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Chicxulub: Not an impact structure?
Dear List Members!
Here are two articles from the ‘earlier days’ of the K-T impact theory
1) Possible Yukatan Impact Basin (S&T, Mar 82, pp.249-250):
Did the impact of a small asteroid cause the extinction of the dinosaurs
and other life forms 65 million years ago? Scientists began to consider
the possibility when iridium-rich deposits dating to that time were
discovered in Italy in 1978. Since then similar deposits have been found
in several locations around the world. Iridium is rare in terrestrial
rocks, but much more common in meteorites.
If an asteroid had struck the Earth, scientists reasoned, a large
quantity of iridium-bearing dust would have been ejected into the
atmosphere at high altitudes, blanketing much of the world as it settled
out.The dust could have blocked out the Sun for the better part of six
months, cutting off photosynthesis and starving many animals to death.
If the impact had occurred in the ocean, the results would have been
similarly devastating. Immense tidal waves would have resulted. In
addition, the enormous volume of water thrown into the upper atmosphere
would have intensified the greenhouse effect, raising worldwide
temperatures by perhaps several degrees. This would have killed off not
only dinosaurs, but sea plankton and disrupted the food chain.
Critics of the theory have cited the lack of recognizable impact craters
dating to that time.One researcher, however, believes such a feature may
lie beneath a kilometer of rock in Mexico's Yukatan peninsula.
At a meeting of the Society of Exploration Geophysiscists last October
(1981) Glen Penfield, a geophysiscist with a Houston-based exploration
firm, and a Mexican scientist reported the results of a 1978 geomagnetic
survey of the Yukatan peninsula. The survey revealed in buried marine
deposits a circular pattern of anomalous magnetic signatures 60 km
across, surrounded by a second ring of anomalies 180 km wide. The
feature is centered on the town of Progreso (21.2° N, 89.4° W).
When Penfield checked an earlier survey of gravity signatures in the
region, he found a similar bull's-eye pattern.
At first puzzled by the extraordinary results, Penfield says he "lit up"
when he read of the asteroid collision theory arising from the discovery
of the iridium-rich layers. He believes the feature, which lies within
rocks dating to Late Cretaceous times, may be the scar from a collision
with an asteroid roughly 10 km across. He notes that the gravity
signature bears a striking resemblance to those of other terrestrial
impact features, such as Quebec's Manicouagan.
In this case, however, the bull's-eye is even more distinct, for unlike
Manicouagan there are no dense crustal rocks at the same depth to
disrupt the gravity signature. Penfield adds that, geolo-gically, the
Yukatan has been relatively stable for the past 160 million years, so
that a buried impact crater would remain fairly well preserved.
If the buried feature proves to be such a crater, scientists debating
the dinosaurs' plight will want to know how old it is. Dating it will be
possible once samples have been obtained and analyzed. At present, only
a few pieces of reddish, fine-grained rock have been brought up by
drilling. They resemble a terrestrial volcanic product called andesite,
but Penfield notes there is no evidence for any volcanic activity in
that area. Furthermore, he points out, andesite can be difficult to
distinguish from impact melt, a form of refrozen rock found in impact
craters, and analysis of the samples may add to the evidence for a
buried crater.
Penfield and his collegues have conducted computer modeling studies
based on the gravity and magnetic data. The results indicate 2
components of the feature that would be expected from an impact event.
The first is a zone of dense and highly magnetic uplifted strata at the
center, which the scientists believe may be the result of crustal
rebound following the impact.
The second is a ring of low-density rock roughly 80 km from the center;
Penfield suspects that this is a deposit of broken and melted material
ejected from the proposed crater.
Penfield believes no other geologic explanation accounts for the feature
as well as an impact origin does. He is looking forward to obtaining
more samples, and says the feature's 1100-meter depth will not be a
great deterrent.
In addition, the layer of low-density rock is an especially interesting
target because similar deposits in another impact crater proved to be a
reservoir of oil.
2) Shocking Impacts, S&T, Jul 87, p.13:
Three years ago, tiny grains of strongly shocked quartz were discovered
in ancient clay sediments in Montana. This finding supports the notion
that a gigantic impact 66 million years ago covered the Earth with dust
and doomed many plants and animals to extinction. In the May 8th issue
of Science, Bruce Bohor and two collegues report finding shocked quartz
in similar sediments from New Zealand, five European sites, and the
north-central Pacific Ocean floor.
The grains endured pressures of 90,000 to 300,000 atmospheres - far
greater than could be produced in a volcanic event. An impact is the
only reasonable cause for this global dusting of altered material, and
Bohor's team thinks the event took place on a continent rather than in
the ocean.
Regards, Bernd Pauli