Unlikely Impact

The unexpected discovery of the Paleogene-Cretaceous impact crater

In January 1975, I was a senior studying geology at Oberlin College in Ohio, and Dr. Norman Grant suggested I travel to Guatemala, borrow a jeep from the Instituto Geográfico Nacional and go to Lake Atitlán to sample volcanoes. During the previous months, I had been using black-and-white air photo stereo pairs to create what became an unusual map of the three Lake Atitlán volcanoes. Their viscous andesites formed a fiendishly complex series of overlapping flows like the successive drippings of three enormous candles. Since I had developed a map and complex stratigraphy of the flows, Grant suggested I go to Guatemala and sample as many of them as I could during a semester break.

I worked the Atitlán volcanoes solo for four weeks, climbing the 3,400-meter Volcán Atitlán and creating the first detailed geologic map of the area (a decade later the GSA-Special Map and Chart Series published this undergraduate map). I returned with more than 100 samples from the stratigraphy I had developed, graduated five months later and left my Atitlán work and samples to Oberlin and Michigan Tech, where they were put to good academic use for a number of years and several publications.

Immediately after graduating in July 1975, I joined the Aero Service Division of Litton’s Western Atlas International in Houston. As the saying goes, “I was born elsewhere, but got to Texas as soon as I could.” Grad school was never a plan of mine. As the child of working-class parents, my goals were international travelling, studying the planet, and getting well paid for it. I was trained at WAI in the latest geophysical industry techniques and spent the bicentennial year in the Bering Sea basins as a quality-control geophysicist on major airborne surveys. My job was largely to look at the magnetometer ground station for solar disturbances that could compromise the airborne magnetometer records. Later, back in Houston, I interpreted the digital data for the St. George, Navarin, and Chukchi Sea basins with maps and Werner deconvolution profiles. During this period, I also worked on integrating airborne with other geological and geophysical data for major oil and mining surveys in Iran and the Americas. And, I joined the AAPG.

My geoscience mentors at Western were Jeffrey Frieberg, Wim Schuur and Peter Brennan. I studied X-band synthetic aperture radar data as Western/Litton flew the first commercial system aboard a Caravelle jet, used in the nationwide Radam Brazil program and in the dense jungle cover of the Petén of Guatemala, where we recognized bright white “corner reflectors,” which turned out to be remnants of Mayan walls and settlements never previously recognized.

The PEMEX Years: Finding Chicxulub

In January 1978 I moved to Mexico as the quality-control geophysicist for two aircraft crews, also charged with doing “field interpretation” and client liaison (to try to enlarge the program). The two crews began in the Sabinas Basin on the Texas border and Merida in the Yucatan. I would travel between them and the various PEMEX headquarters offices for the next three years as PEMEX enlarged the original 175,000-line-kilometers survey to over 510,000 line-kilometers, essentially a national survey. This took us to every state in Mexico.

We began the Yucatan survey grid by flying offshore north of Progresso with nearly 300 kilometers-long east-west traverse lines at a 450-meter altitude. Daily we would collect two to four east-west lines beginning north of the Alacranes Reef and moving south with each flight. By April, we had flown more than half of the offshore portion. The magnetic field was “quiet,” with only long wavelength, medium amplitude anomalies associated with deep igneous-metamorphic basement.

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In January 1975, I was a senior studying geology at Oberlin College in Ohio, and Dr. Norman Grant suggested I travel to Guatemala, borrow a jeep from the Instituto Geográfico Nacional and go to Lake Atitlán to sample volcanoes. During the previous months, I had been using black-and-white air photo stereo pairs to create what became an unusual map of the three Lake Atitlán volcanoes. Their viscous andesites formed a fiendishly complex series of overlapping flows like the successive drippings of three enormous candles. Since I had developed a map and complex stratigraphy of the flows, Grant suggested I go to Guatemala and sample as many of them as I could during a semester break.

I worked the Atitlán volcanoes solo for four weeks, climbing the 3,400-meter Volcán Atitlán and creating the first detailed geologic map of the area (a decade later the GSA-Special Map and Chart Series published this undergraduate map). I returned with more than 100 samples from the stratigraphy I had developed, graduated five months later and left my Atitlán work and samples to Oberlin and Michigan Tech, where they were put to good academic use for a number of years and several publications.

Immediately after graduating in July 1975, I joined the Aero Service Division of Litton’s Western Atlas International in Houston. As the saying goes, “I was born elsewhere, but got to Texas as soon as I could.” Grad school was never a plan of mine. As the child of working-class parents, my goals were international travelling, studying the planet, and getting well paid for it. I was trained at WAI in the latest geophysical industry techniques and spent the bicentennial year in the Bering Sea basins as a quality-control geophysicist on major airborne surveys. My job was largely to look at the magnetometer ground station for solar disturbances that could compromise the airborne magnetometer records. Later, back in Houston, I interpreted the digital data for the St. George, Navarin, and Chukchi Sea basins with maps and Werner deconvolution profiles. During this period, I also worked on integrating airborne with other geological and geophysical data for major oil and mining surveys in Iran and the Americas. And, I joined the AAPG.

My geoscience mentors at Western were Jeffrey Frieberg, Wim Schuur and Peter Brennan. I studied X-band synthetic aperture radar data as Western/Litton flew the first commercial system aboard a Caravelle jet, used in the nationwide Radam Brazil program and in the dense jungle cover of the Petén of Guatemala, where we recognized bright white “corner reflectors,” which turned out to be remnants of Mayan walls and settlements never previously recognized.

The PEMEX Years: Finding Chicxulub

In January 1978 I moved to Mexico as the quality-control geophysicist for two aircraft crews, also charged with doing “field interpretation” and client liaison (to try to enlarge the program). The two crews began in the Sabinas Basin on the Texas border and Merida in the Yucatan. I would travel between them and the various PEMEX headquarters offices for the next three years as PEMEX enlarged the original 175,000-line-kilometers survey to over 510,000 line-kilometers, essentially a national survey. This took us to every state in Mexico.

We began the Yucatan survey grid by flying offshore north of Progresso with nearly 300 kilometers-long east-west traverse lines at a 450-meter altitude. Daily we would collect two to four east-west lines beginning north of the Alacranes Reef and moving south with each flight. By April, we had flown more than half of the offshore portion. The magnetic field was “quiet,” with only long wavelength, medium amplitude anomalies associated with deep igneous-metamorphic basement.

One day, two adjacent lines showed weakly magnetic, but high frequency (shallow) anomalies in a small section of the middle of each profile, though they were flown more than an hour apart. At first suspecting two diurnal magnetic events, I closely checked the ground magnetometer record, one “bump” might have been diurnal noise but for the second line … nothing! The next day and each succeeding day, this zone of 2 to 5 nanoteslas anomalies grew in diameter, remaining centered in the same area.

Later, as the survey approached the coastline, anomalies of 50 to more than 250 nanoteslas began appearing at the center of the feature, with the small anomaly zone to the east and west. I depth-estimated and mapped the feature, and when we suspended flying to move positioning transponders for the upcoming onshore portion of the survey, consulted PEMEX colleagues and requested additional data from their files.

Gravity mapping of the Yucatan began in the 1940s, which led to the drilling of the first exploration wells in the early 1950s. The old data showed a large concentric set of onshore gravity anomalies. When I laid it next to my No. 2 pencil mapping of the offshore magnetic anomalies, the fit was perfect: a shallow, 180-kilometer diameter gravity-magnetic bullseye on the almost non-magnetic, uniform carbonate background of the Yucatan platform!

The detailed symmetry was completely unlike any volcanics I had studied in the previous three years, so despite the nearest wells – Chicxulub No. 1, Sacapuc No. 1, etc. – bottoming in shallow breccias and vitric-andesites, I knew this was not a volcanic edifice, but an impact like those I had observed on the moon as a hobby since childhood. We recognized the crater as the likely Cretaceous-Paleogene boundary event.

‘Glen’s Sky Rock’

My undergraduate acquaintance with continental vulcanism and the following three years looking at many volcanic signatures around the world with ultra-high sensitivity alkali vapor magnetometer data was critical to being able to recognize the Yucatan crater as an impact, despite learned volumes and well logs that said it was the “Merida Volcanics.” This “Merida Volcanics” claim was cited first by PEMEX geoscientists, and then for more than 12 years by the academic impact community and NASA experts (except for one) as reason to ignore or ridicule my 1978 assertion that the igneous center in the northern Yucatan was in fact a pristine, buried 180-kilometer wide impact crater.

This recognition of the significance of the crater preceded the Alvarez and others’ description of the iridium anomaly (and the subsequent search for a crater) by two and a half years, and resulted from familiarity with some of the work of Nobel Prize-winning physicist Harold Urey who had speculated on mass extinctions and cometic collisions in Earth history.

With regard to its Cretaceous-Paleogene age, rather than being a slightly older feature as well data suggested (and as Princeton academics have agonized over for more than 25 years), I adopted right from the beginning the Texas/Oklahoma saying (sometimes seen in remote places) that “There’s no other place like this place, anywhere near this place, so this must be the place.”’ This was a paraphrase of Einstein’s maxim that “Everything should be made as simple as possible, but not simpler,” or of Occam’s Razor from the 1300s. It was the simplest, most elegant explanation for extinctions, as Urey had suggested in his 1973 Nature article.

In August of 1978, I presented the Yucatan field report to PEMEX, outlining the diameter, anomaly zones, central basement-rebound uplift, and likely Cretaceous-Paleogene age of the feature, all to good-natured laughter about “Glen’s Sky Rock,” even from Tony Camargo, my PEMEX project supervisor, later credited as a co-discoverer. One reason was the recent appearance of the authoritative Volume III of Lopez-Ramos’s “Geologia de Mexico,” which with well logs and maps immortalized (temporarily, as it has turned out) this feature as the “Merida Volcanics.” Another reason for their rejection was the opinion sought by PEMEX from a world-famous seismologist who, after reviewing the sparse 2-D seismic in the area, reportedly said, “I don’t see any crater there.”

Going Public

By 1981, the work by Alvarez and others on iridium had the world looking for 200-kilometer-wide craters. I thought Tony Camargo might co-author a presentation as a way for him to go to the Society of Exploration Geophysicists annual meeting in Los Angeles, so I offered two options: “the crater,” which he still suspected was wrong-headed, and another spectacular discovery made in 1981 during detailed study of the PEMEX digital data in Houston. Using proprietary software, I identified the precise location and depth of a more than 300-kilometer buried segment of the Western Main Transform Fault. The WMT is a dextral fault like the modern San Andreas, on which the Gulf of Mexico opened in the Middle to Late Jurassic. I sent abstracts of both topics to Tony; the answer was that we could present the Yucatan material, leaving the word “crater” out of the title and not “giving away” any PEMEX data.

Response on the other option was quite exceptional. Tony’s bosses reportedly said, “You must promise to never mention this feature again, nor ever publish anything about it!” I’ve kept this secret for 38 years, but thanks to recent brilliant geophysical detective work at the University of Houston (Nguyen and Mann, 2016) it’s out of the bag. The WMT passes through basement of the Papaloapan Basin, going offshore near a place called Laguna Verde. It can be located as precisely on our original work as if exposed at the surface. Laguna Verde, still under construction in 1981, is today the only operating nuclear electric plant in Mexico, and indeed it is located almost exactly above one of the largest (inactive) strike-slip faults on the planet.

Our SEG session made the Sunday front page of the Houston Chronicle, Dec. 13, 1981. In March 1982, an interview in Sky & Telescope magazine laid out the discovery and its significance. I brought the SEG materials to the attention of NASA and others within academia. There was no positive response – only amused smiles. After all, the “Geologia de Mexico” said it was volcanics.

There was one exception: Dr. Bill Phinney, curator of the lunar rocks at the Johnson Space Center, encouraged me to find samples from the 30-year-old wells, because the “vitric-andesite” described by the PEMEX well-site geologists of the 1950s was precisely the sort of “impact melt rock” expected from the Yucatan Platform rocks.

Vindication

A two-year odyssey began as I returned to Mexico in search of samples. I was told that the repository of cores for southeast Mexico in Coatzacoalcos had burned down the year before and the site was bulldozed. Also, so-and-so had “a strange looking volcanic core” from the Yucatan wells he used as a doorstop in his office for years but had retired, taking his doorstop with him. Finally, I returned to Chicxulub and Sacapuc, but neither visit yielded anything. Even the original low-altitude, ultra-high-resolution magnetic data were lost forever shortly afterward as a Mexican government agency produced an upward continued “National Map” at more than 3,500 meters altitude and rewrote the 450 meter acquisition tapes.

I left this quest for eight years until a grad student, Alan Hildebrand from the University of Arizona, called me in 1990. He was about to publish a paper with his adviser on the Cretaceous-Paleogene crater being perhaps in the Colombian Basin in northern South America, but he added a new final sentence after I assured him that we knew – and knew exactly – where it was on the Yucatan.

With Alan’s help, through the academic grapevine, we located some samples. I recovered them from a closet at the University of New Orleans, where they’d been sitting since 1965. I sent them to Alan to identify the shocked quartz that confirmed the impact. My Mayan wife, Erendira, and I, along with Alan, decided to name the crater after the town of Chicxulub located near its center, partly to give the academics and NASA naysayers a challenging time pronouncing it after a decade of their dismissals, since “Yucatan crater” was too easily pronounced.

Incidentally, “Chicxulub” is a Yucatec Maya word from the roots “ch’ik,” meaning “flea” or “tick,” and “xulub,” meaning “devil,” “demon,” or “horns.”

We originally submitted the article with the shocked quartz evidence to Nature in the United Kingdom in 1990, where it was rejected. I later learned that a “prominent NASA expert” was one of the rejecting reviewers, with his comment that “the only thing original in this paper is the unpronounceable name ‘Chicxulub.’”

GSA’s journal Geology published it shortly afterward, and Chicxulub is now widely accepted as the site of the impact that caused the Cretaceous-Paleogene extinction event.

To widespread surprise, the crater was discovered with no computer visualization, no colored maps, or supercomputers, not by NASA or academic scientists, but instead was found by a 26-year-old field geophysicist with a No. 2 pencil, drafting triangles, rolls of 1:30,000 chart recorder paper, a Texas Instruments hand calculator, and dual passions for astronomy and petroleum industry ultra-high-quality geophysical data.

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