Geotimes

From the Editor

Earth’s three largest know impact structures are not simply big circular patterns with tributary topographic and subsurface geologic features. The Vredefort Dome in South Africa is partly ringed by the Witwatersrand gold deposits (Geotimes, December 2003), the largest accumulation of that metal on Earth. The Sudbury structure in Canada coincides with one of the world’s greatest nickel districts, and the Chicxulub crater in Mexico is of the same age as the most famous extinction event that the planet has known.

As self-evident as these coincidences are, their geological significance is the subject of intense research and debate. Add to these structures the Chesapeake Bay crater, with its associated world-class harbors and fisheries, and it seems not unreasonable to conclude that extraterrestrial bodies have had more than passing impact on Earth’s geology and its significance to us. In this issue, we survey current understandings of some of these collisions that are forging a new body of geologic knowledge.

“Unraveling the Chicxulub Case” is Staff Writer Megan Sever’s account of the activities of the world-wide research consortium that planned, executed and is now interpreting the results from the 2001-2002 drilling program into the Chicxulub crater. Researchers believe that 65 million years ago a 10- to 15-kilometer-wide extraterrestrial projectile slammed into the Gulf of Mexico with “the force of more than 100 million megatons of TNT,” producing a crater 180 kilometers in diameter. As close as we can tell, this event coincides temporally with the disappearance of 75 percent of all species and 50 percent of genera, including the dinosaurs at the Cretaceous/Tertiary (K/T) boundary. The recovered drill core is providing a wealth of geologic complexity to be resolved with “ejecta” from equally complex computer simulations of the impact. So begins the serious search for the most-likely climate scenario to account for so much death and destruction.

Chesapeake Bay is now widely recognized as the site of a 36-million-year-old impact. In “Coring the Chesapeake Bay Impact Crater,” C. Wiley Poag describes various aspects of this impact, the sixth-largest on Earth and the largest in the United States. He summarizes the results of core drilling within the crater that identified the dominant fill as the colorfully named slumpback, surgeback and washback deposits. These relatively new terms to the earth sciences come to life in computer models of the first several seconds following the impact. Research now focuses on understanding the effects of the structure on modern aquifers in southeastern Virginia, where the crater marks an area of unusually salty groundwater.

In sharp contrast to the on-site studies of the Chesapeake Bay and Chicxulub impact structures, John Warme pieces together evidence of a 370 million-year-old impact based on widely dispersed breccia sediments, with no crater yet in sight. In “The Many Faces of the Alamo Impact Breccia,” he describes the dawning recognition of the unusual nature of a previously recognized but unappreciated limestone breccia in the Devonian platform carbonates of southern Nevada and adjacent Utah. Energized by discoveries and interpretations surrounding the K/T boundary and the thickness, coarseness and aerial extent of their breccia, Warme, together with his colleagues and students, looked for and ultimately found compelling evidence of impact: shocked quartz and then limestone lapilli and bombs formed ballistically in the impact cloud during the Alamo event. Continuing work is building a framework for interpreting processes related to a wet impact on a continental shelf, and finding the elusive crater.

No geological discussion of impacts would seem complete without recognizing the role of Gene and Carolyn Shoemaker. In this instance, Lisa Pinsker, our managing editor, interviewed Carolyn, an astronomer at the Lowell Observatory in Flagstaff. Not only was she co-discoverer with her husband Gene and David Levy of the Shoemaker-Levy 9 comet, which impacted Jupiter in 1994, she has discovered more comets that anyone else alive today. Carolyn spoke with Lisa about the years of field work she and Gene conducted in Australia, during which they identified 22 structures. Gene died in a car accident while conducting field work near Alice Springs, Australia. The Shoemakers are a rich, warm and nostalgic part of the heart of astrogeology.

As rare and improbable as large impacts are on Earth, they seem to have disproportionate significance in Earth’s history and in our sense of what has happened. Perhaps it is nothing more than the heuristics of improbable and recent events. Whichever, this is one geologic event we should “thank our lucky stars” we never see.

Believe your compass,

Samuel S. Adams
Geotimes Editor-in-Chief


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