THE EFFECT OF VOLCANIC ASH FALLS ON THE EVOLUTION OF DINOSAURS ON THE WEST SIDE OF THE WESTERN INTERIOR SEAWAY IN LATE CRETACEOUS TIME

Recent publications have demonstrated the existence of Paleocene dinosaurs in the San Juan Basin of northwest New Mexico and southwest Colorado and date the final demise of this Alamoan fauna at 64.5 Ma - about one million years into the Paleocene. It has been suggested that this Lazarus fauna may have survived the devastation following the end-Cretaceous, asteroid impact event in the Yucatan peninsula area as buried eggs that protected some of these embryonic dinosaurs after the impact. Because discrete ash beds and bentonitic shales are abundant in Upper Cretaceous strata in western North America, it is clear that dinosaurs (and other life forms) dwelling on the long, narrow, spit of land west of the Western Interior Seaway during Campanian time, especially those in the San Juan Basin area, were being subjected to a continuing succession of violent volcanic eruptions from eruptive centers a few hundred km to the west. The frequency of these eruptions is not known with certainty because many (most) of the resultant ash deposits are not been preserved in the rock record. Seven altered ash beds from 385 m of continental strata in the basin have been precisely dated from 75.76-73.04 Ma. These ash beds range from 18-46 cm thick; some ashes in the basin are more than 55-cm thick. Assuming even a modest compaction ratio of 10:1 for fresh-volcanic-ash/devitrified clay it is clear that these ash falls were meters thick – as much as 5.5 m for the thickest ash observed in these rocks! If we assume that the 7 dated ashes represent one-tenth of the eruptions that occurred during the 2.72 m.y., seventy such eruptions could have occurred during that time interval resulting in a frequency of one of these devastating events every forty-thousand years or so! These events, and their inevitable evolutionary consequences, must clearly have prepared the dinosaurs for the much more devastating end-Cretaceous event allowing some of them to live on into the Paleocene.