K-T Boundary Layer - Madrid Site
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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. You will be measuring the clay boundary layer, so please bring a ruler, or measuring tape. Parking is along the road, and please pull over as far as possible since the site is close to a curve. This EarthCache is not difficult, but you will be hiking a short distance up a very steep slope underlain with eroded clay and siltstone. Please wear good footgear and watch your step.
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”, 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.
The coordinates will lead you to an area at the side of the road under a portion of the Raton Formation. The bottom 20’-25’ 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 2” coal layer, and underlain by an 12”-16” brownish-orange coal layer. The clay layer is a pale silvery gray.
To log this cache, answer the following questions: 1.) With your ruler, measure the clay boundary layer. What is the thickness of the clay boundary layer at this location? 2.) Additionally, estimate the thickness of the overlying sandstone layer.
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:
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.
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