Pollen, Iridium, and Asteroids
by R. Farley Fleming

One goal of the Denver basin project is to locate the Cretaceous-Tertiary (K-T) boundary. This boundary marks a mass extinction event in earth history that includes the extinction of dinosaurs. In many places in western North America, the boundary is also marked by the presence of an unusual concentration of iridium, which is thought to have been deposited after an asteroid hit the earth 65 million years ago. Many scientists attribute the mass extinctions at the K-T boundary to this asteroid impact.

As we were drilling at Kiowa, I collected a suite of samples for palynology—the study of fossil pollen and spores. Some species of fossil pollen disappear precisely at the K-T boundary and these microscopic fossils have proven very valuable in locating the K-T boundary. Because of the importance of the K-T boundary in our project, I focused primarily on locating this boundary during the drilling at Kiowa.

We designed an on-site sampling protocol for locating the K-T boundary. Samples were collected every 25 feet throughout most of the core, and every 10 feet when we were drilling through the interval where Bob Raynolds predicted the K-T boundary would be found. These samples were shipped by express courier to a laboratory in Canada for processing and then shipped back to me for analysis. None of the samples from the upper part of the core contained Cretaceous pollen, but they did contain pollen typical of the early Tertiary. The first sample with Cretaceous pollen came from a depth of 881 feet below the surface. Detailed sampling then revealed that the K-T boundary was located below 878.38 feet and above 880.17 feet. Thus, the K-T boundary is located somewhere in this short interval.

Finding the position of this boundary in the core provides the Denver Basin Project with an important time horizon that can be used to support other studies in the Denver Basin Project. It helps place the core in stratigraphic context, which is critical for developing the stratigraphic framework for our hydrogeologic studies. It also gives us important information that can be used in locating the K-T boundary in other places in the Denver Basin and could lead to the discovery of sites with the K-T boundary claystone, which contains the iridium from the asteroid impact 65 million years ago.

The Kiowa core still holds much potential for palynological studies. Ongoing work focuses on constraining the age of the paleosol that was recovered as well as developing an overall biostratigraphic framework for the core. This framework will be extended throughout the Denver Basin and will provide a foundation for understanding the geologic evolution of the Denver area. In addition, the fossil pollen and spores will help us construct a picture of the vegetation that was growing in the Denver area millions of years ago.

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