DISTRIBUTION OF NOBLE-METALS ACROSS THE CRETACEOUS TERTIARY BOUNDARY AT GUBBIO, ITALY - IRIDIUM VARIATION AS A CONSTRAINT ON THE DURATION AND NATURE OF CRETACEOUS TERTIARY BOUNDARY EVENTS

Shales and limestones were sampled from 2.85 m above to 219 m below the Cretaceous/Tertiary (K/T) boundary from the sections at the Bottaccione Gorge and Contessa Valley, Italy, for determining Ir, Pd, Pt and Au by radiochemical NAA. Ir is concentrated by 63 X in the boundary shales in comparison with the background, whereas other metals are enriched by no more than 2.2 times. The Ir enrichment extends 2 m above and below this K/T horizon with four distinct Ir maxima in addition to the major Ir peak in the K/T boundary shales. Abundance of shocked minerals is stratigraphically coincident with these maxima. Limestones are much lower in noble metals. The time represented by 4 m of Ir-rich section is at least 3 X 105 yr. To sustain an increased Ir flux over this period and to account for Ir distribution near the K/T boundary, intense volcanic activity is a preferred alternative to impact of extraterrestrial material. -L.C.H.