TOWARDS A COMPLETE ISOTOPIC RECORD OF THE LATE DEVONIAN: INCLUDING ALFIE, A NEWLY DISCOVERED POSITIVE CARBON ISOTOPIC EXCURSION

Devonian strata at Pinyon Peak, a few miles east of Eureka, Utah include, from base to top, the Bluebell, Victoria, Pinyon Peak and Fitchville formations. A series of extraordinary paleokarst horizons up to 10 m thick exist in the underlying Victoria Formation. These are associated with an influx of quartz sandstone, particularly towards the top of the karst-rich interval. The paleokarsts record possible high-frequency sea-level changes that repeatedly exposed a carbonate platform and were associated with influx of sand due to base level changes and rejuvenation of distal river systems. Carbon isotope data from below and above the karst interval show no major shifts. The karst may record the Frasnian-Famennian boundary, and if so, the underlying rocks would likely represent the rhenana Zone of the uppermost Frasnian, which is the only part of the Frasnian in which negative isotopes are recorded.

The published record of Devonian carbon isotope data does not include the upper 12 zones of the Famennian. Conodonts recovered from the upper 23 m of the Pinyon Peak Formation indicate an Early expansa Zone age, and a Middle expansa Zone age for the lower Fitchville. Early marginifera Zone and older Famennian strata show positive carbon isotopic values with a trend towards negative values. Our older Famennian strata have negative carbon isotopic values indicating that these were deposited during the younger Famennian zones. Our data considerably extend the known isotopic record to include much of the uppermost Devonian. We recovered poorly preserved partial specimens of a Palmatolepis sp. (possibly Pa. wolskajae) from the base of the heretofore-undated lower Pinyon Peak, and these indicate an early Famennian age, which is at odds with the isotopic data. Regardless, the isotopic data indicate a major positive excursion of 2.5 per mil near the top of the generally unfossiliferous lower Pinyon Peak Formation. This shift is a late Famennian (positive) isotopic excursion (ALFIE). What is ALFIE all about? It likely represents a previously unknown major geochemical event, but is not associated with a lithologic change. Therefore, it is unclear whether this event is associated with a sea level change, such as the basal expansa Zone transgression, or a faunal turnover event. The Early and Middle expansa Zone record rising values up to +2 per mil, more typical of the lower and middle Famennian.