Iridium anomaly at the Cretaceous-Tertiary boundary in the Raton Basin
— Charles J. Orth, James S. Gilmore, and Jere D. Knight

In early 1981, in collaboration with scientists of the U.S.G.S. in Denver, we located an iridium (Ir) abundance anomaly at the pollen-defined Cretaceous-Tertiary boundary in a core taken near the York Canyon Coal Mine, about 60 km west of Raton, New Mexico. The Ir concentration reached a peak of 5.6 ppb (5.6x 10^-9 g/g of rock) over a local background of about 0.010 ppb. Subsequent searching for the boundary around the eastern margin of the Raton Basin turned up numerous similar sites located on outcrops and roadcuts. A boundary exposure in a roadcut that overlooks the city of Raton will be visited during this field conference. Here the Ir concentration is lower than in the York Canyon core (~1 ppb), and the kaolinitic boundary clay and overlying Ir-rich laminar clay layer, found in well-preserved boundary sections in the Raton Basin (also Montana and Wyoming), are mostly absent or diluted with detrital clay. Apparently slowly moving streams were at work here during and soon after boundary time. In the better-preserved localities the Ir abundance reaches a peak of 25 ppb, and the total Ir deposition averages about 50 ng/cm² , which is comparable to amounts that have been reported in marine sections all over the earth.