No. 75: May-Jun 1991
We already know the victims (the dinosaurs and other fauna and flora), and there is considerable evidence that the bullet was a cosmic projectile of some sort. The absence of a smoking gun (a sufficiently large terrestrial crater with an age of 65 million years) has allowed volcanists to deny the cosmic catastrophists a complete victory. However, the recent identification of tektite-like glasses at the Cretaceous-Tertiary boundary (KTB) on Haiti is leading geological detectives closer and closer to the missing crater.
Elsewhere in the world, the KTB is characterized by an iridium anomaly and a thin layer of "impact clay" consisting of tiny bits of shocked minerals. At Beloc, on Haiti, though, geologists find a 55-centimeter-thick layer of glassy debris. Approximately 25% of this stra tum consists of 1-6-millimeter particles of tektite-like glass. Most of the glass particles are spherical, but a few have the splash-forms and dumbbell shapes of bona fide tektites. The thickness of the Haitian deposit and the large sizes of the particles suggest that the smoking gun must be nearby. Ironically, the Haiti stratum was originally classified as of volcanic origin; and we must add that we are presenting here only the conclusions of the asteroid school.
But where oh where is this crater? The Manson crater in Iowa (now buried) is of the right age but too small. The best candidate so far is buried in northern Yucatan. The Chicxulub crater is discernible on gravity- and magneticanomaly maps and is probably of the right age. Only drilling will confirm the guilt of the suspect.
Even if Chicxulub is the culprit, much debate prevails over exactly how the dinosaurs were done in. Was it a "cosmic winter" due to dust intercepting sunlight? Or perhaps a "cosmic summer" resulting from a super-greenhouse effect caused by: (1) impact-released methane trapped in sediments, and (2) the CO2 from zapped carbonate rocks. (Smit, Jan; "Where Did It Happen?" Nature, 349:461, 1991, and Sigurdsson, Haraldur, et al; "Glass from the Cretaceous/Tertiary Boundary in Haiti," Nature, 349:482, 1991.)
Reference. Section ETC in catalog Carolina Bays, Mima Mounds is devoted entirely to impact craters. Ordering data here.