14-5 Geographic Differentiation of Conodont Biofacies in the Early Mississippian (Kinderhookian) Siphonodella sandbergi Zone Marine Sequence in the Williston Basin, Northwestern United States

Abstract:

The regionally widespread Early Mississippian (Kinderhookian) marine sequence in the Williston Basin that comprises the Upper Bakken Member of North Dakota, The Upper Sappington Member of Montana and Wyoming, and the Mickelson Member of South Dakota have been previously correlated based on conodonts and were deposited during the S. sandbergi Zone. A review of new and previously published quantitative conodont abundance data revealed significant variations in the conodont assemblages that are preferentially distributed in different parts of the Williston Basin. Four conodont biofacies are recognized: The Siphonodella obsoleta and Polygnathus inornatus group biofacies is most common in the carbonates in South Dakota; the Polygnathus inornatus group biofacies is most common in the shale and siltstones of northcentral Wyoming; the Polygnathus communis group biofacies is most common in the shale and siltstones of northwest Wyoming; and the Siphonodella cooperi, Pseudopolygnathus, and Bispathodus biofacies is most common in the black shales of western Montana. These spatial biofacies variations within the same temporal interval (i.e. the same regional depositional sequence and biozone) likely represent ecological partitioning of conodont groups into different parts of the basin and depositional profile. Although most conodont biofacies focus on the genus-level this dataset display some species level differentiation, most notably within Siphonodella where species with smooth rostral platforms (e.g. S. obsoleta) dominate carbonate facies in South Dakota but species with ridged rostral platforms (e.g. S. cooperi, S. quadruplicata) dominate black shale facies. This compiled conodont data set is limited in the amount of other data types available that would be required to thoroughly test depositional environment and paleocological hypotheses, but the broad patterns observed here warrant further study that could refine our understanding of Kinderhookian conodont paleoecology.

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