Abstract

The F-F extinction is known on four continents by the sudden disappearance of shelly benthos, although there remain questions regarding precisely when and how suddenly it took place. Studies show that this extinction occurred at the gigas/triangularis zone boundary in Utah and Nevada, U.S.A., the Hony and Sinsin sections, Belgium, Montagne Noire, France, and Bad Wildungen, Germany. In Medicine Lake and Trout River, Canada, this extinction event has been bracketed between the gigas and Upper triangularis zones. The correlation with an iridium anomaly in the Canning Basin, Australia, is suspect and an extinction within the crepida zone in Hunan, China, appears to be facies related.

Major biomass reductions at the F-F extinction are supported by sudden decreases in δ¹³C values at this horizon at Trout River and Montagne Noire. δ¹⁸O values in several sections indicate sudden cooling followed by a warming trend. Several F-F extinctions also coincide with a regression and world-wide anoxic event.

Five sections are geochemically anomalous in transition metals at or near the extinction boundary; one (Canning Basin) is weakly anomalous in Ir; and another (Medicine Lake) displays highly positive δ³⁴S values in sedimentary pyrite. None of the minerals examined thus far in samples of the F-F extinction in the Canning Basin, Trout River and Medicine Lake display shock metamorphic features.

The evidence to date indicates that the F-F extinction probably occurred at or near the gigas/triangularis zone boundary, and corresponded to a globally synchronous regression and anoxic event. This anoxic event explains rapid facies changes to carbonaceous sediments, positive δ³⁴S values in pyrite and high contents of chalcophile elements which have been measured at the F-F boundary in several sections. Although poisoning of epicontinental seas with anoxic waters was the most likely immediate cause of mass extinction, the global synchroneity, magnitude and apparent suddenness of the mass killing and its association with a world-wide regression are all consistent with meteorite impact as the ultimate cause.