The Colorado Geological Survey Asks, “Did You Know?”

Title: The Colorado Geological Survey Asks, “Did You Know?”
Author: Vince Matthews
Publication: The Outcrop, February 2002, p. 12

Did you know how important Colorado is to the Cretaceous/Tertiary boundary story? In the 1940’s South Table Mountain was the first place in the world where the K/T boundary was described in terrestrial rocks; dinosaur bones below, only mammals above. This site is so significant that the National Science Foundation held its 50th birthday party on the outcrop at South Table Mountain.

Once the K/T boundary was proposed to be the result of an impact, Colorado sites and geologists played key roles in proving the impact hypothesis, finding the crater, and proving that Chicxulub was the source of the impact debris. The existence of the iridium anomaly at the K/T boundary in coal-swamp deposits in Colorado proved that the iridium anomaly was not just some phenomenon created by seawater. Indeed, Colorado has the highest iridium anomaly ever measured in terrestrial rocks. Colorado sites also showed the presence of shocked quartz grains that could only be caused by impact. The size of the shocked quartz fragments and the existence of two layers in Colorado sites also indicated to crater searchers that the impact must have been located near North America.

Once Chicxulub was proposed as the site of impact, Colorado once again provided key evidence confirming it. The nail was put in the coffin by comparing the lead-loss of zircons from Chicxulub’s rocks with the lead-loss of zircons in the K/T boundary layer from a number of sites. Most of these samples were from Colorado. Colorado also provided the world with its first known shocked zircon grains.

The dozen sites in southern Colorado that preserve the K/T boundary layer are so important that the Smithsonian Institution archived a 2 1/2-ton sample of the K/T boundary from south of Trinidad and has it on display in Washington, D.C. A good place to observe the K/T layer is in Trinidad Lake State Park, where a new information sign is being erected on the Long Canyon Trail.

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