Recent Achievements of the K-T (Cretaceous -Tertiary) Boundary Researches: Review and discussion

Abstract

The discovery of a gigantic crater, of which buried circular structure is located at Chicxulub, on the north coast of Yucatan, Mexico, has gained wide acceptance that the crater is probably the long-sought K-T boundary impact site. Its size and shape were disclosed by Bouguer gravity anomalies, showing a 180-km-diameter circle associated with a central high and two internal concentric lows (Hildebrand et al., 1991). Shocked quartz grains and spherules from core samples, together with the anomalous concentration of iridium (Ir), are the best evidence to show that all these are the impact products. Moreover, ⁴⁰Ar/³⁹Ar ages obtained from glassy melt rocks of cores are 65.2±0.4 Ma (Sharpton et al., 1992) and 64.98±0.05 Ma (Swisher et al., 1992) respectively, irrefutably displaying the time of K-T boundary. Some unusual sedimentary structures at the K-T boundary sections around the Gulf of Mexico are compatible with the theory of nearby (Chicxulub) impact.
The cause of the dramatic extinction event at the K-T boundary has been ascribed either to the impact of a large extraterrestrial material or to the extensive volcanism. However, characteristics of lamellar deformation in shocked quartz are clearly distinguishable from those in quartz of volcanic (eruptive) origin (Alexopoulos et al., 1988). Critics who prefer the world-wide volcanism to the bolide impact have been unable to explain reasons why the eruptive-type deformed quartz grains do not exist at the boundary sections.
Although mass extinctions have occurred many times during the Phanerozoic time, some workers now believe that the K-T boundary extinction might have been the only probable case by the bolide impact (e.g., Hallam, 1989). Other mass extinctions, like P-Tr (Permian-Triassic) extinction, might have been caused by the substantial sea-level change, variable sea water temperature and CO₂ content in atmosphere. These factors are essentially controlled by plate tectonics, but not by the bolide impact.