No. 85: Jan-Feb 1993
Only a few years ago astronomical catastrophism was denied as a major factor in geological change. Now one reads everywhere of huge terrestrial impact craters, iridium layers, and tektite deposits. Even so, one necessary consequence of 70% of the large impacts has been neglected until recently: the giant tsunamis and marine incursions that must have swept over the coasts of the continents following impacts at sea. This oversight is now being rectified.
100,000 BP. The South Pacific. A longstanding geological enigma of the New South Wales coastline is the curious distribution of sand dunes. Those headlands less than 40 meters high have lost most of their dunes, leaving only raw, unweathered rock. On the other hand, the higher headlands have retained these dunes. Australians B. Young and T. Bryant hypothesize that a tsunamis 40 meters high swept the lower headlands clean about 100,000 years ago. They can even plot the incoming wave's direction, because a few remnants of the coastal dunes still cling to the southwest corners of the headlands along the NSW coast south of Newcastle. In their scenario, the tsunamis came from the northeast, smashed into the Solomons, southeastern Australia, and northeastern New Zealand. The Great Barrier Reef protected northeastern Australia from the full force of the wave. Young and Bryant favor a Hawaiian landslip as the initiator of the tsunamis, but acknowledge that an asteroid impact could also have done the job. If the wave began near Hawaii, it would initially been about 375 meters (about ¼ mile) in height. (Davidson, Garry; "A Tsunamis Tale from Sydney," New Scientist, p. 17, October 17, 1992.)
65,000,000 BP. Northeastern Mexico. The date mentioned is, of course, that of the Cretaceous-Tertiary boundary. This is the time when, many scientists believe, a very large asteroid slammed into northern Yucatan, forming the now-buried Chicxulub crater and wiping out the dinosaurs. Since the impact site was covered with ocean at the time, a powerful tsunamis should have surged out from this area. Indeed, debris attributable to a tsunami has been found on the U.S. Gulf Coast and on some Caribbean islands. J. Smit et al now report finding a layer of debris up to 3 meters thick in northeastern Mexico. This layer was apparently deposited in water about 400 meters deep as the giant wave wreaked havoc along Mexico's shore and its backwash piled up debris offshore. This interpretation is supported by the presence of tektites, microtektites, glass spherules, abundant plant material, an iridium anomaly, and near the top ripple beds. (Smit, Jan, et al; "Tektite-Bearing, Deep-Water Clastic Unit at the Cretaceous-Tertiary Boundary in Northeastern Mexico," Geology, 20:99, 1992.
Reference. Puzzling deposits that may have been created by marine incursions are covered in ETM12 in our catalog: Neglected Geological Anomalies. For details, visit: here.