USING SEQUENCE STRATIGRAPHY TO FIND CONODONTS IN DARK SHALE SUCCESSIONS

Constructing a conodont biostratigraphy from deep-water dark shale successions is not a common practice. This is mainly because finding conodonts in these successions can be a “needle in a haystack” search. It is relatively simple to dissolve or disaggregate large volumes of carbonate rocks and light-colored soft shale, and through the use of sieves and heavy liquids, concentrate conodonts. These methods effectively make the “haystack” smaller. Most organic-rich dark shale is difficult to disaggregate, and so these methods of concentrating loose conodont elements are not effective. We propose that it is possible to use a first principle, sequence stratigraphic approach to find conodonts in many dark shale successions. Periods of relatively rapid sea-level rise tend to trap siliciclastic material in shallow water, and these flooding events manifest themselves in deep water settings as organic-rich, splintery back shale. Thus, these event beds result in smaller “haystacks” and a much greater number of conodonts versus volume of rock than the rest of the succession. We successfully tested this idea in the Mississippian Fayetteville Shale, and used this model for targeting conodont-rich beds to construct an integrated conodont-graptolite biostratigraphy of the upper Womble Shale and lower Bigfork Chert (Ordovician, Mohawkian). A conodont biostratigraphy constructed in this way is likely to have a high degree of fidelity because it is unlikely to contain the reworked conodonts that occur within carbonate turbidites. An added advantage to this method is that conodont bedding plane assemblages are commonly present that because these black, splintery shale deposits accumulated in relatively quiescent deep water.