUBJACENT BEDS, BELLEFONTAINE OUTLIER, EAST LIBERTY, OHIO, USA

For many years, the dolomitized Devonian carbonates of the Bellefontaine Outlier appeared to lack evidence for definitive correlation and zonal placement; thus, geologists to put forth wildly differing proposals based on lithology, on interpretations of pre-dolomitization lithology, on few positively-identified macrofauna, and on speculation. Recent work has shown the dolostone of the East Liberty Quarry does contain a limited conodont fauna, though as elsewhere on the Wabash platform it is a shallow-water, low-diversity, Icriodus-dominated fauna that lacks zone-defining Polygnathus species.

A focused analysis of the uppermost nine dolomitic beds (1.5 m thick) revealed a punctuated depositional history, within three non-contiguous conodont zones from the Eifelian, Givetian, and Frasnian. The lower five beds (1-5) contain long-ranging species (I. stephensoni, I. orri, and I. angustus) constrained to the Eifelian. The last occurrence of Latericriodus robustus resides 8.4 meters below Bed 1, indicating the top of the costatus zone. Interestingly, the overlying sample contains the first occurrence of I. angustus. Taken together these taxa constrain beds 1-5 to deposition within the australis to ensensis zones. Additionally, the same sequence (last occurrence of L. robustus followed by the fist occurrence of I. angustus) marks the transition from the Columbus to the Delaware Limestone on the northeastern margin of the Wabash Platform, which suggests Beds 1-5 correlate to the Delaware Limestone, probably within the kockelianus zone. Bed 6, a laterally discontinuous unit that truncates portions of Bed 5, contains both I. ensensis and I. brevis, which only co-occur within the middle and upper portion of the timorensis zone of the Givetian. The Plum Brook Shale along the northeast margin of the Wabash Platform also possesses this co-occurrence. The lacuna between beds 5 and 6 spans at least one (hemiansatus), and probably as many as three (eiflius, ensensis, and hemiansatus) zones. The overlying three beds (7-9) comprise the East Liberty Bone Bed and have a fauna, including Palmatolepis bogartensis, indicative of the MN13 Zone (and so correlates to the majority of the Upper Olentangy Shale) of the Frasnian. Thus, a lacuna of 18 conodont zones exists between beds 6 and 7 and as many as 22 conodont zones between beds 5 and 7.