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 worlds 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 Earths 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 Severs 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
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 Earths 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
Believe your compass,
Samuel S. Adams