Features of the K-T breccia layer

Norm King

Several weeks ago the significance of the breccia layer at the K-T boundary received some discussion. It has been claimed to be the result of one or more tsunamis produced by the presumed K-T impact at Chicxulub. However, critics of the impact hypothesis attribute it to erosion caused by a drop in sea level; it is claimed to be a simple sequence boundary.

I have just returned from the national GSA (Geological Society of America) meeting in Denver, where there was a poster session on impact breccia layers. Examples were from the Upper Devonian of Nevada, the Tertiary Chesapeake Bay crater, and the K-T boundary "megabreccia." It was called megabreccia because in the Yucatan it contains clasts up to 10 m in measured dimension (i.e., could actually be larger). I have seen many sequence boundaries -- sequence boundaries are my friends; this is no sequence boundary (with apologies to Lloyd Benson)! Sequence boundaries are often difficult to pin down. But this is a spectacular deposit of the coarsest-grained chaotic material I've ever seen.

The breccia layer is up to 100 feet thick, containing striated pebbles, cobbles, and boulders, in a matrix of finer material including sub-spherical clasts of green recrystallized glass. The striations are believed to have formed by a partially melted exterior scraping across country rock, or abrasion by smaller, high-speed projectiles. The striations include the "nail-head" variety (they look like the impressions of nails, with a short crosspiece on one end), which previously have been claimed to be diagnostic of glacial deposits (but by a circular argument). Some of the clasts are pitted by numerous small (1 cm) apparent impact pits, again from smaller projectiles. The exhibitors had samples for us to handle and look at close up.

There was no Late Cretaceous glaciation in the Yucatan. And clearly we now must re-evaluate certain supposed tillites from the Precambrian and other ages (like Late Devonian), and review the paleoclimatic inferences based upon them.

The Yucatan K-T breccias also contain crystalline dolomitic lapilli of spherical, ovoid, and tear-drop shape up to about 3 cm in maximum diameter, some with peculiar bright green outer rims. These are new features that have not been found previously in volcanic deposits on the Caribbean region. They did not come from India, either. You've really got to see these things in person folks! It's easy to be skeptical until you actually see the data; in the history of geology, many people have made fools of themselves by pronouncing on ideas they lack personal familiarity with -- this is not the mindless bandwagon phenomenon that Officer claims in his book (The Great Dinosaur Extinction Controversy). There may be an alternative explanation besides near-by impact, but I think we will soon reach the limit of our ingenuity in finding ways to explain such features as the result of something other than an impact. As more evidence accumulates, we have seen that it continues to be consistent with a major impact in the Yucatan area at precisely the end of the Cretaceous, while alternative explanations for the same data are becoming ever more scattered, careless, and labored.

I had the opportunity of discussing these features with Walter Alvarez [an originator of the K-T impact theory], Michael Rampino, and Kevin Pope, among others. They had virtually nothing to say about Officer's book, feeling that it was pointless at this time, in view of the fact that Officer's arguments had already been refuted or shown to be equally consistent with the impact scenario before the book came out.