K-T Boundary Layer - Long's Canyon Site

The aim of this EarthCache is to get an up-close view of the Cretaceous-Tertiary (K-T) Boundary and get an overview of where the clay layer is located in relation to nearby rock strata. Parking for the trailhead is in a gravel parking area at the trailhead for the Watchable Wildlife area in Trinidad State Park, and is located at N 37 07.104, W 104 36.221. There is a $6.00 per vehicle daily usage fee. The trailhead begins at the parking area and heads along a small stream for about 0.25 mile to the Trinidad Lake K-T Boundary Natural Area. The K-T boundary layer is marked by a trailside sign.

Geologic Time is measured in intervals where the remains of certain plants and animals are found in one time period but not in an adjacent time period. This is especially true at the Cretaceous-Tertiary Boundary, where an event occurred approximately 65 million years ago which caused the extinction of over half of the species on Earth. Among the causes examined for the extinction of such a great number of species was climate change, sea level change, a rise in solar radiation levels, and extensive volcanic activity. In 1980, a group from the University of California at Berkeley, proposed an extraterrestrial cause for the mass extinctions. Their hypothesis was that an asteroid approximately 6 miles in diameter struck the earth, causing a large amount of rock to be blasted into the atmosphere. This material, heated to incandescence, would start huge forest fires as it rained back down to earth. Fine particles in the form of ash and dust would stay suspended in the atmosphere for years, causing “nuclear winter”-like conditions, and dropping temperatures far below their norms. The Asteroid Impact Theory is currently the most accepted theory for the K-T mass extinction event.

Evidence for an asteroid impact at this time comes from the claystone layer that marks the K-T boundary. This clay layer comes from altered glass ejecta material that was formed from the impact. This material is from the vaporized meteorite, along with material blasted out from the impact site. The clay layer contains high concentrations of Iridium, with up to 8,000 times the background level, and is well known as the “Iridium Layer.” Shock metamorphosed quartz is also found in the claystone layer. This occurs from extremely high pressures associated with the high speed impact, and has only otherwise been observed in conjunction with nuclear explosions.

K-T boundary layers with the claystone component are rather rare worldwide, although there are at least 17 known sites in the Raton Basin. In this area, the K-T boundary interval is found in the Raton Formation, which consists of sandstone, claystone, siltstone, shale, coal, and conglomerate. The material forming these layers was deposited on an alluvial plain. At the time of the impact, the alluvial plain was an ideal environment for deposition and preservation of the impact ejecta and fallout.

As you approach the site, the rock containing the K-T boundary interval is on your left as you reach the park sign. This is considered one of the best exposures of the K-T boundary sequence in the world! The bottom 25’-30’ of the slope consists of interbedded siltstone and mudstone, and is highly erodible. The K-T claystone layer is just beneath the sandstone ledge at the top of the slope, and is overlain by a thin 1”-2” coal layer, and underlain by an 8” shale and coal layer. The clay layer is a pale silvery gray.

To log this EarthCache, please email me the answer the following questions: 1.) According to the sign at this site, where did the asteroid strike, and how wide was the crater? 2.) What percentage of life on Earth was killed? 3.) Besides Iridium, what else is included in the boundary layer as evidence for an asteroid impact?

Please consider posting photos of yourself, or the local geology, when you log this EarthCache. Photos can be an additional rewarding part of your journey, but posting them is not a requirement for logging this EarthCache, and is strictly optional.

The above information was compiled from the following sources:

Trinidad State Park sign located at site.

Trinidad State Park informational pamphlet.

2000. Morgan, M.L. The K/T Boundary Impact Layer of Southern Colorado and its Relation to the Chicxulub Crater, Mexico; in, Field Trip Guidebook A Dash with the Dinosaurs: A Mountain Bike Trek to the Purgatoire River Dinosaur Trackway and the Cretaceous-Tertiary Boundary Impact Layer of Southeastern Colorado, La Junta and Trinidad, Colorado.

2006. USGS. Online Guide to the Continental Cretaceous-Tertiary Boundary in the Raton Basin, Colorado and New Mexico. Adapted from Pillmore, C.L., Nichols, D.J., and Fleming, R.F., 1999, Field Guide to the Continental Cretaceous-Tertiary Boundary in the Raton Basin, Colorado and New Mexico: Geological Society of America, Field Guide 1.

Thanks to the Trinidad State Park for allowing placement of this EarthCache!