Iridium abundance measurements across the Cretaceous/Tertiary boundary in the San Juan and Raton Basins of northern New Mexico

GSA Special Papers
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Abstract

During the past year we have been measuring trace element abundances and searching for anomalously high iridium (Ir) concentrations in continental sedimentary rocks that span the Cretaceous-Tertiary boundary in the Raton and San Juan Basins of northern New Mexico and southern Colorado. Using neutron activation and radiochemical separations, we have identified anomalous concentrations of Ir in samples from two sites in the Raton Basin: in a drill core at York Canyon, about 50 km west of Raton, New Mexico, and in a road cut near the city of Raton. In both cases the anomaly occurs essentially at the base of thin coal beds, across a thickness span of only a few cm and at the same level at which several species of Cretaceous pollen become extinct and the ratio of angiosperm pollen to fern spores drops sharply. The Ir surface density ranges from 8 to 40 × 10−9 g cm−2. In the York Canyon core the Ir concentration reaches a value of 5.6 × 10−9 g/g of rock over a local background of about 10−11 g/g; the Pt abundance distribution is similar to that for Ir, while Au reaches its maximum concentration about 10 cm below the Ir peak. Se, V, Cr, Mn, Co, and Zn are about two-fold more abundant at the anomaly zone than in adjacent zones, and mass spectrometric 244Pu analysis showed the 244Pu/Ir atom ratio ⩽ 1 × 107. In the San Juan Basin we have located a small Ir spike (55 × 10−12 g/g over a local background of 8 × 10−12 g/g) that is accompanied by high concentrations of Co and Mn. It is thought to be due to geochemical enrichment processes.

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Additional publication details

Publication type Article
Publication Subtype Journal Article
Title Iridium abundance measurements across the Cretaceous/Tertiary boundary in the San Juan and Raton Basins of northern New Mexico
Series title GSA Special Papers
DOI 10.1130/SPE190-p423
Volume 190
Year Published 1982
Language English
Publisher GSA
Contributing office(s) Central Energy Resources Science Center
Description 11 p.
First page 423
Last page 433
Country United States
State Colorado, New Mexico
Other Geospatial Raton Basin, San Juan Basin
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