SEARCH FOR IMPACT REMAINS AT THE FRASNIAN-FAMENNIAN BOUNDARY IN THE STRATOTYPE AREA, SOUTHERN FRANCE

In order to detect whether the end-Frasnian worldwide biotic crisis is related to an extraterrestrial impact, the global stratotype section of the Frasnian-Famennian boundary and auxiliary sections within the s tratotype area have been examined for impact indicators: iridium, Ni-r ich spinel bearing spherules and glassy microtektites. This area is pa rticularly well suited to the search for discrete events because it ex hibits biostratigraphically continuous sections of sedimentologically homogenous off-shore deposits. Different environmental settings on oxy genated deep-water seamounts, such as the stratotype section at Coumia c, and in oxygen-depleted depressions (La Serre section) are available . The latter is investigated in more detail because it is the least co ndensed across the boundary, which is determined by the first occurren ce of the typical morphotype of Palmatolepis triangularis, the indicat or of the first Famennian conodont biozone. Samples from the biostrati graphically defined boundary and adjacent levels failed to provide sig nificantly high Ir values and no Ni-rich spinel or microtektite has be en recovered. This is in contradiction with the results of. earlier in vestigations carried out by H. Geldsetzer on the same section. In cont rast, the values of Ir concentrations that we measured are always very low or not delectable. The small overabundances observed in some samp les, which are about two orders of magnitude lower than what is curren tly observed at the Cretaceous-Tertiary boundary, are probably due to the accumulation of the normal flux of cosmic dust during periods of r elatively low depositional rates or to a terrestrial origin, At presen t, we have no evidence that an extraterrestrial impact occur-red at th e F-F transition. (C) 1997 Elsevier Science B.V.