The three most Ir-rich marine Cretaceous/Tertiary boundary clays are found in Denmark, Spain, and New Zealand. The clays have many features in common, the most interesting being a thin, basal red layer that, together with an overlying organic-rich zone, contains enhanced concentrations of elements such as Ir, Ni, Co, Cr, Zn, Cu, As, and Sb. At first glance, the global correlation of element enrichments may seem to be strong evidence for a dust-cloud origin of the basal red layers. However, detailed layer-by-layer major-element analyses (e.g., Si, Al, Mg, Ca, K, and Na), together with studies of trace-element phase partitioning, reveal that the elastic allogenic fraction of the red layers at each site is predominantly locally derived, water-transported material, and that concentrated metals exist in authigenic phases. The red layers most likely represent precipitation horizons and not fallout layers. Metal precipitation was induced by various redox-controlled processes in connection with decomposition of the abundant algal matter that settled on the end-Maastrichtian sea floor. The concentrated trace elements derive from sea water; however, the ultimate origin of some of the elements may have been an Earth-impacting asteroid.