Geoscience Reference
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and colleagues placed the K-T boundary at the appearance of new
Tertiary foraminifera. This is the way geologic boundaries have tra-
ditionally been set: at the level of first appearance of one or more
abundant new species. However, a mass extinction caused by impact
might delay the evolution of new species for tens of thousands or
hundreds of thousands of years, until conditions were again hos-
pitable. Therefore, Smit argued, to use the first appearance of new
species to mark the boundary in the case of impact will always place
the boundary too high, making it appear younger than it really is
and by definition making the rocks immediately below it appear
older than they really are. In contrast, ejecta and turbidites produced
by impact would settle within a few hours or days of the event and
would have exactly the same age as the impact.
On the contrary, Stinnesbeck and company argued, the deposits
at Mimbral and elsewhere formed through normal geologic pro-
cesses, possibly as coastal sediments slumped into deeper water. In
that view, the Gulf of Mexico K-T sections, including Mimbral,
were not impact-generated, but are of pre-K-T boundary age and
were probably deposited by turbidite or gravity flows.
3
5
Thus the
debate turns on whether the Gulf K-T deposits, as Alfred Fischer
put it in an address at the Snowbird III conference, "formed in
100,000 seconds [1.15 days] or 100,000 years."
3
6
The controversy resembles the one over the sharpness of the
iridium peak at Gubbio. In both cases, experts viewing the same evi-
dence come to opposite conclusions and debate the matter at length
in the literature. But having seen the example of the blind test at
Gubbio, the solution to the Mimbral controversy became obvious,
though it needed a modification: Since the outcrop could not be
brought to the geologists, the geologists had to be brought to the
outcrop. Accordingly, a field trip to Mimbral was organized at the
time of the 1994 Snowbird III conference, held not in Utah but in
Houston. In the party were four former presidents of the Society for
Sedimentary Geologists, including Robert Dott of the University of
Wisconsin, regarded by many as the dean of American sedimentolo-
gists. According to Richard Kerr, the
Science
magazine reporter who
has made the impact controversy a specialty, Dott spoke for these
experts on sedimentary rocks: "We were impressed with the evi-
dence that this sequence was very rapidly deposited. [It must have
taken] closer to 100,000 seconds than 100,000 years."
3
7
According to Kerr, the assembled sedimentologists concluded
that the most likely cause of what they had seen was an impact-
generated tsunami. Keller, ignoring Mark Twain's advice never to get
into a contest with a man who buys ink by the barrel, took vigorous
