IMPROVED ISOTOPIC RECORD OF THE LATE DEVONIAN, AND ALFIE, A NEW POSITIVE CARBON ISOTOPIC EXCURSION OF THE LATE FAMMENIAN

A thick Frasnian–Fammenian (Devonian) carbonate succession at Pinyon Peak, a few miles east of Eureka, Utah exposes the Simonson, Victoria, Pinyon Peak and Fitchville formations. The Victoria contains remarakable paleokarst horizons up to 10 m thick that are associated with the sudden stratigraphic introduction of quartz sandstone, particularly towards the top of the karst-rich interval. These paleokarsts are believed to record high-frequency sea-level changes that repeatedly exposed a carbonate platform and caused an influx of sand due to base level changes and rejuvenation of distal river systems. A carbon isotope chemostratigraphic curve shows no major shifts spanning the karst interval. 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.

There is no published carbon isotope curve for the upper 12 zones of the Famennian. Our isotope data fills in part of this gap; specifically, most of the Early expansa Zone in the upper Pinyon Peak Formation, and the Middle expansa Zone from the lower Fitchville Formation. The underlying lower and middle Pinyon Peak strata are not dated at this locality, but these have negative carbon isotopic values, which suggest that they are relatively younger Fammenian strata (post-Early marginifera Zone). 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, isotopic data from the top of the lower Pinyon Peak show a major positive excursion of 2.5 per mil, despite a uniformity of lithologic composition. This shift is a late Famennian (positive) isotopic excursion (ALFIE). What is ALFIE all about? It likely represents a previously unrecognized major geochemical event, although 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 Zones record rising values up to +2 per mil, more typical of the lower and middle Famennian.