The Death of the Dinosaurs:
UNO Department of Geology's Contribution

by Dale H. Easley¹
Department of Natural and Applied Sciences
University of Dubuque

8/31/2000

A widely accepted hypothesis about the cause of the extinction of the dinosaurs at the end of the Cretaceous Period is an impact of a meteorite or comet (bolide) in the Yucatan Peninsula, Mexico. The bolide is believed to have been about 6 miles in diameter, impacting the Yucatan at a low angle, vaporizing rock, setting off catastrophic fires and tidal waves, and blanketing the Earth with dust, which led to acid rain and a plunge in global temperatures. The angle was such that central and western North America were particularly ravaged. As the dust settled out of the atmosphere, large amounts of water vapor remained, causing a rapid rise in temperature to follow the cooling. Thus the dinosaurs and other Cretaceous life were hit with a one-two punch that caused massive annihilation. The acceptance of this hypothesis is relatively new---the previous edition of our historical geology book doesn't even mention it. Evidence stored at the UNO Geology Department helped provide the support that led to acceptance.

Former professor Al Weidie has had a Mexican connection dating back to his graduate school days working on Gulf Coast geology. (As part of his student accomplishments, he brought home a lovely wife, Ana, from Saltillo, Mexico.) After coming to UNO and helping start the Department of Earth Sciences, Dr. Weidie convinced PEMEX, the Mexican petroleum company, to ship him some cores from wells that PEMEX drilled in the Yucatan Peninsula. (Cores are samples of the rock being drilled through.) When Dr. Bill Ward joined the faculty in 1970, the cores were available for a thesis project by his first graduate student, Bob Marshall.

Cores from near the Cretaceous-Paleogene boundary proved to be difficult to understand. A breccia there was particularly problematic, causing some to accuse Dr. Ward of looking at concrete that had been drilled out in the wells. However, in the breccia were fragments of anhydrite, limestone, and dolomite, and the matrix was dolomitized. Dr. Ward reasoned that all the anhydrite chunks and splinters in the breccia, which spread across the peninsula, would amount to a lot of anhydrite. Its likely source was the Middle Cretaceous, deeper underground. How could it have gotten to its present location? Anhydrite is very soluble, so it had to have been moved quickly and without a lot of water. The mechanism Dr. Ward hypothesized at the time was block faulting. Years later, electric logs and seismic would show that this explanation couldn't be supported.

In the years after Marshall's thesis, a high concentration of iridium at the Cretaceous-Paleogene boundary was found in many places around the globe. The likely source of iridium is extraterrestrial, in comets or meteorites. Its wide distribution at a single time indicated the possibility of a massive impact. The timing also corresponded with the end of the Age of Reptiles. The hunt was on for a large crater caused by the impact. Geophysical evidence indicated the presence of such a crater in the subsurface of the Yucatan Peninsula.

Geologists seldom believe geophysicists unless there is supporting evidence from rock data. The PEMEX wells were known to exist, including one near the center of the proposed impact crater, but the cores from them were supposedly lost. However, someone came across a reference to Marshall's thesis and contacted UNO. In the cores, they found shocked crystals of quartz that have been impacted violently like hitting them with a hammer. Other evidence soon followed, including finding glass microspherules throughout the Caribbean and as far away as Wyoming. In 1995, Dr. Ward and others authored an article that appeared in Geology, presenting much of the petrographic evidence from the subsurface sedimentary rocks for the bolide impact, including a thin section of anhydrite fragments wrapped in altered glass.

As part of a long series of programs on dinosaurs narrated by Walter Cronkite, there was a segment on dinosaur extinction and its possible relation to the Yucatan impact. A camera crew came to UNO and filmed a geologist going into the lab where the cores were stored, across the hall from Dr. Ward's office. The geologist had a flashlight in his hand, looking like an archaeologist entering King Tut's tomb. Suddenly he discovered the long-missing cores. Of course, the geologist passed up a light switch at the door. It looked as if UNO didn't pay the electric bill. Yet there is no doubt that the missing cores helped shed light on the cause of the extinction of the dinosaurs.

Footnotes:

¹Many thanks to Dr. Bill Ward for his recounting to me the history of UNO's involvement in studying the Yucatan Impact Crater.

Additional Reading:
AGU article on the impact crater
A hypercard representation of the impact
NASA image of the impact crater