Geoscience Reference
In-Depth Information
Albion Island is important not only because it is the nearest Chic-
xulub deposit to ground zero, but because unlike all the others so far
discovered, it appears to have been deposited above sea level, with-
out the stirring and mixing effects produced by deposition in water.
The possibility that the impactor might have struck in the ocean
and generated a tsunami, which in turn would have left behind char-
acteristic deposits, was first suggested in 1985 by Smit and Romein.
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They studied the K-T boundary in the Brazos River country of Texas,
famed in American song and story, and found a rock known as tur-
bidite (from the Latin
turbidus,
or disordered; with the same root as
bioturbation). Turbidites are thought to be produced when sediment,
jarred loose at the head of a submarine canyon by an undersea earth-
quake, cascades down as a submarine "landslide" and flows outward
for hundreds and even thousands of miles. Turbidity currents pro-
duce layers of silt and sand several meters thick, with the coarsest
material, which settles out first, at the bottom, grading upward to the
finest sediment at the top. However, a giant wave, such as would be
produced by a large meteorite striking near shore, would generate
an earthquake which would also stir up sediments and produce a
turbidite-like effect. It struck Smit and Romein as important that the
only occurrence of such a rock type in the Brazos country was exactly
at the critical K-T boundary, especially when that boundary also con-
tained an iridium anomaly. Sedimentologist Joanne Bourgeois of the
. University of Washington concluded that the Brazos River turbidite
was created by a tsunami 50 m to 100 m high.
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Subsequent field-
work identified many other examples of possible impact-generated
sedimentary deposits in the Gulf-Caribbean region. The one that by
now is the most thoroughly studied is at Arroyo el Mimbral, one of
several outcrops in northeast Mexico that expose the boundary layer.
"The significance of the Mimbral section lies in the combination
of altered and unaltered glassy tektites, shocked minerals, anomalous
iridium abundance, continental plant debris, and evidence for deep-
sea disturbances and coarse-sediment transport precisely at the K-T
boundary," according to Jan Smit and co-authors.
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Charles Officer,
Wolfgang Stinnesbeck of the University of Nuevo Leon, and Gerta
Keller of Princeton were quick to disagree.
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They too had studied
Mimbral and "found no evidence of a nearby impact." Each piece of
evidence presented by Smit and his co-authors, Stinnesbeck and
company explained away. A key point in the disagreement was the
position of the K-T boundary, which they placed above the level of
Smit's uppermost unit, inevitably causing them to conclude that the
rocks below it were older than, and therefore could not have been
caused by, the K-T event. According to Smit, however, Stinnesbeck
