Th’ Gaussling drove to the southern border of Colorado to have an up close and personal view of an exposure of the K-T boundary. The coordinates are N 37º 7.335′, W 104° 36.248′. This exposure is perhaps 150 meters in length and is no more than a quarter mile hike from the parking area. The exposure is within Trinidad Lake State Park, so a $7 one day park pass is required for entry. 

The term “K-T boundary” refers to both a layer of sediment and to a stepchange transition in paleoecology. The sediment layer was laid down at a time coincident with an extensive plant and animal extinction event. This period and the sediments put down then make up the boundary between the Cretaceous and Tertiary periods. Nomenclature alert:  According to some sources, the use of Tertiary time or rock is discouraged in favor of Paleogene and Neogene. Under this terminology, it is referred to as the K-Pg boundary.

It is widely accepted that a large impactor collided with the earth forming the Chicxulub Crater approximately 65.5 MA.  The Chicxulub  (CHEEK sheh loob) crater was first observed from gravity mapping by Robert Baltosser in the 1960’s and later rediscovered by geophysicists Camargo and Penfield while doing geomagnetic work for Pemex in 1978.  Pemex would not allow the disclosure of the data supporting the presence of the crater for several years. Eventually, Penfield was allowed to disclose their work at a conference.

The Chicxulub crater is found below the surface along the northern coast of the Yucatan Penninsula in Mexico.  Gravity maps show evidence of a circular feature consistent with an impact crater. Sediment associated with the impact contains tektites, shocked quartz, vitrification and elevated levels of iridium. A common mistake propagated in the popular literature is that the layer consists of iridium. In fact the layer contains variously ppt or ppb levels of this platinum group element.

The theory of asteroid impact arose from the anomolous Ir content of the thin sediment layer found to have been deposited at the time of the K-T extinction. Geologist Walter Alvarez, son of non-other than Manhattan Project physicist Louis Alvarez, determined that significant iridium was found only in the K-T boundary layer and not in the layers above and below.  The determination was had via neutron activation analysis and was carried out at Lawrence Berkeley Laboratories.  After a variety of postulates were considered, the theory of asteroid impact ending the age of dinosaurs was born.  The theory was disclosed in 1980, but was met mostly with derision. However, with the disclosure of the Chicxulub crater in the late 1980’s, the theory has since met with widespread acceptance.

Across the earth in what is now India and roughly contemporaneous (68 to 60 MA) with the Chicxulub event was a prolonged period of extensive vulcanism. The formation from that period that remains today is the Deccan Traps.  According to one source, the word “trap” is a geological term from the Swedish word for “stairs”. This period of vulcanism is thought to have produced enough atmospheric pollutants to have raised the average atmospheric temperature by 2º C and enough lava to have covered half of present day India.

Whether or not the fullness of the transition from Cretaceous to Paleogene is due to the Chicxulub event or in combination with the Deccan vulcanism is unclear. What is clear is that the Chicxulub impactor delivered an estimated 4.0 E17 MJ jolt of energy to the planet, resulting in mega-tsunamis throughout what is now the Caribbean basin and the injection of vast amounts of dust and aerosols into the atmosphere.  

An exposure of the K-T boundary can be found in an outcrop just west of Trinidad Lake in southern Colorado. The thin, off white layer lies within a seam of coal and under a cap of sandstone at this location. Note the rock hammer for scale.

Exposure of the K-T Boundary, Trinidad, Colorado.

Side view of the K-T Boundary sitting under a cap of sandstone. Note Sharpie marker just below layer.

K-T layer in context.

The K-T layer at the Trinidad site is comprised of claystone which is weathered and crumbles easily. If the material contains parts per trillion quantities of iridium, then sending a sample out for GDMS is likely to be futile.

North of Trinidad are the Spanish Peaks. These peaks are of volcanic origin and are associated with a substantial array of dikes, a geat many of them visible from the road. The photo below was snapped from a roadcut during a recent rainstorm. To the west is the Sangre de Cristo Mountain range. This range is what is called a horst, which is an uplifted block of crust. Just west of this range is the San Luis Valley containing the Rio Grande rift formation.

Dike formation south of La Veta, Colorado
Dike formation south of La Veta, Colorado.

Recently on an airplane I sat next to a 1948 chemistry graduate of UC Berkeley. We were enroute to John Wayne airport from Denver.  As we both marveled at the majestic topography of the Grand Canyon below she told me of her experience of having both Luis Alvarez and Glenn Seaborg as professors. Alvarez, she said, gave an exam with 7 % as the high score. She shook her head, laughed, and asked, “can you imagine”?

 

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