EVIDENCE OF SOUTHWESTERN QUATERNARY VOLCANISM AT CHACO CULTURE NATIONAL HISTORIC PARK, NEW MEXICO

The mysteries of the people who inhabited hot, arid Chaco Canyon just over 1000 years ago have long stumped archaeologists. Although the large masonry structures, known as Great Houses, suggest a relatively large population, it is difficult to imagine that the marginal environment of the canyon could support the agriculture necessary to feed such a population. Intensive analysis of a sedimentary core recovered from the lower end of the canyon provides a small clue in understanding this mystery. The entire core, which is 260 cm, contains variable quantities of smectitic clay and relatively unweathered euhedral mineral grains such as zircon, apatite, and biotite, all evidence of ash fallout from felsic volcanic eruptions in the southwestern United States. The mineralogy of these grains, in combination with a thorough investigation of well-documented and widely dispersed volcanic eruptions in the region reveal that the volcanogenic material is likely a mixed and reworked combination of materials from the 0.759±0.002 Ma Bishop Ash of the Long Valley Caldera in California, the 2.09 ± 0.01 Ma Huckleberry Ridge Ash Bed and 0.60 ± 0.01 Ma Lava Creek B Ash bed of the Yellowstone Volcanic Field, and the 1.14 ± 0.02 Ma Tshirege Member of the Bandelier Tuff from the Jemez Mountain Volcanic Field. While these eruptions would have occurred far before Chaco Canyon was inhabited and would not have had a direct effect on daily life, small quantities of volcanic ash are known to improve the soil fertility. This, in combination with a series of complex water management planning and control features, could have improved the fertility of the soil at Chaco Canyon and produced a marginally better environment for agriculture to support a larger population. Thus, large volcanic eruptions which could have had a harmful effect on a thriving culture had they occurred during habitation instead provided a highly enriched soil that permitted the development of a strong and successful agricultural base some thousands of years later.