BIOMARKER AND PALEONTOLOGIC INVESTIGATION OF THE UPPER AND LOWER WOODFORD SHALE, SOUTH-CENTRAL OKLAHOMA

While the cause of the Late Devonian extinctions at the Frasnian-Famennian (F-F) boundary and the Devonian-Carboniferous (D-C) boundary is not agreed upon, it is often thought that the same mechanism caused both extinctions. Here, we report on an investigation of the F-F and D-C boundaries in the Woodford Shale of south-central Oklahoma using organic geochemical, bulk geochemical, petrographic, and paleontologic techniques. Three sections were collected, two outcrop sections in the Arbuckle Mountains, and one measured core section from the western Arkoma basin, which together span the F-F and the D-C extinctions. Despite previous assumptions that the two extinctions share a common cause, the biomarker and fossil data are not similar across the two boundaries. The ratio of C₂₉ steranes to C₂₇ and C₂₈ steranes are higher across the F-F boundary than across the D-C boundary, indicating a different change in algal communities around each extinction. High concentrations of isorenieratane and gammacerane at the F-F boundary indicate periods of euxinia and stratification, while the absence of isorenieratane at the D-C boundary is suggestive of stratification with oxic deposition. These conclusions are supported by the microfossils present, as at the F-F boundary the abundances of the algal cyst Tasmanites are elevated, while the fossils recovered from the upper Woodford Shale show an increase in diversity of brown-algae-type microfossils and low diversity benthic faunas dominated by scolecodonts and agglutinated foraminifera. These combined microfossil data and biomarker data suggest a restricted basin environment for the Woodford Shale at and above the F-F horizon and a period of oxygen-poor waters caused by upwelling during the Hangenburg event below and at the D-C boundary. Thus, unlike previous scenarios explaining the F-F and D-C eutrophia as a result of a single cause, these data suggest that it is caused by different processes at each boundary.