COLORADO MASS EXTINCTION: WEIRD FACIES AND COOL FOSSILS FROM THE END-DEVONIAN DYER FORMATION (Invited Presentation)

The Dyer Formation of west-central Colorado is spectacularly exposed along the White River Plateau and spans the end-Devonian Hangenberg Event, one of the most enigmatic extinctions in earth history. Whereas the Dyer is known among fossil collectors for its diverse invertebrate and vertebrate assemblages, its geochronology and facies are poorly constrained. Recent chemostratigraphic analyses of the Dyer reveal a regionally extensive six to eight per mil positive δ¹³C zenith in the upper member of the unit. This excursion occurs at several localities and is interpreted to represent the Hangenberg isotopic excursion. New conodont specimens from the Dyer support this interpretation. Youngest detrital zircons from sands within and above the Dyer and ⁸⁷Sr/⁸⁶Sr values from the unit are also internally consistent with deposition of the Dyer through the Hangenberg interval. Concordant with these observations, strata representing this interval immediately overlie a firmground-like facies which bears abundant rugose corals. Elsewhere such fossils have been interpreted to represent Lazarus taxa that return after near-decimation at the end-Frasnian extinction. Most of the lower Dyer in the southern portion of the White River Plateau exhibits sedimentologic, paleontologic, and diagenetic features consistent with deposition in a shallow wave-influenced stenohaline tropical platform whereas the upper portion of the unit exhibits features more consistent with deposition in a protected mud-dominated euryhaline setting. In contrast, the Hangenberg interval facies in the Dyer are characterized by microbialites and bedded cherts – features consistent with a short-term perturbation in environmental conditions at or prior to an extinction event.