The K-T Boundary In The Hell Creek Formation
Eastern Montana, United States

By Phillip Bigelow

Modified from Retallack (1996) and Fastovsky (1987)

In the Hell Creek Formation, the boundary section that represents the transition from the Cretaceous period to the Paleogene is surprisingly thin (as well as being surprisingly unremarkable in it's overall appearance; a person who isn't a geologist would probably be unable to recognise it in the field).

The strata that defines this transition differs somewhat from place to place around the world, but in all localities the duration of this period of change appears to have been quite short. In fact, evidence bolsters the long-held belief that the change was sudden (One of many such studies is: Mukhopadhyay, S., K. A. Farley, and A. Montanari. 2001. A Short Duration of the Cretaceous-Tertiary Boundary Event: Evidence from Extraterrestrial Helium-3. Science 291:1952-55.)

In the Hell Creek Formation of eastern Montana, the so-called 'boundary bed' (see illustration) is (quoting from Retallack, 1996) "2 centimeters thick, pink to white, kaolinitic, micro-spherulitic, and vuggy". It appears to represent a highly acid-leached layer from impact-caused air-fall shortly after the impact. Retallack (1996) says " is now regarded as an early-settling fraction of altered ejecta from bolide's distinctive kaolinitic composition requires reaction with acid...." Retallack believes that this leached layer is a result of acid-rain that was produced by the impactor (bollide)1 hitting anhydrite target rock at Chicxulub2, Mexico. Anhydrite rock (a form of dehydrated gypsum) produces sulfur dioxide when vaporized. When sulfur dioxide mixes with atmospheric water, it creates sulfuric acid (Howells, 1990) (see also Whitmore, 1985 and Sigurdsson et al. 1992). The subsurface of the Chicxulub area is rich in anhydrite rock (Brett, 1992).

It has also been noted by various workers that this particular clay layer is very plastic when wet, which distinguishes it from other Hell Creek Formation tonsteins (a tonstein is an acid-leached clay which is dominated by the clay mineral kaolinite).

Overlying the "boundary bed" is another impact-related layer (the so-called "impact bed"; see illustration). Quoting from Retallack (1996), the 'impact bed' is "one centimeter thick, gray, smectitic, and layered, with shocked quartz and an iridium anomaly." This layer is not acid-leached as the underlying layer was, and it probably settled-out of the atmosphere over a longer period of time than did the sediment found in the boundary bed, thereby avoiding the prolonged effects of acid-rainfall.

Overlying the 'impact bed' is a layer of low-grade coal (called lignite), which represents quiet-water deposition, possibly in swamps, shallow lakes, oxbows, or a combination of the above. These various paleoenvironments are sometimes lumped into the category "peat mires". Compared to the lower parts of the Hell Creek Formation, the roughly 10 centimeters of rock and lignite that overly the impact bed contain an unusually large amount of fossil fern spores. The scientific literature refers to this sudden increase in fossil fern spore abundance as the "fern spike". In modern environments that have experienced forest fires, fern "blooms" are often the first re-colonizers in the devastated area.

From the illustration above, note that the very top of the Hell Creek Formation is Paleocene in age, not Cretaceous. This rock formation boundary is, for all practical purposes, arbitrary, and was defined in the early part of this century as "the lowest persistent bed of lignite" that is above the highest in-place dinosaur (Calvert, 1912; Brown, 1952). Once a definition of a formation boundary has been published, it becomes authoritative and has priority. Only in recent years (since the 1980s) has it been discovered that the Cretaceous-Tertiary boundary (a time boundary) does not coincide exactly with the Hell Creek Formation-Tullock Formation boundary (a rock boundary). Depending on where you look, the stratigraphic distance (vertical thickness) between the iridium anomaly and the lignite layer can vary from as little as a couple of centimeters to over a meter. In most places, the iridium anomaly is entirely absent. Paleocene erosion of the iridium-containing boundary clay layer is probably the most likely culprit, although the effects of burrowing animals (called "bioturbation") during the earliest Paleocene also could destroy any sign of the clay layer.

Further complicating the stratigraphic picture is that this "basal" lignite is not found in all places where the K-T boundary exists. And in some cases, the "lowest persistent lignite" occurs below the the K-T boundary. In North Dakota, there is one instance of a Triceratops skeleton occuring in the overlying Fort Union Formation (Johnson et al.), however, the fossil was still 1.4 meters below the K-T boundary. This means that the "lowest persistent lignite" bed cannot be used as a reliable indicator of the position of the K-T time boundary. It is, however, an excellent criterion for recognizing the contact between the Hell Creek Formation and the overlying Tullock ("Fort Union") Formation.

Most of this infomation was compiled from Fastovsky (1987), Retallack (1996), Fastovsky et al., (1989), Izett (1990), and from my own unpublished field observations (see selected K-T boundary references for these references and more sources of information on the K-T impact and it's effects).


1The most widely-accepted estimate of the energy released in the impact is roughly 75 million megatons to 100 million megatons. In contrast, the energy released in the Meteor Crater impact in Arizona was only about 10 megatons. Do the calculations yourself. Radiometric dating methods (40Ar -39Ar) show that the melted rock at the bottom of the Chicxulub crater is 65.07 +/- 0.1 million years old (Swisher et al., 1992). This date is extremely close to two separate age estimates for the K-T boundary in the North American interior (65.00+/- 0.04 Ma; Gradstein et al. (1995:102)) (65.4 +/- 0.1 Ma; Obradovich (1993).

2Chicxulub is pronounced "CHICH-a-loob" (Mayan language). The crater was named after the nearby village of Chicxulub, Mexico. Coincidentally, and quite appropriately, "Chicxulub" means "fire of the beast" in Mayan. The structure was discovered by G.T. Penfield, a geologist who worked for the Mexican state oil company. Later, A. R. Hildebrand learned of Penfield's discovery and was the first to hypothesize the connection between the crater and the K-T extinction boundary. There is some disagreement as to who actually proved that the Chicxulub structure was an impact crater. Both V. L. Sharpton's team, and A. R. Hildebrand claim that they were first!

Hell Creek Life © 1997-2010 Phillip Bigelow
Revised 3/11/2010