The cretaceous/tertiary boundary: An extraordinary geological event

Sunday, May 24, 1992

Art Sweet

The extinctions of the dinosaurs, ammonities, reef building rudists, inoceramid clams and some plants have historically been used to mark the end of the Cretaceous 66.4 million years ago. However, the vagaries of the macrofossil record allowed considerable latitude in both the placement of the Cretaceous/Tertiary (K/T) boundary and in hypothesized causal events. Was the change in the biota gradual or abrupt? Was the causal event sudden or long term? Was the change synchronous worldwide? Without a worldwide time-line defined independent independently from changes in the biota these questions were unanswerable.

The breakthrough in answering these questions was the discovery by Louis Alvarez and associates (Berkeley University) of a geochemical (iridium) anomaly at Gubbio, Italy representing a geologically instantaneous event associated with a dramatic faunal change. Subsequently a thin claystone layer associated with the K/T extinction event and containing anomalous elemental abundances, shocked quartz, soot, microspherules and evidence of carbon and oxygen isotope excursions, has been recognized worldwide, confirming the universality of a physically and biologically defined K/T boundary.

The above features have been used to infer that one or more extraterrestrial bodies impacted the earth at the close of the Cretaceous, although this idea is not without detractors who would favour high energy, eruptive K/T events originating within the earth. Recently tektite-containing K/T boundary deposits have been described from the Caribbean region and Mexico and a 180 km diameter subsurface crater, of possible K/T boundary age, has been located in the Yucatan Peninsula by Alan Hilderbrand (GSC, Ottawa) and co workers.

A historical perspective and research on the K/T contact/boundary in western Canada

G.M. Dawson in 1875 wrote "There is, perhaps, no more interesting problem in American geology, than that presented by the later deposits of the interior region of the continent, and especially by the part of them supposed to represent the closing epoch of the Cretaceous and the introduction of the Tertiary. This question is at the same time one of considerable difficulty, so much so, that the systematic position of a great series of beds, in some places several thousand feet in thickness, and including the greater part, if not the whole, of the valuable lignite deposits, is at present in doubt." Although the questions about the K/T boundary have become more refined, Dawson’s 117 year old sentiments are still applicable.

An example from central Alberta highlights the evolution of thought on the position K/T boundary. Geological observations reported by A.R.C. Selwyn in 1874, and J.B. Tyrell in 1887, resulted in the introduction of the terms "Paskapoo Series" and "Edmonton Series" for rocks exposed along the Red Deer River of Tertiary and Cretaceous (dinosaur-bearing) age respectively. J.A. Allan and J.O.G. Sanderson in 1945 defined the contact between the Paskapoo and Edmonton Formations to coincide with the base of a massive buff sandstone and interpreted this contact to represent a K/T disconformity or gap in time.

The perpetuation of the idea that the K/T contact coincided with this disconformity was aided by C.M. Sternberg’s 1949 report of dinosaur bones to within several meters of the base of the Paskapoo Formation. More recently an error was found by J.F. Lerbekmo (U. of Alberta) in C.M. Sternberg’s recorded stratigraphic position of these dinosaur fossils. The stratigraphically highest dinosaur bones are now accepted as being about 33 meters below the base of the Paskapoo Formation (4 meters below the lowest (Nevis) coal in the "upper Edmonton". These revisions allowed the search for the K/T boundary to focus on an interval of conformable strata contiguous to the Nevis coal.

For the last 25 years range truncations in the upward extent of fossilized angiospermous (flowering plant) pollen and changes in the relative abundance of fern spores have been used to recognize the position of the K/T contact within terrestrial sediments in midcontinental North America. These abundant (about 10,000 per gram of rock) microscopic fossils provide finer stratigraphic control than is possible with the erratic occurrence of rarer dinosaur fossils. Applying the above approach and experience gained from examining the K/T boundary in Southern Saskatchewan and the foothills of Alberta, the optimum horizon for the boundary in the Red Deer River Valley was identified in 1984 to be at the approximate base of the Nevis coal by A.R. Sweet (GSC, Calgary). Here (Fig. 1) the uppermost limit in the stratigraphic range of several late Maastrichtian angiosperm pollen species truncate and major changes in spore and pollen dominances occur in overlying beds. Confirmation of this being the K/T boundary came with the finding of a claystone, an iridium anomaly (J.F. Lerberkmo) and shocked quartz (B.F. Bohor, USGS).

As fossil pollen and spores have proven to provide high resolution and reliable chronostratigraphies at the K/T boundary, associated coal seams can be correlated throughout the extent of the Western Canadian Sedimentary basin. Since the mid 80’s over 25 K/T boundary localities have been found over an area extending from along the MacKenzie River, N.W.T. to southwestern Manitoba. At these localities, the K/T contact is usually within a conformable sequence of terrestrial sediments and generally coincides with, or occurs just above, the base of a coal (inferring a regional pre-and/or post-boundary increase in wetness).

The detailed studies of physical and biotic changes across the K/T boundary is ongoing through a series of cooperative studies (initiated by J.F. Lerbekmo; A.R. Sweet and D.R. Braman, Royal Tyrrell Museum of Palaeontology) and these will be enhanced with the recovery of unweathered material under the auspices of the Canadian Continental Drilling Program. Less than centimetre sampling intervals and multidisciplinary approaches are now required to make a contribution to K/T boundary research. In both marine and terrestrial sections in North America, there is a compound boundary claystone. The lower layer is considered to represent the impact ejecta projected through the atmosphere and a second stratigraphically higher layer the supra-atmospheric fallout and possibly redeposited material. At a resolution of millimeters, ongoing research has shown that a floristic shift occurs within the boundary claystone – an observation critical to understanding the cause and effect relationships between the K/T boundary biological crisis and the casual event, and one providing the impetus for further K/T boundary research.

Ed. note: if funded, the first scientific target of the Canadian Continental drilling Program will be the K/T boundary strata mentioned above. We shall keep you informed of progress on this project when (or if) drilling takes place later this decade.

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