Archive - Cretaceous-Paleogene Boundary

October 06, 2019

CPGoldenDSC05707 copy (6)


Each Sunday we present a notable item from our archives. This EPOD was originally
published October 1, 2013.

Photographer: James Van Gundy
Summary Author: James Van Gundy

The Cretaceous-Paleogene (K-Pg) boundary, formerly known as the Cretaceous-Tertiary (K-T) boundary, was first described in terrestrial (non-marine) rocks at the above site on South Table Mountain in Golden, Colorado back in 1943. This boundary actually isn't visually well-defined here but was recognized on the basis of fossil evidence long before the more recently applied evidence of the boundary was imagined. It occurs somewhere within the eroded slope of the badlands topography in the lower-right portion of the photograph. This site was judged to be so important to the history of science that the National Science Foundation held its 50th birthday party here in 2000. Note the tall buildings of downtown Denver in the distance, about 9 mi (14 km) directly to the east. Photo taken on January 30, 2011.

The K-Pg boundary is far more visually recognizable at a number of sites in southern Colorado and northern New Mexico. The lower photograph was taken this summer at Trinidad Lake State Park near the southern Colorado town of Trinidad. Not only is the Trinidad Lake site one of the best places in the world to view the famous K-Pg boundary layer clay, it's also one of the most accessible. A quarter-mile (0.5 km) level trail takes you from the trailhead to the site which is marked by a nice explanatory sign. Here the boundary is clearly exposed just below a layer of protective sandstone caprock. The boundary layer clay here contains abundant evidence of the catastrophic asteroid impact that likely caused the extinction of the non-avian dinosaurs along with perhaps 75 percent of all other species living at that time, roughly 66 million years ago. Such evidence includes high levels of the rare element iridium, shocked quartz grains, glassy microtektites, a pronounced decrease in the pollen of higher plants, and a sharp spike in fern spores. Photo taken on August 24, 2013.

Photo Details: Top - Camera: SONY DSC-W7; Focal Length: 7.9mm; Aperture: f/5.6; Exposure Time: 0.0025 s (1/400); ISO equiv: 100. Bottom - Camera: SONY DSLR-A100; Focal Length: 40.0mm (35mm equivalent: 60mm); Aperture: f/5.6; Exposure Time: 0.017 s (1/60); ISO equiv: 250; Software: DSLR-A100 v1.02.