COMPARISON OF THE LATE UPPER FAMENIAN ISOTOPIC RECORD OF GERMANY AND SOUTHWEST U.S

Three sections of Devonian strata in west-central Germany were analyzed for carbon isotope chemostratigraphy to test whether a Late Fammenian Isotopic Excursion (ALFIE), recently recognized in the southwest US, is also present in Europe. ALFIE is a 2.5 per mil positive excursion identified in the Pinyon Peak Formation, at Pinyon Peak, a few miles east of Eureka, Utah, in either the upper postera or lower Early expansa Zone of the Fammenian. It likely represents a previously unknown major geochemical event possibly 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.

Samples taken from sections in Germany at the town of Oese, and at quarries in Ballberg and Effenberg, cover the marginifera through expansa conodont Zones (including postera). These sections contain relatively deep-water shale, marl, and micritic limestone beds. These fine-grained lithologies are thinly interbedded and generally lack internal sedimentary structures, but some beds contain megafauna such as ammonites. The German sections are more condensed, were deposited in deeper water, and are presumably stratigraphically complete, relative to those in the southwest U.S., which are dominated by carbonate and contain discrete and prominent paleokarst horizons.

Chemostratigraphic curves for the three sections in Germany show carbon isotopic values that generally vary monotonically from +2‰ to ~0.5‰ and can be correlated with reasonable certainty between the three localities. None of the sections exhibit a prominent positive excursion in the conodont zones that correspond with ALFIE. Therefore, the ALFIE excursion is likely a regional Laurentian event, and not a global perturbation. Such an event could represent a change in the geochemistry of the epicratonic shallow Devonian sea, which was made possible by short-term reduction in circulation with the seaway in the present-day southwest.