Online guide to the continental Cretaceous-Tertiary boundary in the Raton basin, Colorado and New Mexico

Introduction

Geologists have recognized for nearly a century that a remarkable mass-extinction event occurred at the end of the Cretaceous Period, about 65 million years ago. Thousands of species of plants and animals that lived during the Cretaceous no longer existed during the Tertiary Period immediately following. This disappearance has led to much speculation and controversy among scientists as to possible causes. Among the causes suggested are changes in climate or sea level, neither of which has been universally accepted. In 1980, researchers from the University of California at Berkeley advanced the startling hypothesis that a large asteroid about 6 miles in diameter struck the Earth about 65 million years ago, causing a worldwide biospheric catastrophe (Alvarez and others, 1980).

The Berkeley team found anomalously high concentrations of iridium (Ir) and other noble elements in a claystone layer that marks the K-T boundary in certain marine rocks in Italy, Denmark, and New Zealand. Ir is extremely rare in the Earth's crust relative to its abundance in certain types of meteorites. Because of this crustal Ir deficiency, the team proposed that the source of the anomalously high Ir concentrations in the boundary layer was extraterrestrially derived and that the layer was formed from fallout of ejecta after the asteroid impact occurred. Since this original proposal, more than 100 sites scattered around the globe have been reported where abundance anomalies of Ir occur in rock layers at the major extinction horizon of marine invertebrate and continental plant and animal fossils that defines the K-T boundary.

Significantly, anomalously high concentrations of Ir were first discovered at the K-T boundary in continental rocks in 1981. A team consisting of chemists from Los Alamos National Laboratory aided by geologists from the U.S. Geological Survey (USGS) reported an Ir abundance anomaly precisely at the palynological K-T boundary in nonmarine rocks in the Raton basin, New Mexico (Orth and others, 1981). The discovery was made in core samples from a hole drilled specifically for palynological and chemical analyses at the York Canyon coal mine about 35 miles west of Raton. Prior to this, all reports concerning the hypothesized asteroid impact at the K-T boundary were from marine rocks, and some geologists had suggested that marine processes concentrated the anomalous Ir; therefore, the Ir need not be extraterrestrial. The discovery of an Ir abundance anomaly at the K-T boundary in nonmarine rocks in the Raton basin removed this objection and provided important supporting evidence for the Berkeley team's hypothesis. In later studies, shock metamorphosed quartz grains were found coincident with the Ir anomaly (Bohor and others, 1984; Pillmore and others, 1984; Izett and Pillmore, 1985a, b); these unusual grains are further evidence of a major impact event. Shock-metamorphosed mineral grains are found only at known meterorite impact sites or nuclear bomb craters, never in volcanic rocks.

Figure 1. Click image to see a larger version.

Since the original Raton basin discovery, many more K-T boundary sites have been found in the east central and southern parts of the basin (Fig. 1), and other continental K-T sites have been located in Wyoming, Montana, North Dakota, Alberta, and Saskatchewan. On this web site we will explore several of the K-T boundary sites in the Raton basin to observe similarities and differences in the character of the boundary sequence and the enclosing interval of rocks.

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