J. S. Gilmore J.D. Knight C. L. Pillmore R.H. Tschudy J.E. Fassett C. J. Orth 1981 <p>An iridium abundance anomaly, with concentrations up to 5000 parts per trillion over a background level of 4 to 20 parts per trillion, has been located in sedimentary rocks laid down under freshwater swamp conditions in the Raton Basin of northeastern New Mexico. The anomaly occurs at the base of a coal bed, at the same stratigraphic position at which several well-known species of Cretaceous-age pollen became extinct.&nbsp;</p> application/pdf 10.1126/science.214.4527.1341 en American Association for the Advancement of Science An iridium abundance anomaly at the palynological Cretaceous-Tertiary boundary in northern New Mexico article