NASA/Spacelink
5/9/91: NASA DISCOVERS IMPACT LIKELY TIED TO DINOSAURS' DEMISE
RELEASE: 91-71
The first surface evidence of a buried impact crater formed
by a comet or asteroid which may have caused the extinction of the
dinosaurs has been discovered by NASA researchers.
The scientists believe a ring of sink holes in the
northwestern corner of the Mexican state of Yucatan outlines the
largest known impact crater on Earth. The crater, which is more
than 125 miles in diameter, is a prime candidate in the search for
an impact that may have caused the planet-wide extinctions of
dinosaurs and other species about 65 million years ago.
Charles Duller of NASA's Ames Research Center, Mountain
View, Calif., discovered the ring formation in 1987 while searching
satellite imagery for water sources used by ancient Mayan cities.
Two other members of the research team -- Dr. Kevin Pope,
formerly of Ames and now with Geo Eco Arc Research in La Canada,
Calif. and Adriana Ocampo of NASA's Jet Propulsion Laboratory,
Pasadena, Calif. -- considered many other geological explanations
before concluding the formation was caused by a buried impact
crater.
"The apparent age, location and size of the proposed Yucatan
impact make it one of the best candidates for the global
catastrophic event, although multiple impacts remain a possibility.
Regardless, the Yucatan impact alone would have had a devastating
impact on the climate, animals and plant life of the Earth," Pope
said.
Some scientists believe such an impact pushed so much dust
and debris into the atmosphere that it blocked sunlight,
interrupting the growth of plants, starving dinosaurs and other
animals and freezing much of the Earth.
The results of the study by Duller, Pope and Ocampo were
announced in the current issue of Nature magazine.
The team's findings agree with the work of other scientists
who have found unusual circular gravity and magnetic patterns and
quartz fractured by an impact, all suggesting a buried crater in
the Yucatan.
The circular hydro-geological feature, which they named the
Cenote Ring (cenote is the local Spanish name for sink holes),
provides surface evidence of the buried crater's precise location
and size. It is centered near the town of Chicxulub, for which the
buried crater is named.
Duller mapped hundreds of water-filled sink holes which form
an almost perfect semicircle that marks the crater's buried rim.
Fresh water springs well up beneath the surface where the Cenote
Ring meets the shore line.
The sink holes are found in clusters at some places along
the rim and spaced up to a mile apart at others. They average 300
- 500 feet in diameter.
Duller and Pope determined the half-circle of sink hole and
severely fractured limestone outside and along the rim area. This
conclusion was verified through independent hydrogeological
research conducted by Dr. Luis Marin, now of the University of
Mexico, during his work on a doctoral thesis under Dr. Eugene Perry
at Northern Illinois University.
"As the buried crater rim settles over millions of years,
the rock on top slumps and cracks. Underground water flows
through the cracks on its way to the ocean. As the water is forced
around the unfractured rock in the center, the flow dissolves the
limestone, causing cave-ins that create the sink holes," Pope said.
Pope and Ocampo examined core sample data taken from nearby
exploratory oil wells and found they geologically date the buried
crater's floor at Late Cretaceous about 65 million years ago. The
crater floor has younger Tertiary sediments on top.
According to the fossil record, more than half of Earth's
plant and animal species, including the dinosaurs, disappeared
about 65 million years ago. This abrupt change in evolutionary
history occurred between the Cretaceous and Tertiary periods in
Earth's geologic history and is called the Cretaceous-Tertiary
(K-T) boundary.
Unusual amounts of the rare element iridium -- more abundant
in comets and asteroids than on Earth -- have been found in the K-T
boundary in many locations worldwide, leading scientists to believe
that a large extraterrestrial impact caused the planet-wide
extinctions.
Rock and melted glassy fragments, "blown out" by an impact,
have been found in the Caribbean and Gulf of Mexico region, causing
scientists to concentrate the search for the suspect crater in this
area. Deposits and erosion patterns produced by a gigantic tidal
wave have been found at the K-T boundary in Texas, Mexico and in
cores from the Gulf of Mexico.
"Our research," Duller said, "adds one more piece of
evidence to a complex and intriguing jigsaw puzzle. Many
researchers in different scientific fields have contributed to our
understanding of the Yucatan impact. Each additional piece brings
us closer to understanding one of the great mysteries in the
evolution of life on Earth."