FRASNIAN/FAMENNIAN BOUNDARY ANOXIA IN THE GREAT BASIN, WESTERN UNITED STATES

The extinctions at the Frasnian Famennian boundary constitute one of the “big 5” crises of the fossil record. The extinction interval is associated with two organic-rich limestones, the “Kellwasser” beds, in condensed, deep-water German boundary sections: consequently marine anoxia has been proposed as a cause of the mass extinction. Evidence for these anoxic events has been sought in the Great Basin of the western United States using integrated techniques of pyrite framboid assay and traditional facies analysis. Until now, an equivalent of the terminal Frasnian Upper Kellwasser Horizon has not been identified in the Great Basin. It has previously been suggested that a discreet anoxic event began during the penultimate Frasnian conodont Zone, but ended 100 Ky before the F-F boundary. However, new data from several sections, representing a variety of depositional settings, suggests that the main anoxic pulse occurred during the topmost Frasnian, coeval with both European anoxia and the main pulse of extinction. The manifestation of this event varies according to location: in basinal locations, anoxic-dysoxic deposition persisted through much of the Frasnian, intensifying near the boundary. In slope settings the anoxic event is a discreet pulse which punctuated oxic / dysoxic background conditions. These events are perfectly contemporaneous with those identified in the Old World and global anoxia is therefore considered to have been a main cause of the Frasnian-Famennian marine extinctions.