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K-T breccia layer
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 boundries
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,
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.
Norman R. King tel: (812) 464-1794
Department of Geosciences fax: (812) 464-1960
University of Southern Indiana
8600 University Blvd.
Evansville, IN 47712 e-mail: email@example.com